3DP Mold Making PT.1

This last three weeks i have been sitting in on Steve Reber’s mold making and casting course at SAIC. Partly to brush up on my mold making skills and partly to learn some of the work flows our students at SAIC use and deploy in this area.

Silicone mold making from a 3D printed model of a Louis H. Sullivan decorative ornamental terracotta.

I have always loved mold making and working with materials like plaster and wax. I enjoy how these materials can be made to “read” a surface or object and produce a mold which in turn makes a facsimile. During this class I also got to work with silicone and urethane which was a first for me. Again i love the colors of the Smooth On silicones and other flexible materials. They are like a candy that cannot be eaten.

One of the interesting overlaps between 3DP and mold making for me are the interchangeability of terms or concepts. Overhangs and undercuts are problems for both processes. In mold making an undercut often requires a multi part mold, in 3D printing an overhang requires support material. The two are basically the same. In the case of plastic extrusion 3DP an unsupported overhang will lead to filament not adhering to the build as intended and falling down like a wispy pasta beard. With mold making a model with undercuts and insufficient mold parts simply will not open up without damaging the cast. Undercuts require three, four, five, six parts to successfully cast the part.

Prepping an acorn squash for a three part plaster mold, clay “barrier” defines parting line

There are other analogies too. The parting line on the model determines how many parts each mold might have. This idea of splitting the object seems close to the idea of the build platform in 3DP where something must touch the surface in order to provide the friction against a plane to define the beginning of the object.

Between Steve and I we started wondering if a 3D printed mold could work with casting materials like silicone, wax or plaster. If we could establish a workflow from a directly 3d printed mold to a wax cast we would be able to easily translate CAD into Bronze with using the established pathways at SAIC.

Two part molds designed in Rhino, “mold box” on the left, and “shell” on the right

I quickly threw together a couple of revolved profiles in Rhino and printed a some very basic two part molds, (download .stl and Rhino files here). The first form was the more successful of the tests – and sports a “traditional” mold box with four keys. It is lacking as decent pour spout something i’ve been adhoc-ing with oil clay but future iterations need to dial in on this. The second form was a nice idea, eschewing the mold box for a simple shell. Alas the best laid plans of mice and men – while the shell prints a wee bit faster the profile I created for the actual cast turned out to be a total nightmare in terms of demolding. So more or less everything was a #fail from that mold.

Urethane cast in 3D printed two part PLA mold

Over the course of the last week i’ve cast the two forms in silicone, urethane, plaster and foundry wax. Needless to say the silicone and urethane more or less popped out of the molds by themselves, if it wasn’t for the cost of silicone and the toxicity of the urethane i’d say we have a winner! The plaster forms on the other hand absolutely refused to leave their happy homes. Partly due to the plaster embedding itself in the “low res” 0.3mm layer height print and partly due to the form just having too many overhangs and gnarly bits. I plan to revisit this form soon with a 4 part mold. i think this will solve the problem, and the nice thing about 3d printing the mold is that it’s more or less the same time as printing a two part mold.

Wax cast from 3D printed two part LayBrick mold

Today I tried out two different experiments. In the blue corner I used LayBrick, a pretty awesome FDM filament from material wizard Kai Parthy that combines polymers with chalk. It prints really smooth, and smells a bit like casein. Anyhow my logic was that as some of it is made of chalk – then it’s going to be porous like plaster. And as such i can soak it in water before hand (like a plaster mold) and then use the water left in the mold a the release to ease the wax cast out. in theory. in practice it sort of worked great at first. One side popped off no worries. w00t! But then the other side refused to budge. I broke it out in pieces :( and in the process learned that the water had really done a number on the LayBrick and nobbled one half of the pour spout in the process. #fail

Wax cast from 3D printed two part PLA mold

In the red corner i had my two PLA box molds prepped and ready for some hot wax. Both molds had/have pre existing issues. The first one has some big holes due to the lack of support material used in the printing process – holes that the wax clung too. Both of them suffer from some severe lack of draft. In short neither offered up a complete cast upon attempted demolding. BUT this will work. As proof of concept goes. This will work. The PLA does not deform despite the wax being poured in at ~180°C which is maybe 10-20°C below that which it extruded at. The casts do release – but they need a bit more encouragement. So back to the drawing board (Rhino).

I’ve also been experimenting with the XTC-3D which on face value sounds like an old skool rave anthem from Skegness or adult fun lube. In reality it’s a “Smooth On” product that claims to smooth away the striations from FDM prints. To be fair i’ve only used it one (today) and i may have mixed it up wrong, but it seems more like a varnish than something that is actually smoothing out the plastic like an acetone vapor bath would/might/does. Anyhow – i’m going to try “smoothing” out the inside of some 3D printed molded with some XTC-3D and see how it helps or hinders the demolding process.

Anyhow next week i’ll update with, hopefully, some successful wax casts from PLA molds. I also plan to try printing some HIPS (High Impact Polystyrene) waste molds, some LayFomm flexible two part molds and some Nylon 2, 3 or 4 part molds.

Here are some useful links for other folks working on 3D printing molds, etc

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