The door latch on my small, inexpensive (inexpensive is really no excuse) GE model JES738WJ 01 microwave oven sticks when it gets hot from long usage or when the temperature in the kitchen reaches the high 80s. This is the common pushbutton door release mechanism. When it sticks, you have to pull on the door while pushing the release button in to get the door open.
A little eyeball investigation revealed that the door latch consists of a couple of spring-loaded hooks on the door which engage a couple of latch pawls on/in the body of the oven. The latch pawls are fixed. The release button opens the door by lifting the hooks so they clear the latch pawls. The problem is, it only lifts the bottom hook. The top hook is supposed to follow the bottom hook and allow the door to open.
Apparently, (I haven't yet bothered to tear the door apart) the two hooks are molded in one piece out of plastic. The hooks and their connecting part are relatively thin. When the bottom hook is lifted by the button mechanism, the entire hook assembly flexes slightly. The spring is apparently at the top of the upper hook.
At normal room temperature, the plastic is rigid enough so that the upper hook follows the lower and the door is unlatched. But when the temperature rises -- due to room ambient temperature above 80F or heavy oven usage -- the hook assembly loses its rigidity and bends. The lower hook is released by the positive lifting action of the release button, but the upper hook, due to the flexing of the now-warm plastic, does not follow sufficiently to allow the door to open.
The simple fix (for the user) would be to file 30 thou to 50 thou off the top hook, which will probably work. But that shouldn't be necessary. Maybe monkeys didn't make my microwave oven, but they sure did design it!
This entry was submitted by Brooks Lyman and edited by Rob Spiegel.
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That certainly helps with clarity, Dave. That's a major change in stiffness with just a relatively small change in temperature. Also seems odd that stiffness alone would affect the latch.
@Rob: It sounds like the problem isn't expansion of the plastic but, as Brooks says in the article, the change in stiffness with temperature.
I'm not sure what material this door latch is made from, but often plastic parts that are designed to flex in operation are made from polypropylene. The elastic modulus of polypropylene changes by a factor of five between room temperature and 100°C, i.e. it is five times less stiff at 100°C than at room temperature. I'd guess that, even from room temperature to 90°F, it might change by as much as 50%.
The mechanical properties of plastics are highly dependent on temperature and strain rate. Design engineers need to keep these effects in mind. When designing a steel part, it's usually safe to assume that the elastic modulus, yield strength, tensile strength, and ductility are the same from room temperature to at least 300°F. With plastics, you can't assume that the properties listed on the datasheet are the same properties the material will have in your application.
Several years ago, Joseph Ogando wrote a great article for Design News regarding the use and abuse of plastics datasheets. It points out many of the parameters that affect the properties of plastics. I've sent it to a lot of people over the years as a reference.
Sounds like the hook should have been designed to be free of the plastic even as the plastic expanded. Even so, should the plastic on this microwave -- which is expected to get hot -- expand so easily? At merely 80 degrees? Seems the plastic may be part of the culprit.
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