Plastic Injection Molding Design Considerations

Plastic Injection Molding Design Guidelines

Simple and complex shapes can be injection molded, however certain design guidelines should be followed:

• Make sure the shape will cool properly by avoiding thick areas.
• Avoid thick 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.
• Use an approximately uniform wall thickness throughout your design.

• Keep walls thin - typically between 1/32" and 1/10". This allows for proper cooling and reduces cost by minimizing use of material. Thin walls also reduce problems with material shrinkage. Although some unevenness will occur due to shrinkage, walls as thick as 1/5" can be used. Keep wall thickness at least wall length / 50. Keep 90 deg walls under 0.25" high. Keep thickness of ejection pin surface wall at least .07".

• To strengthen parts, instead of using thicker walls, use additional structures such as ribs. Use fillets at the base of ribs.

• When using a rib make it about half the main wall thickness. 

• Round corners and edges wherever possible.

• For easy release of the part from the mold, add a slight taper to the sides (typically ~ 2 deg) - especially for textured walls and walls higher than 0.25".

Lighter colors hide flow patterns better than dark colors. Choose the right material from the table. Drawing dimensions should be of the final part - material shrinkage will automatically be considered in the design of the mold. Use raised text instead of recessed text when possible. Where walls meet at a 90 angle, round inside and outside to at least .05" radius - sharper outside corners can create molding problems and sharper inside corners will increase tooling cost. Keep holes at least .015" from edges. It should not be possible to fully hide a 0.3" diameter ball anywhere inside the material.