Printing Plastic: Works, stinks.

We have recently succeeded in fabricating durable thermoplastic objects on the CandyFab. That means that we can produce strong, waterproof-- even dishwasher safe(!) -- forms. What's the downside? It smells like melted plastic when you print, not like melted sugar.

Read on for more about the materials, shapes and process that we used.

For each layer that we are going to print, we need to put down a fresh layer of plastic media. However, the layer really only needs to be in the areas that will be fused together. So, we poured down a flat layer of plastic pellets in the center part of the build volume (where our parts will be made). Outside of that area, we can fill in the layer with sugar, which will serve to support the smaller plastic volume as we go to higher layers in the fabrication.

After fabrication is complete, the plastic pellets can be separated from the sugar with a sieve, assuming that they are of a characteristic size larger than that of the sugar grains. (If we were to use granulated plastic that could not be separated from the sugar with a sieve, it would still be possible to recover and reuse the unfused media by dissolving away the sugar in water and allowing the plastic grains to dry.)

The plastic pellets that we used for this purpose are Dow HDPE 30460M Injection Pellets; mixed black and green, which when melted and mixed together are supposed to make dark green injection-molded parts. (Hence the black and green pellets mixed in with the white.) HDPE is excellent hard plastic, typically used to make milk bottles, cutting boards, playground equipment, and tupperware. It is potentially food safe and dishwasher safe. It complies with FDA regulation 21 CFR 177.1520 (c) 3.2(a) for food contact applications. It has a density of 0.96 g/cc -- it floats--and has a typical melted process temperature of 190 C. (More data here.)

We have several observations about using this material. First, the pellet size is huge-- 3-5 mm irregular pieces. It does not make sense to use a layer thickness smaller than the characteristic grain size, so we set the vertical step size to 0.175", which turned out to be acceptable. Using such immense layer thickness means that we have to be able to melt deeply, over a large area, so we used Heater 1.0. (It turns out to be quite convenient to be able to swap out heaters for different applications.)

While the old heater had too much airflow to be ideal for sugar, it works quite well with the larger plastic pellets which don't blow around with that amount of air flow. As the plastic beads melt, they turn from opaque to clear and expand greatly. This turned out to be a real issue, because they would contract again as they cooled down. Because of this, the edges of a printed layer had a tendency to curl up-- where the material in the middle cooled much more slowly. Fusion between layers becomes critically important then, as a means of preventing curling, which further pries the layers apart. (I believe that this would not be a significant issue with much smaller plastic pellets. May be a good use for a blender....)

The plastic gives off a funny odor when it melts. The scent is actually not what I would expect of melting plastic, but rather like melted wax, particularly like that of cheap restaurant candles. We might have less scent with lower nozzle temperature and slower builds. One additional issue is that we had some contamination from sugar in the melted plastic. This is because the plastic accumulates static electricity that can hold a few sugar grains, and after recycling the pellets through the machine a few times, they tend to have a small amount residual sugar contamination. The browning of the melted plastic in the picture above is due to the sugar contamination-- the pure plastic cools to be opaque white again. So, it will be important when using this method to clean off the plastic with water, compressed air, or a sieve combined with static electricity control if the small amount of sugar contamination poses any issues.

The shape is strong and sturdy. Viewed from one the edge of one side, you can see the individual layers, much like the coarsest prints that we made in sugar-- a characteristic result of Heater 1.0. Obviously we could make objects with much higher resolution by starting with finely powdered plastic instead of the giant pellets.

One other shape that we made in HDPE is a large fruit bowl:

The walls of this piece are thin enough to see light through-- it's a nice effect. See also the smaller sugar version.

I will mention in passing that we have also fabbed in polypropylene, which smells much, much worse than HDPE-- good ventilation is essential. Polypropylene pellets can easily be obtained at craft supply stores in 2 lb. or 25 lb. bags ($50). (They are intended for use in creating weighted beanbag dolls.)

Since our ventilation is not that good, we have not given polypropylene a sufficient workout yet to be sure how to recommend it. It produces structures that are more translucent than that of HDPE, and it comes in smaller, more uniform pellets. Problems with interlayer curling were not nearly so pronounced, and it does seem to be generally more forgiving material.

The next types of plastic that we would like to try are granular, and we are still on the lookout for good sources of granular media. As always, if you have comments or suggestions about media, let's discuss them in the CandyFab Media forum.