Recycling your old urethane molds and resin cast parts!

So I'm starting to accumulate a lot of one-time use molds. Some projects I've done, I've needed 4 or 5 two part molds, one for each stage of casting/cleaning/polish. I've ended up with a LOT of cured urethane rubber molds that I no longer need, and I'm looking for ways to try to recycle it into ground powder to reuse in other molds.

I picked up a meat grinder (something like this Amazon.com: Weston #10 Heavy Duty Manual Tinned Meat Grinder: Kitchen & Dining ) years ago and used it for a while, but it's not a very good method. You have to cut the mold rubber into strips, and feed each one in individually, otherwise it's too much for the meat grinder to handle. Also, you have to crank it hard, and end up rubbing metal on metal, causing some serious wear on the crank and grinder body to the point that the hole in the body for the crankshaft becomes more like an oval. This also causes you to get metal shavings in your mold rubber. You can slow down the wear on the meat grinder by spraying it with a lubricant, but then you get that lubricant all over your mold rubber grindings. There's gotta be a better way to do this. Does anyone else have any techniques?

Also, I have a lot of cured urethane resin pieces left over, either from screw-ups, or from extra resin (poured and mixed 20ml and I only need 13), and I'm hoping to find a way to break these parts down to reuse as a ground resin powder. I'm thinking maybe a disc sander with a vacuum and a hopper to push the resin parts into. Thoughts?

I usually just use large pieces when I need fill for either material. I've found it's not worth the time to try reducing the material size uniformly.

Yeah, just chop them in to little cubes and plop them down into the new mold.

I always wondered if a coffee grinder would work well for this.

You beat up tbe coffee grinder...and thats a sin.
Ive been wantin to motorize my old grinder. But i have an older one made of cast iron and dont have the metal shavings problem

Maybe a dumb suggestion, but what about those heavy duty confetti paper shredders that will cut up staples and floppy disks? If you had strips small enough it would probably work.

Blender. Though don't you run the risk of contaminating your mold or delamination issues?

I've never done this yet, but I think the idea is to use scraps of the same brand RTV. That way you probably won't have any issues.

I don't know about different types, but silicone will bond to the same type just fine.

I've seen it done many times, presumably without issues, but I posed the question at a molding/casting seminar a few weeks ago at a Smooth On distributor and was told it was a bad idea.

In a perfect world we'd all run new material every time with large overlaps and big keys.

In my world silicone's $100 a gallon, the molds die after 20-30 pulls and I've got space to fill.

I've combined pieces of smooth-on's Oomoo with a few different types of their Mold Max and some other silicone from Polymeric Systems and haven't had any negative experience. I do try and keep at least .25" of new material around whatever I'm casting though.

How do you keep the .25" layer intact though? Wouldn't the chunks just sink or slush around wherever they want to go?

They generally stay where they are though I don't often put them on top of a piece. I more use it to fill in the corners of a box mold and such.

I didn't have any old molds on hand when I started of course, but having now made a few I have some old ones available. My plan for the next mold is to make a small mold box, with about 1/8in clearance around the part. I'll pour rubber and let it cure. Then I'll take the box apart and build a larger box the size I want for the finished mold, and pour a new batch of rubber mixed with bits of an old mold.

robn1 said:

I didn't have any old molds on hand when I started of course, but having now made a few I have some old ones available. My plan for the next mold is to make a small mold box, with about 1/8in clearance around the part. I'll pour rubber and let it cure. Then I'll take the box apart and build a larger box the size I want for the finished mold, and pour a new batch of rubber mixed with bits of an old mold.

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I've done this a fair few times, works a treat.

Hey, thanks for the input! I don't want to crap up this thread with replies to every comment, so I'll put them all in here:

Cutting them into little chunks works great, if 1)you have time to cut 50lbs of urethane rubber into .75"x.75" cubes. It's doubly hard if your molds are thicker than .75" in places and you have to cut them to length and width or 2)you can actually use large cubes as filler. I've used ground rubber and cubes and they both have their pros/cons. The cubes are great for sort of blocking out big open spaces in your mold box and the grounds are great for filling odd shaped spaces.

robn1 has the right idea when he says to pour the good, clean rubber over just enough of the piece in the box to cover the surface with the good stuff, and then fill in the rest with the ground mix. It works great, I've done it. The PMC121 pot life and cure time are very forgiving and you can easily pour a liquid/ground filler mix over the original clean mold rubber before it cures fully and still get a good tight bond between the two.

What you don't want to do is mix your ground with some liquid rubber and pour it directly over your prop in the mold box. You end up with either trapped air bubbles, or places where the mold rubber density is different, and you end up with a bumpy surface on your mold, which shows up on every resin pull you do from those molds.

I'm starting to think that I might want to invest in a Blendtec blender as a "business expense" for grinding resin and mold rubber. Because yes, yes it will blend.. Will It Blend? - Hockey Pucks - YouTube

Also, if you're making something for a client, don't skimp, use fresh mold rubber!