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

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:

https://www.freemansupply.com/catalo...manCatalog.pdf

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...

--
Ed Huntress