Materials for making a mold

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. 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?

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

That's almost twice what I'd expect the shop to pay for that piece.

If you're cutting a large cavity in one side of a piece of 6061 plate it's not going to stay flat. Cast tooling plate (e.g. Alcoa MIC6) is stable, but is relatively gummy, so will be more difficult to machine and finish to a polish. Also more expensive.

Eh, I forgot about Rayite MDM:

I don't think you can machine it, but it makes really good molds, with extremely high accuracy. It may not be for this project but it's something to remember for inexpensive fiberlass molding.

====================== As you already have the wireframe/3d drawing you might consider getting the mold/plug 3d printed. I don't know the details, but from the web it appears this is becoming increasingly common.

Given the size you most likely would print in several sections and glue into the complete mold/plug.

Some URLs of interest

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.

I knew about the issue with 6061 -- that's part of the reason that I'm asking questions here!

I wonder if one could just pour a couple of gallons of epoxy casting resin into a box, then send that off to the machine shop -- if you can do it without bubbles you certainly have something that you can polish to a high shine.

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 are: 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.

One trick for pouring resins with few bubbles is to pour a long thin stream from a long distance above the forms. However I am not sure if epoxy would work well for what you are doing. I've found epoxy pours thicker than about

1/2" tend to scorch from their own reaction heat. Even 1/2" is pushing it. I've had the small amount left in a mixing cup scorch once or twice if it was still fairly thick. I've read that you can get past the issue with epoxy by making multiple pours, but I seem to recall that you either need to have your timing just right, or you need to let it fully cure and then prep the surface before making the next pour. Sounds like a long tedious process. There are other casting resins that might work better for thick pours, but I really don't have any first hand experience with them.

If you go an with aluminum mold clamping/bolting it to a heavy backing plate can help it retain its correct shape.

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.

I LIKED IT!

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.

You could pour your own. But I think it would be cheaper to buy something ready made. The patternmaker suppliers have a number of products designed for CNC machiningto make patterns, plugs or molds.

A pattern shop in your area would probably be your best bet for the machining and help in getting type of finish you need.

But why aluminum? Nearly all fiberglass boats, for example, are made using a fiberglass mold. Nearly always a female mold.

Because machined fiberglass takes a huge amount of hand work to polish up. To date, I know of no 3D printing technology that'll lay down polished gel-coat.

The one quote that I got (from Freeman) was more than the cast aluminum slab!

======================== After reading the replies to this post, I have some questions:

(1) What is your projected volume?

(2) Why fiberglass? How about alternatives such as vacuum formed thermoplastic?

(3) Did you get quotes from enclosure/case vendors? The size/shape appears close to a gun case.

A few dozen a year.

First, it's the devil I know, and I know it'll be strong enough (I want the thing to easily survive a drop from table height onto concrete). Second, I have a strong personal tie with the vendor.

Vacuum-formed thermoplastic is certainly a contender, but I think that by the time I get all the other properties I want, given my anticipated production volumes, that I wouldn't be happy.

A hinged case isn't a good starting point, I don't think. I've been down this sort of road before, and I just don't see finding a case that'll meet my needs that isn't custom-made.

(Enclosures are a bitch. With the exception of PC builds I have yet to do an electronics project where the cost of a nice-looking case didn't exceed the cost of the stuff inside it.)

+1. Isn't that just f'n CRAZY? And it's not just for electronics. Carry cases in general are gawdawful overpriced, too.