Moulds from FFFF

From: Jon Hunwick (Delcam  International PLC)
To: Gregory Pettengill (Cote Art & Engineering)
Date: Thursday, July 28, 1994
Forwarded to RP-ML by Greg, 94 08 03
Subject: Moulds from FFFF
Greg,
Thanks for last nights mail. I will attempt to answer your questions.

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. The first step is to make simple wooden boxes (with slight 
tapers on the vertical sides). These form the two main halves of the mould. 
The part is coated in a releasing agent, and suspended so it sits neatly in 
the lower box to the correct depth (i.e. the parting line is level with the 
top edge of the box). The resin is poured in, and allowed to cure. The top 
face of the resin is then treated with releasing agent, and the second box 
placed atop the first. Resin is again poured, so as to completely submerge 
the part.
When it has fully cured, the box can be split open and the solid resin 
pieces removed. To finish the 'tool' holes have to be drilled to allow 
filling (and air to get out), but this is simple enough.

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. 

Another alternative I have seen (which works well for parts with no 
re-entrant features) is to simply cover the whole thing in some form of 
synthetic rubber. I don't know what the stuff is called I'm afraid, but I'll 
try to find out. 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. 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.

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!


Second question - what is the most flexible material?

Well, that depends on what you mean by flexible - bendy or versatile?

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.

As to versatility, my preference is for the LOM (which incidentally is the 
most environmentally friendly of the FFFFs as it takes paper and turns it 
back into bits of tree!). The finished part can be treated in almost exactly 
the same way as wood, and can be sanded, drilled (even tapped), stained, 
painted, polished, etc. If push comes to shove it can also be sawn up (with 
care) if that is necessary to make the mould. 

Talk to you again soon.
Jon Hunwick