effect on part quality when using recycle material 1

As always, that depends.... One of the faults you can have is that fire retardancy is severely compromised. At 50% almost all properties are likely to be degraded. For "ordinary" non-critical parts I usually specify no more than 7-10% regrind.

As insideman says, it all depends, to such an extent that in some cases material can be moulded reground run again, 5 times over with negligible drop in properties, whereas any regrind can turn some mouldings to junk. It really does all depend

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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Thanks for the advise. Will too much of regrind material used causing inconsistency in part diemnsion significantly? Could you recommend a practical test method to determine the content of the regrind material.

Best regards,
ct

Also, one polymer over another may be more allowable in percentages of regrind. I have seen many parts which are acceptable from 100% regrind. On the other hand I have seen a good number of parts fail when using just 5% regrind.



Frank M.
Tradewind Resources

There is no reliable test for %age regrind used, as said several times already, IT ALL REALLY DEPENDS ON A LOT OF FACTORS.

Do you really want to know what is causing your problem or are you looking for someone to agree with the opinion you have already formed?

Give us details like polymer type, filler and reinforcements if used, colour, a quick description of the part, including size, section thickness, gate position and gate size.

Also an indication of moulding conditions would help, as would some indication of the machine age, control system, condition and size.

Where, when and how you measure the part would also help as some materials shrink and or expand for a considerable time after moulding

Regards
pat pprimmer@acay.com.au
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.

The material is general purpose red in colour ABS, the part with a rectangular tower of cross section 35mmx26mm (at tip of tower and draft angle of 1 deg taper down) and height 50mm. It's found that the tolerance on 35mm (measured at the tip of the tower)varies in the range of 0.2mm in different run. The molding is done on the same machine and the parameter has been no change.
The readings obtained are 35 and 34.8
Wall thickness is 1.5mm



Best regards,
ct

OK

Now we are talking.

Unfilled ABS is normally a very dimensionally stable material and fairly forgiving of most things that vary shrinkage.

My thoughts are it could be any one of the following

1) There was a change to the colour formulation
2) There was a change in the base resin (different supplier or change in spec from the same supplier).
3) The ring check valve on the machine was worn and not holding pack.
4) Dare I say it, regrind material might be used and is not consistently charging the barrel of the moulding machine.
5) The melt viscosity of the material is varying due to use of regrind or due to suppliers variation and this is affecting the pack of the moulding.
6) The gate might be to small, giving less than optimum pack.

My main suspicion is that the mould is not fully packed, as other variables such as degree of crystallisation do not have much effect on ABS, and there are no fillers to complicate the situation.

If you are normally getting optimum pack plus a safety margin, you have a repeatable process, but if you are getting less than optimum, normal variations within the process can have a large effect.

Try weighing the parts to see if the smaller parts are also lighter. Make sure you always weigh parts from the same cavity if the mould is multi cavity.

If it is multi cavity, are all cavities the same, or does one cavity vary more than another?

You say the parameters are no change. While the parameters might be set the same, this can only be done within the accuracy of the controls of the parameters, and some parameters are often not controlled, like cooling water flow and temperature, or electrical supply voltage variations etc etc.

It is not that uncommon for two different die setters to get a different result with the same mould in the same machine with the same set up sheet, as they might set things differently when those things are not specified in the set up sheet.

Regards
pat pprimmer@acay.com.au
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.

Highly stressed engineering parts will not benefit from high % regrind. One automotive molder, under price pressure, I believe, resorted to high % regrind. The customer became pissed off from numerous field failures after several years without problems.

The job was pulled, and it was no surprise.