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Voids That Are Trapped Air
In the last problems and solutions blog we discussed vacuum voids. These are holes formed in the areas of the last plastic to cool. A description of how this happens is contained in our Injection Molding Problems and Solutions training course.
There is another type of hole inside a molded part, quite rare, that is called trapped air (or air bubbles) in the plastic melt. How can you tell the difference between a vacuum void and trapped air? A vacuum void will cause a sink mark (depression at the plastic surface). If you’re still in doubt about which it is, raise the mold temperature or heat the plastic part and a the sink mark will develop from the vacuum. An air bubble will cause a dimple on the surface.
Air can be trapped between the melting plastic granules in the injection barrel and be carried all the way into the mold during mold filling. In most plastic parts air bubbles escape out the mold parting line. But in thin-walled parts the air can be trapped. Its pressure causes the dimple. This is likely to cause part ejection problems. Trapped air is rare in molded parts but when it occurs it can be very baffling to the molder. What can you do about it? I’ve given you the cause. Tell us what you think the solution is and we’ll tell you if you are right.
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Could an increase in shear heat in the hotrunner tips (due to obstruction) cause gass bubbles.I am asking this question because in the past few weeks I came across trapped air problem in one or two cavities in an 8 cavities mould. and only purging through the hotrunner system as a last resort worked. Nothing else would. Could it be that the material was degrading due to excessive shear heat in the tips and producing gass as a result. you thoughts on this please.
The answer is yes. Degradation can cause air to become trapped. Excessive shear in the hot runners might be degrading the plastic. Since 2 of the 8 cavities are having the problem, you could swap out those tips and put them in other cavities to see if you can re-create the problem. You should also confirm (if you haven’t already) that the bubbles are actually trapped air and not voids. An easy way to do this is to heat the plastic part in the area of the bubble. If it expands, it is air. If it contracts, it is a void. Thanks for your question.