Rubber Casting Experiments

Hey everybody!

I’ve got a quick one for you today. Its the documentation of a quick experiment I did of trying to cast rubber in a 3D printed mold. This started from me seeing a Flex-seal commercial, ya know, that spray rubber stuff? Well the infomercial guy poured a can of the stuff into a mold and it solidified into a block of rubber. I figured this may be useful to know how to do to make say a button cover or a custom O-ring or something to that effect. Yes you can get TPU filament but at the moment my printer cant print that. Also if you need a lot of parts you can print a few molds in plastic and cast them quicker than printing them in rubber.

Lets start with how I made the mold:

Should be pretty self explanatory for most people already familiar with 3D printing things but lets go through the exercise anyway.

Start with what you want to cast and draw it in your chosen CAD program. I’m using Autodesk Fusion 360 here.

 

The starting shape. Some kind of O Ring to test the method.
The starting shape. Some kind of O Ring to test the method.

Add your riser to get the rubber into the cavity.

O-ring with a riser. Sorta looks like a wedding ring...
O-ring with a riser. Sorta looks like a wedding ring…

Now that we’ve got our negative shape lets make the positive one.

The mold body around the ring.
The mold body around the ring.

This will be a 2 piece mold so we need screw holes to attach the two halves together.

Screw hole addition
Screw hole addition

Now we need to subtract the negative body out of the positive one.

Subtracting the negative out of the mold body
Subtracting the negative out of the mold body

Like I said this will be a two piece mold so lets split it! Create a plane halfway through the mold and split the body along the plane.

Splitting the mold body
Splitting the mold body

So you could print this at this point and maybe get a good result. I decided to go one step further and added some posts and holes to align the two halves.

Top half of mold with female locating features
Top half of mold with female locating features

On the bottom half notice the drafted cylinders and the radius at the top.

Bottom mold half with 2 degrees of draft around the columns and a small radius on the top to help printing and aligning
Bottom mold half with 2 degrees of draft around the columns and a small radius on the top to help printing and aligning

 

Alright! We’ve got our mold!

So now you can print it out on your friendly neighborhood 3D printer for the next steps. I printed this test model out with pretty fine settings to get a smooth finish.

Printed test model
Printed test model

So what I did to get the finish even smoother was acetone smoothing. I use the no heat method (outlined here and here) which takes longer, but is easier for me and less dangerous.

The mold after smoothing
The mold after smoothing

Cool we’ve got a smooth mold. Lets get casting!

The first medium I tried was this stuff that comes in a caulk gun package called wet patch cement. I went to Home Depot and went through the caulking aisle and this stuff looked vaugely rubbery so I bought a tube.

First test medium
First test medium

This way is really easy to pour in the mold since its got an integrated tip.

Pouring wet patch cement
Pouring wet patch cement
The rubber curing in the mold
The rubber curing in the mold

So I let the rubber cure overnight, and turns out this stuff is for quick roof repairs so it remains liquid if not in contact with air. Sooo only the top little bit cured out of the whole mold. Probably just as well since this stuff is super messy.

Very nasty stuff to use indoors
Very nasty stuff to use indoors

Since I was inspired by the Flex-Seal commercial anyway… lets get some of the actual stuff!

My can of Flex-Seal I used for this test
My can of Flex-Seal I used for this test
Flex-Seal poured in the mold
Flex-Seal poured in the mold

This stuff took a rather long time to cure. I left mine sit over a long weekend and the whole thing cured.

Now for the best type of rubber that I have found thus far. Shockingly, its stuff designed to be cast…

The 2 part silicone I used
The 2 part silicone I used

So with this stuff you mix some goop from each jar together in equal parts and it will set up in roughly a half hour.

The finished O-ring in the mold
The finished O-ring in the mold
The complete O-ring with the riser cut off
The complete O-ring with the riser cut off

 

Final Thoughts

The 2 part silicone was the best casting material of the ones I tested. After it came out so good I looked more into the company and they make a whole bunch of other products including cast-able plastic which sounds pretty cool. The main thing I am interested in is they offer silicone in a handful of different duro values (rubber hardness measure). While the Flex-Seal did cure, it had almost no tensile strength. I think it would work far better as a coating than a whole part.

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