The Process That Forces Hot Plastics Through Machine Dies to Form Continuous Shapes is Called

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Manufacturing processes create finished goods from various raw materials. This article limits its look at manufacturing processes to the transformation of metals and plastics into usable forms. Obviously, this is a gross simplification in that just about every product--from hot dogs to circuit boards—goes through a series of manufacturing steps that transform it from its constituent ingredients. But it offers a good place to start. Generalizing again, manufacturing processes can be thought of in terms of primary and secondary processes, where primary processes are used in creating basic forms and secondary processes are used to alter or add features to these forms. This article will concern itself with the following processes.

• Metal deformation
• Metal casting
• Sheet metal forming
• Polymer processing
• Machining
• Finishing
• Assembly

Metal deformation

Metal deformation is used to transform bulk materials in the form of billets, blooms, and slabs as they come from a mill into other shapes such as pipe or bars. Extrusion is one such process, where ductile metals such as copper and aluminum are forced through dies to produce common shapes such as copper tubing or aluminum angles. Tubing manufacturing typically uses a mandrel in addition to a die to produce a hollow cross-section. Many extrusions are made in 40-ft. lengths to enable their transport by a trailer.

Forging uses hydraulic die sets or open dies and hammers to plastically deform usually hot metal into net shapes, oftentimes starting with a rough approximation of the finished shape called a blocked preform. Forging can produce moderately complex shapes in parts that are up to 3 ft. long. Forging can be used to apply beneficial changes to the grain structure of metals.

Rolling transforms mill products into finished raw materials such as I-beams, plates, and sheets. The process may be performed hot or cold, with cold rolling achieving higher yield strength and better surface finishes than hot rolling but requiring much more work. Typically, ingots are rolled into blooms, slabs, or billets which are then rolled further to make structural shapes, sheet metal, or bars and rods.

Bar drawing is used to further reduce bar stock and improve surface characteristics and strength through a cold, die puling process. Straight lengths of circular and rectangular bar stock are produced in this manner with cross-sectional sizes up to 6 in. possible.

Wire drawing continues the process of bar drawing by pulling ductile materials through increasingly smaller dies to wind up with steel, aluminum, and copper wire. The resulting wire is usually small and ductile enough that it can be wound onto spools of significant capacity.

Casting

Casting creates complex shapes from molten metal. Sand casting creates a two-piece sand mold around a pattern. The resulting mold is then split apart, the pattern removed, and reassembled with risers, gates, and sprues added to direct the flow of the molten metal. After the pour, the metal cools and solidifies, following which the mold is broken away to reveal the finished casting. Die casting uses permanent molds into which low melt point metals such as zinc are injected under pressure. Investment casting creates intricate wax patterns that are coated with slurry, the wax melted out, then filled with molten metal. The process was originally invented for making jewelry and, sometimes referred to as the lost-wax process, has become a method for casting complex parts such as turbine blades. Other casting methods include permanent mold casting and centrifugal casting.

Sheet metal forming

Sheet metal operations can be grouped as shearing, blanking, drawing, punching, embossing, and bending. Sheet metal is cut into smaller straight-edged pieces by shearing. Shearing can be done manually by inserting the piece in a metal shear, or, in the case of coiled material, continuously as the material is drawn off a roll. Automated operations will often pull this narrower strip through a progressive forming die where the parts are formed sequentially as they index through each station of the die. Drawing gradually pushes the material into a die cavity that deepens with each step through the die. Punching creates holes and slots where needed. Bending creates tabs and other features that run perpendicular to the plane of the original material. Blanking shears the finished part from the remaining coil material that has served to carry the forming part through the die.

Any of these operations can of course be done individually: parts can be blanked on one press station and loaded into a second press for forming, bending, etc.

Polymer processing

Polymer processing involves the forming of both thermoset and thermoplastic materials usually by molding but also by subtractive methods such as machining. Of the various molding methods, compression, blow, and injection molding are the most common. All three use metal dies whose cavities are shaped in the form of the desired plastic part. In compression molding, an elastomer charge is placed between heated die halves which are subsequently closed to force the material into the shape of the cavity. This is a common method of making tires. Transfer molding is another compression molding technique in which the heated polymer is injected into the closed mold. Blow molding is a common method for making plastic bottles. Here, a softened parison is filled with air to force it against the walls of the closed mold halves. Injection molding uses an auger to soften plastic pellets in a barrel and inject the resulting "shot" under high pressure into a usually multi-cavity mold.

Thermoforming is another polymer processing method that shapes sheets or films of thermoplastic into cavities or over plugs usually using vacuum or air to pull or push the softened material against the mold surfaces. Familiar shapes such as food packages and kiddie pools are made in this manner. Rotomolding is used to produce large hollow shapes such as kayaks by relying on the centrifugal force imparted to molten plastic as it spins within a rotating mold. Polyurethane is often cast by pouring it into open silicone rubber molds.

Machining

Machining uses various cutting tools, abrasive wheels, as well as some unusual media such as water or sparks, to remove material from round and bar stock, castings, etc. to produce accurate finished goods. Machining methods include sawing, turning, boring, reaming, etc. and are oftentimes performed as secondary operations to clean up parts or to create surfaces that are suitable for assembly. In some instances, the part is moved and coordinated with the motion of the tools, such as turning, and in other situations, the part is held stationary and the tool moves over it, such as sawing. Machine tools have come a long way from the days of belt-driven lathes and now almost invariably take the form of multi-axis computer-controlled milling and turning centers. Additional information on machining may be found in our related guides on the Different Machining Processes and the Types of Machining.

Finishing

Finishing encompasses many final operations that make a part ready for assembly. Finishing steps occur after assembly as well, such as post-weld heat treating. Finishing operations include plating, painting, sprue removing, polishing, deburring, etc., depending upon prior manufacturing operations and the intended application of the finished part. Finishing can range from simple manual polishing to sophisticated surface treatments such as shot peening. Heat treating is an important step in the finishing of many metal parts as the primary manufacturing processes can impart undesirable characteristics such as brittleness which need to be baked out.

Assembling

Assembly is where the different parts that compose a finished product come together. Various forms of fastening are often used, including mechanical forms such as screws and rivets, fusion methods such as welding, bonding techniques such as brazing and gluing, and interference methods such as press and shrink fitting. Some assemblies are more permanent than others, as in weldments, which are often called "fabrications" rather than "assemblies."  Other assembly features may be built into the part itself, for instance, plastic tabs and slots made during molding that allow for parts to be snapped together. Assembly often involves quality control checks which frequently have followed along through the entire manufacturing process of a product's constituent parts. Good engineering practice takes into account the ease and accuracy with which parts are assembled.

Summary

This article presented a brief summary of manufacturing processes as they pertain to metals and plastics. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.

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