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Cause of Plastic Container Deformation in Microwave Oven

  1. Oct 5, 2006 #1

    Earlier this afternoon I managed to ruin a perfectly good plastic container in a microwave oven by unwittingly deforming it. I curious as to why it happened.
    All I did was warm it (and the chilli inside it) for about a minute with the lid firmly secure. I've been told this was a stupid move and to never attempt warming things with the lid in place ever again. But I'm not convinced. Upon removing it, I noticed that it had imploded. Depressurization doesn't make sense in this case, unless my knowledge of EM interactions is laughably bad. A lot of Googling turned up molecular deformation of the plastic itself as a possibility (but none of the aforementioned lid warning), but why would the plastic deform in the shape of a cavity and not with the lid removed?
  2. jcsd
  3. Oct 5, 2006 #2


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    A lot of containers are thermoplastic, they are formed by heating the plastic and injecting into a mold.

    Water gets heated by microwaves and a high pressure steam will form in a closed container.
    For example, don't put uncracked eggs in the microwave.
    They will explode.

    The steam is hot enough to make the plastic soften.
    Some escapes the container, if it dosn't blow the lid off and make a mess in the first place. When the microwave shuts down the steam cools quickly creating a vacuum.
    Then the container cools below its forming temp giving the imploded shape.

    With lid off the hot gas quickly leaves the container so that it doesn't get a chance to heat the plastic.
  4. Oct 5, 2006 #3
    Smart guy ^ ^ ^
  5. Oct 5, 2006 #4
    Thanks a lot, that makes perfect sense.
  6. Oct 5, 2006 #5
    That was a very practical response, you must be an engineer. :wink:
  7. Oct 5, 2006 #6
    Or a Chef! ;)
  8. Oct 5, 2006 #7


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    I manage to do a little of both.
    Thanks for the kind words :smile:
  9. Oct 6, 2006 #8


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    Some plastic containers (like styrofoam that to go food is often served in) will just start to melt due to the heat.

    In the cases where a closed lid makes a difference, the other factor is that a lot of heat will be trapped inside the container, causing it to soften quite a bit.

    If the container is micro-wave compatable, then a nearly closed lid might seal up and become concave as the air/steam inside cools, but not the rest of the container which is stiffer than the lid. With a fully closed lid, the lid will be popped loose (hopefully), or the container will expand.
  10. Oct 6, 2006 #9


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    There are microwaveable plastic containers, if heating with a lid on is important (as it si in the case of chilli). TupperwareTM makes a bowl with a lid that has a vent in it and will release pressure without splattering all over the place. That one's called an "Oval Cooker", but there are many kinds of microwavable plastic bowls.

    It's true you should never nuke any plastic bowl without the words "microwave safe" associated with it. This time you just got deformed plasticware; next time it might explode in there. And you probably ended up eating some plastic with that chilli.
  11. Oct 6, 2006 #10


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    Personally, I am worried about chemically altering the plastic and having chemicals leach into the food.
  12. Oct 6, 2006 #11
    Why? What is leaching? It is thermoplastic deformation.

  13. Oct 6, 2006 #12


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    Could be a surface treatment or other compound mixed in with the plastic that vents if heated.
  14. Oct 6, 2006 #13
    Eh, its so minute its not going to kill you. :tongue2:
    Last edited: Oct 6, 2006
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