strength issues with 3d printing parts are quite often not helped by people not thinking about the design being built with layers. a flat printed arm will always break easily at the hingepin, as the hingepin is running in the same direction as the laminated layers. build your 3d model to be printed vertically so that the layers are running at 90 degrees to the hingepin and strength will be a lot better, downside is you do end up having to clean up more support material from the part and remove the mouldings that helped keep the part in position, but it will be a lot stronger in use for just a little bit more work
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