Design for additive manufacturing

Design for additive manufacturing (DfAM or DFAM) is design for manufacturability as applied to additive manufacturing (AM). It is a general type of design methods or tools whereby functional performance and/or other key product life-cycle considerations such as manufacturability, reliability, and cost can be optimized subjected to the capabilities of additive manufacturing technologies.
This concept emerges due to the enormous design freedom provided by AM technologies. To take full advantages of unique capabilities from AM processes, DfAM methods or tools are needed. Typical DfAM methods or tools includes topology optimization, design for multiscale structures (lattice or cellular structures), multi-material design, mass customization, part consolidation, and other design methods which can make use of AM-enabled features.
DfAM is not always separate from broader DFM, as the making of many objects can involve both additive and subtractive steps. Nonetheless, the name "DfAM" has value because it focuses attention on the way that commercializing AM in production roles is not just a matter of figuring out how to switch existing parts from subtractive to additive. Rather, it is about redesigning entire objects (assemblies, subsystems) in view of the newfound availability of advanced AM. That is, it involves redesigning them because their entire earlier design—including even how, why, and at which places they were originally divided into discrete parts—was conceived within the constraints of a world where advanced AM did not yet exist. Thus instead of just modifying an existing part design to allow it to be made additively, full-fledged DfAM involves things like reimagining the overall object such that it has fewer parts or a new set of parts with substantially different boundaries and connections. The object thus may no longer be an assembly at all, or it may be an assembly with many fewer parts. Many examples of such deep-rooted practical impact of DfAM have been emerging in the 2010s, as AM greatly broadens its commercialization. For example, in 2017, GE Aviation revealed that it had used DfAM to create a helicopter engine with 16 parts instead of 900, with great potential impact on reducing the complexity of supply chains. It is this radical rethinking aspect that has led to themes such as that "DfAM requires 'enterprise-level disruption'." In other words, the disruptive innovation that AM can allow can logically extend throughout the enterprise and its supply chain, not just change the layout on a machine shop floor.
DfAM involves both broad themes (which apply to many AM processes) and optimizations specific to a particular AM process. For example, DFM analysis for stereolithography maximizes DfAM for that modality.
Background.
Additive manufacturing is defined as a material joining process, whereby a product can be directly fabricated from its 3D model, usually layer upon layer. Comparing to traditional manufacturing technologies such as CNC machining or casting, AM processes have several unique capabilities. It enables the fabrication of parts with a complex shape as well as complex material distribution. These unique capabilities significantly enlarge the design freedom for designers. However, they also bring a big challenge. Traditional Design for manufacturing (DFM) rules or guidelines deeply rooted in designers’ mind and severely restrict designers to further improve product functional performance by taking advantages of these unique capabilities brought by AM processes. Moreover, traditional feature-based CAD tools are also difficult to deal with irregular geometry for the improvement of functional performance. To solve these issues, design methods or tools are needed to help designers to take full advantages of design freedom provide by AM processes. These design methods or tools can be categorized as Design for Additive Manufacturing.
Methods.
Topology optimization.
Topology optimization is a type of structural optimization technique which can optimize material layout within a given design space. Compared to other typical structural optimization techniques, such as size optimization or shape optimization, topology optimization can update both shape and topology of a part. However, the complex optimized shapes obtained from topology optimization are always difficult to handle for traditional manufacturing processes such as CNC machining. To solve this issue, additive manufacturing processes can be applied to fabricate topology optimization result. However, it should be noticed, some manufacturing constraints such as minimal feature size also need to be considered during the topology optimization process. Since the topology optimization can help designers to get an optimal complex geometry for additive manufacturing, this technique can be considered one of DfAM methods.
Multiscale structure design.
Due to the unique capabilities of AM processes, parts with multiscale complexities can be realized. This provides a great design freedom for designers to use cellular structures or lattice structures on micro or meso-scales for the preferred properties. For example, in the aerospace field, lattice structures fabricated by AM process can be used for weight reduction. In the bio-medical field, bio-implant made of lattice or cellular structures can enhance osseointegration.
Multi-material design.
Parts with multi-material or complex material distribution can be achieved by additive manufacturing processes. To help designers to take use of this advantage, several design and simulation methods has been proposed to support design a part with multiple materials or Functionally Graded Materials . These design methods also bring a challenge to traditional CAD system. Most of them can only deal with homogeneous materials now.
Design for mass customization.
Since additive manufacturing can directly fabricate parts from products’ digital model, it significantly reduces the cost and leading time of producing customized products. Thus, how to rapidly generate customized parts becomes a central issue for mass customization. Several design methods have been proposed to help designers or users to obtain the customized product in an easy way. These methods or tools can also be considered as the DfAM methods.
Parts consolidation.
Due to the constraints of traditional manufacturing methods, some complex components are usually separated into several parts for the ease of manufacturing as well as assembly. This situation has been changed by the using of additive manufacturing technologies. Some case studies have been done to shows some parts in the original design can be consolidated into one complex part and fabricated by additive manufacturing processes. This redesigning process can be called as parts consolidation. The research shows parts consolidation will not only reduce part count, it can also improve the product functional performance. The design methods which can guide designers to do part consolidation can also be regarded as a type of DfAM methods.
Lattice structures.
Lattice structures is a type of cellular structures (i.e. open). These structures were previously difficult to manufacture, hence was not widely used. Thanks to the free-form manufacturing capability of additive manufacturing technology, it is now possible to design and manufacture complex forms. Lattice structures have high strength and low mass mechanical properties and multifunctionality. These structures can be found in parts in the aerospace and biomedical industries. It has been observed that these lattice structures mimic atomic crystal lattice, where the nodes and struts represent atoms and atomic bonds, respectively, and termed as meta-crystals. They obey the metallurgical hardening principles (grain boundary strengthening, precipitate hardening etc.) when undergoing deformation. It has been further reported that the yield strength and ductility of the struts (meta-atomic bonds) can be increased drastically by taking advantage of the non-equilibrium solidification phenomenon in Additive Manufacturing, thus increasing the performance of the bulk structures.
Thermal issues in design.
For AM processes that use heat to fuse powder or feedstock, process consistency and part quality are strongly influenced by the temperature history inside the part during manufacture, especially for metal AM.
Thermal modelling can be used to inform part design and the choice of process parameters for manufacture, in place of expensive empirical testing.
Optimal design for additive manufacturing.
Additively manufactured metallic structures with the same (macroscopic) shape and size but fabricated by different process parameters have strikingly different microstructures and hence mechanical properties. The abundant and highly flexible AM process parameters substantially influence the AM microstructures. Therefore, in principle, one could simultaneously 3D-print the (macro-)structure as well as the desirable microstructure depending on the expected performance of the specialized AM component under the known service load. In this context, multi-scale and multi-physics integrated computational materials engineering (ICME) for computational linkage of process-(micro)structure-properties-performance (PSPP) chain can be used to efficiently search an AM design subspace for the optimum point with respect to the performance of the AM structure under the known service load. The comprehensive design space of metal AM is boundless and high dimensional, which includes all the possible combinations of alloy compositions, process parameters and structural geometries. However, always a constrained subset of the design space (design subspace) is under consideration. The performance, as the design objective, depending on the thermo-chemo-mechanical service load, may include multiple functional aspects, such as specific energy absorption capacity, fatigue life/strength, high temperature strength, creep resistance, erosion/wear resistance and/or corrosion resistance. It is hypothesized that the optimal design approach is essential for unraveling the full potential of metal AM technologies and thus their widespread adoption for production of structurally critical load-bearing components.


Barış Tan

Barış Tan is a Turkish business and engineering academic. He is a Professor of Operations Management and Industrial Engineering at Koç University, Turkey. He is most known for his research in the areas of production systems, supply chain management, and operations management. He served as the vice president of academic affairs, dean of College of Administrative Sciences and Economics and Director of Graduate School of Business at Koç University.
Tan has published over 100 papers and book chapters. He is the recipient of several best paper awards, and has been awarded Turkish Academy of Sciences Distinguished Young Scholar Award, TUBITAK fellowship, and NATO Science Fellowship. He is the manufacturing area editor of "Flexible Services and Manufacturing Journal" and associate editor of IISE Transactions. He also serves in for-profit and non-profit organizations as a board member.
Education.
Tan received his Bachelor’s degree in Electrical and Electronics Engineering from Boğaziçi University in 1990. He then moved to the United States, earning his M.E. degree in Industrial and Systems Engineering in 1991, M.S.E. Graduate Certificate in Manufacturing Systems in 1993, and Ph.D. in Industrial and Systems Engineering in 1994, at the University of Florida.
Career.
Tan began his academic career as a Research Assistant in Industrial Research Laboratory at the University of Florida in 1991 during his PhD studies. In 1994, upon moving back to Turkey, he joined Koç University as an Assistant Professor in College of Administrative Sciences and Economics. He was promoted to Associate Professor in 1998, and became a Professor in 2002. He has also been serving there as a Professor of Industrial Engineering since 2014.
During his tenure at Koç University, Tan held several administrative appointments. He was the Dean of the College of Administrative Sciences and Economics during 2007-2013. Followed by this role, he served as Vice President for Academic Affairs from 2013 till 2021. He served twice as Director of Graduate School of Business (2003-2005 and 2008-2010).
Tan is a member of the advisory boards of Nottingham University Business School (United Kingdom), and Kyoto University Graduate School of Management (Japan). He is also an independent board member of Anadolu Efes, Migros, and Anadolu Isuzu. Since 1993, he has been a member of INFORMS. Moreover, he has served as the chair of the ISM University of Management and Economics Senate and on the boards of CEMS Global Alliance in Management Education, European Foundation for Management Development, EQUIS, Turkish Quality Association, and Turkish Operations Research Society.
Tan was a Fellow at the University of Cambridge – Judge Business School in 2013, and was a Visiting Professor at the University College London – School of Management from 2017 till 2019. Currently, he serves as a Visiting Professor at Politecnico di Milano, and Senior Research Fellow for ERA Project IN4ACT at Kaunas University of Technology. He has also been a visiting faculty at Harvard University and MIT
Research.
Tan has focused his research on design and control of manufacturing systems, business model innovation, supply chain management, and stochastic modelling. His research has been supported by TUBITAK, European Union Horizon 2020 and Horizon Europe programs.
Design and Control of Production Systems.
Tan developed analytical methods to determine performance measures (such as the throughput, distribution of amount of materials produced and cycle time related to material flow in production systems) to design and control of production systems. He developed analytical methods for the analysis of systems in which the material flow in production systems is modeled as a fluid. Tan also focused his research onto developing policies to control production resources dynamically to meet a random demand.
In 2005, he co-edited a book, "Stochastic Modeling of Manufacturing Systems", and explored the development and analysis of performance evaluation models of manufacturing systems while utilizing decomposition-based methods, Markovian and queuing analysis, and inventory control approaches. His 2013 edited book presented the state of the art in stochastic modeling of manufacturing systems, with a particular emphasis on critical stochastic performance analysis as well as integrated optimization models of these systems.
More recently, he focuses on the modeling and data-driven control of production for energy efficiency. He developed an approach for data-driven modeling and analysis of manufacturing systems that combines analytical modeling, optimization, simulation and machine learning methodologies using high-performance computing.
Supply Chain Management.
Tan’s research in supply chain management focused on developing dynamic control policies for the use of production resources at different costs to meet variable demand, data-driven inventory management, and developing and analyzing cooperation-based business models. In early 2000s, he investigated the strategy of increasing production capacity temporarily through contingent contractual agreements with short-cycle manufacturers. He devised optimal control policies to use subcontractors together with the existing resources.
Tan has also worked on developing and analyzing cooperation-based business models. In his work related to data-driven inventory management, he developed methods to manage inventory accounting for substitution by using POS data. Tan also investigated the multi-product newsvendor problem while utilizing Value at Risk (VaR) as the risk measure in a newsvendor framework.
Stochastic Modelling.
Tan developed stochastic models and analysis methods for the solution of different decision problems. Tan's research in the areas of stochastic modelling includes models developed for determining the risks of accidents caused by oil tankers passing through the Bosphorus, determining the agricultural planting planning that takes into account the risks of harvest, yield and demand, developing new methods for testing the efficiency of stock markets, developing a new modeling approach to compare the growth dynamics in emerging markets, and presenting modeling and analytical approaches for different decision problems such as choosing energy saving methods in buildings, developing new business models for financing energy saving investments, and launching and pricing new products.


Hobby injection molding

Hobby injection molding machines, also known as benchtop injectors, hold molds on a smaller scale. Benchtop injectors have become more common as inexpensive CNC milling machines have reduced the cost of producing molds in a home workshop.
In hobby injectors injection pressure is generated manually by the operator, with a lever or gear translating the operator's effort to the required pressure. The most common hobby injection machine uses a handle to press down with. This enables the user to generate roughly of downward force, through the use of leverage.
History.
It is not known when the first hobby injection molder was constructed. Before the development of inexpensive CNC milling machines, producing a metal mold was prohibitively expensive for most hobbyists. With a small CNC mill and personal CAD tools, though, even complex shapes can be cut easily and accurately.
Applications.
Hobby injection molding has a variety of applications including the creation of low cost prototypes, new inventions, replication of lost or broken parts, and provides homeowners the opportunity to build anything. Hobby injection molding is a low cost method of repeatable production.
Materials.
Polyethylene (both LDPE and HDPE), polypropylene, and polystyrene (including HIPS) have all been used successfully with lever-actuated benchtop injectors.
Equipment.
Benchtop injectors are smaller and simpler than their larger industrious counterparts because they rely on the operator to manually inject melted polymer into the mold and remove the finished part from the mold. Production injectors automatically inject melted polymer at a prescribed rate into the mold, cool the mold to rapidly solidify the polymer, then eject the part from the mold once it's cool. The two halves of the mold must be pressed together with great force to prevent a flash in the part where the two halves meet, and the nozzle of the injector must be pressed tightly against the inlet port of the mold to prevent the escape of melted polymer and a defect in the finished part. In a benchtop injector this is done manually by clamping or bolting the mold together and clamping the complete mold into the injector. In a production injector this is accomplished with hydraulic or pneumatic actuators, which increase the cost of the machine but dramatically reduce the labor required to produce a finished part.
Molds.
Metal molds.
Low cost benchtop CNC milling machines allow home enthusiasts to machine molds out of softer metals. Rather than P20 tool steel, most grades of aluminum can be machined into working molds capable of 1000 plus cycles. Mic 6 cast aluminum is more stable post machining and during cycles than hot extruded grades like 6061 and is easy to machine however it has worse mechanical properties. 7000 series like 7050 and 7075 are preferred for the best mechanical properties in aluminum, they are comparable to low to mid carbon steel molds. Copper alloys, like pewter, or bismuth alloy molds can be cast around a model to create strong molds with higher molding temperatures than epoxy molds. The casting around a model to create each mold part produces complex molds quickly. The parts can also capture detailed surface finishes.
Epoxy Molds.
Epoxy molds typically mix epoxy with a metal powder (generally aluminum) to form a mold. Atomized aluminum allows for the distribution of heat from the mold surface outward toward the edges. This typically preserves the surface quality for 50-100 cycles on a single epoxy mold.
Due to the nature of oxygen entrapment in epoxy during the pouring and curing period it is common to have distortions and cavitation in the final injection mold. Pressurizing the epoxy during the curing period is a form of surface quality retention. External pressures can be created with the use of a pressure pot connected to an air compressor to crush air trapped inside the epoxy mold during curing. As time passes over a 24-hour period the oxygen bubbles will not be able to escape and will cure directly inside the mold. With sufficient pressure these small cavities will be invisible to the naked eye.
Degassing the epoxy during the curing period can also be done using a vacuum chamber and will require a pressure of 100 kPa (29 inHg) in order to create near vacuum conditions. This can be achieved with the use of a 2-stage vacuum pump that is capable of 2 Pa (15 μmHg).
Single use molds.
Single use injection molds can be achieved through the use of plaster of Paris. The mold breaks down after the first shot and will rarely allow for the injection of a second shot.


Forming (metalworking)

In metalworking, forming is the fashioning of metal parts and objects through mechanical deformation; the workpiece is reshaped without adding or removing material, and its mass remains unchanged. Forming operates on the materials science principle of plastic deformation, where the physical shape of a material is permanently deformed.
Characteristics.
Metal forming tends to have more uniform characteristics across its subprocesses than its contemporary processes, cutting and joining.
On the industrial scale, forming is characterized by:
Forming processes.
Forming processes tend to be categorised by differences in effective stresses. These categories and descriptions are highly simplified, since the stresses operating at a local level in any given process are very complex and may involve many varieties of stresses operating simultaneously, or it may involve stresses which change over the course of the operation.
Compressive forming involves those processes where the primary means of plastic deformation is uni- or multiaxial compressive loading.
Tensile forming.
Tensile forming involves those processes where the primary means of plastic deformation is uni- or multiaxial tensile stress.
Combined tensile and compressive forming.
This category of forming processes involves those operations where the primary means of plastic deformation involves both tensile stresses and compressive loads.
Bending.
This category of forming processes involves those operations where the primary means of plastic deformation is a bending load.
Shearing.
This category of forming processes involves those operations where the primary means of plastic deformation is a shearing load.


Titanium powder

Titanium powder metallurgy (P/M) offers the possibility of creating net shape or near net shape parts without the material loss and cost associated with having to machine intricate components from wrought billet. Powders can be produced by the blended elemental technique or by pre-alloying and then consolidated by metal injection moulding, hot isostatic pressing, direct powder rolling or laser engineered net shaping.
Blended elemental technique (BE).
The traditional technique of titanium production is via the Kroll process which involves chlorination of TiO2 ore in the presence of carbon and reacting the resulting TiCl4 with magnesium to produce titanium sponge. These processes take place at temperatures as high as 1040 °C. The sponge particle range in size from 45 to 180 μm, with particles ~150 μm termed 'sponge fines'. These fines are irregularly shaped and porous with a sponge-like morphology. The fines are then blended with alloy additions; cold compacted into a green compact at up to 415 MPa then vacuum sintered at 1260 °C to produce a 99.5% dense component. Hot isostatic pressing (HIP) can further increase the density of these parts and produce components more economically than cast or wrought parts, but the porosity present in the material degrades fatigue and fracture properties. The BE approach has been used to produce valves for the Toyota Altezza, golf club heads and softball bats. More recently, close to 100% dense Ti Grade 5 parts has been achieved using a hydrided powder along with 60:40 Al:V master alloy. The mechanical properties compare well with those exhibited by cast-and-wrought products. A cost estimate of less than $3.00 for a 0.320 g automotive connection link has been made.
Pre-alloyed powder production.
Several techniques exist to produce pre-alloyed powder, such as Grade 5. In the hydride-dehydride process feedstock such as solid scrap, billet or machined turnings are processed to remove contaminants, hydrogenated to produce brittle material then ground under argon in a vibratory ball mill, typically at 400 °C for 4 hours at a pressure of 1 psi for Ti Grade 5. The resulting particles are angular and measure between 50 and 300 μm. Cold compaction after dehydrogenation of the powder, followed by either vacuum hot pressing (in this case the dehydrogenation process can be bypassed as hydrogen is removed under vacuum) or HIP and a final vacuum anneal, produces powders with hydrogen below 125 ppm. The possible presence of contaminants makes these powders unsuitable for use in critical aircraft applications.
In the plasma rotating electrode process (PREP), the feedstock, such as Ti Grade 5, is in the form of a rotating bar which is arced with gas plasma. The molten metal is centrifugally flung off the bar, cools down and is collected. The powders produced are spherical; between 100 and 300 μm is size, with good packing and flow characteristics, making the powder ideal for high quality, near net shapes produced by HIP, such as aviation parts and porous coatings on hip prostheses.
In the titanium gas atomisation (TGA) process, titanium is vacuum induction skull melted in a water cooled copper crucible, the metal tapped and the molten metal stream atomized with a stream of high pressure inert gas. The tiny droplets are spherical and measure between 50 and 350 μm. The TGA process has been used to produce a wide variety of materials such as commercially pure (CP) titanium, conventional alpha-beta and beta alloys.
In plasma atomization (PA) process, a titanium wire is atomized by 3 inert gas plasma jets to form spherical metal powders. The distribution of diameter obtained in the PA process ranges between 0–200 μm and the powders obtained is very pure. The PA process specializes in the production of high temperature melting material as titanium (CP-Ti, Ti-6Al-4V), niobium, molybdenum, tantalum and many more.
Powder consolidation.
Several metal consolidation techniques are used to produce the final product. Metal injection moulding (MIM) otherwise known as powder injection moulding is a well-established and cost-effective method of fabricating small-to-moderate size metal components in large quantities. It is derived from the method plastic injection moulding, whereby mixing of a metal powder with a polymer binder forms the feedstock, which is then injected into a mould, after which the binder is removed via heat treatment under vacuum before final sintering. With titanium however, the binders used in MIM results in the introduction of carbon into the matrix due to insufficient binder removal prior to sintering and/or deleterious reactions between the decomposing binder, the debinding atmosphere, and the metal phase. This results in titanium parts with mechanical properties unsuited for critical aerospace applications, but suitable for parts where tensile and impact properties are less important. Recently, work has been carried out to reduce the binder to < 8% volume fraction, resulting in the complete removal of the binder from the moulded component during heat treatment.
In the direct powder rolling (DPR) process BE powder is used to produce sheet and plate and composite multilayered sheet and plates. Sheets between 1.27 and 2.54 mm and 50 to 99+% dense of single layer CP titanium, Ti Grade 5, TiAl (Ti-48Al-2Cr-2Nb) and composite Ti/Grade 5/Ti and Grade 5/TiAl/Grade 5 have been produced by DPR and sintering.
Laser engineered net shaping (LENS) is an additive manufacturing technique for rapidly fabricating, enhancing and repairing metal components directly from CAD data. The processes use a high power solid state laser focused onto a metal substrate to create a ~1 mm diameter melt pool. Metal powder is then injected into the melt pool to increase the material volume and build up the component layer by layer. Experimental gas thrusters (build time 8 hours) and automotive brackets have been manufactured in Ti-Grade 5. Selective Laser Sintering (SLS) is similar, except that the laser selectively fuses powdered material by scanning on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.
In hot isostatic pressing high temperature and pressure are used to consolidate powders to close to their maximum theoretical densities.
Electric current assisted sintering, also known as spark plasma sintering (SPS) relies on fast application of resistive heating and pressure to consolidate powders close to their maximum theoretical densities, without the undesired grain growth effect, thereby retaining close to original grain size and achieving improved mechanical properties in the final product.
Emerging technologies.
Work is progressing on bypassing the conventional route of atomising wrought feedstock or sponge and the inherent cost associated with the traditional Kroll process. Several of these processes, such as the FFC, MER Corporation, OS, Ginatta and BHP Billiton processes rely on the electrolytic reduction of TiO2 (a cheap and abundant material) to form Ti metal. So far, no material from these processes has been sold commercially on the open market, and cost models have yet to be published, but they offer the possibility of inexpensive titanium powder in the near future. The countries that have such facilities to generate Titanium Sponge are Saudi Arabia, China, Japan, Russia, Kazakhstan, the USA, Ukraine and India. The Titanium Sponge Plant in India is the only one in the world that can undertake all the different activities of manufacturing aerospace grade titanium sponge under one roof.


Thermal-transfer printing

Thermal-transfer printing is a digital printing method in which material is applied to paper (or some other material) by melting a coating of ribbon so that it stays glued to the material on which the print is applied. It contrasts with direct thermal printing, where no ribbon is present in the process.
Thermal transfer is preferred over direct thermal printing on surfaces that are heat-sensitive or when higher durability of printed matter (especially against heat) is desired. Thermal transfer is a popular print process particularly used for the printing of identification labels. It is the most widely used printing process in the world for the printing of high-quality barcodes. Printers like label makers can laminate the print for added durability.
Thermal transfer printing was invented by SATO corporation. The world's first thermal-transfer label printer SATO M-2311 was produced in 1981.
Thermal-transfer printing process.
Thermal-transfer printing is done by melting wax within the print heads of a specialized printer. The thermal-transfer print process utilises three main components: a non-movable print head, a carbon ribbon (the ink) and a substrate to be printed, which would typically be paper, synthetics, card or textile materials. These three components effectively form a sandwich with the ribbon in the middle. A thermally compliant print head, in combination with the electrical properties of the ribbon and the correct rheological properties of the ribbon ink are all essential in producing a high-quality printed image.
Print heads are available in 203 dpi, 300 dpi and 600 dpi resolution options. Each dot is addressed independently, and when a dot is electronically addressed, it immediately heats up to a pre-set (adjustable) temperature. The heated element immediately melts the wax- or resin-based ink on the side of the ribbon film facing the substrate, and this process, in combination with the constant pressure being applied by the print-head locking mechanism immediately transfers it onto the substrate. When a dot "turns off", that element of the print head immediately cools down, and that part of the ribbon thereby stops melting/printing. As the substrate comes out of the printer, it is completely dry and can be used immediately.
Carbon ribbons are on rolls and are fitted onto a spindle or reel holder within the printer. The used ribbon is rewound by a take-up spindle, forming a roll of "used" ribbon. It is termed a "one-trip" ribbon because once it has been rewound, the used roll is discarded and replaced with a new one. If one were to hold a strip of used carbon ribbon up to the light, one would see an exact negative of the images that have been printed. The main benefit of using a one-trip thermal transfer ribbon is that providing the correct settings are applied prior to printing, a 100% density of printed image is guaranteed, in contrast to a pre-inked ribbon on a dot-matrix impact printer ribbon, which gradually fades with usage.
Variants.
Color thermal printers.
Thermal-printing technology can be used to produce color images by adhering a wax-based ink onto paper. As the paper and ribbon travel in unison beneath the thermal print head, the wax-based ink from the transfer ribbon melts onto the paper. When cooled, the wax is permanently adhered to the paper. This type of thermal printer uses a like-sized panel of ribbon for each page to be printed, regardless of the contents of the page. Monochrome printers have a black panel for each page to be printed, while color printers have either three (CMY) or four (CMYK) colored panels for each page. Unlike dye-sublimation printers, these printers cannot vary the dot intensity, which means that images must be dithered. Although acceptable in quality, the printouts from these printers cannot compare with modern inkjet printers and color laser printers. Currently, this type of printer is rarely used for full-page printing, but is now employed for industrial label printing due to its waterfastness and speed. These printers are considered highly reliable due to their small number of moving parts. Printouts from color thermal printers using wax are sensitive to abrasion, as the wax ink can be scraped, rubbed off, or smeared. However, wax-resin compounds and full resins can be used on materials such as polypropylene or polyester in order to increase durability.
Tektronix/Xerox solid-ink printers.
So-called "solid ink" or "phaser" printers were developed by Tektronix and later by Xerox (who acquired Tektronix's printer division). Printers like the Xerox Phaser 8400 use rectangular solid-state ink blocks (similar in consistency to candle wax), which are loaded into a system similar to a stapler magazine in the top of the printer. The ink blocks are melted, and the ink is transferred onto a rotating oil-coated print drum using a piezo inkjet head. The paper then passes over the print drum, at which time the image is transferred, or transfixed, to the page. This system is similar to water-based inkjets, provided that the ink has low viscosity at the jetting temperature 60 °C (140 °F). Printout properties are similar to those mentioned above, although these printers can be configured to produce extremely high-quality results and are far more economical, as they only use the ink needed for the printout, rather than an entire ribbon panel. Costs of upkeep and ink are comparable to color laser printers, while "standby" power usage can be very high, about 200 W.
ALPS MicroDry printers.
MicroDry is a computer printing system developed by the Alps Electric of Japan. It is a wax/resin-transfer system using individual colored thermal ribbon cartridges and can print in process color using cyan, magenta, yellow, and black cartridges, as well as such spot-color cartridges as white, metallic silver, and metallic gold, on a wide variety of paper and transparency stock. Certain MicroDry printers can also operate in dye-sublimation mode, using special cartridges and paper.
Uses.
Usage of TT printers in industry includes:
Barcode printers typically come in fixed sizes of wide. Although a number of manufacturers have made differing sizes in the past, most have now standardised on these sizes. The main application of these printers is to produce barcode labels for product and shipping identification.


Digital image correlation for electronics

Digital image correlation analyses have applications in material property characterization, displacement measurement, and strain mapping. As such, DIC is becoming an increasingly popular tool when evaluating the thermo-mechanical behavior of electronic components and systems.
CTE measurements and glass transition temperature identification.
The most common application of DIC in the electronics industry is the measurement of coefficient of thermal expansion (CTE). Because it is a non-contact, full-field surface technique, DIC is ideal for measuring the effective CTE of printed circuit boards (PCB) and individual surfaces of electronic components. It is especially useful for characterizing the properties of complex integrated circuits, as the combined thermal expansion effects of the substrate, molding compound, and die make effective CTE difficult to estimate at the substrate surface with other experimental methods. DIC techniques can be used to calculate average in-plane strain as a function of temperature over an area of interest during a thermal profile. Linear curve-fitting and slope calculation can then be used to estimate an effective CTE for the observed area. Because the driving factor in solder fatigue is most often the CTE mismatch between a component and the PCB it is soldered to, accurate CTE measurements are vital for calculating printed circuit board assembly (PCBA) reliability metrics.
DIC is also useful for characterizing the thermal properties of polymers. Polymers are often used in electronic assemblies as potting compounds, conformal coatings, adhesives, molding compounds, dielectrics, and underfills. Because the stiffness of such materials can vary widely, accurately determining their thermal characteristics with contact techniques that transfer load to the specimen, such as dynamic mechanical analysis (DMA) and thermomechanical analysis (TMA), is difficult to do with consistency. Accurate CTE measurements are important for these materials because, depending on the specific use case, expansion and contraction of these materials can drastically affect solder joint reliability. For example, if a stiff conformal coating or other polymeric encapsulation is allowed to flow under a QFN, its expansion and contraction during thermal cycling can add tensile stress to the solder joints and expedite fatigue failures.
DIC techniques will also allow the detection of glass transition temperature (Tg). At a glass transition temperature, the strain vs. temperature plot will exhibit a change in slope.
Determining the Tg is very important for polymeric materials that could have glass transition temperatures within the operating temperature range of the electronics assemblies and components on which they are used. For example, some potting materials can see the Elastic Modulus of the material change by a factor of 100 or more over the glass transition region. Such changes can have drastic effects on an electronic assembly's reliability if they are not planned for in the design process.
Out-of-plane component warpage.
When 3D DIC techniques are employed, out-of-plane motion can be tracked in addition to in-plane motion. Out-of-plane warpage is especially of interest at the component level of electronics packaging for solder joint reliability quantification. Excessive warpage during reflow can contribute to defective solder joints by lifting the edges of the component away from the board and creating head-in-pillow defects in ball grid arrays (BGA). Warpage can also shorten the fatigue life of adequate joints by adding tensile stresses to edge joints during thermal cycling.
Thermo-mechanical strain mapping.
When a PCBA is over-constrained, thermo-mechanical stress brought about during thermal expansion can cause board strains that could negatively affect individual component and overall assembly reliability. The full-field monitoring capabilities of an image correlation technique allow for the measurement of strain magnitude and location on the surface of a specimen during a displacement-causing event, such as PCBA during a thermal profile. These "strain maps" allow for the comparison of strain levels over full areas of interest. Many traditional discrete methods, like extensometers and strain gauges, only allow for localized measurements of strain, inhibiting their ability to efficiently measure strain across larger areas of interest. DIC techniques have also been used to generate strain maps from purely mechanical events, such as drop impact tests, on electronic assemblies.


Cross section (electronics)

In electronics, a cross section, cross-section, or microsection, is a prepared electronics sample that allows analysis at a plane that cuts through the sample. It is a destructive technique requiring that a portion of the sample be cut or ground away to expose the internal plane for analysis. They are commonly prepared for research, manufacturing quality assurance, supplier conformity, and failure analysis. Printed wiring boards (PWBs) and electronic components and their solder joints are common cross sectioned samples. The features of interest to be analyzed in cross section can be nanometer-scale metal and dielectric layers in semiconductors up to macroscopic features such as the amount of solder that has filled into a large, 0.125in (3.18mm) diameter plated through hole.
Preparation.
Cross sections can be prepared by several methods typically chosen based on the scale of the feature of interest because the technique affects the smoothness of the final polish. Smoother polishes allow an analysis of smaller features but can also take longer or be more expensive to prepare. Cross sectioning hard materials such as alumina might require a different technique than a soft material like gold or soft plastic.
Mechanical grinding and polishing.
Mechanical grinding and polishing is a common method of preparation to analyze features on the order of 1s to 10s of microns to macroscopic features. Samples may first be cut down in size, for example, around a via in a PWB or around a ceramic capacitor soldered to a PWB. Samples may be prepared by encapsulation in a rigid material such as epoxy to keep the sample intact during grinding and with a vacuum step to fill in air gaps and create a solid sample with no voids. However, cross sections of some samples can be prepared with no encapsulation.
Encapsulated samples are prepared using a rough grinding medium to remove material from the sample until just before the plane of interest is reached. Equipment can help automate the process by holding grinding and polishing media firm and then spinning it so a sample can be pressed against it. Typical grinding media are silicon carbide and diamond, which can be in the form of disposable discs impregnated with the grinding media or a slurry applied to a reusable pad. Successively finer media are used to finish grinding to the plane of interest and to polish at the plane of interest. Each successively smaller grit is used to remove the scratches and damage caused by the previous grit.
Mechanical cutting or milling.
Some equipment allows for preparation of cross sections by direct cutting or milling.
Other techniques.
Focused ion beam, ion beam milling, and cleaving are common techniques in the semiconductor fabrication industry.
Printed wiring boards.
Manufacturers of substrates used in electronics prepare cross sections of a final product for quality assurance. In cross section, the quality of drill holes can be assessed and the plating quality and thickness in vias can be measured. Voids in the substrate materials may be seen which show the quality of the lamination process.
Electronic components.
Viewing the internal structures of electronic components by cross section can reveal problems with manufacturing and material quality. In integrated circuits, a cross section can reveal the die, and its active layers, the die paddle, and 1st level interconnect (wire bonds or solder bumps).
Solder joints.
Cross sections of component solder joints are commonly prepared to assess the quality and extent of the metallurgical bond. This analysis can be used to help determine any issues during the soldering processes that could lead to solder fatigue and failure. Solder joint cross sections are also commonly prepared during failure analysis to see cracks in the solder. Crack morphology can lead to identification of the type of stress and ultimately the root cause of the solder joint failure.
Analysis techniques for cross sections.
Analysis of polished cross sections can be performed with a variety of techniques. Images are commonly taken with optical microscopy and scanning electron microscopy. Chemical analysis can be done with energy dispersive x-ray spectroscopy (EDS). Hardness testing can also be performed.


Dye-and-pry

Dye-n-Pry, also called Dye And Pry, Dye and Pull, Dye Staining, or Dye Penetrant, is a destructive analysis technique used on surface mount technology (SMT) components to either perform failure analysis or inspect for solder joint integrity. It is an application of dye penetrant inspection.
Method.
Dye-n-Pry is a useful technique in which a dye penetrant material is used to inspect for interconnect failures in integrated circuits (IC). This is mostly commonly done on solder joints for ball grid array (BGA) components, although in some cases it can be done with other components or samples. The component of interest is submerged in a dye material, such as red steel dye, and placed under vacuum. This allows the dye to flow underneath the component and into any cracks or defects. The dye is then dried in an oven (preferably overnight) to prevent smearing during separation, which could lead to false results. The part of interest is mechanically separated from the printed circuit board (PCB) and inspected for the presence of dye. Any fracture surface or interface will have dye present, indicating the presence of cracks or open circuits. IPC-TM-650 Method 2.4.53 specifies a process for dye-n-pry.
Use in failure analysis of electronics.
Dye-n-Pry is a useful failure analysis technique to detect cracking or open circuits in BGA solder joints. This has some practical advantages over other destructive techniques, such as cross sectioning, as it can inspect a full ball grid array which may consist of hundreds of solder joints. Cross sectioning, on the other hand, may only be able to inspect a single row of solder joints and requires a better initial idea of the failure site.
Dye-n-pry can be useful for detecting several different failure modes. This includes pad cratering or solder joint fracture from mechanical drop/shock, thermal shock, or thermal cycling. This makes it useful technique to incorporate into a reliability test plan as part of the post test failure inspection. It is also a useful method to inspect or diagnose failures due to manufacturing defects or design flaws. This includes defects such as black pad for PCBs with ENIG surface finishes or early failures due to excessive board flexure from depaneling or In-circuit test (ICT).


Rolling (metalworking)

In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is similar to the rolling of dough. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is known as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is known as cold rolling. In terms of usage, hot rolling processes more tonnage than any other manufacturing process, and cold rolling processes the most tonnage out of all cold working processes. Roll stands holding pairs of rolls are grouped together into rolling mills that can quickly process metal, typically steel, into products such as structural steel (I-beams, angle stock, channel stock), bar stock, and rails. Most steel mills have rolling mill divisions that convert the semi-finished casting products into finished products.
There are many types of rolling processes, including "ring rolling", "roll bending", "roll forming", "profile rolling", and "controlled rolling".
Iron and steel.
The earliest rolling mills in crude form but the same basic principles were found in Middle East and South Asia as early as 600 BCE. The invention of the rolling mill in Europe may be attributed to Leonardo da Vinci in his drawings. Earliest rolling mills were slitting mills, which were introduced from what is now Belgium to England in 1590. These passed flat bars between rolls to form a plate of iron, which was then passed between grooved rolls (slitters) to produce rods of iron. The first experiments at rolling iron for tinplate took place about 1670. In 1697, Major John Hanbury erected a mill at Pontypool to roll "Pontypool plates" – blackplate. Later this began to be rerolled and tinned to make tinplate. The earlier production of plate iron in Europe had been in forges, not rolling mills.
The slitting mill was adapted to producing hoops (for barrels) and iron with a half-round or other sections by means that were the subject of two patents of c. 1679.
Some of the earliest literature on rolling mills can be traced back to the Swedish engineer Christopher Polhem in his "Patriotista Testamente" of 1761, where he mentions rolling mills for both plate and bar iron. He also explains how rolling mills can save on time and labor because a rolling mill can produce 10 to 20 or more bars at the same time.
A patent was granted to Thomas Blockley of England in 1759 for the polishing and rolling of metals. Another patent was granted in 1766 to Richard Ford of England for the first tandem mill. A tandem mill is one in which the metal is rolled in successive stands; Ford's tandem mill was for hot rolling of wire rods.
Other metals.
Rolling mills for lead seem to have existed by the late 17th century. Copper and brass were also rolled by the late 18th century.
Modern rolling.
Until well into the eighteenth century, rolling mills derived their power from water wheels. The first recorded use of a steam engine directly driving a mill is attributed to John Wilkinson's Bradley Works where, in 1786, a Boulton and Watt engine was coupled to a slitting and rolling mill. The use of steam engines considerably enhanced the production capabilities of the mills, until this
form of power was displaced by electric motors soon after 1900.
Modern rolling practice can be attributed to the pioneering efforts of Henry Cort of Funtley Iron Mills, near Fareham in Hampshire, England. In 1783, a patent number was issued to Henry Cort for his use of grooved rolls for rolling iron bars. With this new design, mills were able to produce 15 times more output per day than with a hammer. Although Cort was not the first to use grooved rolls, he was the first to combine the use of many of the best features of various ironmaking and shaping processes known at the time. Thus modern writers have called him "father of modern rolling".
The first rail rolling mill was established by John Birkenshaw at Bedlington Ironworks in Northumberland, England, in 1820, where he produced fish-bellied wrought iron rails in lengths of 15 to 18 feet. With the advancement of technology in rolling mills, the size of rolling mills grew rapidly along with the size of the products being rolled. One example of this was at The Great Exhibition in London in 1851, where a plate 20 feet long, 3 feet wide, and 7/16 of an inch thick, and weighing 1,125 pounds, was exhibited by the Consett Iron Company. Further evolution of the rolling mill came with the introduction of three-high mills in 1853 used for rolling heavy sections.
Hot and cold rolling.
Hot rolling.
Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semi-finished casting products, such as ingots, slabs, blooms, and billets.
If these products came from a continuous casting operation, the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations, the material starts at room temperature and must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces; for smaller workpieces, induction heating is used. As the material is worked, the temperature must be monitored to make sure it remains above the recrystallization temperature.
To maintain a safety factor a "finishing temperature" is defined above the recrystallization temperature; this is usually 50 to 100 °C (90 to 180 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated prior to additional hot rolling.
Hot-rolled metals generally have little directionality in their mechanical properties or deformation-induced residual stresses. However, in certain instances non-metallic inclusions will impart some directionality and workpieces less than thick often have some directional properties. Non-uniform cooling will induce a lot of residual stresses, which usually occurs in shapes that have a non-uniform cross-section, such as I-beams. While the finished product is of good quality, the surface is covered in mill scale, which is an oxide that forms at high temperatures. It is usually removed via pickling or the smooth clean surface (SCS) process, which reveals a smooth surface. Dimensional tolerances are usually 2 to 5% of the overall dimension.
Hot-rolled mild steel seems to have a wider tolerance for the level of included carbon than does cold-rolled steel, and is, therefore, more difficult for a blacksmith to use. Also for similar metals, hot-rolled products seem to be less costly than cold-rolled ones.
Hot rolling is used mainly to produce sheet metal or simple cross-sections, such as rail tracks. Other typical uses for hot-rolled metal:
Shape rolling design.
Rolling mills are often divided into roughing, intermediate and finishing rolling cages. During shape rolling, an initial billet (round or square) with edge of diameter typically ranging between 100 and 140 mm is continuously deformed to produce a certain finished product with smaller cross section dimension and geometry. Starting from a given billet, different sequences can be adopted to produce a certain final product. However, since each rolling mill is significantly expensive (up to 2 million euros), a typical requirement is to reduce the number of rolling passes. Different approaches have been achieved, including empirical knowledge, employment of numerical models, and Artificial Intelligence techniques. Lambiase et al. validated a finite element model (FE) for predicting the final shape of a rolled bar in round-flat pass. One of the major concerns when designing rolling mills is to reduce the number of passes. A possible solution to such requirements is the slit pass, also called split pass, which divides an incoming bar in two or more subparts, thus virtually increasing the cross section reduction ratio per pass as reported by Lambiase.
Another solution for reducing the number of passes in rolling mills is the employment of automated systems for Roll Pass Design as that proposed by Lambiase and Langella. subsequently, Lambiase further developed an Automated System based on Artificial Intelligence and particularly an integrated system including an inferential engine based on Genetic Algorithms a knowledge database based on an Artificial Neural Network trained by a parametric Finite element model and to optimize and automatically design rolling mills.
Cold rolling.
Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used. Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.
Cold-rolled sheets and strips come in various conditions: "full-hard", "half-hard", "quarter-hard", and "skin-rolled". Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction. Cold rolled steel is then annealed to induce ductility in the cold rolled steel which is simply known as a "Cold Rolled and Close Annealed". Skin-rolling, also known as a "skin-pass", involves the least amount of reduction: 0.5–1%. It is used to produce a smooth surface, a uniform thickness, and reduce the yield point phenomenon (by preventing Lüders bands from forming in later processing). It locks dislocations at the surface and thereby reduces the possibility of formation of Lüders bands. To avoid the formation of Lüders bands it is necessary to create substantial density of unpinned dislocations in ferrite matrix. It is also used to break up the spangles in galvanized steel. Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required.
Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small. Cold rolling shapes requires a series of shaping operations, usually along the lines of sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing.
If processed by a blacksmith, the smoother, more consistent, and lower levels of carbon encapsulated in the steel makes it easier to process, but at the cost of being more expensive.
Typical uses for cold-rolled steel include metal furniture, desks, filing cabinets, tables, chairs, motorcycle exhaust pipes, computer cabinets and hardware, home appliances and components, shelving, lighting fixtures, hinges, tubing, steel drums, lawn mowers, electronic cabinetry, water heaters, metal containers, fan blades, frying pans, wall and ceiling mount kits, and a variety of construction-related products.
Processes.
Roll bending.
Roll bending produces a cylindrical shaped product from plate or steel metals
Roll forming.
Roll forming, roll bending or plate rolling is a continuous bending operation in which a long strip of metal (typically coiled steel) is passed through consecutive sets of rolls, or stands, each performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing parts with long lengths or in large quantities.
There are three main processes: 4 rollers, 3 rollers and 2 rollers, each of which has as different advantages according to the desired specifications of the output plate.
Flat rolling.
Flat rolling is the most basic form of rolling with the starting and ending material having a rectangular cross-section. The material is fed in between two "rollers", called "working rolls", that rotate in opposite directions. The gap between the two rolls is less than the thickness of the starting material, which causes it to deform. The decrease in material thickness causes the material to elongate. The friction at the interface between the material and the rolls causes the material to be pushed through. The amount of deformation possible in a single pass is limited by the friction between the rolls; if the change in thickness is too great the rolls just slip over the material and do not draw it in. The final product is either sheet or plate, with the former being less than thick and the latter greater than; however, heavy plates tend to be formed using a press, which is termed "forging", rather than rolling.
Often the rolls are heated to assist in the workability of the metal. Lubrication is often used to keep the workpiece from sticking to the rolls. To fine-tune the process, the speed of the rolls and the temperature of the rollers are adjusted.
For thin sheet metal with a thickness less than , the rolling is done in a "cluster mill" because the small thickness requires a small diameter rolls. To reduce the need for small rolls "pack rolling" is used, which rolls multiple sheets together to increase the effective starting thickness. As the foil sheets come through the rollers, they are trimmed and slitted with circular or razor-like knives. Trimming refers to the edges of the foil, while slitting involves cutting it into several sheets. Aluminum foil is the most commonly produced product via pack rolling. This is evident from the two different surface finishes; the shiny side is on the roll side and the dull side is against the other sheet of foil.
Ring rolling.
Ring rolling is a specialized type of hot rolling that increases the diameter of a ring. The starting material is a thick-walled ring. This workpiece is placed between two rolls, an inner "idler roll" and a "driven roll", which presses the ring from the outside. As the rolling occurs the wall thickness decreases as the diameter increases. The rolls may be shaped to form various cross-sectional shapes. The resulting grain structure is circumferential, which gives better mechanical properties. Diameters can be as large as and face heights as tall as . Common applications include railway tyres, bearings, gears, rockets, turbines, airplanes, pipes, and pressure vessels.
Controlled rolling.
"Controlled rolling" is a type of thermomechanical processing which integrates controlled deformation and heat treating. The heat which brings the workpiece above the recrystallization temperature is also used to perform the heat treatments so that any subsequent heat treating is unnecessary. Types of heat treatments include the production of a fine grain structure; controlling the nature, size, and distribution of various transformation products (such as ferrite, austenite, pearlite, bainite, and martensite in steel); inducing precipitation hardening; and, controlling the toughness. In order to achieve this the entire process must be closely monitored and controlled. Common variables in controlled rolling include the starting material composition and structure, deformation levels, temperatures at various stages, and cool-down conditions. The benefits of controlled rolling include better mechanical properties and energy savings.
Forge rolling.
Forge rolling is a longitudinal rolling process to reduce the cross-sectional area of heated bars or billets by leading them between two contrary rotating roll segments. The process is mainly used to provide optimized material distribution for subsequent die forging processes. Owing to this a better material utilization, lower process forces and better surface quality of parts can be achieved in die forging processes.
Basically any forgeable metal can also be forge-rolled. Forge rolling is mainly used to preform long-scaled billets through targeted mass distribution for parts such as crankshafts, connection rods, steering knuckles and vehicle axles. Narrowest manufacturing tolerances can only partially be achieved by forge rolling. This is the main reason why forge rolling is rarely used for finishing, but mainly for preforming.
Characteristics of forge rolling:
Mills.
A "rolling mill", also known as a "reduction mill" or "mill", has a common construction independent of the specific type of rolling being performed:
Slabs are the feed material for hot strip mills or plate mills and blooms are rolled to billets in a billet mill or large sections in a structural mill. The output from a strip mill is coiled and, subsequently, used as the feed for a cold rolling mill or used directly by fabricators. Billets, for re-rolling, are subsequently rolled in either a merchant, bar or rod mill. Merchant or bar mills produce a variety of shaped products such as angles, channels, beams, rounds (long or coiled) and hexagons.
Configurations.
Mills are designed in different types of configurations, with the most basic being a "two-high non-reversing", which means there are two rolls that only turn in one direction. The "two-high reversing" mill has rolls that can rotate in both directions, but the disadvantage is that the rolls must be stopped, reversed, and then brought back up to rolling speed between each pass. To resolve this, the "three-high" mill was invented, which uses three rolls that rotate in one direction; the metal is fed through two of the rolls and then returned through the other pair. The disadvantage to this system is the workpiece must be lifted and lowered using an elevator. All of these mills are usually used for primary rolling and the roll diameters range from .
To minimize the roll diameter a "four-high" or "cluster" mill is used. A small roll diameter is advantageous because less roll is in contact with the material, which results in a lower force and power requirement. The problem with a small roll is a reduction of stiffness, which is overcome using "backup rolls". These backup rolls are larger and contact the back side of the smaller rolls. A four-high mill has four rolls, two small and two large. A cluster mill has more than four rolls, usually in three tiers. These types of mills are commonly used to hot roll wide plates, most cold rolling applications, and to roll foils.
Historically mills were classified by the product produced:
Tandem mill.
A tandem mill is a special type of modern rolling mill where rolling is done in one pass. In a traditional rolling mill rolling is done in several passes, but in tandem mill there are several "stands" (>=2 stands) and reductions take place successively. The number of stands ranges from 2 to 18.
Tandem mills can be either of hot or cold rolling mill types.
Cold rolling mills may be further divided into continuous or batch processing.
A continuous mill has a looping tower which allows the mill to continue rolling slowly the strip in the tower, while a strip welder joins the tail of the current coil to the head of the next coil. At the exit end of the mill there is normally a flying shear (to cut the strip at or near the weld) followed by two coilers; one being unloaded while the other winds on the current coil.
Looping towers are also used in other places; such as continuous annealing lines and continuous electrolytic tinning and continuous galvanising lines.
Defects.
Thickness changes along length.
In hot rolling, if the temperature of the workpiece is not uniform the flow of the material will occur more in the warmer parts and less in the cooler. If the temperature difference is great enough cracking and tearing can occur.
The cooler sections are, among other things, a result of the supports in the re-heat furnace.
When cold rolling, virtually all of the strip thickness variation is the result of the eccentricity and out-of-roundness of the Back-up Rolls from about Stand 3 of the Hot Strip Mill through to the Finished Product.
The Back-up Roll eccentricity can be up to 100 μm in magnitude per stack. The eccentricity can be measured off-line by plotting the force variation against time with the Mill on creep, no strip present, and the Mill Stand below face.
A modified Fourier analysis was employed by the 5 Stand Cold Mill at Bluescope Steel, Port Kembla from 1986 until that Cold Mill ceased production in 2009. Within each coil, the exit thickness deviation times 10 for every meter of strip was stored in a file. This file was analyzed separately for each frequency/wavelength from 5 m to 60 m in steps of 0.1 m. To improve the accuracy, care was taken to use a full multiple of each wavelength (100*). The calculate amplitudes were plotted against the wavelength, so that the spikes could be compared to the expected wavelengths created by the Backup Rolls of each Stand.
If a Mill Stand is fitted with Hydraulic Pistons in series with, or instead of the electrically driven Mechanical Screws, then it is possible to eliminate the effect of that Stands Back-up Roll eccentricity. While rolling, the eccentricity of each Back-up Roll is determined by sampling the roll force and assigning it to the corresponding portion of each Back-up Roll’s rotational position. These recordings are then used to operate the Hydraulic Piston so as to neutralize the eccentricities.
Flatness and shape.
In a flat metal workpiece, the flatness is a descriptive attribute characterizing the extent of the geometric deviation from a reference plane. The deviation from complete flatness is the direct result of the workpiece relaxation after hot or cold rolling, due to the internal stress pattern caused by the non-uniform transversal compressive action of the rolls and the uneven geometrical properties of the entry material. The transverse distribution of differential strain/elongation-induced stress with respect to the material's average applied stress is commonly referenced to as shape. Due to the strict relationship between shape and flatness, these terms can be used in an interchangeable manner.
In the case of metal strips and sheets, the flatness reflects the differential fiber elongation across the width of the workpiece. This property must be subject to an accurate feedback-based control in order to guarantee the machinability of the metal sheets in the final transformation processes. Some technological details about the feedback control of flatness are given in.
Profile.
Profile is made up of the measurements of crown and wedge. Crown is the thickness in the center as compared to the average thickness at the edges of the workpiece. Wedge is a measure of the thickness at one edge as opposed to the other edge. Both may be expressed as absolute measurements or as relative measurements. For instance, one could have 2 mil of crown (the center of the workpiece is 2 mil thicker than the edges), or one could have 2% crown (the center of the workpiece is 2% thicker than the edges).
It is typically desirable to have some crown in the workpiece as this will cause the workpiece to tend to pull to the center of the mill, and thus will run with higher stability.
Flatness.
Maintaining a uniform gap between the rolls is difficult because the rolls deflect under the load required to deform the workpiece. The deflection causes the workpiece to be thinner on the edges and thicker in the middle. This can be overcome by using a crowned roller (parabolic crown), however the crowned roller will only compensate for one set of conditions, specifically the material, temperature, and amount of deformation.
Other methods of compensating for roll deformation include continual varying crown (CVC), pair cross rolling, and work roll bending. CVC was developed by SMS-Siemag AG and involves grinding a third order polynomial curve into the work rolls and then shifting the work rolls laterally, equally, and opposite to each other. The effect is that the rolls will have a gap between them that is parabolic in shape, and will vary with lateral shift, thus allowing for control of the crown of the rolls dynamically. Pair cross rolling involves using either flat or parabolically crowned rolls, but shifting the ends at an angle so that the gap between the edges of the rolls will increase or decrease, thus allowing for dynamic crown control. Work roll bending involves using hydraulic cylinders at the ends of the rolls to counteract roll deflection.
Another way to overcome deflection issues is by decreasing the load on the rolls, which can be done by applying a longitudinal force; this is essentially drawing. Other method of decreasing roll deflection include increasing the elastic modulus of the roll material and adding back-up supports to the rolls.
The different classifications for flatness defects are:
It is important to note that one could have a flatness defect even with the workpiece having the same thickness across the width. Also, one could have fairly high crown or wedge, but still produce material that is flat. In order to produce flat material, the material must be reduced by the same percentage across the width. This is important because mass flow of the material must be preserved, and the more a material is reduced, the more it is elongated. If a material is elongated in the same manner across the width, then the flatness coming into the mill will be preserved at the exit of the mill.
Draught.
The difference between the thickness of initial and rolled metal piece is called Draught.
Thus if formula_1 is initial thickness and formula_2 is final thickness, then the draught is given by
The maximum draught that can be achieved via rollers of radius with coefficient of static friction between the roller and the metal surface is given by
This is the case when the frictional force on the metal from inlet contact matches the negative force from the exit contact.
Surface defect types.
There are six types of surface defects:
Surface defect remediation.
Many surface defects can be scarfed off the surface of semi-finished rolled products before further rolling. Methods of scarfing have included hand-chipping with chisels (18th and 19th centuries); powered chipping and grinding with air chisels and grinders; burning with an oxy-fuel torch, whose gas pressure blows away the metal or slag melted by the flame; and laser scarfing.


Strut bar

A strut bar, strut brace, or strut tower brace (STB) is an automotive suspension accessory on a monocoque or unibody chassis to provide extra stiffness between the strut towers.
With a MacPherson strut suspension system where the spring and shock absorber combine in one suspension unit, which also replaces the upper control arm, the entire vertical suspension load is transmitted to the top of the vehicle's strut tower. This is different from a double wishbone suspension where the spring and shock absorber may share the load separately. In general terms, a strut tower in a monocoque chassis is a reinforced portion of the inner wheel well and is not necessarily directly connected to the main chassis rails. For this reason, there is inherent flex within the strut towers relative to the chassis rails.
Function.
A strut bar is designed to reduce this strut tower flex by tying the two strut towers together. This transmits the load off each strut tower during cornering which ties the two towers together and reduces chassis flex. The transmission of load provides an increase in steering control accuracy going into a corner, reducing the possibility of the vehicle sliding or losing traction. To accomplish this effectively (especially on MacPherson strut suspensions), the bar must be rigid throughout its length. Many manufacturers have fitted strut braces to performance models as standard or optional equipment.
Types.
Most strut bars follow one of two design types. These designs include:
A single-piece strut bar is typically more durable and provides more rigidity as compared to the hinged type strut. However, the hinged type strut can allow for easier fitment of engine components due to its ability to move or pivot.
Benefits.
Beyond reducing chassis flex and increasing steering control accuracy in a corner, other benefits of strut bars include:
Disadvantages.
Some manufacturers have avoided the use of a strut bar due to a drawback in having the strut towers connected. The force from a significant impact or collision to one side of a vehicle would be distributed across the two struts leading to possible damages on both sides of the vehicle. This results in a higher repair cost.
Although a strut bar is useful for improving the handling of a vehicle, in motorsports applications, the added weight from a traditional steel or aluminum strut bar can come as a disadvantage. To offset this issue, alternative materials for strut bars are being researched with carbon fiber being the main focus, as it can provide more strength in relation to its overall weight compared to most materials.


Automotive hemming

Hemming is a technology used in the automotive industry to join inner and outer closure panels together (hoods, doors, tailgates, etc.). It is the process of bending/folding the flange of the outer panel over the inner one. The accuracy of the operation significantly affects the appearance of the car’s outer surfaces and is therefore a critical factor in the final quality of a finished vehicle.
Hemming processes.
Press hemming.
Hemming presses are widely used in automotive manufacturing for hemming of sheet-metal body components. The process uses traditional hydraulically operated ‘stamping presses’ to hem closure parts, and, being the last forming process in stamping, it largely determines the external quality of such automotive parts as doors, hood and trunk lid.
Table top hemming.
Tabletop hemming machines are utilised for the manufacture of medium to high production volumes, with the ability to achieve cycle times as low as 15 seconds.
Robot (roller hemming).
Robot hemming is utilized for the manufacture of Low to medium production volumes. It uses a standard industrial robot integrated with a roller hemming head to provide a flexible method for forming closures. The flange of the outer panel is bent over the inner panel in progressive steps, by means of a roller-hemming head.
One advantage of this process is that it can use the robot-controlled hemming head to hem several different components within a single cell. Another is that minor changes or fluctuations in panel-hemming conditions can be quickly and cost-effectively accommodated. If equipped with a tool-changing system, the robot could serve a variety of additional functions within the same assembly cell, such as operating dispensing equipment for adhesives and sealants, or carrying out panel manipulations, using a gripper unit.


Flexure bearing

A flexure bearing is a category of flexure which is engineered to be compliant in one or more angular degrees of freedom. Flexure bearings are often part of compliant mechanisms. Flexure bearings serve much of the same function as conventional bearings or hinges in applications which require angular compliance. However, flexures require no lubrication and exhibit very low or no friction.
Many flexure bearings are made of a single part: two rigid structures joined by a thin "hinge" area. A hinged door can be created by implementing a flexible element between a door and the door frame, such that the flexible element bends allowing the door to pivot open.
Flexure bearings have the advantage over most other bearings that they are simple and thus inexpensive. They are also often compact, lightweight, have very low friction, and are easier to repair without specialized equipment. Flexure bearings have the disadvantages that the range of motion is limited, and often very limited for bearings that support high loads.
A flexure bearing relies on the bearing element being made of a material which can be repeatedly flexed without disintegrating. However, most materials lose strength and eventually fail with repeated flexing and bending. For example, most metals will fatigue with repeated flexing, and will eventually snap. Thus, one part of flexure bearing design is the careful consideration of material properties to avoid fatigue with normal use.
Flexure bearings can give very low friction and also give very predictable friction. Many other bearings rely on sliding or rolling motions (rolling-element bearings), which are necessarily uneven because the bearing surfaces are never perfectly flat. A flexure bearing operates by bending of materials, which causes motion at microscopic level, so friction is very uniform. For this reason, flexure bearings are often used in sensitive precision measuring equipment.
Many types of flexure bearings are not limited to low loads, however. For example, the drive shafts of some sports cars replace cardan universal joints with an equivalent joint called a rag joint which works by bending rubberized fabric. The resulting joint is lighter yet is capable of carrying hundreds of kilowatts, with adequate durability for a sports car.
Because flexure bearings do not rely on sliding or rolling motions, they do not require lubrication. Consequently, they can be employed in abrasive environments and environments hostile to lubricants: underwater, in a vacuum and at elevated temperatures.


Parallel task scheduling

Parallel task scheduling (also called parallel job scheduling or parallel processing scheduling) is an optimization problem in computer science and operations research. It is a variant of optimal job scheduling. In a general job scheduling problem, we are given "n" jobs "J"1, "J"2, ..., "Jn" of varying processing times, which need to be scheduled on "m" machines while trying to minimize the makespan - the total length of the schedule (that is, when all the jobs have finished processing). In the specific variant known as "parallel-task scheduling", all machines are identical. Each job "j" has a "length" parameter "pj" and a "size" parameter "q"j, and it must run for exactly "pj" time-steps on exactly "q"j machines in "parallel".
Veltman et al. and Drozdowski denote this problem by formula_1 in the three-field notation introduced by Graham et al. P means that there are several identical machines running in parallel; "sizej" means that each job has a size parameter; "C"max means that the goal is to minimize the maximum completion time. Some authors use formula_2 instead. Note that the problem of parallel-machines scheduling is a special case of parallel-task scheduling where formula_3 for all "j", that is, each job should run on a single machine.
The origins of this problem formulation can be traced back to 1960. For this problem, there exists no polynomial time approximation algorithm with a ratio smaller than formula_4 unless formula_5.
Definition.
There is a set formula_6 of formula_7 jobs, and formula_8 identical machines. Each job formula_9 has a processing time formula_10 (also called the "length" of "j"), and requires the simultaneous use of formula_11 machines during its execution (also called the "size" or the "width" of j).
A schedule assigns each job formula_9 to a starting time formula_13 and a set formula_14 of formula_15 machines to be processed on. A schedule is feasible if each processor executes at most one job at any given time.
The objective of the problem denoted by formula_1 is to find a schedule with minimum length formula_17, also called the makespan of the schedule.
A sufficient condition for the feasibility of a schedule is the following
formula_18.
If this property is satisfied for all starting times, a feasible schedule can be generated by assigning free machines to the jobs at each time starting with time formula_19. Furthermore, the number of machine intervals used by jobs and idle intervals at each time step can be bounded by formula_20. Here a machine interval is a set of consecutive machines of maximal cardinality such that all machines in this set are processing the same job. A machine interval is completely specified by the index of its first and last machine. Therefore, it is possible to obtain a compact way of encoding the output with polynomial size.
Computational hardness.
This problem is NP-hard even when there are only two machines and the sizes of all jobs are formula_21 (i.e., each job needs to run only on a single machine). This special case, denoted by formula_22, is a variant of the partition problem, which is known to be NP-hard.
When the number of machines "m" is at most 3, that is: for the variants formula_23 and formula_24, there exists a pseudo-polynomial time algorithm, which solves the problem exactly.
In contrast, when the number of machines is at least 4, that is: for the variants formula_25 for any formula_26, the problem is also strongly NP-hard (this result improved a previous result showing strong NP-hardness for formula_27).
If the number of machines is not bounded by a constant, then there can be no approximation algorithm with an approximation ratio smaller than formula_4 unless formula_29. This holds even for the special case in which the processing time of all jobs is formula_30, since this special case is equivalent to the bin packing problem: each time-step corresponds to a bin, "m" is the bin size, each job corresponds to an item of size "qj", and minimizing the makespan corresponds to minimizing the number of bins.
Variants.
Several variants of this problem have been studied. The following variants also have been considered in combination with each other.
Contiguous jobs: In this variant, the machines have a fixed order formula_31. Instead of assigning the jobs to any subset formula_32, the jobs have to be assigned to a "contiguous interval" of machines. This problem corresponds to the problem formulation of the strip packing problem.
Multiple platforms: In this variant, the set of machines is partitioned into independent platforms. A scheduled job can only use the machines of one platform and is not allowed to span over multiple platforms when processed.
Moldable jobs: In this variant each job formula_9 has a set of feasible machine-counts formula_34. For each count formula_35, the job can be processed on "d" machines in parallel, and in this case, its processing time will be formula_36. To schedule a job formula_9, an algorithm has to choose a machine count formula_35 and assign "j" to a starting time formula_39 and to formula_40 machines during the time interval formula_41 A usual assumption for this kind of problem is that the total workload of a job, which is defined as formula_42, is non-increasing for an increasing number of machines.
Release dates: In this variant, denoted by formula_43, not all jobs are available at time 0; each job "j" becomes available at a fixed and known time "rj". It must be scheduled after that time.
Preemption: In this variant, denoted by formula_44, it is possible to interrupt jobs that are already running, and schedule other jobs that become available at that time.
Algorithms.
The list scheduling algorithm by Garey and Graham has an absolute ratio formula_45, as pointed out by Turek et al. and Ludwig and Tiwari.
Feldmann, Sgall and Teng observed that the length of a non-preemptive schedule produced by the list scheduling algorithm is actually at most formula_46 times the optimum preemptive makespan.
A polynomial-time approximation scheme (PTAS) for the case when the number formula_8 of processors is constant, denoted by formula_25, was presented by Amoura et al. and Jansen et al.
Later, Jansen and Thöle found a PTAS for the case where the number of processors is polynomially bounded in the number of jobs.
In this algorithm, the number of machines appears polynomially in the time complexity of the algorithm.
Since, in general, the number of machines appears only in logarithmic in the size of the instance, this algorithm is a pseudo-polynomial time approximation scheme as well.
A formula_49-approximation was given by Jansen, which closes the gap to the lower bound of formula_4 except for an arbitrarily small formula_51.
Differences between contiguous and non-contiguous jobs.
Given an instance of the parallel task scheduling problem, the optimal makespan can differ depending on the constraint to the contiguity of the machines. If the jobs can be scheduled on non-contiguous machines, the optimal makespan can be smaller than in the case that they have to be scheduled on contiguous ones.
The difference between contiguous and non-contiguous schedules has been first demonstrated in 1992 on an instance with formula_52 tasks, formula_53 processors, formula_54, and formula_55.
Błądek et al. studied these so-called c/nc-differences and proved the following points:
Furthermore, they proposed the following two conjectures, which remain unproven:
Related problems.
There are related scheduling problems in which each job consists of several operations, which must be executed "in sequence" (rather than in parallel). These are the problems of open shop scheduling, flow shop scheduling and job shop scheduling.