I use plastic as metal will wear out plastic casings also if you have a thread sensor it also needs to be able to sense when thread is getting low so it needs a clear bobbing, its why paper prewounds don't work with sensors. I haven't had any problems with my bobbins breaking unless i accidentally step on one. But I also bought a package of like 50 for 5 bucks so I have plenty and am not constantly using the same one