Acetal Hypothesis…Confirmed!

As you might have read in a recent post, I had a gut feeling that the reason Acetal filament fails so catastrophically compared to Nylon might be a function of the thermal conductivity of the print bed. The standard aluminum bed of the printrbot simple, while great for a heated setup, works as a heatsink for materials. Acetal/POM, which has a low thermal conductivity easily shifts its heat over to the printbed, and cools down too quickly.

However, that’s just a guess. The only way to confirm this would be a trial with a less thermally conductive bed. Thanks to the wonderful folks over at the Metrix Create:Space over on Capitol Hill (in Seattle), I was able to get some acrylic sheet and composite board cut to replicate the bed of my simple[1].

After attaching the bed to the simple, I ran some trials with PLA to get everything calibrated in and setup.

Then I began running the Acetal.

I think the white acrylic is pretty slick looking.

Results were interesting. The material initially printed and laid down well, and was actually beginning to look quite great. As is the case with POM/Acetal, the filament was extruded as a clear plastic, and stayed clear for some-time. It began to cool down, and return to its natural white color.

Unfortunately, the print began to curl at its edges, and at about the 7th layer, it curled up enough to catch the print head and get pulled off the bed.

The “finished” piece is below:

So yes, there was a failure, but there was also forward progress. This is the farthest I’ve gotten in a POM Print, and it confirms that one of the limiting factors in getting accurate prints with this material is its ability to control the rate at which this material cools.

Next steps, are to begin sourcing parts and materials for a heated bed, and put together a heated bed setup. Also, to get a large rubbermaid container to place the printer in during prints, to try and limit the amount of formaldehyde produced from printing this material at high temperatures.

[1] eps file for printbed 

[2] Printed at 245C for First Layer, 235C for Subsequent Layers. Print layer height of .4 MM


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!

Experiments with POM

As mentioned before, the standard white masking tape I’ve been using lately has been a nice improvement over my prior tape of choice. It’s bond is so strong, and the filament adheres so well to it, that the slight amount of warping PLA creates has been more or less eliminated for me, without requiring a heated bed.

Given such great results, I thought it’d be interesting to try a run with some of the Acetal/POM filament I have on hand, and see how that works out.

Unfortunately, results were significantly less positive. These runs are admittedly a bit nicer than prior attempts, and at least the pieces are solid.

I’m inclined to believe that there isn’t really a way for me to get around the necessity of a heated bed for this one. Unlike nylon, which I’ve had minimal success in keeping adhered to the print-bed, the POM continues to pop off.

Polyoxymethylene, commonly reformed to as POM, or Acetal or Delrin has about the same shrinkage factor as PLA (approx. 2% give or take the filler content) so on its surface, you wouldn’t expect such issues, compared to a higher shrinkage rate polymer like ABS. Especially when PLA has to be heated to only about 185 C for good results, whereas POM requires 215 C minimum.

However, there’s a second factor coming in to play. As many of us have experienced, PLA retains a degree of heat after printing, often remaining relatively pliable and soft for some period after printing. This is because it has a relatively low crystallinity; it melts sooner, and it retains heat longer, because it is an amorphous material. It doesn’t set into an organized structure.

POM is highly crystalline by comparison; it naturally falls into a more “organized” state, which is part of why it has so many great properties (low friction, high durability) but is also why it is so hard to work with. It sheds heat quickly, cooling down into a solid within seconds of printing. When it is being printed, it becomes amorphous, and clear; upon cooling it becomes an opaque white as you can see in the photos above.

Because of this, it cools off so rapidly, that all 2% of its shrinkage occurs at once, instead of over a period of minutes like PLA. In turn, the material shears off the build plate, and curls inward due to contraction along the perimeter of the print.

As a result, there are likely only 2 ways to get solid reliable prints with PLA.

  1. Utilize a heated bed to provide enough heat to the print to slow the cooling rate down (likely somewhere around 80 to 140 C)
  2. Utilize a heated build chamber to slow the print’s cooling rate down.

The latter is what the $10,000 plus Stratasys machines do, and they have quite the nice patent on it. It’s also a lot harder to design. So, I think my shopping requirements for the next few weeks are pretty sorted out.

In the meantime, I will probably continue to see how it performs on other substrates. I’ve heard some mentions that it prints extremely well onto particle boards like OSB, so I might have a small wooden build plate laser cut for the purpose. I haven’t had a good opportunity yet to go up to the Metrix Create Space in town.

Chinese Sample Boxes

You might not know it, but that spool of filament setting on your desk, or the one hooked up to your printer has probably traveled further than you might realize.

As it stands, most of the filament used in the consumer printing market is manufactured in China, and then sold to a vast network of resellers that you purchase from, whether you’re buying from Amazon or somebody else. There are some US manufacturers, but so much of the material is being manufactured abroad.

Earlier this year, I reached out to a few suppliers, looking to see if I could save money by buying directly from the source, and was fortunate enough to come into possession of some samples of material I wanted to try.

Namely, Nylon, POM, TPE and Polycarbonate.

They had a lot of other materials (wood, glow-in-the dark PLA, ABS, etc), but really these 4 were what I wanted to experiment with, because they all have unique and valid uses.

Nylon is of interest because of it’s extraordinary strength and heat resistance.

POM is an incredibly durable material, with a low coefficient of friction (it’s more slippery than nylon) and higher degree of crystallinity which would make it a great choice for clasps and other items where constant friction is part of an item’s service life.

TPE is interesting, because it’s basically printable rubber; there are so many uses for this material as an interface material in various parts and objects.

Polycarbonate was interesting mostly because a lot of my experience in the chemicals industry is focused around acrylics. Now acrylics have a lot of great properties, and whenever I get around to writing a sort of omnibus post about materials selection, I’ll probably devote a section to them, but PC has something truly special that makes it stand out over acrylics in many application. It’s very impact resistant, and it’s got excellent optical properties.

Over the last few weeks I’ve been experimenting with these materials, but under a strange constraint. My printer doesn’t have a heated bed. Adding a heated bed is of course next on the docket, and one of the things I’m doing this January; what I’ve learned from trying to use such challenging materials without a heated bed is useful nonetheless for the insight in how these materials perform in the 3-D printing process.

Stay tuned, and I look forward to hearing from others that are out there trying to push the limits of 3-D printing.