Injection moulding

Injection moulding (United States Injection Molding) is a manufacturing technique for making parts from thermoplastic material. Molten plastic is injected at high pressure into a mold, which is the inverse of the desired shape. The mold is made by a moldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is very widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. It is the most common method of production, with some commonly made items including bottle caps and outdoor furniture.

The most commonly used thermoplastic materials are polystyrene (low-cost, lacking the strength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a co-polymer or mixture of compounds used for everything from Lego parts to electronics housings), nylon (chemically resistant, heat-resistant, tough and flexible - used for combs), polypropylene (tough and flexible - used for containers), polyethylene, and polyvinyl chloride or PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser).

Mould

Considerable thought is put into the design of moulded parts and their moulds, to ensure that the parts will not be trapped in the mould, that the moulds can be completely filled before the molten resin solidifies, to compensate for material shrinkage, and to minimize imperfections in the parts, which can occur due to peculiarities of the process.

Moulds separate into at least two halves (called the core and the cavity) to permit the part to be extracted; in general the shape of a part must be such that it will not be locked into the mould. For example, sides of objects typically cannot be parallel with the direction of draw (the direction in which the core and cavity separate from each other). They are angled slightly; examination of most household objects made from plastic will show this aspect of design, known as draft. Parts that are "bucket-like" tend to shrink onto the core while cooling and, after the cavity is pulled away, are typically ejected using pins. More complex parts are formed using more complex moulds, which may require moveable sections, called slides, which are inserted into the mould to form particular features that cannot be formed using only a core and a cavity, but are then withdrawn to allow the part to be released.

The resin, or raw material for injection moulding, is usually in pellet form, and is melted by heat and shearing forces shortly before being injected into the mould. The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame. This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection. The sprue and runner system can be cut off and recycled. Some moulds are designed such that it is automatically stripped from the part through action of the mould.

The quality of the moulded part depends on the quality of the mould, the care taken during the moulding process, and upon details of the design of the part itself. It is essential that the molten resin be at just the right pressure and temperature, so that it flows easily to all parts of the mould. The parts of the mould must also come together extremely precisely, otherwise small leakages of molten plastic can form, a phenomenon known as flash. When filling a new or unfamiliar mould for the first time, where shot size for that particular mould is unknown, a technician should reduce the shot size and nozzle pressure so that the mould fills 90-95%, thus creating a "short shot". Then, using that now-known shot volume, pressure can be raised without fear of damaging the mould. Sometimes factors such as venting, temperature, and resin moisture content, can effect the fomation of flash as well.

Traditionally, moulds have been very expensive to manufacture therefore they were usually only used in mass production where thousands of parts are being produced. Moulds are typically constructed from hardened steel or aluminium. The choice of material to build a mould is primarily one of economics. Steel moulds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made in the mould before wearing out. Aluminium moulds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for moulding hundreds or even tens of parts.

The process of Spark erosion has become widely used in mould making. As well as allowing difficult shapes to be formed by first machining the inverse, the process allows pre-hardened moulds to be shaped so that no heat treatment is required. Changes to a hardened mould by conventional drilling and milling normally require annealing to soften the steel, followed by heat treatment to harden it again. Spark erosion is a simple process in which a metal shape, often made of copper or graphite, is very slowly lowered onto the mould (over a period of many hours), which is immersed in parafin oil. A voltage applied between tool and mould causes sparking at the closest point, and consequent erosion.

Injection process

Heated plastic is forced under pressure into a mould cavity; it is then clamped together and solidifies into the shape of the mould creating the part.

Resin pellets are poured into the Feed hopper, a large open bottomed container, which feeds the granules down to the screw. The screw is turned by hydraulic or electric motor that turns the screw feeding the pellets up the screw's grooves. The depths of the screw flights decreases towards the end of the screw nearest the mold. As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets. Heaters on either side of the screw assist in the heating and temperature control around the pellets during the melting process. The screw travel limit switches set the distance the screw moves.

The hydraulic system pumps oil from the oil tank to firmly close the male and female mold parts, that run along the tie bar; the liquid resin is then injected into the mould. Since the molds are clamped shut by the hydraulics, the heated plastic is forced under the pressure of the injection screw to take the shape of the mold. Some machines are run by electric motors instead of hydraulics or a combination of both. The water-cooling channels then assist in cooling the mould and the heated plastic solidifies into the part. The cycle is completed when the mold opens and the part is ejected (with the assistance of ejector pins within the mold).

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Plastics industry

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Mould

Injection process

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