Casting custom O-Rings

Our products employ the use of an O-Ring to provide for a low pressure seal (60 psi). With all of the constraints we have, there is no standard size/material O-Ring available that meets our specifications.

I purchased the new elastic resin and an LT Tray, and printed several tests. The durometer is good, but the support removal is virtually impossible on a 5"+ O-Ring.

We have the specs defined and have received quotes from a local vendor for sample quantities for testing. Not inexpensive! I was explaining our dilemma to my son who used to run a plastic sales and fabrication shop and he asked me “Why not cast your own?” He pointed out that materials are available in silicone and urethane. He used a piece of acrylic with a groove machined into it, of the appropriate cross-section and diameter with an acrylic lid, (square profile O-Ring). BUT he was building a vacuum chamber, not a for-sale product!

I first considered using that process, but I then decided to print the mold. Why not?

This is what I came up with:
Two piece mold and one removal device. It works in my mind! :blush:

The holes are for resin conservation. After seeing the Preform results I’m thinking that 1/8" will work, so I’ll shave an 1/8" off of the removal device and the mold.

Removal device: Hex for orientation, pins to press the O-ring and the filler ring out of the mold.
I’m printing this in rigid, hopefully the pins will be strong enough.
If not, I’ll print one with holes and we’ll epoxy in steel pins.
The hex registers before the pins engage

The mold: you can see two of the 18 removal holes, matching the pins above

The ring: fills the mold except .139” for the cast of the actual O-ring.


Clean the supports and smooth the bottom of the filler ring.
Apply mold release to mold & ring
Place the filler ring into the mold
Apply mold release
Mix the silicone or urethane and degas it
Carefully pour or inject the mixed material into the mold
Let it cure
Trim the top with a razor blade
Place the mold onto the removal platform
Ensure that all 18 pins are aligned with the holes
Gently press the mold down forcing the O-ring out of the mold
Trim any flash from the O-ring
Clean the mold parts
Repeat as desired/required

Comments/suggestions encouraged.

We’ve done that, works great! 60 Shore custom odd shaped gasket. Performed well over temperature.

However, only condensation cure silicone and urethane worked. Without mold release but just a bit of grease the urethane will stick to the mold and you can’t remove it; the silicone released quickly.

The addition cure silicone didn’t work: it simply didn’t want to cure. Maybe something in the resin prevented it. So you need to apply a coating.

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I assume that you meant that the urethane won’t release without mold release or grease. I consider urethane to be an adhesive so I wouldn’t try it without mold release or maybe a thin coating of Vaseline applied with a cotton swab.

I’ve cut the bottom plates down to 1/8" and added a bevel at the bottom of the 18 holes in the mold. I’ll begin the printing in the morning after I return from my aquatic PT (stroke). At high resolution, to keep it smooth, printing will take some time. I think I’ll try Platinum silicone first, then urethane. Wish me luck, I’ll post results.

Yes, the urethane won’t release without mold release. I added some grease as well but it still adhered to the print material.

The addition cure silicone I tried is a platinum cure silicone. This one didn’t want to cure inside the mold without any coating. The supplier mentioned that something in the resin most likely reacted, preventing it to cure and said this is common for SLA printed parts. They suggested applying a coat of polyvinyl alcohol but I haven’t tried this yet.

Forget the pins- If you use tin cured silicones, all you need is a single port that you can squirt high pressure air into and the silicone part will rocket out of there.

Tin cured silicones should release clean from acrylics with no separation agent…

Secondly, the rigid ring you are sticking down in there is overly complex and fragile for what you are doing.
The cavity side mold should be a solid plate with a slot exactly the eight you want… and you could use a simple sheet of glass on top to seal the top of the cavity.

Or- make Two mold halves with either half the depth of the rectangular section ring inset to each side.
Or where the outside separation line is at the low edge of the rectangular section, and the inside separation line is level with the upper, inside corner of the cross section. so that the mold has a “step”- for easier demolding.

lastly- if this ring will be subjected to high temperatures- tin cured silicones are not suitable- they swell when heated and will not stay in their place.- in that case you want to shop around for a platinum cure silicone that has a very low cure inhibition… and run some tests on a surface separation agent that will enable the silicone to cure in your tool…

OR- print a negative of the actual tool parts, and Cast the functional mold halves in a material the Silicone will not be inhibited by.

After review of the comments above and more research, I’ve tweaked my process. I’ve simplified the mold and incorporated the ability to inject the material on one side with a vacuum on the other side while applying pressure to the mold top (flat). While completing my mold models, I thought that maybe injecting elastic resin into the mold, with no top and then placing it in the cure chamber might work. I think that I’ll try it, but only after a spray of PVA on the mold. Luckily my mold is only 5.75" in diameter which enables me to print it flat, keeping my groove clean of supports and it fits neatly in the cure chamber. The o-ring is only 0.74 cc so 1.0 cc syringes are perfect. We’ll see. :slight_smile: