An air trap occurs where the melt traps and compresses a bubble of air or gas between two or more converging flow fronts, or between the flow front and the cavity wall. Typically, the result is a small hole or a blemish on the surface of the part. In extreme cases, the compression increases the temperature to a level that causes the plastic to degrade or burn.
Air traps are often caused by converging flow fronts which occur as a result of racetrack or hesitation effects, or from non-uniform or non-linear patterns. Even when the part has balanced flow paths, air traps can occur at the end of the flow paths, as a result of inadequate venting.
Use this result
Determine how severe the air traps are and where they can occur on the part. Air traps on surfaces that are visually unimportant can be acceptable.
Use the fill time result with the air trap result to confirm filling behavior. Assess the likelihood of air traps actually appearing at these locations.
Use the air trap plot to determine the probability of an air trap occurring at a particular location. The higher the value, the higher the probability that an air trap can occur.
The Air traps result can reveal the following problems in your plastic part:
- Burn marks
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If the air in an air trap is under enough pressure, it can ignite and burn the plastic, causing a burn mark.
- Short shot
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A short shot is the incomplete filling of the part. Sometimes an air trap is not vented. It is not compressed quickly enough to cause a burn mark. It can cause a short shot or leave bubbles of air or gas.
- Other surface blemishes
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Even if an air trap does not cause a burn mark or short shot, it can still leave a surface blemish on the part.
Things to look for
The following methods prevent air traps:
- Use flow leaders/deflectors.
- Increase the injection speed. Increased injection speed can eliminate air traps caused by converging flow fronts and hesitation.
- Decrease the injection speed. Decreased injection speed reduces air traps caused by poor venting, and prevents burn marks.
- Decrease the part wall thickness ratio to reduce race tracking.
- Move the injection locations so that the air traps form in areas that are easy to vent, such as the parting plane.