How I Do It: Recycling Silicone Mold Rubber

 Introduction

Once again, I find myself at the end of the month and I don’t have any current projects that are close to completion, let alone ready for a write up. But thankfully, I can maintain my streak of blogging at least once a month with an article that I’ve been keeping in my back pocket. Consider this a sort of addendum to my general molding and casting process since I’ve been incorporating it into more and more often over the past few months. This article will cover how I take silicone molds that I know I won’t be using anymore and scraps of excess rubber that I peel from the bottom of mixing containers and recycle them to essentially act as “filler” to cut down on the amount of fresh silicone I need to use to create new molds.

Here’s a few things to note off the top before we dive in:

·        This process mostly applies to platinum cure silicone rubber. It can work for more expensive tin cure rubbers as well but going to the extents to which I’m about to describe probably isn’t necessary. There are a few examples of previous articles wherein I take great big chunks of cut-up molds made with a good tin cure silicone (like Specialty Resin and Chemical’s Cast-A-Mold 25T), pour fresh silicone over the chunks, and experience no delamination with the fresh silicone fusing with the chunks of already cured silicone with absolutely no issues whatsoever. But in my experience, things aren’t quite so straightforward with cheaper and more readily available platinum cure silicone and additional processing of the recycled silicone is required.

·        Avoid mixing different brands of platinum cure silicone. In a perfect world, any platinum cure silicone should fuse to other platinum cure silicones because they’re technically the same type of silicone but (once again speaking from my own experience) that simply isn’t the case. I experimented with this by taking bits of a mold created with a different brand of silicone than the one I used for a small test batch and the chunks did not fuse with the fresh silicone once it cured. So unless you’re exceedingly lucky in finding two compatible brands of platinum cure silicone, you probably don’t want to roll the dice with this.

·        In fact, I would recommend only recycling molds made with the same silicone mold making kit. Basically, if you’re working with a gallon of platinum cure silicone, only recycle molds made with that same gallon. This might sound limiting, and in a way it is, but I’ve been able to stretch out a half gallon kit of silicone rubber using this recycling process for far longer and I would have previously thought possible.

My silicone mold recycling process breaks down into the following phases:

1. Cut up the old molds into chunks.
2. Clean the chunks with isopropyl alcohol (91% or more).
3. Grind the chunks into even smaller chunks with a meat grinder.
4. Mix ground up chunks into fresh silicone.
5. Vacuum degas silicone before pouring.
6. Cure the new mold(s) in a pressure pot.

I’ll be going through each one, some in more detail than others.

Phase 1: Cut the Molds into Chunks

Using a sufficiently sharp pair of kitchen shears, take any molds you don’t use anymore and plan to recycle and cut them into strips. Then cut the strips into small chunks. I emphasize the word “small” because they’re going to be fed to a meat grinder in Phase 3 (spoiler alert) and chunks that are too big might impede the blade or make it difficult to turn the meat grinder’s handle. But I’ll get more into that later.

Phase 2: Clean the Chunks

Now that the silicone has been chunkified, you’ll want to clean it off to get rid of any lingering casting material/resin, mold release, and other debris that might have accumulated over time. What I do is put all the chunks in a plastic deli container with a lid (like the kind you put soup in), pour in some isopropyl alcohol (91% or more), secure the lid and shake up the container until all the chunks have been submerged. I think you will agree that’s much faster than scrubbing each and every tiny chunk of silicone with a brush.

After washing the chunks, remove the lid and strain out the excess isopropyl alcohol.

Transition the chunks from the strainer to a tray or baking pan and let them sit for a while to give the isopropyl alcohol a chance to dissolve on its own. Even though the chunks might appear dry and even feel dry (though I would minimize touching the chunks with your hands after this to reduce the chance of contaminating them all over again), I still like to bake them in my oven for 30 minutes at around 275 degrees Fahrenheit to get rid of any excess moisture. I use 91% isopropyl alcohol and the remaining 9% is water which I don’t trust to evaporate on its own, at least in a reasonable amount of time.

Phase 3: Grind Up the Chunks

Once you take the chunks out of the oven and let them cool down, it’s time to break out the meat grinder. This was the cheapest heavy duty metal one that I could find on eBay (it was about $8 or $9) and you’ll want to use it specifically for grinding up silicone and nothing else. While it is a manual hand crank grinder, I’ve seen a few videos on youtube of someone using an electric meat grinder for this though I can’t comment on its effectiveness personally. Do your grinding over a plate or a mat since tiny pieces of minced silicone tend to get away from you once the handle starts cranking and the blades start chopping.

Feed in the chunks a little at a time so as not to gum up the blades and make turning the handle too strenuous. After I mince all the chunks, I will feed them back through the grinder for a second time to mince them into even smaller, finer chunks for good measure. The smaller the chunks, the easier they will be to mix with fresh silicone in the next step and the less you have to worry about them not fusing with the rest of the mold. Again, that has only ever happened to me when I mixed chunks from a different batch of silicone with a test batch from a different brand, but you still don’t want to take any chances. So mince that silicone as finely as you can.

Sweep up and dispose of any chunks that get on your floor or any other surface that isn’t clean, since they might be subject to contamination and not adhere to the fresh silicone when curing. Better to toss them out than risk ruining the whole batch. I keep my minced silicone in a sealed deli container like the one in which I washed the larger chunks, but you will want to store them in any airtight container to keep them from getting all dusty (or accidentally knocked over).

Phase 4: Mix Chunks with Fresh Silicone

When you’re ready to make a new mold that incorporates the minced silicone (see my general molding and casting process for the skinny on setting up mold surrounds and estimating the material needed), first measure out and mix the fresh silicone according to the manufacturer’s instructions. After the fresh silicone has been fully mixed, carefully scoop (or dump) some of the recycled silicone from its storage container into the mixing container with the fresh silicone. Then mix the contents again to evenly distribute the recycled chunks, since they will settle on top of the fresh silicone and you will want to get them fully integrated.

As far as how much recycled material you should use, I typically keep it around 1/3 of the overall weight or volume. For the platinum cure silicone that I’m using here, it’s a 1:1 mix of Part A and B by weight instead of volume. So if the fresh silicone is 60 grams of Part A and 60 grams of Part B, I would mix in approximately 60 grams worth of recycled silicone chunks (measuring everything out on my digital scale). If you silicone goes by volume, then go by the same ratio, following the marks on your measuring cup or just eyeball it if that’s something you feel comfortable doing. Either way, the mixing container will need to be at least twice the size of the overall silicone mixture to account for the expansion of air bubbles that will take place in the next phase and prevent any spillage.

While I was experimenting, I did make a marginally successful mold that was nearly 50/50 fresh silicone with recycled chunks, though it was an incredibly simple open-face mold and so much recycled material made the overall consistency of the silicone mix more of a paste that I had to spread with a popsicle stick rather than something I could pour. I was really shocked that it managed to cover the surface of the object that I was testing it on. What’s more, the underside of the 50/50 mold was incredibly rough from the preponderance of chunks (imagine a surface at the bottom of a Nestle Crunch bar). This adversely affects casting since its best if molds are flat on the bottom. But I found the 1/3 ratio of recycled to fresh to be the optimal combination since it still remains liquid enough to easily pour and prevents the chunks from interfering with the outside surface. You can go a little over or a little under 1/3. It doesn’t have to be exact.

Don’t pour the silicone into the mold surround just yet, since all that thorough mixing requires an equally thorough degassing in the vacuum chamber.

Phase 5: Vacuum Degas and Pour Silicone

For general instructions on the use of a vacuum chamber to degas silicone, I once again defer to my general mold making process article. Place the silicone mix in the vacuum chamber, secure the lid, and pump it up to around -35 Hg and leave it for about 5 to 10 minutes or more depending on the amount of silicone that was mixed and the potlife you have to work with. Here’s a picture of a batch vacuum degassing for your viewing pleasure:

Once the material has been degassed, pour it into the mold surround (or mold box) like you would any other batch of silicone. You’re probably okay to walk away here but I like take things just one step further to ensure the mold’s success…

Phase 6: Cure New Molds in a Pressure Pot

I used to not cure my silicone molds in my pressure pot since I figured it was enough to just vacuum degas the silicone. And for the most part it is, but I feel that the pressure pot is necessary here. When you pour resin (or most any other type of casting material) into a mold and pressure cast it, the pressure pot allows the material to get into all the nooks and crannies in the mold’s negative space, ensuring that the mold fills properly and produces a complete casting. The same principle applies to all the nooks and crannies in the silicone mix that is created by the addition of all those little chunks, making sure the entire surface of the master part gets covered by the fresh silicone that needs to work its way between all the bits of recycled filler. I think it goes without saying at this point that I cover pressure casting in better detail in my general molding and casting article.

After the allotted cure time, take the mold out of the pressure pot and you’re done. Cut it open, remove the master part(s), tape it back up, cast with it. You know the drill. Here an example of a completed mold made with recycled silicone rubber. The fused chunks tend to make the molds made with translucent silicone look a little grungy but it saves me so much on materials that I’m willing to look past that.

One final note on this process: for parts like this action figure leg that has a socket, I’ll take the time to pour a small batch of just fresh silicone that is enough to rise up over and cover the area of the part with the socket hole (which I position at the bottom of the mold surround for reasons that should be readily apparent here). I have also started getting into the habit of pressure casting my silicone molds of parts with sockets in them because, again, the pressure pot is that extra bi of insurance that the silicone will get all the way inside the socket. Once the initial silicone pour is cured, I can pour the rest of the mold with the fresh/recycled mix and not have to worry about chunks possibly preventing the socket holes from being fully molded.