View Full Version : tips on foam mold creation, types and cutting tools

Fri 08 April 2011, 19:04
I was contact by an old friend of the forum today and asked for suggestions on how to make foam molds with the MM in hopes of casting fiberglass parts.

The 'tip' that I can offer is experience:

I have machined Open cell (white) EPS (expanded polystyrene), Extruded Polystyrene *styrofoam* Pink and Blue insulation sheet, and fine-celled high-density polyurethane foam (brown) all successfully.

It's important to note, that all Foams are toxic in various ways. It's important to know what type will react while machining. My lawyers won't let me tell you all the specifics, but get the MSDS forms out and realize that they all will produce toxic fumes, particulate matter, melt, burst into flame or be perfectly inert and be awesome to work with. PLEASE know the difference. This is true with the bonding/painting and sculpting agents you use too.

Foam is supplied in 4, 6, 8, 10, 12, 15, 18, 20, 30 and 40 pound per cubic foot densities, in sheet thickness up to 12 inches. Standard sheet sizes: 48" x 48" and 48" x 96". 30 pound per cubic foot densities are available in 24" x 48" and 24" x 100" and about any sheet size if you willing to go and find the local manufacture.

A couple of tricks to cheap easy stuff.

A positive statue, sign or otherwise can be cut out of White EPS foam or blue/pink foam and then coated with non-reactive agents to produce a hard, paintable, finish.

Some of my favorite coatings are:

Styropoxy from Hotwiredirect.com. The 7045-.25 series (http://www.shop.hotwiredirect.com/index.php?main_page=product_info&cPath=2&products_id=97) is machinable/sandable. The trick with this if using for a Negative mold, is to overcut your part by 1mm or so, then coat in this "putty like substance" then cut again while still on the table and have a hard mold when done. Good for at least a couple of pulls if you use good release agent. If a postive, then just do the same, but no part will be pulled. The foam hardcoated is the part.

My other favorite "non-reactive" coating is from volitle free. (http://www.volatilefree.com/qwik_spray_system.asp)

This is applied as a binary agent with just compressed air and makes a great paintable hard coat.

A hotspray like frogskin (http://3dcutting.com/products/frogskin) is awesome too....just expensive and requires the right equipment to apply and spray.

Cutting bits - use standard carbide 2 or 4 flute bits - ball nose.
Rate - 250 to 500 ipm
step over rate 25% for roughing, 75% or higher for finishing.
You can get the really nice - perfect for this use bits at frogtools (http://3dcutting.com/products/frogtools) or make your own.

For really deep molds I will purchase a 14" x .5" ti coated standard spiral bit and BULLnose the end of the drill bit with a bench grinder to make it. Trial and error will get you the right shape and correct balance. Remember, this will be spinning 9-15K RPM when you done.....take the time to balance it.

Marc - here's the phone conversation we had today.....Happy mold making.


Sat 09 April 2011, 01:07
Thanks for the tips, I was also looking into fiberglass molding. And this is some good info about it.

Sat 09 April 2011, 02:37
Sean , Show some pic`s for us that would like to try it

Sat 09 April 2011, 10:49
I don't have many more photo's to share than what's up under the MM #5 construction thread. I will dig in the archives at the office and see what I can come up with later this week.

Sat 09 April 2011, 21:46
Thanks Sea,
Great to hear from someone with experience.
Are you using spindles?
Do you have to slow your cuts to prevent light EPS melting?

Sun 10 April 2011, 12:39
Not really slower for reasons noted. I had the 1/2" bit at 9K RPM and 90-300 ipm depending on how fast the dust collection could keep up. I really never had a melt issue, more of the Collector keeping up with the huge amount of material coming off the bit.

Mon 11 April 2011, 11:23
Just to put my 2 cents in, everyone, please follow the advice about learning about the material safety for each of these products. I used to do foam carving and hardcoating for a living until the polyurethane/polyurea stopped me. Take the time to look up the health hazards of isocyanates before you get too deep. A supplied air respirator is recommended, but in my opinion is absolutely necessary. Unlike polyesters, the nasty stuff in polyurethane is odorless, and doesn't make you dizzy, so you don't know it's doing bad stuff.