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Jobber Instructions


#1

On several occasions now, with two different casters, I requested
they make an RTV mold of my wax and cast several pieces in different
metals, the idea being to get them identical in size to each other
and retain critical measurements. What I get back is one piece in
original size and the extras are 10% smaller. So apparently they are
casting the wax and then making vulcanized rubber molds of the
resulting cast piece.

Am I being unclear in my instructions? Is there some caster’s lingo
that will get the idea across without misinterpretation? BTW, these
are both otherwise good casters with good reputations.


#2
What I get back is one piece in original size and the extras are
10% smaller. 

I’m going out on a limb commenting because my experience is limited,
but…

I would say that casters don’t like to make a mold of anything less
sturdy than metal. That’s the mindset, so it will take a very
explicit conversation on the phone with the actual caster, with lots
of repetition, before you can be sure you are communicating.

I don’t mean to impugn casters-- not at all-- it’s just a pretty
firm habit of thought and behavior.

Can you bow to the inevitable and make your original 10% oversized?

Noel


#3

If you ask them to make a “cold” mould from your wax first, and the
use that to do the rest you should find they all come out the same
size.

Chris Mead


#4
What I get back is one piece in original size and the extras are
10% smaller. So apparently they are casting the wax and then making
vulcanized rubber molds of the resulting cast piece. 

Sounds as if they even clean the casting to make a new mould. I don’t
see the advantage with not just doing it the way you asked. I never
let castors keep my moulds and if I don’t make them myself I have an
outsider make them. Somebody isn’t giving or taking instructions
right.


#5

Hi Neil,

I’m probably late in this response since I’ve gone to Orchid Digest.
I don’t think you’re being unclear in your instructions at all. I
think your casters are just terrified of messing up the RTV mold,
destroying your original wax and having nothing to show for it.
They have a reliable casting systems so they are pretty much
assured of successfully

casting your wax and having a metal model to work with. That’s
obviously the route they’ve taken.

Many of us have worked with complicated metal models that took
several attempts before a good production mold was made. Just
imagine having just one shot to make a production mold with a
precious hand-carved wax or a single milled or RP produced wax that
won’t survive the molding process. Also, a lot of the RTV molding
materials aren’t as easy to cut as vulcanized rubber. They can’t be
pulled and stretched as much or cut in corkscrews as easily.
Complex molds can be very difficult to guarantee in RTV.

You have several options:

(1) You can have them use a ZERO shrink rubber and mold the single
accurate casting and go from there and see if you can get more
acceptable results than the 10% smaller castings. In addition, you
might look into more technical investments like those used in dental
casting since those have very, very minimal shrink since casting
crowns and dental appliances require exacting fit.

(2) You can absolutely absolve them of any responsibility if they
don’t successfully RTV mold your wax. You will have to take on ALL
the risk and not penalize them for destroying your wax (the
assumption is that you use good jobbers and they do everything
professionally possible to insure success). If you’re running on a
deadline, again, you accept all the risk if things don’t work out.
If you’re not willing to accept this risk, how can you expect them
to? It makes perfect sense for them to hedge their bets to the best
of their ability and to at least be assured of giving you
something.

(3) You can carve your wax larger to compensate for the 10%
shrinkage so your job shop can cast your wax, mold the resulting
casting and produce the end result castings you desire in the
proper size, assuming you’ve compensated correctly in making your
"master" wax.

(4) If you can produce waxes via CAD/CAM and send multiples as
backups for the RTV mold making process, your jobbers may feel more
secure in attempting to follow your instructions to RTV mold the
original wax and make subsequent waxes and casting directly as you
desire. (NOTE: Not all RTV products are zero shrink so you might
want to find out what RTV material your jobbers use.)

(5) If none of these options are acceptable, you’ll have to do
everything yourself. At least you’ll have total control and complete
responsibility and I don’t mean this is a sarcastic, judgmental
way. I do a lot of things myself simply because I don’t want to be
in a position to regret an outcome that I didn’t retain control of.
(It’s sort of like the old question, “Do I set the horrendously
expensive center stone that could destroy my business if I crack it
or do I send it out to a setter?” ) Also, it doesn’t help work
relations to blame someone else when bad things happen because they
do from time to time. Whenever we job something out, we ultimately
accept the risk. In the final analysis, I think your jobbers are
doing what they think is best and most responsible. They think they
are looking out for your interests as well as their own.

I hope my suggestions help in some way and good luck with future
endeavors.

Donna Shimazu


#6

Hi Noel;

I would say that casters don't like to make a mold of anything
less sturdy than metal. 

Actually, it’s common to make an RTV mold directly from a wax model.
There is less risk, since castings sometimes don’t work out and RTV
molds, if you know what you’re doing, are a pretty safe bet. And
there’s less shrinkage taking a mold directly from a wax than casting
the article and having the metal shrink when it cools, then the
additional reduction due to finishing and polishing.


#7

Hello Chris;

If you ask them to make a "cold" mould from your wax first, and
the use that to do the rest you should find they all come out the
same size. 

I have to disagree, with caveats. There are compounds which claim to
have “near zero” shrinkage, and injection waxes making similar
claims, but in reality, even if the RTV mold produces a cavity
exactly like the model, the injection wax, being hot when injected,
will shrink at least a miniscule amount when it cools. The metal,
when poured into the investment, will also shrink minimally when it
cools. Together, these two phenomenon will result in a measurable,
albiet minimal, amount of shrinkage. Finally, when you finish the
casting, you will be removing some metal, hence, more reduction.
When making a prototype, this shrinkage/reduction must be considered
and compensated for, even when using RTV molds directly from a wax
model.

David L. Huffman


#8

Perhaps the design of the piece does not allow for the mould to be
cut while the master is made from wax, as it is much easier to cut a
mould with a metal master. We dont do this but we do inform the
customer that there may be a chance of having to remake the wax
master if the mould does not work. They also may be worried that the
wax is dirty and may react with the silicon mould. This can happen
if you handle the wax too much or clean it with some chemicals. The
other thing is to allow for the shrinkage when making the master.