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.
- You
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
volumes.
- 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.
Plastic Injection Molding Design Considerations
- Use rounded corners.
- Add draft to walls to allow easy ejection.
- When the design does not allow for additional structures to improve strength, consider using a stronger material, such as glass fiber filled plastic.
- Consider specifying a fire retardant material when necessary.
- A small rough spot will appear at the gate; a small line will occur at the parting line; and a round mark will occur at ejector pins.
- Consider specifying where to place the gate and parting line and what surface finish to use - polished, matte, textured.
- Since a seam between two halves of a box is difficult to fully hide, consider making the joint pronounced - to make it look like it is decorative.
- Contoured parts warp less than flat parts.
- Review existing injection molding components for additional ideas and techniques.
- See additional tips and the material selection table linked in the sidebar
