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Old June 17th 15, 08:06 PM posted to rec.crafts.metalworking
Bob La Londe[_7_] Bob La Londe[_7_] is offline
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Default Materials for making a mold

"Carl Ijames" wrote in message
"Tim Wescott" wrote in message

On Tue, 16 Jun 2015 21:41:25 -0400, Ed Huntress wrote:

On Tue, 16 Jun 2015 19:25:47 -0500, Tim Wescott

I'm working on a project that may well end up having a case made of
fiberglass. As I'm working on a 3D model that will use mostly to make
pretty PDFs for my fiberglass guy and my customer, It occurs to me that
I could just make a 3D model of a mold and send it to a machine shop.

A 1.25 x 12 x 48 inch piece of 6061 costs $430 from Online Metals. Is
that pretty much what I would expect to pay for materials from a machine
shop? Is there any material that's significantly cheaper, that can be
tossed into a CNC mill and made into something mold like, and then hand-
polished to a high shine?

There is a polymer-modified, machinable gypsum that is made for CNC
test-cutting, which has been used for molds. Compared to aluminum, it's
dirt cheap. And there is another one, which I haven't seen for 10 years
and the name of which I forget, that is made for casting highly-finished
molds for fiberglass molding. Maybe someone else here knows brand names.

I'm sorry I can remember the product names. One thing to watch for with
polymer-modified gypsum cements: they don't dry very well. The trick to
using them to make a polished mold is to wipe them with acetone and then
to spray them quickly with lacquer before water migrates back to the
surface. They can take months to dry completely but you can use them the
next day with the acetone trick.

With the kinds of work you do, you should have the Freeman catalog:

I love plaster because I'm a real cheapskate. g And I like styrofoam.
I have made models from ordinary pink styrofoam insulation board, glued
together with a thin coat of white glue; painted it with three coats of
house paint; and then sanded them, buttered them with bondo (screeding
it on is tricky), sanded it, coated it with one-part polyurethane paint,
waxed the hell out of it, shot it with PVA, and then pulled a fiberglass
part off of it.

If you're a little less cheap than me, use high-density polyurethane
foam. It takes less finishing and you don't need the housepaint.
Polyester won't dissolve it. You probably won't need the bondo, either.
Just spray on some external gel coat, which you probably know well.

In fact, you probably know all of these processes well. So why are you
asking us? g

BTW, the machinable plaster is pretty shiny after it's machined. Did I
mention that it's cheap? Just like me...

I know one process for making molds. It starts with a high-quality plug
(which is often an original Ford body part that's been modified for the
purpose) and makes a fiberglass mold around it.

I'm working on a project with my brother that starts with a 3D model and
ends up with a part. Rather than having him make a high-quality plug,
then a mold, I'm thinking that it would be nice to send a 3D model to a
machine shop and get back a hunk of aluminum or whatever, polish the snot
out of it, and make parts.


Tim Wescott
Wescott Design Services
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I've never done it, but seems to me you could glue together sheets of the
thickest MDF you can buy to get up to say double the height of your piece,
find someone with a cnc router to cut the mold cavity, sand it, then paint
with epoxy resin. Now you have the same surface you would if you were
pulling a mold off of a body part, so sand and polish like you do those.
If 12x48" is big enough, buy a 4x8' sheet and cut that into up to 8 strips
to glue together to get plenty of thickness so it won't warp. I think you
said that this is for the carrying case. If so, you could even put in a
line or divot pattern and logo with the router and not sand at all, to get
a nice surface texture that would hide future dings and scratches. Have
to keep the sealing coat very thin and uniform. Anyway, just a suggestion
from up here in the cheap seats :-).

Carl Ijames


You an actually machine a pretty decent surface finish in MDF, although I
don't use it much except as glued backers for special projects.