Casting With Soft Mold Constructed From Sheet Materials




About: Wei Li is an artist and designer that currently lives and works in San Francisco. Li uses her work to explore fantasies and desires with visual, visceral and whimsical language. Incorporating pure sensations...

This project started with an interest in exploring unconventional casting techniques, especially in applications where the repeatability of the shape is not the primary focus. In traditional casting, you make a big block mold from your original, and then use this mold to produce multiples of the same form. In this process you'll normally use a lot of materials to construct the mold, and you'll try to make it perfect so that it lasts longer.

But what if you want to make multiples of different forms? Then constructing a big block mold for each of the forms will not be economical. In this instructable, I'll show you how to cast with molds constructed from cheap sheet materials, so that you can quickly cast different forms without investing much material and money.

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Step 1: Cut / Score Patterns

The basic idea is to construct a shape using paper folding (origami) techniques. If you know anything about origami, you should know that you can create a million different 3d forms from 2d sheet materials. Here I'm not using paper because paper will be too easily penetrated by any casting material. I used polypropylene sheet from Tap Plastics, which comes in 24.5" x 45" x .020'' with one side matte and one side smooth.

I cut / scored a pattern with triangles on one sheet of polypropylene on an Epilog laser cutter. If you don't have access to a laser cutter you can also score by hand using an exacto knife. With a pattern with equilateral triangles you can fold the sheet into a closed shape very easily.

Step 2: Fold Pattern Into Casting Container

I used duct tape to close the shape. Then blue tape and duct tape to close the bottom of the container. Blue tape is magical! It is sticky but it doesn't really stick to a variety of casting materials. They can be very easily removed later on.

With the pattern I have, I can push the mold in different spots to make it more dynamic. This also means that I can create different shapes with the same mold.

I used the smooth side of the polypropylene sheet for the inside of the container. It is very smooth and release agent is not totally necessary.

Step 3: Casting: Materials & Techniques

You can do either solid casting or slush casting. Solid casting means that you'll pour in materials to completely fill up the void. As long as your mold is strong enough to hold the weight of the casting materials, you can create a solid casting using the mold you have. To save materials, you can also do slush casting. Slush casting is a casting technique to create a hollow casting. In the process the material is poured into the mold and allowed to cool until a shell of material forms in the mold.

In this example I'm doing slush casting, by pouring in casting materials and then rotating the mold to get an even coating on the inner surface of the mold. I did it by hand, but you can also make a rotating gig to help get a consistent coating.

I tried two different materials: hydrocal (a kind of plaster with higher strength than plaster of paris) & Smoothcast 65D (semi-rigid urethane resin). Both materials set super fast, so you don't need to worry about holding a mold and rotating forever until it sets. In the resulting casts, hydrocal has a beautiful matte surface and a nice weight, while Smoothcast 65D has a shiny plastic feel. One advantage of Smoothcast 65D over hydrocal is that you can do multiple coats, building up the cast little by little, since Smoothcast 65D sticks to itself super easily. Hydrocal, on the other hand, works best in one batch. This is because that when hydrocal is curing, it expands a little bit, which will crack earlier layers.

You can also color your castings. With hydrocal you can use liquid tint or coloring powder. With Smoothcast 65D you can use resin pigment and dyes.

Step 4: Demold & Finish

Once the casting cured, you can remove the tapes and peel off the polypropylene sheet easily. If you're careful enough not to destroy the mold, you can keep using it to do a few more castings. If your sheet mold is destroyed, no big deal, it only costs a few bucks.

Sand to smooth down the seams on your casting. Both hydrocal and Smoothcast 65D can be sanded down easily.

Step 5: Possibilities & Limitations

This technique provides new possibilities to casting, but it also has its limitations. Since we're folding from sheet materials, it is easier to do geometric / faceted shapes than organic shapes.

1. As I mentioned before, we can do so much with origami. Check out Paul Jackson's Folding Techniques for Designers to get more inspirations.

2. Using Pepakura, a software that exports a paper folding pattern for almost any 3d model you might have, you can make anything!

3. Designer Phil Cuttance has used this technique to create lamp shades and vases.

4. Large-sclae, architectural explorations of this technique: Crease, Fold, Pour: Advancing Flexible Formwork with Digital Fabrication and Origami Folding, at the University of Michigan Taubman College.

5. Crumpling instead of folding: I tried to use aluminum foil as a casting container for Smoothcase 65D. It failed since I was not able to remove the aluminum foil later. But I think with the proper release agent, this can succeed and the result will be interesting.

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13 Discussions


3 years ago

Hi Wei, we miss you!!!!

Hey, I'm working on a new series of molds and want to use your idea and mark my folded edges on the polypropylene in the laser cutter. Can you tell me- did the poly warp at all? I'm wondering if it would make it easier to build a fixture to hold the poly flat before I laser.

Thanks! XOXOXO

1 reply
wei weiJenniferBerry

Reply 3 years ago

Hey Jen!! I miss you too!!! What are you working on these days?
I'm jealous you're still doing works. It's been hard for me to do work or even buy materials here in China....

When I lasercut the popypropylene sheet It didn't warp, though I imagine if you're cutting very intricate shapes it might. I was just scoring big triangles and it wasn't a problem.

I do remember the sheet itself was a bit arched since I rolled them up once. I just used some blue tape to secure the edges. And the lasercutter bed also sucks the material down, so when it cuts the material was pretty flat.

Hope this helps a bit! :)


3 years ago on Step 5

Nice project! I'm currently working through the Paul Jackson book, rendering some of the simpler projects in sheet metal. An elaboration that one can do in metal (and probably the plastic also) is replacing the straight line folds with shallow arcs. It really changes the "feel" of the final object.

With regard to the crumpling experimentation, in addition to the release agent, you will likely have to also solve the problem of undercutting. To some extent it can be solved by opening the crumpled sheet more than the book suggests, but then you lose some of the crumpled texture.


4 years ago on Introduction


I'm in absolute awe of your ingenuity. Also, have been wanting to make some "nesting" concrete planters for outdoors and have been struggling with a design. The idea of making origami planters, though, is now solidly in my mind. I will have to start figuring out how to make a mold for concrete using this technique.

The number of applications and mediums I'm envisioning for every room in my house using this guide are almost too many with which to keep up! I've been in a creative block lately so I can't tell you how much I appreciate the inspiration. Love it.

Thanks so much for sharing this with us, and cheers!

1 reply
wei weikirnex

Reply 4 years ago on Introduction

Thank you so much for your comment!

I wish I was really that clever but I actually learned this technique from a few other people who shared their works online. See the last step where I listed other examples. There's that video showing concrete casting but in larger, architectural scale. I'm glad you're having many ideas now and best luck with your planter project! :)

wei weirandofo

Reply 4 years ago on Introduction

Actually I was not sure where I was going with all happened in the last minute