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8 Design Rules for Injection Moulded Products

Injection moulding is a versatile process and can be applied to almost any product. Although injection moulding is the industry standard for fabricating parts for products, it is not without its holdups. There are a few basic limitations to be taken into account. Here’s eight rules to follow when designing your product to ensure quality and durability:

Maximum wall thickness. The wall thickness of your part is directly proportionate to both the total materials needed to make the part and the cooling time required. By reducing the maximum thickness of the wall of your part, you reduce both these factors, resulting in lower cycle time, thus lower production costs. If the wall of your part is too thick or is inconsistent, problems can be caused involving sinkage and warpage, resulting in rejects and costly redesigns. Ensure your wall thickness is matched to the capabilities of the machine.
Corners. They can be a problem in a mould and will not always come out flush. It is almost impossible to force plastic into a perfect corner, and the result will look messy and amateurish, not to mention the strength of the part could be compromised. Round all corners where possible to enhance aesthetics and durability.
Applying a draft. A draft is a tiny angle — usually one or two degrees — applied to the mould on the face perpendicular to the parting line. This will allow for easy removal of the piece from the mould. Not including a draft in your design will mean the automatic ejection system of the injection moulding machine will not operate.
Ribs. Ribs are structural elements for your part, used for overall stability control. They are thin wall protrusions that extend perpendicularly from a wall or plane. Adding ribs rather than thicker walls will offer greater structural support.
Bosses. Bosses are hollow, cylindrical protrusions usually included in a design for accepting screws or other mating components of your deign. Ensuring these are secured by either attaching them to a wall or adding ribs will mean the bosses will remain straight and accept the part it was designed for without a problem.
External undercuts. A protrusion or depression in the outside of your mold — the cavity half — can create problems when trying to separate parts from the mold. Adjust your parting line to accommodate this.
Internal undercuts or overcuts. Similar to external undercuts, these protrusions or depressions are on the inside of your mold — on the core half. Adjust your parting line to accommodate this.
Threads. If your mould contains a thread, always arrange it perpendicular to the parting line. This will ensure that the fragile thread is not damaged. It is better, if possible, to not include a thread at all in your design. Simplifying your design will lower the chance of something going wrong.

Injection moulding design ensures a quality product and the countless possibilities far outstrip the limitations. Designing for a quality injection moulded product is the essence of the design process, and these limitations are the guidelines for creating a versatile end product.