All voids are trapped gas? 1

[kunc]

I heard the argument again that "All voids in injection molded parts are due to trapped air or gas, and not due to shrinkage. Shrinkage causes internal cracks, but not voids". For this discussion, voids are defects that are roughly spherical. Any comments?

 

[patprimmer]

Voids, by definition are void of anything.

bubbles in mouldings can be trapped gas, but are more likey voids, especially when in the middle of a thick section only. They form if the shrinkage is to high, and occurs after the surface has set hard and the gate has frozen, or holding pressure is dropped off, but the core of the moulding is still molten.

A simple test is to heat the surface near the bubble untill it is soft, and see if it sucks the surface in, or blows it out. Regards
pat

 

[ornerynorsk]

shrink most definitely will also cause voids. Thick sections will almost always have voids due to this.

 

[TheAnswerGuy]

Voids can be caused by moisture in the material.
Due to improper drying.
I have witnessed this problem mostly with PVC and Nylon.

You can also find voids in components where the molder has used a foaming agent.

Voids will typically be spherical but I have seen many different shapes. The non-spherical shape (void) is likely caused by flow patterns or cooling.

 

[chaving]

Voids are a vacuum and are caused by differential skrinkage, they are affected by hold pressure, hold time, melt temp, mould temperature and gate size.

Air bubbles, comonly incorectly are refered to as voids and are caused by volitiles in the melt, this could be in the form of water vapour or indeed some polymer additive that has been over heated.

Regards

Chaving