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btlbcc
User Rank
Gold
Re: Is it the plastic?
btlbcc   8/21/2012 11:11:36 AM
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Dave,  If I were designing that latch, I would attempt to have no flexing at all.  That is to say, the hook shouldn't flex.  That it will flex, is obvious, and needs to be designed around - which is the problem, really, since it apparently wasn't, or at least, not sufficiently.

 

Agreed, that a reinforced plastic would probably eliminate the problm.  Of course, the oven cost $35 at retail, so the manufacturer had to cost reduce it as much as possible....and overdid it.

 

Brooks Lyman

DetailDevil
User Rank
Iron
Re: Inexpensive often means poor quality
DetailDevil   8/21/2012 8:54:17 AM
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While low-cost may (or may not) ensure lower quality, high price certainly does not necessarily ensure higher quality. Witness the Keurig coffee makers... a weak point in their design is a plastic hinge in the coffee canister handler portion. The higher priced model employs the same plastic hinge. If product life expectancy is limited by this part, the less expensive model is a smarter consumer choice.

Charles Murray
User Rank
Blogger
Re: Is it the plastic?
Charles Murray   8/20/2012 8:54:19 PM
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I agree, Dave. A 20% change in the elastic modulus over just 20 degrees F is very significant, especially if the designer is unaware of it.

Ann R. Thryft
User Rank
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.

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.

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....

btlbcc
User Rank
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.

Dave Palmer
User Rank
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.

Charles Murray
User Rank
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.

William K.
User Rank
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.

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