Okay, looks like Printbus was right. The bed is warping and closing up the gap. I have my heat bed hard-mounted to the wood with standoffs, since I'm using auto-bed leveling. I put a machinist's dial indicator on one corner of the bed and turned on the heat. The relative expansion of the bed and the wood mount are causing the issues. Here's what's happening:


  1. The bed starts heating.
  2. The heat bed expands horizontally, pushing on the standoffs, causing the wood to bow downward, lowering the bed .012"
  3. The printer probes the bed and establishes the zero point.
  4. As the print runs, the heat soaks through into the wood, causing it to expand over the next hour. This flattens out the warp and pushes it the other way, raising the bed by an additional .021".


The rising bed compresses the print in progress by a total of .033", or .84mm over the course of an hour or two, causing the parts to appear massively overextruded.

I think the solution is going to involve relieving the stress by soft-mounting the heat bed in a way that it can expand. I'll try the springs. That may be enough. If the bed is warping because of the heat gradient across it, another material might be called for. Aluminum wouldn't build up a gradient because of its conductivity.

I'll post as I learn more.