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The turntable support part in my microwave has broken. It is a three armed part, with small wheels at the end of each arm.

support part

I'm confident I could print a replacement, and reuse the existing wheels (since they can be removed).

The heat of the food would be unlikely to conduct through to the PLA, but I'm concerned that it might get heated up by the microwave radiation. I can't find any clear information online about whether PLA absorbs microwaves, or if it is in any other way unsuitable for this.

Will this be a disaster, or should I give it a go?

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    $\begingroup$ This raises the question of what the original part was made of, and whether or not that material be used in a 3D printer. $\endgroup$ – John Sensebe Aug 23 '16 at 14:12
  • $\begingroup$ I would AVOID plastics and anything involving heat, and foods. A lot of plastics are food safe, but their additives are not. Or they are cut with cheaper plastics that are not. $\endgroup$ – StarWind0 Aug 23 '16 at 16:23
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I would say PLA itself should not be heated up by microwave. It's because microwave oven creates oscilations which excites water particles (see microwave explanation here) so assuming PLA doesn't contain water, it won't heat up. (removed to not mislead as the water is not only material which heats up by microwaves. Thanx to Tom van der Zanden for being vigilant)

But as usual, it's more complicated.

First. PLA can contain water as while producing it can be cooled down in water bath. Of course well made PLA will have as less water as possible as water has an influence on printing process.

Second. PLA is absorbing humidity so in fact it gets water inside right from the air. This unfortunately causes problems in microwave oven.

Water can be overheated and oven can overheat water above 100C. But even at 100C, PLA will not be hard anymore so your 3 arm star would "collapse". Wheels could get oval or start sticking to their axis.

Eventually if high power is delivered to very "wet" PLA, I think it can... well maybe not explode but break.

Here goes a test which shows it can be used to defrost things on PLA plate in microwave

But here Daan Snijders claims PLA gets soft in microwave during the test

Will it be a disaster?

In my opinion it will work only for short uses of MW. Heating up a glass of milk or so. But for longer sessions when there will be much more heat (out of heating dish) it won't work.

SHORT TEST

  • 20sec and 950W gives no effect on my sample (hotend cooling fan duct)
  • 40sec and 950W caused the sample became a bit warm

Inspite that it's not a good idea to run MW without "proper-absorber" this little test confirmes my suspisious - short sessions are ok.

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  • $\begingroup$ The idea that a microwave only heats up water is completely wrong. Many more materials can be heated by microwaves. $\endgroup$ – Tom van der Zanden Aug 23 '16 at 10:17
  • $\begingroup$ Of course you are right. But microwave is generally designed (oscilation selected) to excite water particles primarily. Many metals get heated up by MW (of more prcisely by eddy currents). But in terms of PLA and his 3 arm star it's not applicable :) $\endgroup$ – darth pixel Aug 23 '16 at 10:40
  • $\begingroup$ here is a part of the article you linked: "...and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy...". My understanding of microwave oven is precisely as this citation and my answer is based on this understanding. "Frequecy" means "oscillation" for me... and not only for me (wiki:frequency-This article is about the rates of waves, oscillations, and vibrations.) I have a feeling you are trying to over-generalize questions and then answers ;) no offence $\endgroup$ – darth pixel Aug 23 '16 at 11:47
  • $\begingroup$ Heat IS rotation/oscillation so obviously that will be involved at some point. However, you are making the claim that microwaves are designed to heat up water and only heat up water and therefore items not containing water do not heat up. This is not true. It could very well be (or not) that PLA has dipoles as well. The exact mechanic by which the heating happens is secondary to this (if you continue reading beyond the quoted portion you'll notice that they mention that heating through resonance is a common misconception which I thought your argument about oscillation was referring to). $\endgroup$ – Tom van der Zanden Aug 23 '16 at 11:54
  • $\begingroup$ :) I've read whole article in EN and in my native lang again to not argue without arguments. As I said - you are right but it's not really applicable to his question. $\endgroup$ – darth pixel Aug 23 '16 at 12:12
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You really shouldn't

  1. PLA "glass point" (the temperature where it starts to get soft) is approximately 60C, even if it doesn't heat up from the microwaves it's way to likely the ambient temperature in the microwave will reach that level and cause the part to collapse (even if you "never" get it hot - it only have to get to 60C once during the lifetime of the part).

  2. PLA is not safe for cooking (or even for food storage) and your typical PLA filament isn't even pure PLA, only god knows what chemicals there are in your specific brand of filament and what will be released into the air inside the microwave when it's heated.

In first glance ABS seem to be a better option because it has an higher glass point and food-safe ABS exists - but it's still bad because even food-safe-ABS is only safe for cold and room temperature food.

I don't know about other materials.

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    $\begingroup$ 1. I doubt it gets that hot in a microwave. Do you have anything to back this up? The plastic part is sandwiched between the metal case and glass plate, both of which are heat sinks and stay quite cool to the touch - in my microwave anyways. 2. I'm pretty sure PLA is not inherently food-unsafe, but in any case, in his application this is not a problem because the plastic isn't anywhere near the food. There are lots of commercial uses of PLA in food applications. $\endgroup$ – Tom van der Zanden Aug 25 '16 at 15:04
  • $\begingroup$ @TomvanderZanden, I agree. Unless heating up PLA produces some off-gasses, I don't see any reason not to try it out. Worst case scenario the structure will bend and not function - which really isn't that big of a deal. $\endgroup$ – Tormod Haugene Aug 26 '16 at 18:57
  • $\begingroup$ @TomvanderZanden - the way I see it, especially with cheap Asian filament that contains god knows what additives, unless the manufacturer specifically specified the filament does not produce poisonous gas when heated it should not be used in a Microwave (or anywhere else there's a combination of heat and food) $\endgroup$ – Nir Aug 28 '16 at 11:03
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Yes. You can. I would think ABS would be a better idea, as it melts at a higher temp. http://www.thingiverse.com/thing:385288

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  • $\begingroup$ ABS does not cause any issues when heated? $\endgroup$ – Jash Jacob Aug 24 '16 at 3:56
  • $\begingroup$ You should elaborate a bit more on this. Why is it okay to use PLA, and why is using ABS presumably better? $\endgroup$ – Tom van der Zanden Aug 24 '16 at 11:44
  • $\begingroup$ Just because someone uploaded something to Thingiverse doesn't make it safe. If you were to attempt this, though, it'd probably be safest to use uncolored ABS because its glass point is higher than PLA and you never know what's in the colors. $\endgroup$ – John Sensebe Aug 26 '16 at 15:41
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There are Food Safe PLA filaments. These PLA filaments share similar qualities with ABS, in regards to temperature extremes et al. There are several on the market. (Eg. https://shop.germanreprap.com/en/pp-plastic-600g-3mm-black and http://www.formfutura.com/hdglass/ -- I've not used the HD glass personally but have been told it works well)

When shopping for food safe PLA filaments check for the Material Safety Data Sheet (MSDS) that comes with the filament. It will contain all the normal chemical warnings, some composition information, etc. but will also indicate if the material is Food and Drug Administration(FDA) approved.

PLA, food safe or not, can also have bacterialogical issues--the material is pourous enough to allow for bacterial growth. I would suggest using a polyurethane to seal the printed part to limit bacterial growth.

You will need to check what sort of extruder you have on your pinter. Many brass extruders also contain lead which can leach into the printed product. Switch to a stainless steel print head to avoid this issue.

General care--warm soapy water and handwashing.

I would not, as yet, recommend any printed product as a vessel for food--cup/bowl. This element is still somewhat new and I don't personally trust the materials for extended contact with food. (Knives/spoons et al. have much shorter contact with food vs. a plastic mug of hot coffee).

For your intended application, the piece in question would not come into direct contact with food to be consumed. As such, you should be fine with the correct filament.

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  • $\begingroup$ This seems a very generic answer about food safety of plastics, while the question is about using the plastic in an application where it won't be in contact with food at all, and the main concern is how it would hold up to microwaves. You could have skipped the entire part about why PLA might not be food safe (which is irrelevant) completely and then the answer boils down to "Yes, you can do this" - completely ignoring whether microwaves might affect PLA. $\endgroup$ – Tom van der Zanden Aug 26 '16 at 21:35
  • $\begingroup$ As the application of the material is in an environment in which food is prepared, some of the context of why PLA, and even food grade(FDA approved) PLA, may not be food safe is completely relevant. $\endgroup$ – WarOrdos Aug 28 '16 at 1:21
  • $\begingroup$ You are making an assumption that the piece won't be in contact with food at all. This is a microwave. Boil-overs et al. happen. Exposure to organic compounds/food etc. is almost inevietable. Therefore, some comments on prep(sealing the PLA piece) and clean-up(warm soapy water) also were justified. I did not comment with specifics as to Food safe PLA reactions with microwaves as that would, depend on which product the user elected to go with. Such information would normally be contained in the MSDS sheets of said product(s) and would be available pre and post purchase. $\endgroup$ – WarOrdos Aug 28 '16 at 1:38
  • $\begingroup$ You think people should look up the most essential piece of information asked for in this question in MSDS's (and you do not even mention that this could be an issue), even though you could very well make some general statement as to the microwave compatibility of PLA plastic in general? I find that very strange, since food safety is secondary to this question (and there are many good questions and answers about food safety already) and product specific as well (so for food safety your same line of reasoning says that it should be looked up in the product datasheets). $\endgroup$ – Tom van der Zanden Aug 28 '16 at 6:42
  • $\begingroup$ If something in your microwave boils over I don't think any reasonable person would try to eat the portion of the food that boiled over and came into contact with the plastic. In this application, it will never happen that something comes into contact with the plastic and subsequently goes into someone's mouth. I don't see how bacterial growth or trace amounts of lead could be a problem in that case. $\endgroup$ – Tom van der Zanden Aug 28 '16 at 6:46

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