Non destructive testing/inspection for voids in plastic moulds 1

[joeclarke]

I am working with a company that make injection mouldings. Their main concern is internal voids that can lead to failures in the field. Does anybody know of any CHEAP ways of checking for voids. The material is invisible to X-ray. Ultrasound can be used but only on a sample inspection as it has to be sent off site to a university to use their equipment.

 

[TheAnswerGuy]

joeclarck,

I would tackle the cause of the problem first!

I have heard of people who don't fix the problem, use a sensitive scale to weigh their parts.

 

[joeclarke]

Sorry, I was a little unclear. They have found voids during testing and are taking appropriate measures to fix the root causes but need to have a NDT method to measure the sucess of these measures.

 

[dhuffy]

Hmmm...this could be a tough one. You did not mention the material. Is there a clear version of the material you are using?

If so, try molding it to help verify the changes in the molding process and elimination of the voids. Then go to the colored material and see if the changes hold for the actual material. You may have to do some initial destructive testing to verify the actual material but running the clear material would give you a good head start.

Good luck.

Dave

 

[patprimmer]

Weigh parts
Reduce melt temp to minimum that will pack the mould. Increase mould temperature if necessary to obtain a good part at the minimum melt temp
Increase injection pressure until machine max or just short of flash
Weigh new parts
Increase injection hold time
Weigh parts
Keep increasing injection hold time untill parts do not increase in weight.
Physically cut parts to inspect for voids
If voids present, increase gate/runner size
Weigh parts
Repeate process above.
Increase gate/runner etc untill no voids present.
Use final weight as QC test.
Clear material might help, but colours can change moulding characteristics, including tendency to form voids, so once good solid mouldings are achieved clear, mould colours, then test as above

Regards
pat

 

[TomH]

You don't say what material you're molding or how thick it is, but this might work:

Heat the finished items and look for bulges due to expansion of any gas bubbles. Also, a high intensity lamp might reveal lighter colors where any bubbles exist.

 

[plasgears]

Void avoidance is the best path. The key is constant thickness design and packing the mold under time and pressure. In-process sectioning of samples will show the voids nicely.

The Mu-Cell process injects nitrogen into the melt, which results in micro cells instead of voids. Inquire on the internet.