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.

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2 Comments

  1. Regarding that heated build chamber, you don’t have to have something complicated to see improvements. I literally drape a towel over my printrbot and I have measured chamber temperatures get up around 113F (with my heated print bed). Doing this has greatly reduced warping on ABS prints.
    Also, would you mind sharing your source for POM filament?

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    1. I’ll have to give that a try. I was actually thinking of putting the thing in a large rubbermaid tub. Especially since printing acetal creates a potential formaldehyde exposure.

      I’m a bit obligated to not share the exact details of my source, but I can get you a 1 KG reel for very cheap, if you’re interested. I evaluated a few different suppliers materials, and was not impressed with most due to poor packaging practices, poor consistency, and a lot of uncertainty regarding if material was as claimed.

      Shoot me a note: exoticfilaments@gmail.com

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