Post 463 –by Gautam Shah
State or phase of a material is mainly due to the temperature and pressure. Materials have three common states, namely Gas, Liquid, and Solid. At ambient temperature it is the solid phase that has its own shape. Most substances are solid at low temperatures, liquid at medium temperatures, and gaseous at high temperatures. Shape-form of solids can be changed at various temperatures, commonly called ‘cold and hot forming’.
There are essentially three types of changes in form, the desired ones, uncontrolled ones and recoverable ones. The uncontrolled changes occur in the plastic phase, and can have many transient forms. Such altered shape-forms are non recoverable. Some changes occur at a yield stage of a material and can continue even after reduction-removal of the stress. The form of an object recovers after removal of the stress, such as in simple bending, and objects like springs. Alterations of forms of a material also occur due to movement of electrons (e.g. electroplating) and chemical reaction (rusting).
The desired changes in the forms of materials began with home cooking of foods, land preparation for agriculture and handling of clays for ceramics. This was further supported by exploitation metals, woods, grasses and natural fibres.
Shaping and Sizing: Forming processes change the dimensional format of materials. This was first learnt from dressing the kill, and later by rolling and forming the wet clays for pots. Grinding grains, sizing of firewood, shaping of rare stones for adornments and for creating utility tools were material forming processes. Plastic clay forms became stable on drying and then baking. Wood, stones, bones and farm grains could only be shaped by downsizing. Fibers could be formed by weaving. Metal nodules however could be hot and cold forged. Unlike any other material, metals can be reworked and joined by initially beating (forging), and in later periods by melting-casting. Glass beads could be cast-formed but not reshaped by beating.
Shaping by Rolling was the first process for shaping clay, leather, meat, etc. Rolling not only compresses, levels and stretches the material but it can emboss patterns and textures. Hot-rolling fuses several layers of materials. Hot and cold rolling of metal sheets are done to provide specific surface qualities.
Shaping by Drawing and Stretching were results of rolling or pressing processes. Thin sections were drawn by stretching animal guts and vines. Modern day wires, seamless pipes, rods and filaments are drawn products. Drawing is often accompanied by twisting-spinning, threading or rope forming. Extrusion of plastic is also a drawing process. During drawing and stretching constituents like molecules, grains and fibres get rationally realigned, providing greater strength and uniform surface qualities. Drawing and stretching are also carried out to mix new ingredients such as colours, liquids, plasticizers, lubricants, etc. Glass is blown for shaping.
Shaping by cutting, Stamping (die forming), Punching, though are seemingly modern processes, but there were many primitive techniques that achieved similar results for leathers or hides, and furs. Metals were shaped and surface treated by such processes. Ruptures can be performed on materials with substantial stiffness (density) but for softer materials shear cutting or punching is required. Stamping allows stretching the material into a shape cast.
Bending and Folding, are processes used for shaping ductile materials. Bending and folding can be done by controlled hammering. Bending and folding operations are also used for shear cutting. Bending relates to smooth curving, whereas, folding relates to a sharp or angular turning. In sheet metal fabrication bending and folding generally increases the planner stiffness, as in AC ducts, cans and tins, barrels, corrugated sheets, purlins, pipe etc. Bending and folding, have been most essential processes for vessels making.