1980s refrigerator shelf failures

Material-substitution case. Manufacturers transitioned to plastic interiors but kept screw-mounted shelves, designing plastic as if it were steel. Plastic's brittleness at stress concentrations caused three-year failure cycles until KitchenAid and others adopted molded slots.

But what bugged me, because I lived through this transition in the 80s: I would go and buy a nice refrigerator, and within three years it was a piece of junk, because they screwed the shelves into the plastic. That's not how you design plastic, folks. Plastic can't take screw holes and sharp stress concentration; it's a somewhat brittle material. I've shown you about fracture mechanics — you put a little notch in something and it breaks easily. Plastics don't have the toughness of steel. If you're going to design a refrigerator — by the 1990s KitchenAid and other people learned — you slide the shelves in, you don't screw them in. Plastic has this wonderful property: it's easy to mold into complex shapes, so you mold a slot in there. Now you go look at virtually any refrigerator, it's made out of plastic but there're no screws on the inside. The other problem with screws on the inside: after time they would start rusting, and you'd have a little rusty drip on the inside. Your refrigerator didn't look very good. Plastic is great — it's got great room-temperature corrosion resistance.