When plastic is allowed to sit for excess time at melt temperature the risk of thermal degradation is elevated. The length of time that the plastic spends in the barrel of the molding machine is referred to as residence time. The material being molded will determine the maximum allowable residence time, and some materials such as polyethylene and polypropylene are much less sensitive to degradation from long residence time especially when compared to something like a PVC or acetal. Figure 17.3 shows purged material that has been degraded because of excess residence time; if this material had been injected into the mold the part would have had brown streaks and splay.