Am happy to say that as perhaps the leading guinea pig producer of Delrin/Acetal topped blocks that I have yet to hear of a single crack occurring, whether through use, misuse or abuse.
Delrin readily deforms to stress pressure, unlike any clear plastic that I'm aware of. I agree with Jaydee116 that it is possible to attempt to largely mitigate the cracking in clear plastics with careful machining, but it is never a guaranteed thing. It's not something that ever can be treated like a metal, where if the metal fails it's because of a major localised flaw, and in that case is extremely rare.
A large number of plastics can be machined quite successfully without much apparant danger of introducing micro-fractures, the problem is that there are NO clear plastics that fall into that category, which I would imagine is due to the highly crystalline nature of these plastics that enables them to be clear in the first place.
Thermal stresses are the primary source of failure though. Yes - the cracks are introduced at machining time, but it is the thermal stresses of warming up and cooling down which will propagate cracks quite quickly.
As for molding plastics, we have to think of the volumes being done, and in some special cases, the need for very, very precise, accurate and repeatable tolerances which is not that easy to achieve with the natural plastic shrinkage that occurs during the mold cooling process.
Still, even with injection molding, depending on how one swings the figures it would basically take a production run of around 2000-3000 blocks to "break even" with production based CNC milling, and really would take well over 5000 blocks worth of production to begin to make the molding process markedly cost-beneficial.
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