Making molds for tiny parts

[Aternox]

Hey all.. I have been trying to make a mold of some tiny parts for a plastic model that got dropped. The smallest is about half an inch long and about an eight inch wide. Tiny... The other pieces are about the same thickness/width but around 4" long.

I've done quite a bit of research on making molds but the problem I'm running into is that the molds always come out weak. In other words they tend to bend way too easy. I've read that the epoxy resin is exothermic and needs more mass to cure properly so I have put the mold in the oven at 200 F. It's harder but still too flimsy. I'm definitely mixing them in equal quantities... Not sure what I'm doing wrong here...

Starting to think that my epoxy is the wrong product for the job. I'm using the Amazing Casting resin from Aluminite. Is that any good?

 

[PegaZus]

I'm no expert, having just starting playing around with things, but I think what you're saying is that the casts are too weak, right? I'd almost say that something that small isn't going to work with the resin as I'd wager it is just too soft. But I just checked some stuff I cast a few weeks ago, and even the thin stuff is just super strong. Can't scratch it with a fingernail. Not even the little thin stuff. May be different materials, however. My bottles say "Low Viscosity Super Light Liquid Casting Plastic", and smells like fiberglass resin when I mixed it. Just checked their website and they are different from what you've got, but the materials don't seem to be that different in strength. I remember reading that the resins have a shelf life of a year, but that may be just my stuff too. How old is what you have?

The other thing is that you may be taking them out of the mold too soon? When I was playing around with mine (cast a Hershey's bar, FYI), it was very flexible after I took it out. Which was a good thing, as I straightened it. Maybe what you've got is so small that it just seems weak because it hasn't fully set. I'd pull out some of your discards and see if they hardened up over time.

 

[airhead]

I HATE THIS NEW BOARD>>> REPLY BUTTONS THAT BLANK OUT YOUR REPLY >_<

Epoxy adhesive is not casting resin. Casting resin is not adhesive.

Are you in the U.S.? Try www.smooth-on.com

Hints and tricks try www.hirstarts.com

Warning: the cost of making the mold and casting it may be more than buying another kit.

Have you tried contacting the manufacturer and asking if they will sell you just the part?

 

[Shawn R. L.]

I HATE THIS NEW BOARD>>> REPLY BUTTONS THAT BLANK OUT YOUR REPLY >_<

Ditto

I've had great luck using Green Stuff to make the molds with - a press mold and using J B Weld as a filler. Car wax makes a good release agent.

 

[Aternox]

Thanks guys..

It is the actual finished product that is flimsy.. The mold is fine. Green stuff is just to hard for the molds I'm doing.. I think I'll keep trying other epoxy products. I'm not using an epoxy adhesive by the way. It is an epoxy resin. Sorry for the confusion. I think it is possible that I'm just not getting the mold hot enough. I'm going to increase the temperature of the mold to 230 and see if there are changes.

 

[airhead]

Epoxy is a two-component system that causes an exothermic reaction (generates heat when the two parts are mixed).
Adding heat will only speed the reaction - not make the product any harder. Too much heat will damage the epoxy - 230 degrees is way too hot to be pouring epoxy into with normal equipment.

If you want a harder product you need a different resin: Harder epoxy, Urethane, Polyester, etc. (again, look to Smooth On)
You could try adding some stuff to the epoxy to make it harder: cabosil or ground fiberglass strands come to mind. Try West Marine for small quantities.

What are you making and why does it have to be 'hard'.
What exactly do you mean by harder? (sorry, that is the engineer in me coming out.) Resistant to sheer? Resistant to abrasion? Resistant to tension?

 

[Xephan]

Ive had worked with resin for small parts from sculpts i have done to replicate pre-heresy sholderpads. i use smooth on products, i reccomend the pourable starter kit, dont be discuraged you will mess up alot its a learning process. http://www.smooth-on.com/Getting-Started-Po/c4_1217/index.html i perfer the longer set time casting resin from the same line cuz i pressure cast. The end products keep form as well as gw plastics and with some black pigment they are a similir color gray. There is no heat required.
If you have questions pm me.

 

[xMANIGHTx]

If you want "hard", I would go with a gypsum cement product instead of resin.

Easier to work with, cheaper, harder. And by "harder", I mean the "compressive strength" is much greater. "Impact strength", however, is generally much less - more brittle. Look up Ultracal, Hydrostone, etc...

Also, for your issue, it sounds like the very small amount of resin is not generating enough exotherm to properly cure. A gypsum cement will not have this problem.

 

[DXM]

Here is a little trick
Embed a piece of wire in the molding resin. Pour the resin and lay a piece of wire, (2 if the piece is wide and thin) into the resin before it hardens. The wire will hold the resin together and add stiffness to the piece.

Hope this helps.

 

[xMANIGHTx]

Thanks all. I ended up buying a different epoxy (by the same company actually) and it worked great. Much less harder to bend which is exactly what I needed.