Injection Molding

  • Process

    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.

    Applications

    • Gears
    • Enclosures
    • Robot Parts
    • Auto Parts
    • Toy Parts
    • Packaging

    Cost Considerations

    • 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.

    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.