RE: Old SLS powder . . . why does it do that?

From: David K. Leigh (
Date: Tue Dec 10 2002 - 22:12:38 EET

What you are seeing is a variance in shrinkage associated with old/bad
powder. As the powder gets older(more heat cycles), it crosslinks. This
causes messed up shrinkage, so your squares will end up looking like hour
glasses. If you have "marginal" powder, the sidewalls wont totally sink in
but you will get areas of greater shrinkage that will appear like acne on
the part.

To remedy, use good powder. :) Or. . . you can blend the old with some new
at a less than optimal value. This will reduce your material cost and
possibly give you better results. The thing you'll want to ensure is that
the "test" material is the same as the "run" material.

David K. Leigh (254) 933-1000
Harvest Technologies, Inc. fax (254) 298-0125
Rapid Prototyping Services

-----Original Message-----
From: []On
Behalf Of Scott Tilton
Sent: Tuesday, December 10, 2002 9:53 AM
Subject: Old SLS powder . . . why does it do that?

Hi everyone,

I've got a question for the SLS users out there.

I had a job for some big ugly parts . .ones that don't have relatively low
requirements for surface finish and part accuracy.

So I loaded up some Duraform powder that is pretty much only good for
building such parts and ran myself off some test parts to try and get some
calibration values on it.

With old powder, that's not the easiest thing in the world to do actually.

I resort to actually making one of the parts in question .. (or at least a
partial cross section of it) and then using that as the basis for further
parameter tweaking.

I also ran the standard beam coupons and scale pyramids for the heck of it.

They come out with some funky sidewalls.
They seem to be concave in any place where there is a significant amount of
cross sectional area being scanned by the laser.

Can anyone help to explain why this happens or any techniques for lessening
the effect?

Take a beam coupon for instance.
It is essentially a small plate 4" x 1" x .25" (x, y, z directions

With old powder, the top and bottom .025" of the part have a pronounced lip
on them.
They are significantly larger than the middle .200" (in the Z direction) of
the part.

What causes the extra shrinkage or contraction in the middle?

Or . . . if you like . .why doesn't the bottom and the top of the part
contract as much as the middle?

Curious minds want to know.

Thanks in advance.

Scott Tilton

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