Dear Chris, Josh, Mitchell and jialin
Thank you very much. Your reply is very helpful. The flowability of original
powder is quiet good. At temperature of part bed below about 100 degree,
powder levelling is perfect (but warpage is serious when sinter it by laser
because the low part bed temperature). At a higher temperature, powder
clumps and can't level. After I decreased the temperature, the powder can
level again. However, after heat and sieve the powder for several times to
find out the optimal part bed temperature 'sinter window', I found that the
powder can not be leveled even at room temperature. I am not sure if they
add flow enhancing materials in the original powder. If they do, maybe the
flow enhancing material has gone after several heating? I observed the
powder by SEM, no obvious change occured in shape between original powder
and recycle one. The sphericity of the two powders are also very good and
almost near 1. So it must be something changed at the powder surface, as you
said, the surface energy, or static. Then is there any instrument or method
to prove it?
I haven't measure the powder size distribution yet and will do it later. The
powder is as-received from a Japanese Co. We can not tailor the
distribution. :(
We want to develop PE/HA object directly at SLS as biocompatible implant. So
it's better not to add any additive, especially non-biocompatible, in the
powder.
One more question, why DTM use roller but not a doctor-blade? because
anti-rotation roller can compress the powder? From my observation, I think
the roller rotation cause clump of powder.
anyway, thank you very much for your nice reply, they are very helpful and
valuable to our work! I will post my progress later in the list.
Yours sincerely
Dr. YANG Shoufeng
Design Research Center
School of Mechanical & Production Engineering
Nanyang Technological University
Singapore, 639798
Tel: 65-7905332
Fax: 65-7924062 (local) 65-7911859 (oversea)
Email:ysff@hotmail.com; msfyang@ntu.edu.sg
> ----------
> From: Chris Sutcliffe[SMTP:c.j.sutcliffe@liverpool.ac.uk]
> Sent: 2000Äê7ÔÂ11ÈÕ 17:41
> To: 'Yang Shoufeng (Dr)'
> Subject: RE: SLS powder level problem
>
>
> Dear Yang
>
> as you reduce powder size you increase its tendency to clump together as
> you
> have seen in your experiments. This comes about because of an increase in
> the specific surface energy of the particles. You can illustrate this by
> considering the fact that by the fact that two footballs when placed
> together don't want to stick whilst your powder does. This problem is
> rectified in commercial sinterstation by the used of specially developed
> flow enhancing materials which are mixed with the raw powder. I don't
> know
> the specific materials used but perhaps that's the next question you need
> to
> ask.
>
> The other thing is I was wondering was is your powder static free. You
> may
> be able to use some anti static spray mixed in with your powder to reduce
> this.
>
> Dr. Chris Sutcliffe
> Rapid and Micro Manufacturing Research
> The University of Liverpool
> Department of Engineering
> Ashton Building
> Brownlow Hill
> Liverpool
> L69 3GL
>
> Tel. (0151) 794 4316
> Fax. (0151) 794 4675
> Email c.j.sutcliffe@liv.ac.uk
>
>
>
>
>
> -----Original Message-----
> From: owner-rp-ml@ltk.hut.fi [mailto:owner-rp-ml@ltk.hut.fi]On Behalf Of
> Yang Shoufeng (Dr)
> Sent: 11 July 2000 08:58
> To: 'rp-ml@bart.lpt.fi'
> Subject: SLS powder level problem
>
>
> Hi
>
> We tried fine PE powder in the SLS ( average particle size about 10
> microns). However, the powder on the part bed clumped to flake-like clump
> after roller rotated through. Simple test showed that the flowability of
> the
> powder turned bad after several recycles. Does anyone know the mechanism
> of
> anti-rotate roller for powder levelling? Which properity of powder will
> influence the mechanism? Does this problem belong to the field of fluid
> mechanics?
>
> Thanks in advance.
>
> Yours sincerely
>
> Dr. YANG Shoufeng
>
> Design Research Center
> School of Mechanical & Production Engineering
> Nanyang Technological University
> Singapore, 639798
>
>
> For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
>
For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
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