Plastics - Industrial Processing

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Injection Moulding

The plastic injection moulding process produces large numbers of parts of high quality with great accuracy, very quickly. Plastic material in the form of granules is melted until soft enough to be injected under pressure to fill a mould. The result is that the shape is exactly copied. Once the plastic moulding has cooled sufficiently to harden the mould opens releasing the part. The whole injection moulding process then repeats.

  • CLAMPING - the moving and fixed platens of the injection moulding machine holds the mould tool together under pressure.
  • INJECTION - the molten plastic that has been melted from pellet form in the barrel of the moulding machine is injected under pressure into the mould.
  • DWELLING - after the molten plastic has been injected into the mould pressure is applied to ensure all cavities are filled.
  • COOLING - the plastic parts are then allowed to solidify in the mould.
  • OPENING - the moving platen moves away from the fixed platen separating the mould tool.
  • EJECTION - rods, a plate or air blast then aids ejection of the completed plastic moulding from the injection mould tool.
  • The length of time from closing the mould to ejecting the finished plastic moulding is the cycle.
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Extrusion

Extrusion moulding is a manufacturing process used to make pipeshosesdrinking strawscurtain tracks, rods, and fibre. The granules melt into a liquid which is forced through a die, forming a long 'tube like' shape. The shape of the die determines the shape of the tube. The extrusion is then cooled and forms a solid shape. The tube may be printed upon, and cut at equal intervals. The pieces may be rolled for storage or packed together. Shapes that can result from extrusion include T-sections, U-sections, square sections, I-sections, L-sections and circular sections.
One of the most famous products of extrusion moulding is the optical fiber cable.

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Blow Moulding

Blow molding is a manufacturing process by which hollow plastic parts are formed. In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and injection stretch blow molding. The blow molding process begins with melting down the plastic and forming it into a parison or in the case of injection and injection stretch blow moulding (ISB) a preform. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.
The parison is then clamped into a mold and air is blown into it. The air pressure then pushes the plastic out to match the mold. Once the plastic has cooled and hardened the mold opens up and the part is ejected.

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Vacuum Forming

Vacuum forming is a simplified version of thermoforming, whereby a sheet of plastic is heated to a forming temperature, stretched onto a convex, or into a concave, single-surface mold, and forced against the mold by a vacuum (suction of air). The vacuum forming process can be used to make most product packaging and speaker casings. It is also used to fabricate car dashboards. The first commercial manufacturer of vacuum-formed plastics was Robinson Industries of Coleman, Michigan.

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Rotational Moulding

Rotational Molding involves a heated hollow mold which is filled with a charge or shot weight of material. It is then slowly rotated (usually around two perpendicular axes) causing the softened material to disperse and stick to the walls of the mold. In order to maintain even thickness throughout the part, the mold continues to rotate at all times during the heating phase and to avoid sagging or deformation also during the cooling phase.

Rotocasting (also known as rotacasting), by comparison, uses self-curing resins in an unheated mould, but shares slow rotational speeds in common with rotational molding. Spin casting should not be confused with either, utilizing self-curing resins or white metal in a high speed centrifugal casting machine.

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Line Bending

Line bending is a thermoforming process, i.e. it is a method of forming a thermoplastic after it has been heated until it has become soft and pliable.
 

Line bending involves heating a thermoplastic sheet material over a strip heater until it becomes soft and pliable, then bending it, usually over a former.

Thermoplastics may be bent to any angle, using a jig or a former, or if the angle is not critical, simply by bending the thermoplastic sheet by hand and then holding it until it has cooled.

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Compression Moulding

Compression moulding is the most common method of forming thermosetting materials. It is not generally used for thermoplastics. Compression moulding is simply the squeezing of a material into a desired shape by application of heat and pressure to the material in a mould. Plastic moulding powder, mixed with such materials or fillers as woodflour and cellulose to strengthen or give other added qualities to the finished product, is put directly into the open mould cavity.
The mould is then closed, pressing down on the plastic and causing it to flow throughout the mould. It is while the heated mould is closed, that the thermosetting material undergoes a chemical change which permanently hardens it into the shape of the mould. 

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Drape Forming

Drape forming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" when referring to thinner gauges and certain material types, is heated in an oven to a high-enough temperature that it can be stretched into or onto a mold and cooled to a finished shape.

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