(no subject)

From: Gregory Pettengill (Cote Art & Engineering)
To: Jon Hunwick (Delcam  International PLC)
Date: Thursday, July 28, 1994
Forwarded to RP-ML by Greg, 94 08 03
Jon,
Thanks for this morning mail. 

Last you wrote;
>The process for making resin casts from a model is **exactly** the
>same as the methods which would be employed for getting castings from
>a wooden master model.

My point is that you can save the cost of having a mold made by drawing the 
FFFF model as a cavity mold to produce the part.

>If (as in the case of the oil gallery I mentioned before) the part
>cannot be cast as above, because there are overhangs, non-straight
>split lines etc. then things (obviously) get a lot more complex to
>figure out, but with care, patience (and a fair amount of black magic
>help from your friendly, local pattern maker) most things **can** be
>made in this way. However, there is a distinct trade off between the
>amount of effort necessary and the cost of making a FFFF model and
>then destroying it. 

I hear what you are saying.  I guess that it is my background as a Mold 
Maker & Die Designer that makes me think there is little effort in drawing 
in a parting line.  I suppose that this sort of thing could be intimidating 
to someone without that kind of experience.

>Another alternative I have seen (which works well for parts with no
>re-entrant features)

What do you mean by "re-entrant features"?

> is to simply cover the whole thing in some form
>of synthetic rubber.

Urethane and Silicon Rubber are the industry standards.

> I don't know what the stuff is called I'm afraid,
>but I'll try to find out.

There are many, please don't bother on my behalf.

> When cured it is faintly translucent,
>meaning that you can still see the part buried in the middle. To
>extract the part, the rubber is cut open with a bluntish instrument,
>which causes it to tear rather than slice. 
What you describe seems to me to be a technique commonly used in jewelry 
casting.  But a very sharp knife is used, one that has a curved blade, sort 
of what a woodworking gouge is like only much smaller.

>This leaves a very rough
>looking split face, but that can actually be an advantage as it means
>the two halves of the 'tool' always sit together correctly.

The term that I use for this is "register".

>It seems to me that the whole point of RP is to do things RAPIDLY!!
>Before choosing a course of action make sure you choose the most rapid
>method, which is not necessarily the most elegant!

Very true.  I find that the project with the best result is the one that was 
planned the best.  Also, the quickest process is the one with the fewest steps.

>Well, that depends on what you mean by flexible - bendy or versatile?
I meant "bendy".  Something around 40 durometer Shore D would be ideal for 
flexible rubber molds.

>The most bendy I've seen is the polythene type material used by the
>fused extrusion type machines, but the downside is that the models
>themselves aren't exactly spectacular. The best of this type of
>machine is from STRATASYS of Minnesota.

Some of us call it the "Sticky String Method".  I have never seen a piece of 
work from a STRATASYS machine, but I've read that detail is not very good.