Coinjection gives molders a one-step process to build parts which have one material for the skin and another for the core. The core is often foamed for light weight and energy absorption, or it contains recycled material for a substantial cost saving. Still, only 1% of molders use the process, probably because it has been expensive and complex. A new process devised by Twinshot Technologies, Rifton, N.Y., uses coaxial screws to melt both materials in predetermined quantities. This eliminates the need for a secondary hydraulic system and computer controller.

After both materials melt, they are collected in a common space and then injected into the mold in one stroke. The first material coats the mold, creating the skin, while the second layer solidifies in the center. The system is simple and low-cost compared to other coinjection techniques, according to Twinshot, and it has been proven to reduce resin costs - the single greatest cost for custom molders - by 25% or more. The hardware for the Twinshot process can be retrofit onto single-material machines, or designed into new ones.