Re: creep values

From: eschl (
Date: Tue Mar 27 2001 - 21:18:34 EEST

"David K. Leigh" wrote:
> My mom used to do this to encourage me to make up my bed. Things like "I
> guess you really like to live in a pig pen"or "Oh, that's good. Is that the
> best you can do?"
> Anyway, I think it only works with perfectionists.
> Bottom line, SLA resins are the creepiest materials (in more ways than one).
> Our experience with SLS is that what you get -- stays.
Thats true, where as with the reaction the (photo)polymers change their
As long as you do not have 100 % conversion, the reaction especially of
epoxy resins goes on until 100 % are reached. So properties are
When 100 % are reached other aging processes become more considerable.

So it is quite hard to have two SLA samples with the same properties and
to know the creap behaviour of any stereolithography part.

As example for the glass transition temperature Tg the DiBenedetto
Equation can be used:

(Tg-Tg0)/Tg0= a*x/(1-b*x)

Where a and b are material constants, Tg0 is the glasstransition
temperature of the uncured resin and x is the conversion.

With high conversion, there is a rather dramatical change of Tg and so
also the material properties (Stiffness, elongation at break etc.).

The parts you get out of the SLA machine might have a conversion of
60-80 %.

At the Nottingham or european conference back in 1996 we showed the
relation of the youngs modulus and the conversion, which is quite the
same as for Tg.

" Dusel K.-H.; Eschl J.; Wiedemann B.: Improvement of Part Accuracy -
Investigation into the Basics of
    Photopolymerisation; In: 5th European Conference on RPM, University
of Nottingham, 4th-6th of June, 1996. "

>I think LOM varies
> depending upon sealant used and humidity. I'm not sure about the rest. I
> don't have any quantitative analysis. Just a thumbs up, thumbs down
> approach.
> *************************************************************
> David K. Leigh ph (254) 933-1000
> Harvest Technologies, Inc. fax (254) 298-0125
> Rapid Prototyping Services

Johannes Eschl
IKP, Institut fuer Kunststoffkunde und Kunststoffpruefung, Universitaet
Boeblingerstr. 78E, D-70199 Stuttgart
Tel.: (+49) 0711 / 641 23 06 Fax :  (+49) 0711 / 641 22 64
E-Mail :

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