How to make Female molds for vacuforming?

Heavy Melder · Post by **Heavy Melder** » Sun Jun 17, 2012 1:41 pm

OKAY, last resort.

I've been looking high and low for info on how to make good female molds to make vacuform plastic parts. But I can't find a damn thing online. Lots and lots of videos of making parts with male molds but not female molds.

So many questions....
A) How does someone make a female mold for vacuforming? I've done plenty of resin casting and molding, so I'm guessing a similar process.

B) What is used for the mold material? I heard some talk of a air-porous plaster or somesuch, but I can't find much else in terms of information....
The matericals seem to be the clincher on this. Any ideas?

ajmadison · Post by **ajmadison** » Sun Jun 17, 2012 10:25 pm

From what I've heard,

a female mold for a vac-form is actually alot like making a regular mold for a resin casting.

e.g.

1. build a prototype, understanding that you cannot have any undercuts from an individual axis.

2. make a mold of prototype.

3. drill holes in the mold to provide plenty of sources of low-pressure/vacuum to the heated plastic.

4. make copies.

Over simplified, but that is the theory. Hope this helps.

Mr. Badwrench · Post by **Mr. Badwrench** » Mon Jun 18, 2012 5:23 am

I've wondered the same thing myself, and ran into the same trouble - What the heck is this porous media that people use for female buck vacuforming? You got me curious, so I googled "air porous plaster", and this is what it came up with:
http://www.alibaba.com/showroom/porous-plaster.html
I wonder if this stuff would be useful? How faithfully would it hold detail?

*edit*

Funny thing, the fifth google search result for air porous plaster is this very thread.

Ziz · Post by **Ziz** » Mon Jun 18, 2012 6:16 am

The way I understand it, you need to coat your master in a thick layer of resin to create a hard shell, then use the porous plaster as a filler behind it, building the whole thing up in a box just as you would a rubber mold for normal resin casting. Then you have to come to terms with the fact that you'll probably have to destroy your master to get it out of the resin. Make any patches you need to make to the negative mold impression and then drill fiber optic sized holes through the resin until you get to the plaster. That creates the airflow through the mold. How many holes and where is a matter of trial and error though depending on the size of your mold, the shapes you're trying to make and the strength of your vacuum.

DennisH · Post by **DennisH** » Mon Jun 18, 2012 2:30 pm

I've made female vacmolds from hydrocal or ultracal. They're much stronger than regular plaster.

I've got one here that is a shallow cone shape, but my photo program is being uncooperative at the moment or I'd upload a pic of the set up. I'll try to put some up later. I'll describe it for you.

You need to secure your pattern to a base plate with walls for containing the ultra or hydro. It's basically an open faced mold like you would make for RTV. My pattern is double face taped to a sheet of plexi with sidewalls. The pattern and inside of the mold box was coated with vaseline and the hydrocal was mixed and poured into the mold.

After the hydrocal sets up, you pop the pattern out and drill small holes into the inside face of the negative hydrocal mold. It depends on the details and whatnot as to where the holes go, but you'll want to place them so that the plastic is pulled into the all of the areas. (The ultra or hydra will dull your bits super quick, just so you know)

The bottom of the hydrocal mold (which was at the top of the openface pour) gets a frame placed under it to create the void for the vacuum effect. On square or rectangular molds, we'd make a thin wooden frame and duct tape it to the bottom of the mold around the outside edges to create an airtight seal. This one I still have is a round mold, so I used a short length of acrylic tube to make the frame. The whole thing was then duct taped to the vac table around the bottom edges.

Vac away and Bob's your uncle.

modelnutz · Post by **modelnutz** » Mon Jun 18, 2012 5:02 pm

Best vac material I've found is an Aluminum Filled Epoxy from Huntsman Chemicals.

Not cheap...but...rock hard and Uber-durable with a stellar surface

I pour the Epoxy just as you might for an open back mold...let the Epoxy cure...then ( and this is where experience pays off ) I drill very small holes at all points where I feel that max. vacume is required.
The drilled holes have to find their way to the bottom of the mold so that the vacume table can draw air through them to create a vacume at the face of the mold.

Heavy Melder · Post by **Heavy Melder** » Mon Jun 18, 2012 6:24 pm

DennisH wrote:I've made female vacmolds from hydrocal or ultracal. They're much stronger than regular plaster.

I've got one here that is a shallow cone shape, but my photo program is being uncooperative at the moment or I'd upload a pic of the set up. I'll try to put some up later. I'll describe it for you.

You need to secure your pattern to a base plate with walls for containing the ultra or hydro. It's basically an open faced mold like you would make for RTV. My pattern is double face taped to a sheet of plexi with sidewalls. The pattern and inside of the mold box was coated with vaseline and the hydrocal was mixed and poured into the mold.

After the hydrocal sets up, you pop the pattern out and drill small holes into the inside face of the negative hydrocal mold. It depends on the details and whatnot as to where the holes go, but you'll want to place them so that the plastic is pulled into the all of the areas. (The ultra or hydra will dull your bits super quick, just so you know)

The bottom of the hydrocal mold (which was at the top of the openface pour) gets a frame placed under it to create the void for the vacuum effect. On square or rectangular molds, we'd make a thin wooden frame and duct tape it to the bottom of the mold around the outside edges to create an airtight seal. This one I still have is a round mold, so I used a short length of acrylic tube to make the frame. The whole thing was then duct taped to the vac table around the bottom edges.

Vac away and Bob's your uncle.

This sounds like what I'm trying to figure out.

Can you tell what this hydrocal or ultracal material is? Or something similar that I can source locally? I have fairly easy access to Smooth-On products. Is it like Smooth Cast 305 or something?

Once the hydrocal cures, is getting the master out hard?

Also, could mould release be used in the mould box instead of vaseline? just wondering.

Johnnycrash · Post by **Johnnycrash** » Mon Jun 18, 2012 7:19 pm

Here is what you do:

Make your master as you would any other master for casting. Keeping in mind that you can have NO under cuts at all (not totally true, but it's too complicated to get into under cuts for now).

Make a silicone open face mold. Then, apply mold release to that silicone mold (as if you are making a two-part mold), and make a silicone casting of it, as if it were your master.

Now you can pour as many female molds as you need. Make at least one good one, and set it aside as your master for the making of a new silicone mold.

Take the male silicone mold, and insert wires of the diameter of the air holes you want. Sticking them into the rubber to hold them there, and where the plastic will need to be sucked down and into. These wires then get coated with Vaseline and are left long. You will need to grab on to them later.

Pour a hard mold over all this. I use dental stone material (Special Tray by DeTrey Dentsply). It's an acrylic material. Just mix with water, pour, wait.

Once it has hardened up... Using pliers, grab the wires that should be sticking out of the top (really the bottom) of the mold/casting. And pull for all you're worth. It's better to use MORE Vaseline than you think you need. Never can have too much lube. Wait What?? Anyways...

You can substitute the Special Tray for an aluminum filled epoxy resin. There are several that are made specifically for making vacuform molds. The aluminum allows it to absorb the heat from the plastic without cause the mold harm. The mold will last longer that way as well. But for small parts/short runs, you can simply use just about any resin. Use what's on hand first, and see how that goes.