[lcifers]

Hi,

I was wondering if there is any material that I can cast urethane foam in besides silicone rubber? I want to cut the mold on a CNC and cast with it. It's "straight pull" casting, meaning no undercuts. I'll be casting flexible urethane foam from Smooth-On. I need to find a material that can be cut on a ZenBot router. Aluminum is pretty much out of the question. It would have to be something soft like plastics, wax, or even wood.

This is for a prototype. I don't need to be able to cast many of them and surface finish is not critical.

If this is not possible maybe I can use Smooth On's Soma Foama instead. It's a silicone foam. Any thoughts on that?

Thanks in advance for any info regarding the best mold material.

[awerby]

Urethane foam is really sticky stuff; it will stick to just about anything. I've never cast it into anything but silicone rubber myself. You could just carve your form positive and make a silicone rubber mold for casting parts in foam. But if you want to carve the mold instead of the part, do a test in a couple of materials that seem promising - like machinable wax and LDPE. Just make a little cavity in each sample, then pour a little foam in it. See what happens - and let us know, okay?

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[lcifers]

OK good advice. I have some machinable wax so when I get the foam I will try that and will be sure to post up results. I'll also try some other stuff. Do you know of a good source for LDPE? I am hoping with a mold release I can get something to work.

The problem with milling the positive and creating silicone molds is that it has a removable core to create a cavity. I can't mill the cavity. But if worst comes to worst I can cast a negative of the mold and re-cast it in silicone. Just trying to avoid all that...

[lcifers]

Oh yeah I also found that Smooth-on has a flexible silicone expanding foam. It's 15# stuff - a little denser than I would like - but maybe that would cast better then urethane?

[judleroy]

Try using polypropylene for yor molds with a silicon mold release agent. This should work well. At least it has for me in the past. Good luck.
Judleroy

[lcifers]

OK thanks Judleroy!

[dpryor]

We have CNC machined many urethane molds out of RENboard. If you prep the surface with high-perfomance sealant and a little mold release, it works fine.

[eSilviu]

what about soap? I never tried, but it should work

[lcifers]

Polypropylene ordered. Should cut molds within a couple of weeks. Will post results. Thanks for the input.

[lcifers]

Molds still not cut but I mixed some of the foam and tried it on the polypropylene. I realized at the last minute that I did not have any mold release left so I used a thin coat of... well... olive oil

It actually released really well so I imagine silicone mold release will do even better.

Looking forward to cutting some molds...

[MadMax]

Polypropylene may be a pain to machine. I find you need nice sharp cutters with rounded corners. Ball end is a usual choice. Flat end mills often leave hairy bits which are difficult to cut off without scratching the surface. I have casted non foamed Smooth On urethane (70A up to 90A hardness) compounds in acetal moulds with good result. Again, ball end mills work better than flat end mills because they leave very little corner flashing. You can hand stone rads into the corners of HS end mills if you really need a flat bottom.

I find auto body carnuba wax worked well as a release agent on acetal/delrin. It's easy to rub into the mould and it does not appear to adsorb into the casted part. I have found that silicone release agents seem to adsorb into the cast part resulting in a greasy texture which is sometimes impossible to scrub away. Do not sand your mould after machining. I have found that the as machined surface cut with a sharp ball mill released more easily than a sanded surface. I suspect the sharp little scratches left by sanding offer sufficient texture for urethane compounds to bond to.

[suncles]

After CNC machining for cavity, then do a simple polishing for the surface, it will be OK. If you need some woven pattern, do chemical corrode will OK.
If you have any problem in building Mold, donot hesitate to let me know. SUNCLES specialize in mold building and prescision machining.

[lcifers]

I ended up using polypropylene for the molds and it worked great. I was sure I had a can of mold release but couldn't find it. Well I was all ready to cast something so I decided to do the unthinkable. I coated the mold surface with - I know, it's bad - olive oil. I wiped all of the excess off leaving behind only a thin film. It worked great

Getting a can of real mold release for next time though

[cnczane]

@lcifers:

My home-built machine--which ran through pine like it wasn't even there--put its foot down when it came to hard-rock maple, making noises reminiscent of my m-i-l.

Do I understand you to have poured blocks?

If so, can you offer an opinion on how to go about making:

20 x 20 x 2 in blocks (for machining terrain maps)?

I'd love to see a picture of you rig and result?

Thanks for the thread.

cnczane

[lcifers]

Hi Zane,

I did cast urethane foam in the polypropylene molds and it worked very well. I just cut the molds on my Zenbot and poured them. I made the molds where they screwed together to make a tight seal, and had dimples and bosses to align them correctly. I used Smooth-On foam with some color added.

Hope this helps!

[cnczane]

Thanks LC, it does!

In order to get my 20x20x2 blocks, it looks like I'd need to order (2) 24x24x1 polypro sheets and a 1/2, and machine them as you directed for tight fit.
--
the zane man