2 part mold help (probably a really simple answer)

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Lonnie Sweet
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2 part mold help (probably a really simple answer)

Post by Lonnie Sweet » Sat Sep 13, 2014 10:59 am

Hi all, I recently tried my first 2-part mold. I poured the bottom half, including multiple locating pins. Then I coated the entire piece with mold release, let it sit 24 hours to ensure it was dry, and poured more RTV silicone over it (OOMOO 25). Both pieces are firmly stuck together. I had to cut it apart and the mold cannot be used.

Is there a trick or step I missed to ensure this doesn't happen again?
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Post by Ziz » Sat Sep 13, 2014 11:21 am

Pour your first half, let cure. Take a cheap disposable paint brush and paint the exposed surface of the mold where it will mate to the second mold half with vaseline. DON'T make it thick or goopy - it should just be a thin film kind of coating, and don't get any on the exposed surface area of the part you're casting. Then pour your second mold half. When it's cured, they should come right apart. You may have tiny spots where they're stuck together because your vaseline didn't perfectly cover that spot, but just pull gently or cut it with your Xacto.

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Post by Joseph Osborn » Sat Sep 13, 2014 11:53 am

Ya, sounds like your mold release between the halves wasn't up to the task.

May I ask what you used between the mold halves? There is a lot of very bad information floating around on the various internet forums and it's usually perpetuated by people who don't actually know what they are talking about... like using Pam cooking oil spray as a mold release...

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Post by OdysseySlipways » Sat Sep 13, 2014 9:52 pm

i use Smooth-On rubber and resins but have stuck with this brush on barrier from Polytek (have to get more, not sure if they still carry it) that is like a vaseline and mineral spirits (or something). i have found that i need to brush on 3 light coats of it allowing about 15 minutes of dry time between each coat and then pour the second part of the mold.

even then i sometimes get areas little that just bond together and require cutting.

if your looking to start making your own molds for production, give a shout
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Post by Chas » Sat Sep 13, 2014 11:54 pm

A neat trick we use to use when I was in the rapid prototype industry was to heat the Vaseline in an oven till it turned liquid. Much easier to get thin even coats that way.
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Post by Kylwell » Mon Sep 15, 2014 1:18 pm

And typically you don't wait more than an 30 minutes to pour after applying mold release.
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Post by EVApodman » Fri Oct 07, 2016 11:07 am

The kit mold release I used worked fine.
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Post by gsb5w » Fri Oct 07, 2016 6:44 pm

I have always poured my second half shortly after brushing on the mold release. No problems here.

I wonder if the second pour went around the edges and down the sides of the first half and stuck to that. I have had that happen before so I brush a light coat of release agent around the sides of the first pour. I was still able to separate my mold when that happened.
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