The 4" extruder dies are made of 1/2" HDPE (High Density polyethylene). Which is extremely wear-resistant and completely non-contaminating. The top edge of the die cavity is beveled to compact the clay as it passes through, giving a crisp, more solid extrusion. In the hollow dies, there is a bevel on both the interior and exterior portions under each side of the bracket (which holds the center of the hollow dies) that helps the extrusion to "heal" after is has been cut apart by the bracket. This bracket fits into a slot machined into the top of the hollow die and is held in place by the barrel of the extruder. If a hollow die is mounted in place, the center part of the die is in exactly the right place and the wall thickness will automatically be even all the way around. In general, the walls of extrusions 3-1/2 inches OD or less are 1/4 inch thick, while larger pieces have 5/16 inch walls. We have a selection of stock 4” dies available & custom dies are quickly made and inexpensive.