Student Project- Ren Mold for Thermoformed Chair

Hi everyone,

I’m making a chair from 3 layers of thermoformed plastic (ABS, Polycarb, and Acyrlic) for which I need a ren mold. Any advice for keeping the price of the mold down?


Why the 3 materials? Make the mold out of plaster, be sure that it is thoroughly dry. All of the tooling boards will be costly, RenShape will be the most costly.

Thanks! Any limitations/things to be aware of when CNCing plaster?

Three materials:
Polycarb-middle layer, main structure, strong, but flexible so it gives a little when you lean back
abs-doesn’t scratch, strong, supplier only has white and black
acyrlic- color accent for the back

Are you planning on pulling one piece; Then pulling the second part on top of that?
Or pulling each piece individually then assembling later? (Each process will have a few different issues you’ll have to deal with.)

How deep of a draw are we talking about?

Can you show a simple CAD grab of the part your trying to make?

Your question is pretty vague.
Any drawings/cad?
Are you doing the mold, CNCing or pulling parts yourself?
If you’re not doing it yourself then it’s probably best to do a good CAD model and sit down with 2 or 3 Thermoforming vendors and see what they recommend. It will be a good learning experience.

I’ve seen metal, wood and ren molds. Sometimes they glue several pieces together so watch out for glue lines. There’s also male and female molds depending on which side/features you want to control.

Are they pretty flat shapes or some intricate details?

The legs of the chair have since been updated, but here’s a visual. There’s only one thermoforming vender in the area as far as I’ve been able to find. I’m making the mold and mold base, the vendor is heating, forming, and rough cutting the plastic. Each piece will be formed individually. They are 1/8th thick sheets so the slight radii change from layer to layer is basically unaffected.

I have a meeting with the vendor tomorrow to learn how to make the mold and base. Last time I talked to him, I was told to make a male mold with either 450 or higher ren or mahogany. I actually just found a really good deal on some quality ren so it looks like that’s what I’ll be doing.


I know you have not asked for feedback on the design, but here is some for you to consider in any case.

These are the issues you will face with the design:

The change in radius no matter how small will affect the layers and the fit. Maybe you could use a silicone sheet 1/8 inch thick and then 1/4 inch thick to simulate the change of radius on the single mold, not sure.
Bonding the three layers together will be tricky, maybe a PU adhesive, something with some flexibility.
If you do not bond the layer together, there will be very little stability and a lot lot of squeaking.
If bonded, there will be issues with the shape changing at different temperatures, the coefficients of expansion are all different.
You are building a sandwich, in a sandwich construction the outer layers carry the load, you have split the outer layer at the rear lower back of the chair, creating a big stress line at the highest stress point.

Feedback on the design is welcome. So the outer layer should be one piece?

For the maximum amount of strength in the structure you have designed yes. Currently it is only two layers strong at that split point.