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.
@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.
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 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....
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.
@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.
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: 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.
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.
@btlbcc: Actually, what I said was that a reinforced plastic might solve the problem of excessive flexing, but might also make the latch more likely to fail in fatigue. I suspect that would be a much worse failure mode, since it would render the latch completely inoperable.
For what it's worth, mineral-reinforced plastics are often cheaper than their unfilled counterparts.
A couple of years ago, I was interested in a new plastic that I had read about in a trade magazine. I called the supplier, and he came to our plant and gave a presentation. After reviewing the information he provided us, I told him that we liked the material, but the heat deflection temperature and stiffness were too low for our application, the mold shrinkage was too high for our molds, and the price was too high for our budget.
He came back a few months later with a mineral-filled version of the same plastic. The mineral filler increased the heat deflection temperature and stiffness, and reduced the mold shrinkage. It also brought the price down by more than a factor of two. The impact strength is lower, but still quite high. Needless to say, we were very impressed. For what it's worth, this is the grade he came up with.
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.
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.
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.
I can think of many examples of expensive consumer products that were built of inadequate materials or poorly designed. So yes, expensive doesn't necessarily mean well built or well designed. Unfortunately, though, I can think of far more that are inexpensive and that fail, either immediately or far too quickly, because of bad design or inadequate materials. This didn't used to be the case, but in the last decade or so it seems to have become the rule in consumer products.
Ann R. Thyft, Actully, my experience (admittedly limited; see next paragraph) with cheap microwave ovens has been quite good; I've never had one fail to function in it's basic task. I've given a few away because they were too small or didn't have a turntable to help equalize the heating, but never had to actually dispose of a non-functioning oven.
The door latch problem, while annoying (and I'll eventually find time to file down that upper latch hook), has not affected the proper functioning of the oven. That said, I am a very light user of the machine, using it for warming coffee, thawing food and similar tasks. I prefer not to actually cook in the microwave, a personal preference. So someone who does serious cooking in a microwave would probably soon discover all sorts of weak points - or outright failures - in the cheaper models.
If I might make a general personal comment about buying stuff online: It's certainly one way to get a lower price on things, and I often take advantage of it. On the other hand, one usually has to pay shipping, which on larger items can be a significant amount, and then the package gets beat up by UPS/Fedex/etc. before it gets to you (with heavier items taking a worse hit). So often, checking out something online (those reviews are useful, and many of the online retailers post all the reviews, including the ones that mercilessly pan the product!) and then buying it at a local store (like Walmart, Target, etc.) is actually a less expensive overall and less-risky, shipping damage-wise than buying it online.
Brooks, I don't use a microwave to cook, either. I don't consider that cooking. But I do use it a lot for reheating, melting or softening butter, peanut butter and honey, etc. I consider it one of my sous-chefs. So it needs to be dependable. As I wrote, until recently I bought microwave ovens at Kmart, and had only bought 2 in the last 15 years because they lasted and I don't need a lot of fancy options. Then I bought one more there for about $50 and was disgusted when it stopped working, in stages, over a couple of weeks less than 30 days after purchase. That's when I went to Amazon and had my excellent experience. Since I live way out in the country, there are no local stores that sell them, except for that Kmart.
I'm shopping for a new microwave to replace my high-end GE. The touchscreen (similar to an iPhone, not membrane keys) failed under warranty and now the replacement screen is failing due to a glass-flex circuit connection. Repairs ever 3-6 months have stopped being effective.
After lots of shopping and reading consumer reviews and info, it seems like nobody makes a reliable microwave. I feel lucky my GE lasted 6 years of moderate use. Brand or price point seem to be irrelevant to quality...some very high-end microwaves have numerous, bad, user reviews.
I've found 2 specific models that have fewer reliability complaints (both mid-price) so I'm going that route and will regard a microwave as a 2-4 year purchase.
The idea that price point and brand have no relevance is a complaint we're hearing more often on these message boards. The fact that some brands and models have fewer complaints may be a result that fewer of them were sold. I guess I'm luck, my GE microwave has lasted many, many years without any trouble.
@Rob- Hang onto your "golden oldie" as long as possible! I've been a GE appliance fanboy for years. My first GE microwave was going strong after 15 years when I sold that house. The one in the new house was fine when I remodeled after 16 years. The current one went 4 years but I was able to repair it. Current reviews leave an expectation of 2-4 years.
Another observation- My GE range and dishwasher have been 100% reliable. Both are made in Louisville. The fridge (Mexico) and microwave (Malaysia) have been nothing but trouble. Also, many microwaves, regardless of brand, are made in Malaysia. There must be a gigantic magnetron factory over there!
Kennish, it could be that my microwave was built before the outsourcing trend started to include appliances. There seems to be a pattern (which you identify in your posting) that many of the problems can be traced back to outsourced manufacturing.
I grew up in the 70's with an Amana Radar Range that made the lights in the kitchen dim when it cycled and required its own 20 amp breaker. I've had good luck with the Sharp line of carousel microwaves over the years but I've revived my range hood Jenn-air uWave that came with the house by resoldering the interconnects on the control board which gets flexed by aggressive finger mashing. On the hooks, there must be two for UL redundancy and to verify the door is latched for microwave emission safety. I would not start filing down their clearance as you may get unintended consequences. Like a lightshow on your glasses frame if you lean in too close. You can't stiffen the vertical coupling bar with metal but I would look to reduce its flex before I would file down materials. I buy online all the time and rarely have shipping nightmare stories to relate. Don't ever buy the cheapest and you usually don't get that much better with the Audi or Lexus grade. Just something to polish and fret over.
Loadster, I had a colleague who had one of the original Amana Radar Ranges; you are right about the current draw. But I never heard any complaints from him (or his wife!) about the oven. Talking about kitchen appliance current draw, I have another colleague who has an induction range in his kitchen. The temperature control seems to be some sort of zero-cross switching with relatively long on-off times, so the lights in his house are constantly dimming and brightening when the range is being used....
As for my filing down the upper hook, don't worry, I only need to take off 20-25 thou to allow it to clear the latch pawl. That leaves the better part of a quarter inch to maintain latching integrity (not only are there two hooks, but they have good engagement).
I know that when I manage to release the bottom hook, but the top hook is still holding the door closed, that there is no change in the door position, so I doubt that I would have any trouble with RF leakage. If filing the hook fails, I have the choice of scrapping the oven or pulling the door apart. A hook assembly cut out of G10 board (without copper) should be rigid enough, but probably not worth the time to make it....
As for reinforcing the hook, actually the vertical part of the hook assembly is inside the door - completely surrouned by metal, and the hooks and their latching pawls, etc. also seem to be outside the RF zone. So you could probably use metal to reinforce (or replace) the hook assembly. I'm sure that the hook assembly is plastic for economical reasons, not because of RF. I believe that the old Amana used metal latches. If the plastic hooks were subject to RF, there's a serious possibility that they would soften, melt or even ignite during long cooking cycles.
Inexpensive does sometimes mean poor quality, and it is sound reasoning that a product costs less should be inferior and have a shorter life span than a more expensive model, but sometimes this is not the case. As a kid, I wanted to new pair of fishing pliers. My choices were either $3 for a needlenose set in the hardware aisle or $7 for needle nose pliers in the fishing aisle that were marketed to be made exclusively for fishing use. They were hyped to be especially water resistant and designed to easily cut nylon line. I paid the $7 and got the better pliers. The first fishing trip went great, but before going out on the second trip, the pliers had already rusted so bad that they could not close properly, and the cutting section had serious dings from cutting line. I had to go back to the store and buy the hardware pliers that I still have 30 years later. Sometimes the higher cost only pays for the name.
What bugs me is that the cheapest appliances used to work just fine and last a long time; they just didn't have the bells and whistles of the more expensive models. That was before both offshoring/outsourcing and before the throwaway consumer product concept really took hold. Now you have to buy high end on many appliances just to get them to work right and for a decent period of time. Offshored/.outsourced doesn't have to mean poor quality: but it often means poor oversight, resulting in the same thing.
Ann, I agree with your analogy inexpensive etc. I have read so many laments in this Made By Monkeys about applliances that it scares me about buying new. As a result I am ever vigilantly on the look out for appliances at estate sales. We have 6 micowaves (home, office, lake house and all three kids' homes) all purchased at estate sales. None cost more than $25 and all perform well.
As for the faulty latch, I would hesitate to rework the closing mechanism. One of our customers makes commercial grade microwaves and I was shocked at the amount of engineering and testing that goes into the design to assure there is no leakage before it is allowed onto the market.
Tool_maker, thanks for the idea on where to find older, used appliances. Thankfully, I don't need them yet, but had started wondering where I could find them when I do. And that's a really good point about the dangers of reworking the latch: you could fry a lot more than your food if doing so caused leakage.
Offshoring or outsourcing has less to do with it than maximizing profits does. A cheaper plastic, a thinner pin, even if it only saves a 10th of a cent on each item. It doesn't matter who or where the product is assembled. I've came across a lot of garbage 'Made in USA', so that doesn't mean much.
I disagree. The desire to maximize profits is a constant, not a variable. But most consumer products sold in the US are now made in Asia. Offshoring has a lot to do with the quality problem because of the difficulty of managing quality control and other details at a distance. This has been a well-known, ongoing problem in both products and services ever since this trend began in the 80s.
I drive an Asian made car without issues for 12 years now while my neighbor is on his third US made Ford. We drive about the same amount. Maybe he just got a lemon twice in a row, but quality is a matter of business decisions, not production location. Offshoring is mainly done to make use of lower labor costs, not for the sake of producing junk.
I agree that quality is affected by business decisions, but citing the desire to maximize profits means your set includes all companies, so that doesn't explain why some of those companies make bad decisions and others don't. The point about offshoring is that once that started, it became a lot tougher to manage all the details, including quality assurance, for multiple reasons, many of which are obvious. That's still the case, at least for many areas of Asia, to the point that it spurred the reverse trend, onshoring. And I drive Asian cars, too: specifically, Japanese cars. The Japanese are known for very high quality standards, in fact, better than in the US. They listened to Deming going on about TQM and Six Sigma long before we did.
I still use my old Amana. It was a LOT more expensive than $50, back in 1981 - but it has never failed, and I've only replaced the light bulb once to boot. I've never noticed the current draw, but maybe it's always been on a lightly used circuit. I like to think that the defense company that once owned it put the high-rel design and mfg effort into it that made it so expensive, and maybe that's why it turned out to be an incredible money loser for them. Four children have abused this item (and the youngest will turn 20 this year). I'm probably risking the curse by saying this, but it will probably outlast me.....unless the government outlaws it for some reason.
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