5 Plastic Injection Molding Tips

Injection molding is used in the manufacturing industry to produce a wide variety of parts, from small AAA battery boxes to automobile body parts. Every injection molding process begins with a component design and mold creation. The mold is machined precisely to help features in the desired part. During the process, a thermoset or thermoplastic material is fed into a heated barrel, mixed, and allowed to pass through this mold, where it cools and hardens. Thus, when designing a mold, extra care should be taken to ensure high quality and minimum waste of materials. This post provides some plastic injection molding tips that will help achieve desired quality, and optimum results in any injection molding project.

Five Important Injection Mold Design Tips

These mold design tips are beneficial to both – a novice and an experienced engineer. These points will help make injection molding processes beneficial and productive.

  • Appropriate Draft: A proper draft is necessary to avoid friction between the mold surface and part surface during the injection molding process. The choice of draft will depend on various factors such as location, height, and surface texture of the desired feature. The draft is required for surfaces that lie parallel to the mold opening.
  • Uniform Mold Temperature: When heated by steam or oil, the temperature of each half of the mold should be uniform throughout within +/- 5oF. A uniformly heated mold:
    • Is easy to fill
    • Generates parts with less warpage
    • Produces uniform surfaces
    • Improves dimensional stability
  • A properly designed mold will also have uniform temperature throughout.

  • Resin Characteristics: The resin characteristics largely vary across various injection molding projects. Here, we will discuss some important features in detail:
    • Stronger resins require less material. However, weaker resins may need more to meet your material requirements.
    • Shrinkage affects moldability. This is especially applicable for filled resins, which are affected by uneven resin flow.
    • Viscosity largely varies among resins.
  • Uniform Wall Thickness: Uneven wall thickness may hamper the quality of a finished product. The wall thickness depends on the gate placement. The resins shrink on cooling. Also, thick areas are prone to shrink faster than thin ones, which may produce warps or uneven body parts. The ideal wall thickness depends on the type of resin used and the function of the part.
  • Venting: During the molding of thermosets, polymerization takes place and volatile substances are produced. These substances react with air in the chamber, and may raise the temperature to 700°F -800°F (375°C – 425°C). If the gases remain trapped in the chamber, they may leave burn marks on parts, so it is important to let them escape through vents. Improper venting may result in uneven filling of parts, cause dimensional problems, or have lesser electrical/physical strengths. A typical vent design will include :
    • Phenolic Vent: ¼” (6mm) width, depth 0.003″ – 0.0035” (0.8 mm – 0.09 mm).
    • Polyester Vent: ¼” (6mm) width, depth 0.003″ – 0.0035” (0.8 mm – 0.09 mm).

The above features will help you design even and appropriate molds. Also, you can approach an experienced engineer to understand various facets of mold designing in detail.