Less is more

Yesterday I took off my cobbled-together insulation and installed the simple fix that Printrbot sent out — thank you! Since part of the installation process involved running the bed up to 100c to get the thermal expansion right for tightening the rails, I thought I’d take some data on the way, and I found some interesting results.

First, of course, the fix works, and well. The simple reflective sheet kills most of the heat loss from the bottom of the bed, and insulates the rails without getting bogged down in complicated fitting and patching.

However. (Yeah, I’m always going to be picking nits.) The thermistor is inside the reflective zone, so it will report the temperature of the heater, not necessarily that of the bed surface. During the installation process, for example, the thermistor reached 100C in 7 minutes. Temperature of the bed at that time: 43C. (Measured with a cheapo Sear IR thermometer that’s correlated reasonably well with a thermocouple in the past.)

Over the next 20-plus minutes I took a series of readings, finishing up at 30 minutes out when the bed temperature reached 85C. Oh, and the small wing at that point was about 53C. I wish I could figure out how to put a thermal break between it and the bed. (Tried to fit a piece of mylar, but there’s not enough room. UPDATE: made sure the bed was solidly attached, loosened the wings, put in some mylar shims, didn’t make any discernable difference.) The rails were about about 40C.

But. It turns out that tipping the machine on its side is a really bad configuration for heating the bed, because that warm vertical surface is perfect for convection cooling. When I put the Metal Plus back upright, fitted some reflective insulation on the wings and put a piece over the bed while it was heating I got some nicer results. At 7 and a half minutes, when the thermistor registered 100C, the bed said 48. About 13 minutes to get to the 65 I usually use for printing PLA. It reached 85C 20 minutes out, and at 30 minutes it was at 93C. That’s much more plausible. And when my basement warms up (15C now) things will only get better.

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One Response to Less is more

  1. Pingback: Printrbot Metal Plus thermal fixes | proof of concept

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