Bob
We do not agree with Mueller or Jacobs.
Have they heard about measuring critical dimensions on the casting SLA master,
then rerun to make corrections? Stereolithography repeats very well.
Also, if the resulting plastic molded part is not in specification,
modifications
can be made to the rapid tool. Part dimensions are controlled by how the rapid
tool is processed ie. mold temp, stock temp, injection pressure, and speed,
just
to name a few.
We have been and plan continuing to use SLA casting masters in the Albright
Process. We have our difficulty at times holding tolerances, but no more
than a steel mold.
In holding tolerances the toughest part is how much the plastic shrinks,
which is
always a range not a fixed number. On a 10" part if the resin is acrylic
the shrink is 2-8 thousandths per inch, for the part it could be .020 to
.080 inch, and we have
seen it exceed this.
So how do you hold plus or minus .005? By making an "informed" guess, which
for me is based on the combination of knowledge and "real" experience.
Bob, I wonder how many injection molds Paul has built.
I thank you for your reply
At 05:20 AM 10/8/1999 EDT, you wrote:
>
>
>I don't know how many of you were able to attend the recent Modern Mold and
>Tooling show at McCormick Place in Chicago. It was very good for the budding
>rapid tooling industry to be included with the big boys in the tooling
>business. There were many of us their showing our different processes and
>there was much interest. Most of the seminars were very informative. I
>especially enjoyed the one on DTM Rapid Steel 2 and soon to be 3. We finally
>have some friendly competition for the 3D Keltool process which I'm glad to
>see. In the Rapid tooling seminar there were 5 or 6 speakers on the panel of
>which I was one. Each with a different process and all very good. The last
>speaker was Tom Mueller who with Dr. Paul Jacobs put together a presentation
>showing the accuracy limitations of RP/RT methods. The presentation was
very
>slick and seemingly well presented. They went through and did some kind of
>study on a bunch of SLA parts measuring the accuracy. Then using the
>shrinkage and possible warpage of each of the different rapid tooling
>processes they plotted out the accuracy curves of each type of Rapid Tooling
>process. Their conclusion was that SLA could not be used to build rapid
>tooling and that the inherent accuracy of all the Rapid tooling techniques
>were not accurate enough to even hold plus or minus .030" on even a 3 inch
>piece. I beg to differ with them and feel their conclusions were very flawed
>and biased. The study they did on the SLA parts was done mostly on older
SLA
>250 and 500 type machines all with Somos resins. My company builds SLA
Master
>patterns on a regular basis for tooling masters. We have shipped over 300
>jobs in the last year and a half. We do not recommend the use of Somos resin
>for Tooling Masters. We only recommend SL5510 resin it is the only resin
>rigid and stable enough that we know of to do the task. We also only
>recommend making tooling masters on 3500 machines or the newer 7000
machines.
>The 3500 series has the laser mounted directly above the vat and it is apt
to
>give the most accuracy of all the 3D Systems designs. The SLA 7000 with the
>.001 layer thickness gives the best replication over curved surfaces. I'm
>sure 3D Systems and others are working very hard on the next generation of
>machines which will open up whole new avenues for Rapid Tooling as well. We
>have 3D Systems technicians come in and tune our laser every three months.
We
>also do weekly test builds to insure the accuracy of our equipment. In
>practice we find that most SLA master patterns measure within .002-.003 of
>the desired dimensions in the X and Y. There is an inherent difficulty
with Z
>steps that forces the Z levels to only be accurate as the slice thickness.
We
>typically leave stock in areas where Z depth is critical. We attempt to do
>all cavity masters in the center of the vat and try very hard not to mix
>tooling builds with part builds. We have to know at all times the exact
>diameter of the laser spot to insure the correct laser offset values. We
>highly recommend that the Tooling masters be finished only by master
>craftsman. Finishing SLA masters is almost equal to finishing CNC cut EDM
>electrodes the step downs are usually about the same. So the moldmakers have
>no trouble finishing and certifying the master patterns. My family has been
>in the tooling business for many generations as far as we can trace several
>hundred years. As a family legacy I was required to finish several complex
>pieces to a tolerance not to exceed plus or minus .0002" without the use of
>any machine tools of any kind. My father a master craftsman himself
carefully
>inspected all my work and when he was satisfied with my ability gave me his
>cherished tool box and tools. Which I will in turn give to my three sons as
>they will have to pass the same test along with bachelers degrees in
>Engineering. A toolmaker is not worth much if he cannot finish anything to
>precise dimensions. This is all really a moot point anyway because all
molds
>built in the mold industry are fully guaranteed to meet customer
>specifications. This is how the mold business works. The moldmaker does not
>get final payment until the final molded product meets the customers
>specifications. This means if the tolerances required are very loose the
>mold will be relatively inexpensive. If the tolerances are very tight the
>mold will cost much more. Some molds we have build cost in excess of a
>million dollars. Any moldmaker worth his stuff will always know exactly
where
>the critical dimensions are and make sure there is stock there for final
>tweaking and finishing. What we do when presenting 3D keltool process as an
>alternative is we ask the customer to let us know exactly which dimension we
>are required to hold to exact numbers and which dimensions can be relaxed.
We
>are always up front with the limitations and make sure the customer is very
>aware of what he is getting. The number of critical dimensions have a direct
>bearing on the final price. We have built 3D Keltool molds for automotive
>certified production molds. The cost was not very much less than a
>conventional mold because we had to certify each dimension. On the other
hand
>we have built other molds using 3D Keltool Process which the customer was
>willing to accept whatever we were able to provide as long as the part
>functions. These molds were able to be delivered much more quickly and
>inexpensively, aproximatly half the cost of any other method available
today.
> In practice we have found the 3D Keltool process to hold pretty true to the
>advertised accuracy of .002"inch / per inch accuracy. This does not mean
that
>every block that comes out meets these specifications. In reality we have
>nearly 20% losses using our process. We have stringent inspection standards
>in place all developed by 3D Systems, and if the block does not meet the
>specifications it is failed. The customer never sees these blocks however.
>This is all a part of doing business. Any dimensions that have to be held to
>a closer tolerance have machining stock left and are finished via EDM or
CNC.
>As I said earlier all molds are completely guaranteed at all times to meet
>customer specification. If they do not we do not get paid until they do. We
>have at times had to put double or even triple the quoted amount into molds
>to satisfy customer demands. This is our loss however not the customers.
This
>is the way this business works and perhaps if Mr Mueller had understood the
>business a little better he wouldn't have gone belly up. I feel their
>presentation did a great injustice to our struggling industry. I am at a
loss
>as to what gain they had whilst they were sitting there giggling through
>their presentation. But personally I was hurt by it. This industry is
>struggling badly enough without having to endure this nonsense. I really
>don't know the history between these guys and 3D Systems, DTM, or the other
>struggling rapid tooling companies and I don't really care. All I know is
>they threw a devastating blow to our industry with that presentation and it
>appears only to satisfy some sick vendetta. I also am irritated at Modern
>Tool and molding to allow two member of their editorial board to present at
>their own conference. Especially such a devastating and unfounded bashing of
>our industry with no opportunity for rebuttal. Pen in hand you have not
>heard the end of this issue from me. My company has invested millions into
>this RP/RT industry trying to help develop it into something great. And I am
>not about to let a couple of grumpy old men destroy it for the rest of us.
>Bob Morton
>Fusion Engineering
>
>For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
>
>
www.albright1.com
fpt.albright1.com
V 978-422-8051 F 978-422-8058
92 Albright Road
Sterling,MA 01564
For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
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