RepRap: Blog

Once I established that using pads and guy flanges suppressed warping in large dimension objects printed in ABS I decided to see if the same method with HDPE. Basically, I printed a 5x20x145 mm bar guyed to pads on the raft.

Some years ago, when I was working with HDPE on my Tommelise 1.0 Repstrap printer the big problem with HDPE, aside from a tendency to warp profoundly, was finding a surface that it could be printed on. I found that I could print on art store foamboard. This material is about a quarter of an inch thick, typically and consists of plastic foam sandwiched between two sheets of plastic coated paper. HDPE would stick to that quite nicely. Unfortunately, however, this material tended to perish when exposed to the hot extruder head of Tommelise 1.0. It also lacked the structural strength to resist the tendency of HDPE prints to curl at the corners.

Recently, Bogdan Kecman, a Rapman user in Belgrade, has been making a serious effort at working through the problems presented by HDPE and polypropylene as printing materials. Yesterday, he was kind enough to offer me advice about what sorts of materials that he found HDPE would stick to. In the list he gave me it appeared that polypropylene sheet offered the best option for printing HDPE. I happened to have a few sheets of 9.5 mm polypropylene left over from my milling work with Tommelise 2.0 earlier this year, so I quickly converted one of these into a print surface for my Rapman 2.0 printer and got to work.



After a few false starts and a careful go at leveling my print surface, I managed to print my guyed beam successfully with no peeling of the raft and NO subsequent curling or warping of the beam.



The beam was printed using Skeinforge with a 40% hexagonal fill. the raft was printed at 220 C and the beam at 225 C. The extruder head speed was set at 16.8 mm/sec and the feed rate at 46 rpm. I am not completely satisfied with the surface quality of the beam. I suspect that I am extruding a bit too cool. I will be exploring these kinds of settings in coming days.

One of the most important discoveries of this exercise is Bogdan's discovery of polypropylene as a printing surface for HDPE. It is sticky enough to keep HDPE rafts solidly on its surface and strong enough, in thick pieces to successfully resist HDPE's tendency to curl and warp during the printing process. Without Bogdan's discovery, guying would have been impossible with HDPE. Polypropylene sheet is also readily available and cheap as well. I shaped it into a print surface for Rapman with ordinary hand tools. It behaves a lot like acrylic plastic except that it isn't brittle at all, a characteristic which makes it much less risky to work with.

HDPE has several advantages over more conventional plastics like ABS and PLA. It is a single polymer and not a witches brew like ABS. When you print with it you can, if you get close to the print head, detect a mild, pleasant odor from the hot HDPE. Try the same thing with ABS and you get a very different experience. HDPE also can be had for about $5.50/lb {~$12/kg) compared to $9.75/lb for ABS {$21.50/kg} and $10/lb for PLA {~$22/kg}.

HDPE also constitutes a very large fraction of plastic waste worldwide if we ever get around to developing small scale plastics grinding and filament extruding equipment that will get us into recycling.

Please understand that I am not suggesting that everyone run out and buy lots of HDPE right now. We have a lot of practical experience using PLA and ABS and we shouldn't dump that. Different plastics have different uses. What is nice, however, is that it looks like we have another one to add to our list, which, like PLA, has not heretofore been used in commercial equipment.

I am very excited because Bogdan Kecman is going to be testing the pads and guying technique with polypropylene in the next few weeks. If he succeeds, we will have a printable plastic that costs about $4/lb {$8.80/kg}. THAT will be a brilliant development for the Reprap endeavour.