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Letter 11097
Internal Stresses in Plastic
Products
Dear Sirs,
I am working in one of the biggest manufacturer of Plastic
products in Saudi Arabia. In our factory we are encountering a lot of
problems, especially wobbling and concentricity on producing Blower
wheels and Slingerfans. These engineering items are made of ABS and
PP resins with or without glass fiber. The problems with glass fiber
as a reinforcing filler is becoming more after 24 hours.
My questions are:
1) How the internal stresses occur in plastic products?
2) What are the main reasons of creating Internal Stresses?
3) How to completely avoid or reduce the effect of internal stresses
on the above mentioned defects?
4) How to measure the internal stresses imposed in the plastic
products directly after processing?
5) What are the injection processing parameters affecting the
internal stresses?
Mohamed Ibrahim Mohamed
Ali
plastic factory - Riyadh, Saudi Arabia
Hi Mohamed!
I'm not really too qualified in answering your questions although
in 'plastics' (heavy fabrication for 40 odd years) but may I make
some comments.
l. Having used and bought VERY, VERY OFTEN injection moulded
centrifugal fan impellers (Brinkmann, GmbH in Frankfurt) in natural
PP, they never distorted, they never became eccentric but, to obtain
perfect balance, now & then a very small piece of PP was
fused/welded onto the wheel by the manufacturer for a perfect
harmonic balance...
2. I have the feeling, if I may say this, that you do not have
highly trained injection moulders, ie. people who know about
injection temperatures & cooling cycles and, of course, about the
very important and accurate tooling necessary to make these
impellers.
3. Have you not contacted your resin suppliers? Surely they should
offer some words of wisdom?
4. Try approaching a local but friendly injection moulding Company
for some advice
You should not be having these problems. Try Hoechst Chemicals
(Germany) for advice on the PP resins or Union Carbide for data, if
they will impart it, on their excellent ABS resins.
I hope that you will succeed. Best of luck.
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Freeman Newton
- White Rock, B.C. Canada
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August 13, 2008
The glass filled material is always a problem in post moulded
components.
simple example is explained below to understand the effect of GF.
when you burn a paper, it shrinks. why? Bcos the (bamboo)pulp, which
are like lines in structure, tend to converge or contract. Hence the
paper shrinks or frills. The same tendency with the glass fibres. It
cools by the time and tends to contract. This will give you the
distortion in the component. To avoid that, proper mould cooling to
be provided so that the resin is well set in the mould itself and the
cooling time also. The ejection of the component to be even and then
handling and packing of the items are also critical. Try a aging
process before assembly. you can see some result.
bhoopathy
- Chennai, India
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