Archive for April 15th, 2011
Printing ABS objects on an ABS film atop a heated aluminum plate works just about as perfectly as I could want, as witness those calibration objects. It turns out that the thickness of the ABS film makes a big difference in how well the first layer bonds to it.
I’m coating the plates with scrap ABS objects dissolved in MEK, because MEK seems to be less aggressively flammable than acetone. It smells horrible, though, and spreading a layer of toxic gunk with the consistency of honey can’t possibly be good for me. I use dead credit cards as spreaders and wonder if there’s a better way; a brush would clog up almost instantly.
The rough rule of thumb:
- If the ABS layer isn’t obvious, then it’s too thin.
A Companion Cube growing out of a good pink film layer:
Peeling a smaller Cube off a plate shows how well it bonds to clear film. Notice how the film peels off the build plate without leaving any residue except for a tear in the film covering the hollow underside of the Cube.
These outline extrusions show the effect of a too-thin film, where the extrusion simply peeled off the film. Where it’s thick enough, the extrusion is welded right to the surface. Intermediate thicknesses tend to rip on both sides of the extrusion.
In round numbers, the perfectly formed film at the lower left is between 0.05 mm (the darker regions) and 0.09 mm (the deepest pink). The others range from 0.02 mm to 0.05 mm and are too thin for good bonding. Even the thickest film doesn’t add much to the first layer thickness.
The other part of the secret is extruding the first layer at 10 mm/s, which is 25% of the 40 mm/s I use for the rest of the object layers. The platform is at 120 °C, the Thermal Core at 210 °C, and the extrusion sticks like it’s welded… which, in fact, it is.
I think that a too-thin film cools the extrusion before it can bond with the film, while a just-right film melts slightly on contact. Extruding at 10 mm/s guarantees enough contact time for the filament to melt the film and cool down before the nozzle puts any tension on it: corners come out perfectly.
The other part of the puzzle requires an absolutely level build platform at a constant height from the nozzle. The platform leveling described there helps, but it’s a hassle to get everything set up.
This should go without saying, but I got it wrong the first time: protect those little pins that drive the ABP belt motor if you’re not using them, particularly if you have a grounded aluminum build plate a few millimeters away.
I used a pair of jumpers, side by side, that did not connect the two pins, because they were close at hand when I first shorted the build plate to the pin carrying +12 V. A jumper that connects the pins will blow the motor control MOSFET on the Extruder Controller if you (or the G-Code) should happen to ever turn it on by mistake, so you need two.
Better, just put some heatshrink tubing over both pins.
You could print up a little dummy plug for the thing…