"Tim Wescott" wrote in message
...
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?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com


Depending on desired finish quality and tolerances you could machine a
fiberglass mold out of any of a number of things, coat it with something to
seal it and use a good mold release. I imagine you could make it out of
wood if it was stable enough (which in general it is not - although there
are tricks).

There are guys who mold hot plastisol (350F) in plaster of paris molds
sealed with Elmers glue, and I have seen guys mold molten lead for a few
pours before it burns out in Bondo.

Silicone mold material is used for plastisol and lead casting all the time,
although you do need a master to pour a silicone mold from. I am sure if it
was left in a rigid form box to prevent distortion it would work fine for
your fiberglass part. All kinds of things are done quick and dirty with
silicone molds, and you might not even need a mold release. (you probably
want one on your master)

If you are just vacuum bagging then any materials that will managed the
temperature of your catalyst reaction will do. If you are going to
autoclave your parts then it needs to stand the temps in your autoclave.

The price for the piece of aluminum sounds a little high, but then price has
usually stopped me from buy from Speedy Metals or On Line Metals in the
past. I have gotten better prices on large orders from on-line metals by
having them submit a quote, but for me freight is usually a killer. I found
a local metal vendor finally for the the aluminum I use to make molds.

The real questions a How many of these fiberglass boxes do you need to
make? Would a hand made mold do? (wood frame, plywood box, hand filleted
inside corners. Maybe glassed sanded and gelcoated?) Do you have a machine
shop in mind that can make a 48" mold in a reasonable amount of time? Will
they do it for a reasonable price? Will they do it in one setup or multiple
setups? If multiple setups how accurately can they align one setup with the
next? Does it matter?

Something to note: A piece of aluminum that big might hold your mold
cavity, but I've found if you have large areas of thin web from large
cavities or multiple cavities to close together the total work piece can
start to distort from clamping force. It can be tricky to hold the piece
and hold tolerance. I personally don't do anything over about 18-20 inches.
If the piece is longer than the table it can sag causing some distortion.