Brent,
Thanks for the lead-in. I can supply some input on the pros and cons of
powder injection molding into rapid tooling. This has been the subject of
my research at Georgia Tech for the past year and a half.
The advantages include:
* A functional prototype that is representative of the material and the
process can be produced in one week's time.
* Very good surface finish and integrity, as the prototype is not from a
layer process.
* A wide variety of ceramic and metals are available in injection molding
mixes, or can be formulated comparatively easily.
* Multiple parts can be made in almost the same amount of time as other RP
methods.
* When manually molding into a polished ACES mold, you can see the mix
filling the mold, an asset in process development.
* Can be used for low run production - we have molded over 200 parts from
one mold without any signs of wear (compared to about a dozen parts from
plastic injection molding into the same mold.
Please note that I am using LOW pressure injection molding!
Some disadvantages:
* Shrinkage in the range of 10-15% is difficult to predict and can be
anisotropic and has been shown to be related to particle shape, mold
geometry, and gate position.
* Some part geometries can be difficult to mold and/or debind.
* The molding cycle time is not low enough for large volume production,
but advances in rapid tooling could change this.
For more information, see my web page:
http://rpmi.marc.gatech.edu/~judsonb
or email me at gt4684d@prism.gatech.edu
It seems that 3D Systems and Allied-Signal are also interested in this
approach. See their most recent press release on their web page:
http://www.3dsys.com
Beth Judson
Georgia Institute of Technology
Materials Science and Engineering
Atlanta, GA 30332
phone: (404)894-9158
>
> Marshall and the RPML,
>
> As a person who has focused the last 4 years of his life on the merits and
> drawbacks of fabricating green bodies of ceramics and metals using RP,
> I have some thoughts.
>
> For SLA:
> Mixing dark and/or non-reflective powders in your resin won't work,
> so that eliminates most potential ceramics/metals. If you get the ceramic
> and/or metal of choice to work as a green body, you will have a difficult time
> burning out the polymer because there is no porosity in the part and thus the
> burnout phase must proceed very slowly. If you want to do an infiltration of
> your part after polymer burnout, the amount of residue in the part may affect
> the wetting characteristics. On the plus side, SLA has some of the best
> accuracies and surface finishes in the industry.
>
> For SLS:
> There are some distinct benefits to using SLS. First of all, it is a
> powder process, so it lends itself naturally toward green body fabrication.
> When the green body is created there is continuous porosity throughout
> the green body, so the binder burnout phase proceeds at a more rapid
> rate. The trick is finding the right binder that will give the green strength
> you want, sinter in the SLS machine at room temperature and burnout clean
> in the furnace.
>
> For FDM:
> You can use either a ceramic/metal loaded filament or a "syringe"
> full of extrudable binder and ceramic/metal. The solids loading appears to
> be quite good. The drawback is that there is often segregation between binder
> rich and solids rich sections during the extrusion. In addition, FDM often leaves
> internal voids (although this has been addressed in the recent past) and that
> is a big drawback for certain materials. Lastly, we again have all of the area
> between the ceramic/metal particles full of binder, so the binder removal is
> more difficult. The benefit is this is an expensive process.
>
> For LOM:
> You can apply ceramic tape technology. The machines are relatively
> inexpensive. The drawbacks are, again, lots of binder that is difficult to remove.
> In addition, you have a lot of "waste" material that needs to be recycled,
> because, unlike the paper that is thrown away, the extra ceramic/metal tape is
> probably too expensive to just put in the trash.
>
> For 3DP:
> This process, like SLS, is a powder-based process. It has the
> same benefits as SLS. The drawback is that you can't order your own
> machine, yet, and produce your own parts in-house.
>
> For molds:
> This is a good process as well. Georgia Tech has done good work with
> low pressure injection molding of ceramics into SLA tools to make green
> parts. It seems to be a great process. In addition, you could injection mold
> into an SLS nylon part. Also, the "replication" processes such as ExpressTool
> and Keltool use this technique well. For service bureaus, this is a good
> way to create good ceramic/metal parts from RP masters. The main drawbacks
> of these processes are that they entail a number of steps and thus
> tolerance buildup becomes an issue. In addition, they often use binders that
> totally fill the voids between the ceramics/metals and thus they can be
> difficult to burn out.
>
> Those are my two cents worth. I could write a lot more, but I don't want
> to bore anyone more than I already have.
>
> Brent
>
> P.S. For those of you who have been following my work on EDM electrodes,
> there is a company that has recently been licensed to make available this
> new Zyrkon material for EDM electrodes. Contact me for more information
>
> _________________________________________________
> Dr. Brent Stucker
> University of Rhode Island
> Dept. of Industrial and Manufacturing Engineering
> Gilbreth Hall, 2 East Alumni Ave.
> Kingston, RI 02881
> ph: (401)874-5187
> fax: (401)874-5540
> stucker@egr.uri.edu
>
>
> ----------
> From: Marshall Burns[SMTP:marshall@ennex.com]
> Sent: Tuesday, November 11, 1997 4:21 PM
> To: Guy Allen Brady
> Cc: Murphy, Matt; RP Mailing List
> Subject: Green-body fabrication technique comparison
>
> Guy Allen Brady wrote:
>
> > Well, now, this is a nice surprise. I've been working on
> > Stereolithography of Ceramics for the past 2+ years. My suspensions are
> > 50vol% Alumina in a photocurable resin. There are two others groups (at
>
> Dear Allen and others,
>
> I wonder if I might ask a question that could spark some interesting
> technical debate.
>
> What are the relative pros and cons of the various techniques for
> fabricating green bodies of either ceramics or metals? The techniques that must
> be considered include direct fabrication of the green body:
> - Selective photocuring of a photopolymer/powder suspension (SLA)
> - Laser sintering of polymer-coated powders (SLS),
> - Extrusion of a polymer/powder paste (FDM),
> - Lamination of polymer/powder tapes (LOM),
> - and perhaps some other direct techniques.
> And one should also include indirect techniques of:
> - Fabricating a mold or master which is used to mold a powder system
> (old fashioned P/M as well as Keltool, etc.).
>
> What do people think about how these various techniques campare?
>
> Best Regards,
> Marshall Burns
> marshall@ennex.com
>
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