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Rob Spiegel
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Is it the plastic?
Rob Spiegel   8/17/2012 10:43:43 AM
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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.

Dave Palmer
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Platinum
Re: Is it the plastic?
Dave Palmer   8/17/2012 11:31:31 AM
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@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.

Rob Spiegel
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Re: Is it the plastic?
Rob Spiegel   8/17/2012 11:37:56 AM
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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.

William K.
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Platinum
Door latch sticks when it gets warm.
William K.   8/17/2012 2:44:54 PM
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This problem could be caused by other than poor engineering, such as the notorious "purchasing department redesign" that saves money by replacing materials with cheaper ones that may meet one or more of the original requirements, possibly. 

Some plastics stay quite stiff over a higher temperature range, and many reinforced plastics prvide adequate strength even in thin cross sections. But substituting another type that may still hold it's dimensions adequately over the range may not provide enough stiffness. 

It is also possible that the molder modified the plastic a bit, or used to much "regrind", to reduce their cost. I am aware of that happening at least once.

So it may indeed be "made by monkeys", but not designed by monkeys.

Charles Murray
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Blogger
Re: Is it the plastic?
Charles Murray   8/17/2012 5:57:10 PM
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The modulus of elasticity of polypropylene changes by a factor of 50% between room temperature and 90 degrees F? I realize that we don't know if polypropylene was used here, but it stands to reason that this could be a likely culprit. It also seems like the kind of information that might get ignored during the design process.

Dave Palmer
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Platinum
Re: Is it the plastic?
Dave Palmer   8/17/2012 6:59:05 PM
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@Chuck: Okay, 50% was an overstatement.  According to this graph, the flexural modulus of polypropylene falls by about 20% between room temperature and 90°F. Still, it's a significant effect.

btlbcc
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Gold
Re: Is it the plastic?
btlbcc   8/20/2012 11:06:50 AM
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Dave, the particular part isn't meant to flex.  Now it's possible (I am not going to spend the time to tear down a $35 microwave oven) that the spring which forces the latch hooks down (and which I blithely assume is a standard steel compression spring mounted at the top end of the latch piece) may in fact be plastic, and molded integral with the latch.  If that were the case, the use of a plastic such as polypropylene would make sense.  I have nothing against smart design and integrating multiple parts in one - that's the foundry industry's mantra - but let's get the design right.  A hairtrigger latch release where apparently a few thousanths of an inch of flex makes the difference beteween proper operation and failure may be okay in an Olympic-class target pistol sear (made of steel), but is bad design in something like a microwave oven when made of plastic.

btlbcc
User Rank
Gold
Re: Is it the plastic?
btlbcc   8/20/2012 11:19:35 AM
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Rob Spiegel, the latch design makes it sensitive to material flex.  The latch is two hooks tied together by a vertical member (all of which is presumably milded in one piece) and held down by a spring.  The lifting force on the latch is applied to the tip of the lower hook, which lifts the tip of the hook up off the latch pawl; the upper hook follows along when the lower hook is lifted.  Unfortunately, as I said, when the temperature rises, the lower hook bends under theb lifting force, so the upper hook doesn't follow along quite as much as it needs to, this making the door-opening a two-handed "push the button and lift the door" operation.  The whole thing is (poorly) designed to have minimum clearance, so a few thou flex causes the problem.

 

I'm not an expert on how microwave oven doors work, but they all seem to operate in about the same manner - push the button which unlatches the door which then springs open slightly - so there shouldn't be any problem with making a proper design; most of them work fine....

Dave Palmer
User Rank
Platinum
Re: Is it the plastic?
Dave Palmer   8/20/2012 11:44:28 AM
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@btlbcc: Maybe I misinterpreted your comment: "When the bottom hook is lifted by the button mechanism, the entire hook assembly flexes slightly." The hook assembly is plastic, right? Polypropylene is just a guess.

As I understand your description of the problem, the hook assembly is designed to flex slightly, but flexes too much at elevated temperatures, so that the upper hook will not disengage.  That's why I suspect the designer didn't take into account the variation of the modulus with temperature.

As you suggest, a looser fit for the top hook would address this.  I wonder if this is partly an example of designers specifying unnecessarily tight tolerances -- something that has come up in "Made by Monkeys" before.

A mineral-filled or glass-filled plastic would be stiffer and somewhat less sensitive to temperature variations, but would also be subject to fatigue.   That could potentially be an even worse problem.

William K. suggested excessive regrind or substitution of a less expensive plastic as  potential causes.    While this is possible, I tend to agree with you that it's a design issue.

Ann R. Thryft
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Blogger
Inexpensive often means poor quality
Ann R. Thryft   8/20/2012 12:52:26 PM
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Given my recent experiences buying microwave ovens, cheap *is* an excuse. A poor one, I agree: good ones that work right for years shouldn't cost more than $50. But they do. A lot more. After wasting about $100 on two crummy ones from Kmart, I spent about $150 to get a decent one on Amazon after reading a lot of reviews. It works fine, no poor design or materials problems.

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