bubbles in PC

[targetdesign]

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

 

[plastx]

Is it in a thick area or far end of the part from the gate?

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

 

[patprimmer]

Not enough info supplied, but generally bubbles are caused by:-

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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[gambro]

We had experienced a similar problom a few years back in a section of the mold where the melt flow first passed through a thin-walled section, then a thick-walled section and at last through another thin-walled section. The bubbles formed in thick-walled section. We think the cause is air trapped in the thick-walled section during mold filling. We got rid of this problem by tapering off the thick-walled section towards the following thin-walled section in flow direction to ease air passage and by lowering the injection speed a bit.

 

[patprimmer]

gambro

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.

 

[injectionmolder]

Try to use injection profiles since this definitely sound like you are going from a thin wall, to a thick section as the cavity is fill. Boost as fast as you can until you are getting into the thick wall, then slow dramatically down with higher pressure in order to maintain low speed followed by holding pressure. This might help you to set up the thick mass before the gate fezzes off.

Regards