Drawing (manufacturing)

Tensile forces are employed to extend a workpiece.

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Drawing is an essential manufacturing technique that involves the application of tensile forces for the purpose of elongating materials such as metal, glass, or plastic. This process not only stretches the material but also reduces its thickness, allowing it to achieve a specific profile and measurement. Drawing can be categorized into two main procedures: sheet metal drawing and wire, bar, and tube drawing. In sheet metal drawing, a flat metal sheet undergoes plastic deformation along a curved axis, while in the wire, bar, and tube variant, the initial material is drawn through a die to decrease its diameter and increase its length. Typically, drawing happens at room temperature, making it a type of cold working, although hot drawing is utilized for larger wire, rods, or hollow tubes to minimize exerted forces.

Unlike rolling, which relies on the pressurized turning action of a mill, drawing depends on localized force applied near the area of compression. This creates a situation where the maximum drawing force is determined by the tensile strength of the material, a pertinent factor when dealing with slender wires.

The cold drawing process begins with hot-rolled stock that is appropriately sized.

Metals

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The success of the drawing process is contingent upon how well the material can flow and stretch. Common metals utilized in drawing include steels, copper alloys, and aluminum alloys. During sheet metal drawing, a die manipulates a metal blank, compelling it to reshape itself according to the die. The flow of material is moderated by adjusting the pressure applied to the blank and applying lubrication. If the material moves too freely, it can lead to undesirable wrinkles. Hence, to counteract this, more pressure or reduced lubrication ensures that the material stretches adequately without compromising its integrity.

Deep drawing arises when the length of the workpiece exceeds its diameter. Often, the workpiece is subjected to additional forming processes, such as piercing, ironing, and rolling. Conversely, shallow drawing occurs when the drawing depth does not surpass the smallest dimension of the hole.

Bar, tube, and wire drawing operate on a similar principle where the starting material is drawn through a die, resulting in a reduced diameter and increased length. Typically, a draw bench supports the die, and the beginning of the workpiece is streamlined to facilitate its passage through the die.

Furthermore, cold drawing can be utilized to create complex cross-sectional shapes. Cold drawn parts exhibit superior accuracy and surface finish compared to their hot extruded counterparts. This enables manufacturers to utilize less expensive materials instead of costly alloys while still meeting strength requirements due to the inherent work hardening properties involved. It is important to note that the drawn bars or rods cannot be coiled, requiring straight-pull draw benches for processing. Chain drives can handle workpieces up to 30 meters long, while hydraulic cylinders are reserved for shorter items. The reduction area typically ranges between 20% to 50%, as any greater reduction could surpass the tensile strength of the material, contingent on its ductility. Achieving specific dimensions may necessitate multiple passes through increasingly smaller dies alongside intermediate annealing sessions.

Tube drawing parallels bar drawing, except that the initial material is a tube. The purpose of tube drawing is to decrease diameter while enhancing surface finish and precision. A mandrel may be employed based on the process utilized, and in some cases, a floating plug is inserted into the tube's ID to regulate wall thickness. Wire drawing, a long-established method for producing flexible metal wire, involves drawing the material through a series of progressively smaller dies made from materials, with tungsten carbide and diamond being the most common.

The cold drawing procedure for steel bars and wires entails the following steps:

1. Lubrication: The surface of the bar or tube is coated with a lubricant such as oil or phosphate to facilitate cold drawing.
2. Pointing: The leading ends are reduced to allow easy passage through the die, typically achieved through swaging or extruding.
3. Cold Drawing: At room temperature, the reduced end enters the die and grips are activated, pulling the remainder through the die and shaping its profile.
4. End Product: The result of this process is known as "cold drawn" or "cold finished" product, marked by a polished finish, boosted mechanical properties, improved machining attributes, and strict dimensional tolerances.
5. Multi-pass drawing: Achieving intricate shapes may involve drawing the workpiece multiple times through diminutive dies to reach the specified tolerances, with annealing typically implemented in between to enhance ductility.
6. Annealing: This thermal treatment process is aimed at softening the drawn material, modifying microstructure and mechanical properties, and dissipating internal stresses. Annealing may happen before, during, or after drawing depending on the desired results.

Glass

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Analogous drawing techniques are applied in glassblowing and the production of glass optical fibers.

Plastics

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Plastic drawing, frequently termed as cold drawing, mirrors the metal drawing process but is principally designated for plastics. This technique is essential in plastic fiber manufacturing and was initially pioneered by Julian W. Hill while attempting to generate fibers from early polyester materials.

This procedure follows after the material is "spun" into filaments through polymer melt extrusion. During this phase, individual polymer chains begin to align due to viscous flow. These filaments, in their amorphous state, undergo drawing to further align the fibers, thereby enhancing properties like crystallinity, tensile strength, and stiffness. The stretching for nylon fibers can reach up to four times the initial spun length, with crystalline structures formed by hydrogen bonding between various polymer chains. Additionally, polyethylene terephthalate (PET) sheets are drawn in both dimensions to produce BoPET (biaxially-oriented polyethylene terephthalate) which boasts improved mechanical attributes.

See Also

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References

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Further Reading

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• Degarmo, E. Paul; Black, J T.; Kohser, Ronald A., Materials and Processes in Manufacturing, 9th ed., Wiley, ISBN 0-471--4.
• Kalpakjian, Serope; Schmid, Steven R., Manufacturing Engineering and Technology, 5th ed., Upper Saddle River, NJ: Pearson Prentice Hall, ISBN 0-13--8.

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