an apparatus for injection molding two compatible polymeric materials , in which two or more plasticizing zones on a screw are used to simultaneously or sequentially plasticize the two materials into a common accumulation space through separate pathways . the charge comprising multiple layers of material is then injected into a closed mold by means of forward axial motion of the screw with respect to its enclosing barrel . once inside the mold , the first material forms a skin layer , totally or partially surrounding the other material . in this way a part having a plurality of material properties may be produced in a single operation .