Plastic Injection Molding
Injection molding machine
Mixer formed by injection molding
Winch reel side formed by injection molding
Sign letter formed by injection molding
Plastic injection molding produces plastic parts by forcing molten plastic into a mold where it cools and hardens. In plastic injection molding granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are moved forward by a screw-type plunger, the plastic is forced into a heating chamber, where it is melted. As the plunger advances further, the melted plastic is forced through a nozzle into the mold cavity. The mold remains relatively cold so the plastic solidifies almost as soon as the mold is filled. Custom steel tooling is required and adds to the initial cost but is quickly amortized.
Plastic injection molding provides low cost at moderate to large quantities. Plastic molding is an extremely versatile process for producing a wide range of simple or complex plastic parts with a good finish. Almost any 2D or 3D shape can be achieved. However draft is required in most cases as the shape must allow ejection from the mold. Side holes and even threaded holes are possible though they complicate the tooling.
Examples of parts made by plastic injection molding include:
Plastic injection molding can form most plastics including:
Cost reduction options include:
- Minimizing size of part and material volume.
- Minimizing side holes and recesses.
may be able to arrange multiple pieces in one mold by connecting them with small
bars ~0.1", however the sub-components must not have widely varying
- Minimize side features and threaded holes.
- Consider using living hinges.
Several terms are commonly used for injection molding. Ejection pins are rods that push the part out of the mold. The gate is the
location where plastic is injected. The parting line is the location where
the two mold halves meet - a thin line will appear at this location. A living hinge is a thin connection provided between two sections of a
molded part so that it can be used as a hinge, e.g. a box, with a lid,
molded as one piece. Insert molding is where rigid part is inserted into the cavity prior to injection - a screw driver with a plastic
handle is an example of insert molding.