Subtractive Manufacturing – Using Dissolvable Filament to Create Complex Shapes

5/1/2014

3D Printing: Additive Manufacturing

The dowdy technical term for 3D printing is “additive manufacturing.” The term sounds, well, kinda boring – like something your would see on a weather-beaten sign on a prefab building in the industrial part of town. So be it. It is a bit more descriptive, though, and gives one the overall idea that 3D printers produce objects by adding small layers on top of each other to form a whole part – which is how most 3D printing technologies work.

One of the most common 3D printing techs is the fused deposition modeling (FDM) method, also known as fused filament fabrication (FFF). Plastic filament is heated and forced through an extruder head, which moves back and forth to form one layer of the object being printed. When that layer is done, the extruder head moves up (or the build plate moves down), and the next layer is formed. Almost all of the hobbyist and “prosumer” 3D printer models use this technology.

One of the great promises of 3D printing is its ability to create shapes and objects that would be difficult or impossible to create by other manufacturing means. However, as I discovered early on, there are some limitations to the shapes that can be created with a 3D printer. Mostly, this has to do with the fact that each layer needs to be supported by something beneath it, ideally the previous layer. For example, you couldn’t print an upright “T” shape, because when the printer started the bottom layer of the arms of the T, the plastic filament would just drop down to the build plate and make an ugly mess.

Supporting Complex 3D Prints

The way around this is to use supports that are not part of the original 3D model. These are inserted by the “slicing” software that converts the 3D model into the instructions that the 3D printer uses to move all of its parts and gizmos to produce the 3D print. These support objects are handy, but can also be a pain in the fundament, as they need to be manually removed from the final print, sanded down, etc.

When I first started 3D printing way, way back in 2011, I didn’t use supports for my 3D prints. This was because the slicing software I was using was very non-intuitive, and I never figured out how to turn on the support creation function. It became an interesting challenge to figure out what I could get away with as far as creating cantilevered, nonlinear shapes that could successfully be created without supports. I liked to think that it was like being a medieval builder, and experimented with what sort of shapes I could produce within the limitations of the materials and technology. Not surprisingly, a lot of the prints I managed to get to work had a Gothic look to them, like the one below:

Gothic-looking crown object printed without supports.

Gothic-looking crown object printed without supports.

Eventually, I stepped up to more sophisticated slicing software, and was able to start creating objects with supports that allowed much greater flexibility in the shapes that I could create. However, I still ran up against some limitations with using supports in the 3D prints I was making. For objects with complex or delicate internal geometries, it was nearly impossible to manually remove the support structures, and I wound up with broken or sloppy-looking prints.

HIPS and Dissolvable Filament

Fortunately, there is a better way to deal with this – dissolvable support materials. There are several types of plastics that can be used with FFF printers that are dissolvable. Polyvinyl alcohol (PVA) easily dissolves in cold water. My Makerbot Replicator 2X is kind of persnickety about the types of plastic it can use, so I had to go with an alternative. This is high-impact polystyrene (HIPS), which has similar characteristics to the commonly-used ABS plastic (the type for which the Rep 2X is “optimized”). HIPS must be dissolved in limonene, which is a citrus-based solvent. A complex print can be made with the standard ABS, with the HIPS used as the support material. The final product is submerged in a limonene bath, and the HIPS supports dissolve in a few hours. The limonene bath is a little more difficult to work with than one with plain old water, but it does have the advantage of leaving the prints smelling lemony fresh!

I recently got to put the dissolvable HIPS to a stress test when I was approached by a local structural engineering firm to make a model. They wanted to have a model of a complex rebar connection that they could show to a contractor on a project. The model they provided was very complex, with a lot of very thin structural members and fairly complex internal geometry. There was absolutely no way that this could be done with traditional ABS supports; it was just too delicate and complicated to be able to manually remove the support material.

So I put the HIPS to the acid (or limonene) test. It took nine tries to come up with a print configuration that produced the desired results. Coming out of the printer, the shape of the ABS model was nearly unrecognizable – it was completely wrapped in a cocoon of HIPS. It took 14 hours to print and needed nearly as much time in the limonene bath to dissolve the HIPS – not too surprising, as about 60% of the final print was HIPS material. The final product came out looking good, and the engineers were pleased with the result.

Complex structural rebar connection rebar connection model. Image on left shows print in dissolvable filament "cocoon"; right shows finished product

Complex structural rebar connection model. Image on left shows print in dissolvable filament “cocoon”; right shows finished product

Even for the relatively simple FFF 3D printing technology, very complex shapes can be created by using different materials with different properties, such as dissolvable PVA or HIPS. There are more FFF-compatible materials being introduced as well, which will further enhance the flexibility and variety of 3D printing – more excitement and possibilities for this already innovative technology.

For more information on Tesseract Design’s 3D printing services, please see www.tesseract-design.com/3-d-printing

(Postscript – It just occurred to me that my clever little “subtractive manufacturing” title could also be used to describe computer numerical control (CNC) machining. I have little experience with this technology, although there are now consumer-accessible CNC tools available. It’s definitely on my “wanna try” list. Still, I have trouble believing that even a highly sophisticated CNC machine could have produced that rebar connection model, but I don’t know enough to say for sure.)