(For a TL;DR of printing with Opaque HTPLA, see “Dialing In Print Settings” below)
The biggest challenge I regularly face using FDM 3D printing for functional parts is temperature stability, principally for automotive parts. At high temperature, thermoplastics soften and warp, and at car-in-sun-on-hot-day temperatures, a part under load will give way completely.
The biggest issue with printable thermoplastics is that, in general, the maximum usable temperature of a part is proportional to the temperature the plastic must be printed at, so parts tolerant of very hot environments need extreme printing temperature and therefore require exotic machines for successful printing, or at least significant extra effort.
PLA behaves interestingly in that you can actually raise its softening temperature by heat-treating it after printing. Here’s a great video all about it:
The problem is, the heat treating process itself tends to warp and shrink the part. A few manufacturers have made materials with additives to aid dimensional stability during the process, most notably ProtoPasta’s HTPLA series.
My Experience with HTPLA
First off, HTPLA actually comes in a couple varieties: Opaque, Matte Fiber, and Carbon Fiber. The difference is important both because they print differently, but also because they heat treat differently. Carbon fiber, though it doesn’t actually increase part strength, aids tremendously in stiffness and dimensional stability, and (I’m told) results in a part that shrinks/grows very little during heat treating, and warps almost not at all. Matte Fiber presumably has similar characteristics.
I’m cheap, so I did this project with Opaque instead, which ultimately probably cost me a lot of extra time dialing in the details. Here’s the part I’m trying to make – a dashboard mount for a Galaxy Tab A 10.1″ on a 2009 BMW 328i:
You can see how dimensional accuracy is super important on this part because of the interference fit of the tablet in the clip, and also how any temperature weakening would be game-over in the sun. I’ve had a bunch of prototype failures, including:
- The top piece melts and stretches until the tablet tilts forward and falls off the dash
- The main bulk warps away from the tablet, releasing its grip
- the little tab stuffed under the vent cover melts and lets go of the rest of the brace
HTPLA is the right choice here because neither ABS nor PETG would tolerate such a high temperature, and a more exotic material like nylon or polycarbonate is completely untested in my workshop. But dialing in the shrinkage factor was especially tricky.
Unfortunately, I ran into a bug in PrusaSlicer that made it nearly impossible to dial in precise scaling factors, but ultimately with the release of the latest 2.1.0, this was fixed and I was finally able to test properly.
Dialing In Print Settings
So the issue with printing PLA then heat treating it is three-fold: shrinkage in X and Y, growth in Z, and warping. HTPLA’s additives are supposed to (mostly) take care of warping, and you can pre-compensate the shrinkage by just scaling anisotropically in your slicer.
In my case, print settings of X: 102.1% Y: 102.1% Z: 99% worked well enough. Ultimately, the part was a LITTLE tight in Y, but did work, and the tolerances were tight to begin with. Z I actually don’t know about, since it’s not a functional axis of my print and over-extrusion on the top surface made it tough to accurately measure before/after.
Warp is pretty significant even on HTPLA, so if you REALLY need bombproof dimensional accuracy, you might have a better experience with Carbon Fiber HTPLA, which shrinks much less than this and remains more stable. But for this part, it worked out.
Luckily, it only took me maybe 6 tries to get the goldilocks fit 🙂