Friday, October 1, 2010

Getting into polypropylene

In which your narrator begins to familiarise himself with the vagarities of printing with polypropylene.

I cut a 3/8 inch black HDPE print bed for printing polypropylene (PP). PP doesn't stick all that firmly to it but it should be adequate for my purposes. Before getting into designing and printing ligaments, I decided to just do some sample prints to get used to how it behaves and what the settings are.

I found that my ABS temperature settings were quite adequate for PP. PP does, however, tend to swell after leaving the extruder orifice considerably more than ABS. Whereas ABS swells from the 0.3 mm orifice to 0.458 mm, PP at the same temperatures and extruder stepper rates swells to 0.58 mm. This means that you're running 60% more PP through the extruder than you are ABS for identical extruder settings. That quickly became apparent with several trial prints.

Just for fun I printed the middle finger metacarpal.

The print didn't have the fine definition of ABS, but then I don't have the flow compensated completely for the faster flow rate that PP does.  The metacarpals were quite flexible.

I printed a few flat plates of PP several layers thick with no raft and it appears that PP will work very well for ligaments.


  1. Hi Forrest,
    The die swell doesn't alter the volume of plastic or the flow rate, It just means it comes out shorter and fatter. If you don't change the setting from ABS you will have exactly the right amount of plastic, but it will be stretched more, so will tend to cut corners and shrink holes. The only cure for that is to slice for thicker filament so it is not being stretched much, or use a smaller nozzle. But a smaller nozzle is a law of diminishing returns because it swells more.

  2. If you are getting more plastic volume for the same extruder rates it will be because the feed stock diameter is bigger.

  3. Nop: Thanks for the heads up. I'm just starting seriously on PP and can use all the good advice I can get. Yours, of course, is just about the best on offer! I'll check that out. I expect to play with this stuff for a while to get the feel of it. :-)

  4. swelling is nasty with PP, I found that lowering the speed of the print solves most of the problems. I was printing 0.3mm layer with 0.5mm nozzle without a problem (have not tried PP with my 0.25mm nozzle yet) but I had to go down to 6mm/sec + fan (not the one on extruder but one ~0.5m from rapman so you only create a slight breeze over the PP object). Also pushing corners "bit more" (infill over perimeter in skeinforge)helps a lot too.

    I had also one successful test (that I have not repeated) with 16mm/sec and "pause" on the corners (I drive the head to the corner at 16mm/sec then stop for few ms, then continue) .. it was something I wanted to test more but moved to other stuff

  5. I'm pretty much locked out of the bfb forums by some sort of software bug unless I fire up my old PC, so I'll answer your glue question here, if you don't mind.

    I've bought BONDiT B-45TH and B-482TH to see if I can get a good bond between PP and PP and PP and ABS. They are epoxies, viz, two part adhesives. I should know how useful they'll be in a week or two.