Designed for soft plastics. Slow spiral bits are well known for their use in dealing with problematic materials; usually materials with brittle surfaces that require a more gentle cutting action. The cutting helix of slow spiral bits are less intense than standard spiral bits thus producing less shear.
Understanding the characteristics of the plastic type to be routed is the key to success; there are so many varieties and manipulations of plastics. General failures when routing plastic are usually associated with chipping and shattering; "re-weld" occurs when the chips produced get so hot that they literally reattach themselves to the finished edge and thus spoil the finish. Router bits designed for soft plastics should be used on heat sensitive plastics which also tend to be more flexible and bendable at room temperature; router bits designed for hard plastics should be used on the more hard, rigid and brittle plastics.
Good routing technique for plastics is keeping the RPM's up (at approximately 18000 as a start), keep the feed rates fast and minimize dwelling in the same place as this will cause unnecessary heat build up and re-weld; utilize the plunging ability of the router to eliminate dwelling time - ramping in and ramping out. Keeping the bit as cool as possible is the top priority.