Injection moulding

Injection molding is a manufacturing technique for making parts from plastic 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 from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Injection molding is very widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars.

Mold

Considerable thought is put into the design of molded parts and their molds, to ensure that the parts will not be trapped in the mold, that the molds can be completely filled before the molten resin solidifies, and to minimize imperfections in the parts, which can occur due to peculiarities of the process.

Molds 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 mold. 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 molds, which may require moveable sections which are inserted into the mold to form particular features that cannot be formed using only a core and a cavity, but are then withdrawn to allow the part to be released.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 molding, is usually in pellet form, and is electrically melted shortly before being injected into the mold. 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 molds are designed such that it is automatically stripped from the part through action of the mold.

The quality of the molded part depends on the quality of the mold, the care taken during the molding 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 mold. The parts of the mold 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 mold for the first time, where shot size for that particular mold is unknown, a technician should reduce the nozzle pressure so that the mold fills, but does not flash. Then, using that now-known shot volume, pressure can be raised without fear of damaging the mold.

The moulds are very expensive to manufacture therefore they are usually only used in mass production where thousands of parts are being produced. The moulds are constructed from hardened steel.

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.

The PC/ABS granules are poured into the Feed hopper, this is pretty much a large open bottom container, which feeds the granules down to the screw. The screw is turned by hydraulic motor that turns the screw feeding the granules up the screw's grooves. The screw travel limit switches set the distance the screw moves. As the PC/ABS granules move up the grooves of the screw, heaters on either side of the screw heat the granules to high temperatures changing their state from solid to liquid plastic.

The hydraulic system uses pumps and oil from the oil tank to close up the male and female mould parts that run along the tie bar, then the liquid PC/ABS is injected into the mould. Since the moulds are clamp shut by the hydraulics the heated plastic is forced under the pressure to take the shape of the mould. The water-cooling channels then cool the mould and the heated plastic solidifies into the part.

Note: PC/ABS stands for amorphous polycarbonate and acrylonitrile-butadiene-styrene terpolymer blend.

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Categories: Manufacturing