Gating Design
A gate is a small opening through which melted polymer enters the die cavity. The gating strategy is one of the most important considerations when designing a mold for a plastic part. A part can have one gate--- or multiple gates. Moreover, there are different kinds of gates that the designer can choose from.
The preferred strategy is a single gate, because multiple gates exacerbate weld and melt line problems. However, multiple gates may be necessary if the required melt flow is very large.
Rules for optimizing gate design:
- Place the gate(s) in a location that ensures the mold will be filled rapidly and uniformly.
- All things being equal, a single-gate system requires a higher injection pressure than a multiple-gate system.
- Weld lines reduce strength, and affect the cosmetic quality of a part. Therefore, consider how the location of the gate affects the melt flow pattern and position of the weld line. (Only disc-type gates produce parts free of weld lines.)
- Locate the gate in the thickest area of the part.
- Select a gate location that allows the air inside the cavity to escape during the injection process. Otherwise, the finished part will have burn mark. Air trapped inside the cavity can also result in a short shot (a shot which fails to fill the entire geometry of the part).