Custom Molds for Thermoforming -

Custom Molds for Thermoforming

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Custom Molds for Thermoforming

Postby aeshep » Sun Oct 07, 2012 5:54 pm

Hello everyone,
I have never made a mold but now think I might need to. So I have been searching the bulletin board and reading (e.g.“Warm Glass”, “Contemporary Fused Glass” among others). To say I need help and expert advice would be an understatement. I have been thermoforming sheet glass over fiber cutouts of letters and symbols for use in one-off projects. I'm considering producing multiples but cutting (& buying) that much fiber felt spoils the deal for me. (I am in the process of trying Hotline's fiber mold hardener so the cutouts could be reused. The jury's still out on that wonderful process.) But ultimately what I'd like to have are reusable, open face molds. I see in Brad Walker's book under "Model Creation" that “clay is most often used for making a shallow or open faced object like a plaque or medallion”. I also see refractory clay discussed in “Warm Glass”. My questions are these: 1) can the final mold itself be made from this clay? There is a ceramics studio near my home if they need to be fired in a kiln for ceramics. 2) If so, does it make sense to first make a glass “model” (like I am now doing only slightly larger than the desired end product) and then fashion the clay molds from that? Reading all the information about lost wax and investments and such I have gotten completely confused. I feel I must be missing something important. Any suggestions and advice about that process that can point me in the right direction will be much appreciated.

Lynn g
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Re: Custom Molds for Thermoforming

Postby Lynn g » Sun Oct 07, 2012 9:44 pm

I've been using Brad's 110J fiber sheets for this sort of forming. It's like 1/8" fiber paper, except it is more rigid and can be re-used many times if handled carefully. It cuts with a craft knife. I use it for slumping small shallow dishes and pocket vases; it works great! I wrap a piece of it with a strip of Thinfire and use it for the pocket vase insert; it removes easily and stays intact for re-use.

Check it out at under refractory materials in Supplies.
Lynn g
"Patriotism is not enough, I must have no hatred or bitterness towards anyone." - Dame Edith Cavell

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Re: Custom Molds for Thermoforming

Postby Morganica » Mon Oct 08, 2012 2:14 am

Depends on what you're doing. Permanent refractory molds usually require a release (kilnwash or or something else that will take prolonged high heat), and they can't have any undercuts because you'd have to break either mold or glass to get the finished pieces out of the mold. Creating molds also comes with a bit of a learning curve, so expect to have to practice a bit before you get a perfect mold.

The problem with letters of a hardened refractory material (clay, fiber, etc.) is that the glass needs to contract over them as it cools (or pop off before it starts to contract). If the letters are tall enough, they can trap the glass while it's contracting, causing cracks that wouldn't happen with soft fiber paper. More than likely it won't be a problem if the letters are thin, but it's something to think about.

If I want a reusable mastermold of a design, I generally make a silicone negative of my pattern/model, then make the positive, i.e., a silicone of the silicone. Then I pour refractory into/over the positive (flexible) silicone, let it set, and ease it out. I'm left with a mold that I can make as many times as I like, and as long as I don't go crazy with the undercuts the soft silicone will usually flex enough to get the mold out even if the drafts aren't perfect.

The other, probably faster way to do this is to create a flat bed of clay, press your letters into it, then build clay walls around the flat clay and pour a refractory plaster mix into the clay. Let it set, peel off the clay and wash off your mold. It's usually one-time use (unless you use permanent refractory), but it does a good job. Letters you press into the clay will become incised into the glass in the final version; letters you put on top of the clay will be raised.

You can also bisque-fire the finished clay instead of pouring refractory material directly into it. Let it dry completely first, very slowly, to avoid cracks, then bisque-fire it. You'll have to kilnwash it to use it, and you still need to be careful about not trapping the glass, but that would also give you a permanent mold.
Cynthia Morgan
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Re: Custom Molds for Thermoforming

Postby aeshep » Mon Oct 08, 2012 3:43 pm

Thank you Lynn g and Morganica for your quick and well thought out responses. I am always humbled by the generous support and wealth of experience.

Lynn g: I have been considering the ordering the 110J and giving that a go. It's so helpful to hear how well it works in a specific application! But in the course of thinking this through I had hoped to eliminate the tedious, associated task of alignment that comes with using individual letters. I generally prefer abstract work and find a certain "wabi sabi" approach more artful. But this idea of embossed words and short sayings seems to have caught on. I have an expressed opportunity to work at it wholesale: ALL of this is entirely new to me and frankly, I offered it on a lark. Nevertheless, I would be remiss not to explore fully whether it's worth my while (read fun and profitable). Hence the idea about molds.

Morganica, wow! Thank you so much for the detailed info. This all sounds consistent with my readings and thought processes. I'll have much to think about.
Last edited by aeshep on Tue Nov 13, 2012 12:45 pm, edited 1 time in total.

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Re: Custom Molds for Thermoforming

Postby Geo » Tue Oct 09, 2012 9:02 am


I have never used the 110J fiber sheets. Can you please explain how it would work to slump a shallow bowl?


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