Figure 1. Casting process
Moulding is the process in which a pliable raw material is poured or forced into a rigid frame or model called a mould, where it solidifies or hardens. The solid material is then removed from the mould. The material to be moulded may be liquid plastic, glass, metal or ceramic raw materials.
Moulding is an efficient way to make large numbers of an object quickly and inexpensively. Many of daily life use products are used moulding as a manufacturing process such us food containers, metal cans and case of electrical appliances.
Different type of moulding process will be briefly introduced as follows:
Casting
Casting is a mass production process which involves molten materials (such as metals, plastics or clay) being poured into a mould without pressure, allowed to solidify and then extracted for use. Casting can be thought of as a method for reproducing something - whether a mere part or a single unit by itself.
Figure 1. Casting process
Compression moulding
Compression moulding - the moulding material is generally preheated, is first placed in an open, heated mould cavity. The mould is closed with a top force or plug member, pressure is applied to force the material into contact with mould areas, and heat and pressure are maintained until the moulding material has cured. Figure. 2 shows the simple compression moulding procedure.
Figure 2. Compression moulding process
Injection moulding
Injection / Extrusion moulding - plastic material is heated until it reaches a state of fluidity, is then forced into a mould cavity through a system of channels, and then cooled in the mould until it again reaches a solid state, conforming in shape to the mould cavity.
Figure 3. Extrusion moulding machine
Figure 4. Injection moulding machine
Reaction injection moulding (RIM) is a method for rapid production of complex plastic parts. The process is similar to injection moulding except that a reaction occurs within the mould. The process uses thermoset polymers (commonly polyurethane) instead of thermoplastic polymers used in injection moulding.Two liquid reactants flow at high pressure generated by the pump into a mixing chamber. In the mixhead the streams impinge at high velocity, mix and begin to polymerize as they flow out into the mould cavity. Because the mixture is initially at low viscosity, low pressures are needed to fill the mould.
Figure 5. Reaction Injection Moulding process
Thermoforming
Thermoforming is a manufacturing process for thermoplastic sheet or film. The sheet or film is heated between infrared, natural gas, or other heaters to its forming temperature. Then it is stretched over or into a temperature-controlled, single-surface mould. The sheet is held against the mould surface unit until cooled. The formed part is then trimmed from the sheet. The sheet trim is usually reground, mixed with virgin plastic, and reprocessed into usable sheet. There are several categories of thermoforming, including vacuum forming, pressure forming, twin-sheet forming, drape forming, free blowing, and simple sheet bending.
The earliest method was vacuum thermoforming (called simply vacuum forming when it was developed in the 1950s), in which negative pressure is used to draw a preheated sheet into a mould cavity. The process is explained below in its most basic form. The holes for drawing the vacuum in the mould are on the order of 0.8 mm (0.031 in.) in diameter, so their effect on the plastic surface is minor.
Vacuum thermoforming: (1) a flat plastic sheet is softened by heating; (2) the softened sheet is placed over a concave mould cavity; (3) a vacuum draws the sheet into the cavity; and (4) the plastic hardens on contact with the cold mould surface, and the part is removed and subsequently trimmed from the web.
Figure 6. Vacuum forming
Blow Moulding
Blow moulding is the ability to produce hollow objects out of thermoplastic. Plastics that are used for blow moulding include polyethylene (PE) squeeze bottles that could contain washing detergent, polyvinyl chloride (PVC) that could be used for cooking oils, polyethylene terephthalate (PET) for carbonated beverages, our others. A round, solid bottom, hollow tube know as perform is made from the heated plastc by either extrusion or injection moulding. The perform is then positioned between the halves of a split mould, the mould closes, and perform is expanded against the mould by gas pressure. The mould is then cooled and the product is removed. Figure. X shows the schematic diagram of blow moulding.
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