bubbles in PC
bubbles in PC
(OP)
Hi.
We have some PC injected parts that show, in a very specific area of the part (and not anywhere else) bubbles trapped inside the plastic material.
Can anyone tell me what can be causing this?
PC doesn't absorb much water, so my guess is that this isn't a moisture problem....besides if it was, bubbles would be scatered all over the part...
Can anyone help me out?
Thanks
Gerard
We have some PC injected parts that show, in a very specific area of the part (and not anywhere else) bubbles trapped inside the plastic material.
Can anyone tell me what can be causing this?
PC doesn't absorb much water, so my guess is that this isn't a moisture problem....besides if it was, bubbles would be scatered all over the part...
Can anyone help me out?
Thanks
Gerard






RE: bubbles in PC
If so it may be a "sink" where the PC is shrinking as it cools but the outside walls have already solidified causing a vacuum bubble to occur instead of drawing the wall inward.
Mike
RE: bubbles in PC
Insufficient pack in the area the bubble is located, mainly due to changes in section thickness between the bubble and the gate. These are actually contraction voids, not bubbles. They are normally consistent and in the middle of the thick areas and look very clean. If you warm the material, and the surface of the moulding sucks in toward the void, this proves it is a vacuum and not a gas trap
Trapped air can also form bubbles, but this tends to also produce some burn marks at the inside surface of the bubble. Heating the moulding will result in the trapped gas expanding out toward the surface.
Moisture. Depending on a number of factors, moisture may appear as silver streaks, but if there is only a little moisture, it can propagate the formation of a void due to out gassing of the moisture in the material.
Although PC only absorbs a small amount of moisture, it is also very intolerant of moisture and small amounts can cause problems such as voids, silver streaks and a significant reduction in impact strength
Regards
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RE: bubbles in PC
RE: bubbles in PC
The bubble was almost certainly caused by lack of effective hold up pressure as the thin section froze off therefore blocking flow before the thick section contracted as it froze.
Slower injection might have also resulted in a little more pack, as higher pressure could be used without inducing flash.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: bubbles in PC
Regards