I had to replace the insert in one of their 'break' shafts and the way the plastic looks, and the way it machined as I remove the insert remains, makes me think they are using white acrylic (plexiglas). Cast? Extruded? Who knows, but cast acrylic should be stronger, and still not what I'd install as insert in a break shaft. I replaced the Rhino insert with PPSU. PPSU is a very glue friendly engineering plastic originally dreamed up for the healthcare industry, and you can beat it on an anvil with a hammer and it won't break - it will deform under the ball pein hammer test, but not break. Can't say the same for acrylic. PPSU is my go-to insert material now for CF play AND break/jump shafts, unless someone requests something different.