Thermal Conductivity

As part of my interest in printing with POM, I’ve noted before that the largest issue with the material is the incredibly rapid rate at which it cools down. It dumps all of its heat into the environment around it so quickly that it audibly pops off the bed.

Thermal conductivity, is a measure of how quickly something absorbs and transmits heat. If you look at a simple table (such as this one), you can see at a glance, the values for different materials.

Looking at PLA, thermal conductivity is a mere .13 W/m-K, as borne out from the experience any of us have had grabbing a fresh print off the bed without allowing it to cool. It absorbs heat slowly, but also releases it slowly.

Looking at Acetal/POM, it’s only a mere .23, Nylon is .26 and something like Phenolic Resin (a common choice for high temp applications) is only .15

Air itself is only .024. Which, by the way, is the reason insulating gear/clothing for the outdoors is built around having fibers that trap air. Your body heat gets the material “up to temp” and then it takes a very long time for it to cool back down.

So, given all of that, it raises the question of where the heat present in the POM that’s being printed is going. The answer to that is in the bed. Aluminum has a thermal conductivity of 205!

What that means then, is that the aluminum bed allows prints to rapidly cool down, by serving as a heat-sink for the print that you’re making. Now, normally, this wouldn’t be much of an issue. PLA works just fine afterall, and aluminum printbeds are a standard for the ease at which they can be made into heated beds.

However, my hypothesis is, that by using a bed substrate which has low thermal conductivity, it might be able to just barely push the needle to make printing in POM work without adding a heated bed.

Looking at the list, Balsa wood stands out. It’s cheap, it’s the lowest thermal conductivity of any wood, and it’s easy to laser-cut. Acrylic also stands out for similar reasons. The advantage that I believe Balsa wood, or any wood presents is that wood has greater porosity, which means more sites and locations for the extruded plastic to form mechanical bonds with the substrate.

Which means that my next steps are:

1. Create a pattern based on the current Printrbot Aluminum Bed I have.
2. Purchase Acrylic and Balsa Sheet at a size near to the current bed.
3. Have the materials laser cut per the pattern I create. (Seattle has just such a space, and I’ve been meaning to check it out anyways)

Expect to see a post in a week or two outlining how this all goes for me!