Screw boss thread stripped during screwing operation

[DaiLG]

Hi All,

I have an issue with a screw mounting hole in a polycarbonate moulding. The screw is screwed into the hole to a set torque. The screw softens the plastic as it turns forming the thread. The problem is that the thread is being completely stripped and separated from the moulding during this operation. The defect mouldings are dimensionallyy identical to good mouldings.

We have ruled out the screw and all other process inputs as the root cause (torque setting, driver speed, user influence) and determined the issue lies with the moulding itself. Our thoughts are the mechanical properties of the moulding have somehow changed (from what we usually see). We are thinking a loss in strength or lower melting point? The question is how?

Any thoughts / help would be appreciated!

Thanks
D

 

[MikeHalloran]

First thought is that initial testing was done with 'hot' parts, e.g. fresh from the mold, at a temperature somewhat elevated from room temperature, and now you are working with parts that have been in a (possibly cold) warehouse for a while, or were just shipped by truck through a snowstorm, or something like that.

Second thought is that maybe the molder is sneaking in a little regrind, which is known to deteriorate PC mechanical properties.

Third thought; maybe you were using clean, dry screws before, and recent lots of screws are coated with oil or wax or something that attacks PC.

Fourth thought: You got a new assembler, who uses hand lotion, which attacks PC. You probably should demand use of gloves regardless.




Mike Halloran
Pembroke Pines, FL, USA

 

[DaiLG]

Hi Mike,

Thanks for the reply, much appreciated.

Your first, third and fourth thoughts have already been disproved through our investigation so far - though the reply is very appreciated and I'll make a note of these factors for future investigations .

Your second thought is very intriguing though. I did not know regrind could alter the properties of PC and I do know the vendor does chomp up defective mouldings on occasion to re-process them. Do you know of any literature on this I could look at?

I have the data sheet for the PC and am looking at what we can measure. Unfortunately we don't have the equipment available to conduct tensile, impact or hardness testing (at least fairly anyway). The only thing I have been able to measure and confirm is a 2% drop in the density of the defect mouldings when compared to a sample of a good batch. This doesn't tell me much (as I don't have the materials knowledge) other than the good and bad mouldings are different. Any thoughts on this?

Thanks,
David

 

[MikeHalloran]

Your PC supplier should be able to provide some info about mechanical properties vs. % regrind, which is why part drawings or specs may forbid any regrind.

You could approximate the tests you mention, with simple rigs made from stuff you can buy at Harbor Freight. Your results won't satisfy ASTM, etc., but for A|B comparisons, there's no need.


Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

PC which has not been dried correctly prior to moulding undergoes a reduction in molecular weight due to hydrolysis. This happens well before there are any visible signs such as silver streaks on the part.
Are you (op) making these parts?

www.tynevalleyplastics.co.uk

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[DaiLG]

Hi Pud,

No, we buy the parts from the moulders.

Could this result in a reduction of the strength or hardness of the material though? I'm not very clued up on plastics moulding so apologies if it's a bit of a silly question.

Thanks,
David

 

[MikeHalloran]

This would be a good time to spend a day at the moulder's, getting to know them, and asking stupid questions, especially so if they are running your parts or similar parts in the same material.



Mike Halloran
Pembroke Pines, FL, USA

 

[btrueblood]

Does hydrolysis result in a reduction of strength? Yes.
Does the risk of hydrolysis increase with increased levels of regrind? Yes.

Regrind can also be contaminated with other stuff (oils, greases, mold release compounds...) that impair good molding performance.

Just the heating and shearing of being injection molded reduces the molecular weight of the material, i.e. every cycle through the molding machine is degrading the material.

see

https://www.uni-obuda.hu/journal/Ronkay_39.pdf

 

[Compositepro]

Pay attention to the part temperature and room temperature when installing the screws. Temperature will have a significant effect on mechanical properties.