RE: [rp-ml] SLS Powder Life - example situation.

I think what you have proposed in your SLS hypothetical encyclopedia is a
very valid argument. I think the best way to settle the argument is to run
7 layers of a part, take the part cakes. . . mix them. . .then take a sample
and send it to be tested. At the same time, build a 7 layer tall build,
take the part cake, mix it. . . then see what the melt flow for each one is.
Of course, you'll need to start with the exact same material so your results
will correlate.

My contention on powder handling has always been to aim for a steady state
process. The only way to do this is to either fix your melt flow index or
you can get a close approximation by having a consistent remix rate.
Running powder over and over and over does not result in the best surface
finish, build to build consistency, properties, etc.

But, if I were to guess on the results. I would think the part heater
setpoint will have more effect on the powder degradation than the time it is
heated in the part cake. The reason I say this is that the melt flow index
of powder in the overflow cartridges see about the same drop in quality as
that in the part cake. . . not exactly the same, but close.

*********************************************
David K. Leigh (254)933-1000
Harvest Technologies fax(254)298-0125
Rapid Prototyping Services dkleigh@harvest-tech.com

-----Original Message-----
From: Scott Tilton [mailto:stilton@protoprod.com]
Sent: Wednesday, June 23, 2004 3:23 PM
To: rp-ml@rapid.lpt.fi
Subject: [rp-ml] SLS Powder Life - example situation.

So a co-worker of mine is explaining to some folks about the recycleability
of SLS powder.

He started expressing the age of the powder in terms of how many times it
has cycled through the machine. (how many times the powder has been in a
partcake)

When he asked me for confirmation, I said that he was basically right, but
that I thought a measure of HOW LONG the powder had been in a partcake was
more relevant than the number of times it had cycled through the machine.

I got somewhat of a dismissive look and a general blow off with a statement
like "yes, there are many of other minor factors that affect the degredation
of the powder."
(mental note, don't correct boss in front of visitors)

Later I was asked to explain what I meant and I used an example:

Say you are building a bunch of parts that can be arranged in a nice 2"
layer in the partcake.
You need to build as many of these parts as you can with a the material you
have on hand - which happens to be just enough to completely fill the feeds
on your machine (2500+ in my case)

In one extreme, you could load up the machine and run 7 of the two inch
"layers" of parts all in one tall build.

In another extreme, you could just probably run 5 or 6 builds, each of which
only had one "layer" of parts.

My boss says this second example would be stupid thing to do. You'd waste
the powder on the warm up and cool down layers. (and if you use a heat
fence . . . you'd waste those sacrificial parts each time)

I wasn't so sure.

I mean . . . say that the 2 inch tall "layer" of parts requires 5 hours of
build time.
Run that 7 layer partcake and the powder in the bottom of the build has been
in there cooking for somewhere over 30 hours.
30+ hours being maintained at that high partcake temperature.

The individual part layer builds each only subjected the powder to the high
partcake temperatures for around 5 hours.

At then end of the two examples . . . you may have one or two layers more
parts by running them in the single build, but your powder seems like it'd
be more aged / degraded to me.

Yes I know . . . there are other cost factors . . setup time, and the
material wasted by warm up and cool down stages, exactly how hot is your
partcake/cylinder/ piston heaters etc.
But remember . . I was only talking about maximizing the life of your
powder.

Anyone have any opinions on this topic they'd like to share.
Heck, maybe some of you hardcore SLS'ers (like you melt flow indexers out
there) have data to support your opinions.

Assuming you've read this far . .thanks for your attention.
Hope this doesn't open me up to another FLOOD of spam.

Scott Tilton
Received on Thu Jun 24 16:54:15 2004