I have been to the APC factory in Sacramento and seen how they are made, and they are NOT glass filled. They are just an epoxy resin pellet that looks just like grey plastic rabbit food! There are four machines that use injection molds. The pellets are melted down and injected into 1 of many sized molds. 4 machines produce whatever size is needed that day. And the reason I like APC props is because I haven't found a prop that comes more balanced than APC. Very little balancing to do here, even on the 21 inch props!! That's amazing.