Material for casting in a mold

[johnm]

Folks -

Well, progress here, but sometimes it seems inordinately slow.

Gotta question... After taking several hours on one project I was wondering about making a mold of the object, rather than the object, and then just pouring.... something.... to make a copy. We may need to make dozens of a particular part, and casting them in molds may end up being much faster than cutting them out.

I would like to know what I would use for making the mold itself, since it seems like it would need to be flexible to release the casting.

Secondly, what should I use for pouring the casting itself? We are doing Victorian trim, so ideally it would be something light weight and construction-friendly, yet able to withstand the elements. While I'm at it, it would also be nice if the material could be tinted for permanent coloring, and was inexpensive..

I remember back in the printing business, we had a sign that said we provided quality work, fast work and cheap work, but you only had a choice of two.

I'd sure appreciate any leads. Thanks in advance!

John Moorhead

[luke]

RTV Silicone is an excellent and simple mold making compound. It more expensive than latex, but also more flexible and durable for repeated uses. A liquid expanding rigid density polyurethane foam will work for less detailed stuff but you may have consider using a more expensive polyurethane casting resin. Check tapplastics.com. They offer some free quick video tutorials that may take the mystery out of casting for you. I really don’t have any advice on tinting except anything polyurethane can be sprayed with a Gel coat. Gel coating can be tinted, but if you are going to have to spray it save yourself the extra equipment and just paint it. I hope this helps.

[marshawk]

Hi John,

For molds made for use with plaster or other cold liquids, we have had a lot of luck using sign foam coated with epoxy. the trick is getting the epoxy flat enough so it does not cover the details.

Cheers!

Chip

[rookie432]

John,

www.smooth-on.com (http://www.smooth-on.com)
tons of info and products very easy to use. I like the POYO Putty.

Bill

[Brady Watson]

John,
This (http://www.polytek.com/products/litecast-2005.pdf) is what you want.

The biggest challenge of any moldmaking endeavor is creating a quality master. You ideally want something that is tight-grained (or no grain) as a material to machine for your master. I personally like USG Rayite100 because I can mix up just what I need, toolpath, seal with shellac and have a perfect master. You can also use prototyping board if you think that it is worth the investment. The most important part of the moldmaking process is getting it perfect...this means tight stepovers with a small bit and careful finishing of the part. Every part you pull will be based on this master. So...it's gotta be perfect.

You have 2 choices for mold rubber: Silicone RTV & Polyurethane rubber. When you start to get into large castings, silicone gets expensive and lacks the rigidity to keep its shape without a hard shell around it (blanket mold). Urethanes on the other hand are cheaper and stiffer. They do require a release agent (not a big deal...spray silicone 2300 does the trick)

This is not hard folks. Make a perfect master (hardest part), seal it, secure master to open-top box with 4 sides & bottom, wax or apply release, mix & pour your mold rubber, pull off box sides and release master from new mold. To cast part, apply release (if necessary), mix up appropriate plastic, plaster, wax or other substrate, pour into mold, allow it to cure, demold and repeat. For those doing quantity, make several molds and cast parts at the same time.

-Brady

[rookie432]

I have to agree with Brady on this. For less than 100 bucks you can get started in molding parts. That's for basic materials. Try it once and you will see the possibilities. I decided to give poured polyurethane foam a try and fell in love. Perfect for signs, pretty forgiving and is the same thing as 15lb signfoam that you can pour into a mold. Once you get past the "how easy is this" part you'll really start having fun.

Bill