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Drawing is amanufacturing process that usestensile forces to elongatemetal,glass, orplastic. As the material is drawn (pulled), it stretches and becomes thinner, achieving a desired shape and thickness. Drawing is classified into two types:sheet metal drawing andwire,bar, andtube drawing. Sheet metal drawing is defined as aplastic deformation over a curved axis. For wire, bar, and tube drawing, the starting stock is drawn through adie to reduce its diameter and increase its length. Drawing is usually performed at room temperature, thus classified as acold working process; however, drawing may also be performed at higher temperatures tohot work large wires, rods, or hollow tubes in order to reduce forces.[1][2]
Drawing differs fromrolling in thatpressure is not applied by the turning action of a mill but instead depends on force applied locally near the area ofcompression. This means the maximal drawing force is limited by the tensile strength of the material, a fact particularly evident when drawing thin wires.[3]
The starting point of cold drawing ishot-rolled stock of a suitable size.
Successful drawing depends on the flow and stretch of the material. Steels, copper alloys, andaluminium alloys are commonly drawn metals.[4]
In sheet metal drawing, as adie forms a shape from a flat sheet of metal (the "blank"), the material is forced to move and conform to the die. The flow of material is controlled through pressure applied to the blank and lubrication applied to the die or the blank. If the form moves too easily, wrinkles will occur in the part. To correct this, more pressure or less lubrication is applied to the blank to limit the flow of material and cause the material to stretch or set thin. If too much pressure is applied, the part will become too thin and break. Drawing metal requires finding the correct balance between wrinkles and breaking to achieve a successful part.
Sheet metal drawing becomesdeep drawing when the workpiece is longer than its diameter. It is common that the workpiece is also processed using other forming processes, such aspiercing,ironing,necking,rolling, andbeading. In shallow drawing, the depth of drawing is less than the smallest dimension of the hole.
Bar, tube, and wire drawing all work upon the same principle: the starting stock is drawn through a die to reduce its diameter and increase its length. Usually, the die is mounted on adraw bench. The starting end of the workpiece is narrowed or pointed to get the end through the die. The end is then placed in grips which pull the rest of the workpiece through the die.[1]
Drawing can also be used tocold form a shaped cross-section. Cold drawn cross-sections are more precise and have a better surface finish than hot extruded parts. Inexpensive materials can be used instead of expensive alloys for strength requirements, due towork hardening.[5] Bars or rods that are drawn cannot be coiled; therefore, straight-pull draw benches are used. Chain drives are used to draw workpieces up to 30 m (98 ft). Hydraulic cylinders are used for shorter length workpieces.[1] The reduction in area is usually restricted to between 20% and 50%, because greater reductions would exceed the tensile strength of the material, depending on itsductility. To achieve a certain size or shape, multiple passes through progressively smaller dies and intermediateanneals may be required.[6]Tube drawing is very similar to bar drawing, except the beginning stock is a tube. It is used to decrease the diameter, improve surface finish, and improve dimensional accuracy. Amandrel may or may not be used depending on the specific process used. A floating plug may also be inserted into the inside diameter of the tube to control the wall thickness.Wire drawing has long been used to produce flexiblemetalwire by drawing the material through a series of dies of decreasing size. These dies are manufactured from a number of materials, the most common beingtungsten carbide anddiamond.
The cold drawing process for steel bars and wire is as follows:
Similar drawing processes are applied inglassblowing and in making glassoptical fiber.[7]
Plastic drawing, sometimes referred to ascold drawing, is the same process as used on metal bars, applied to plastics.[8] Plastic drawing is primarily used in manufacturingplasticfibers. The process was discovered byJulian W. Hill in 1930 while trying to make fibers from an earlypolyester.[9]
It is performed after the material has been "spun" into filaments; by extruding the polymermelt through pores of aspinneret. During this process, the individual polymer chains tend to somewhat align because ofviscousflow. These filaments still have anamorphous structure, so they are drawn to align the fibers further, thus increasingcrystallinity,tensile strength, andstiffness. This is done on adraw twister machine.[9][10] Fornylon, the fiber is stretched to four times its spun length. The crystals formed during drawing are held together byhydrogen bonds between theamide hydrogens of one chain and thecarbonyl oxygens of another chain.[10]Polyethylene terephthalate (PET) sheet is drawn in two dimensions to makeBoPET (biaxially-oriented polyethylene terephthalate) with improved mechanical properties.
