Another interesting additive in many plastics is glass fibres or beads, they are often used to strengthen the plastic and give it slightly different mechanical properties. The downside of the glass fillers is that they erode the tools much more quickly (glass is very abrasive), kind of the opposite of Tallow which helps to keep the plastic flowing over the surfaces better.

On glass filled parts you will often see a small ring around the "gate" (gates are where the plastic is injected). The gate is under enormous pressure and has a very high flow rate through it, so it tends to erode significantly more quickly. To counter that the gate is often an inserted part that can be changed out easily when servicing the tool, or made from a higher toughened grade of steel. Hence a small ring, a witness mark of where the join is.