UHMWPE inclusion/subsurface defect detection

[McKagan13]

I have recently been tasked with scrap reduction in my manufacturing cell. The reason for most our scrap is through vendors, and we are currently working with them to reduce the levels. In any event, one main scrap contributer in our plastics division is inclusions/subsurface defects (voids, burnt resin ,etc...). Any visual defect is immediately scrapped if on a bearing surface. The problem is, a lot of these defects are not visually noticeable until the device is milled. Does anyone know of any processes that could identify the location of these defects? I would like to be able to scrap these components prior to accumulating the value associated with machining.

Thanks

 

[CoryPad]

Ultrasonic inspection can detect subsurface flaws.

http://www.geinspectiontechnologies.com/en/products/ut/index.html

http://www.olympusndt.com/en/index.html

http://www.sonoscan.com/ami.html

http://www.scanmaster-irt.com/

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

Using black material can remove all defects due to discolouration.

Regards

eng-tips, by professional engineers for professional engineers
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[Pud]

Nice one Pat - giving away trade secrets huh! Great solution is that black masterbatch!!!

Back to the problem.......

Not sure if it's your vendors giving you the problem, but if it is, my approach would be to get them to to do the corrective action. All you will be doing is their final/inprocess quality checks!!

Voids/burns/ are usually process errors - very occasionally part or tool design.

Not very cost-effective all round!

Cheers

Harry

 

[ZiggyK]

The "defect" needs to be verified as 1)already in the material or 2)caused by the machining operations.
Sometimes linear polyolefins (HMWPE, LLDPE, UHMWPE, etc) will crosslink from the heat associated with shearing (machining)the part.

 

[patprimmer]

I have never seen PE, other than cross linkable PE crosslink with short term exposure to air and heat up to it's melting point.

From the insufficient evidence provided I would guess the voids and burns are gas traps.

These can be controlled by moulding conditions, but this is a poor remedy.

Best remedy is to position the gate(s) correctly and have sufficient vents correctly placed and designed.

The concept that for plastic to fill a mould, it must also expel the air is sometimes lost. The fact that air compresses allows people to forget its there, but at high compression, it gets very hot and burns parts.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Demon3]

It's really amazin what compression of air can do. Here is a link to a science kit where you compress air in a syringe and it gets so hot that it ignites paper (the extra concentration of oxygen may help too).

http://www.teachersource.com/catalo.../?id=83c9f4a1d3b1cb9c3e29d544c5ffd3c2#FIR-150

Some really interesting stuff at that site.


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