Re: Keltool

From: Connelly, Rob - BDRC (connelly@bdrc.bd.com)
Date: Thu Nov 07 1996 - 18:11:46 EET


In response to Michael Wilson's reply:

I could give you my $.02 concerning working with 3D-Keltool inserts. Here
at BD we have performed around 10 projects making 3D-Keltool inserts for
injection molds from both SLA and Sanders ModelMaker patterns. We have been
quite pleased with the results.

>Thanks for the article on Keltool it was very informative, up until the
last line,
>which claims a accuracy of 0.001 inch per linear inch.

I have found this to be generally the case, with a few outlying data points.
 Of course, this depends on the shrinkage, as described below.

> there is to my knowledge an 8% shrinkage overall from the
>..STL master.

The A-6 material shrinks .008 inch/inch, which would be 0.8%. As a basis
for comparison, this is on the order of the shrinkage of injection molded
polycarbonate.

>We have found that shrinkage can be the villan in any form of SLA
>replacement material for tool creation.

Now here is where it is all pulled together. The .008in/in shrink in the
3D-Keltool takes place at a point in the process when the insert in
unconstrained, i.e. it is not in a mold or shell, so that all of its
features can *pull in* together. This is one of the things that sets
3D-Keltool apart from investment casting and injection molding, where the
part is being tugged and distorted by metal or ceramic while it is trying to
shrink to its final size. Also, we are talking about a bulk shrinkage (far
less internal stress) as opposed to the additive stresses applied during the
SLA process using some of the old high-shrinkage materials. The result for
3D-Keltool is a very consistent shrinkage anywhere on the part.

To take advantage of this, scale your master up by 1.008 (just like you do
for the plastic shrinkage) and when the 3D-Keltool shrinks, it will shrink
right into your dimensions!!

One final concern, however, is whether 3D-Keltool can hit EXACTLY .008in/in.
 For the majority of my projects, they have done just that, but in a couple
of instances, the shrinkage was more like .012in/in. There are methods to
control this during the 3D-Keltool process, however, and I am confident that
with 3D on board, this issue will be addressed before most of us know it
even exists!

Good luck, and keep up the good work on your impressive polycarbonate
injection into ACES molds!

 - Rob
 --------------
Rob Connelly
Becton Dickinson Research Center
21 Davis Dr.
Research Triangle Park, NC 27709
919-990-2263



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