Design tips for injection molding

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The injection molding process is widely used in the production of large batches of parts, as it produces a comparatively low amount of scrap and has high repeatability. The versatility of the injection molding process demands much broader design considerations. Most of the design considerations will be made in the mold after the product requirements are established.
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Some of the factors that affect injection molding design are: the end use of the part (single product or for assembly), its dimensional and mechanical requirements, and its ability to withstand elements such as chemicals or pressure. Here are some vital tips to keep in mind when designing for injection molding.
1. Carefully choose the right materials for your design
Different injection molding materials offer different properties. For example, some injection molding materials offer more dimensional stability than others. Similarly, some stick better to adhesives than others. Material design takes into account: temperature, pressure, and biological and chemical interactions.
Thermoplastic resins can be broadly classified as amorphous and semicrystalline . While semi-crystalline thermoplastics offer greater chemical and electrical resistance, their amorphous counterparts are much more dimensionally stable and more impact resistant. Material selection may affect the required tolerance level or certain characteristics, such as wall thickness.
2. Consider the tolerance of the part
Tolerances are affected by shrinkage that occurs during the cooling process. Amorphous materials like PLA typically have tighter tolerances than semi-crystalline materials like PEEK.
Close tolerances make it more expensive to produce, but may be necessary for your part to fit or function properly, especially if used in an assembly.
We recommend that you contact your supplier at the design stage to discuss the tolerance standards they use.
For example, DIN 16901 contains a general table of tolerances as a reference for different materials. If your supplier uses this standard and you need tighter tolerances or other standards, they will ask you to provide 2D drawings.
3. Choose the right wall thickness
There are a few key points to keep in mind to ensure you choose the right wall thickness for your injection molding design:
Thinner walls shorten cycle time and lower the cost of your part. For many applications a wall thickness of 1.5-2.5mm is sufficient, but you can also refer to recommended wall thicknesses for different materials.
Unlike CNC machined parts, plastic injection molded parts benefit from consistent wall thickness. If a piece is thicker in one section than another, a sink mark will appear there.
Non-uniform wall thicknesses also cause warping, as these walls cool and shrink at different rates. If non-uniform thickness is required, the thickness change should not exceed 15% of the nominal wall thickness and should always have a smooth or tapered transition to achieve a high quality part.
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