Special Process Molds Stronger Parts In Very Thick, Near-Net Shapes

Very thick, near-net shapes that boast much higher strength than similar shapes produced by other processes are now available to a wider audience of processors that machine shapes to final dimensions. Piper Plastics, Chandler, Ariz., developed the special injection molding process several years ago, but until now has used it only for internal production. Piper formulates high-performance materials and supplies machined parts and semifinished shapes (and occasionally compounded pellets). It was recently acquired by Quadrant Engineering Plastic Products, a global subsidiary of Swiss-based Quadrant AG, which has similar product lines (U.S. office in Reading, Pa.). This acquisition was a factor in making near-net shapes produced by Piper’s proprietary process available to other plastics machining companies, including Quadrant customers, explains Dave Wilkinson, materials engineering manager at Piper Plastics.

“This process yields parts that are typically 15-20% stronger than extruded shapes and 50% stronger than compression molded shapes, plus more consistent mechanical properties,” Wilkinson claims. He attributes this to the anisotropic orientation of polymer molecules and fiber reinforcements in extrusion and to the lack of any distributive mixing in compression molding. Wilkinson cites the example of 30% carbon-fiber/PEEK compound: “The best extruded samples can achieve a flex modulus of 1.2 million psi, and compression molded samples are under 1 million psi, but we can get 2.5 million psi with our process.”

Piper built its own injection molding machine to produce parts typically 1- to 2-in. thick and weighing 2 to 6 lb, under high pressure (an order of magnitude higher than compression molding) but without shear, so as to preserve molecular weight and fiber length. Wilkinson says the process yields a very homogeneous melt and parts with virtually zero porosity, even at weights up to 20 lb, using molding cycles 5- to 10-min long. Piper commonly molds materials such as PEEK, Torlon PAI, or thermoplastic polyimide (TPI). Filler levels can reach up to 65% and fiber lengths to 1 in.

By injection molding a near-net shape, only 10-15% of the material is lost to final machining. In comparison, machining from extruded or compression molded blanks typically removes 60-70% of the initial material. Material savings can be up to 80%, Wilkinson says. He also notes that molds are inexpensive because there are no tight dimensional tolerances on near-net shapes.