I had previously noted the shrinking of the first few layers of a build on my CupCake and attributed it to the ABS shrinking too rapidly after extrusion because the room and the build chamber weren’t terribly warm. Although I’d leaned things against all of the CupCake’s windows so the heated build platform warmed the whole chamber, I thought that too much heat was leaking out when I prepped the print and removed the pre-print test extrusion and that it took a number of layers to heat the interior back up to non-shrinking temperature.
I figured I could retain more heat inside if I didn’t have to move the front “curtain” to remove the test extrusion, so I went to The Yard and picked up a big pair of Kelly forceps. Since I wanted to use them as giant tweezers, I Dremeled off the locking mechanism, leaving myself with a big pair of plain ol’ forceps.
I heated the build platform and the now-more-enclosed chamber for half an hour (longer than the time it takes to print past the warped area), then sneaked the forceps in and snaked out the test extrusion. Whaddya know — the build warped in just the same way as before.
I don’t yet have my build platform’s thermistor connected, so I’ve been running the heater open-loop using a lab power supply to adjust the temperature via current regulation. I remembered a while back when I had it so hot that the bottom half inch of my objects stayed melty-squishy while being built … and although I no longer run it that hot, I’ve been sloppy lately about turning down the current once the first layer is adhered to the platform.
Maybe, I thinks, maybe the first few layers are too warm and pliable and get compressed as the layers above them cool and shrink. I uncover all the CupCake’s windows, start a print, and dial the heated platform current down from 2.7A to 1.8A after the first layer sticks down. I get perfectly straight walls, so I’m finally on the right track and it’s less heat that I need, not more –
but whatever temperature 1.8A delivers isn’t enough to keep the object stuck to the kaptan tape and the bracket I’m building pops loose when it’s almost finished, naturally when I’m in another room, and the last half mile of extrusion doesn’t adhere particularly well to the bracket when the bracket is no longer on the platform.
2.7A for first-layer adhesion, 2.2A after first layer, slight shrinkage, no popping loose from the platform. Now to the point of tweaking for diminishing returns. I should maybe add a fan pointed at the platform like other folks have done, although that has its own dangers (easy to cool the platform too much).
I’m eager to get the thermistor wired up (heater PCB v1.1 will have less restrictive connector spacing); translate these currents into temperatures; and connect the heater and thermistor to the CupCake for closed-loop, PID control. I will not abide the clickety-clack of relays, so I’ll find some big FETs.