Polymer Engineering Center > Research > Process Development and Material Characterization of Microcellular Co-Injection Molding

This proposed project is aimed at developing a new process to produce complex plastic parts with class "A" surfaces, variable wall thicknesses, dimensional stability, high strength-to-weight ratios, and capability to recycle post-consumer plastics. The idea is to combine the aesthetic and processing advantages of co-injection molding with the property attributes and benefits of the emerging microcellular plastics (MCPs). Co-injection molding (also called sandwich molding) comprises sequential or concurrent injection of a "skin" material and a dissimilar but compatible "core" material into a cavity to produce a laminated structure. This innovative process offers the inherent flexibility of using the optimal properties of each while enabling modification of part property or to achieve particular engineering effects. Microcellular plastics (MCPs) are single-phase, polymer-gas solutions by dissolving or saturating a polymer in a suitable supercritical fluid and then triggering nucleation of polymer by adjusting process conditions such as pressure and temperature. In polymer processing, MCPs are plastic foams with cell diameter sizes from 0.1 to 10 microns, cell densities from 10⁹ to 10¹⁵ cells per cubic centimeter, and specific density reductions (relative to the unfoamed plastic) from 5% to 95%. They were initially conceived by Suh at MIT as a means of reducing material consumption in mass-produced plastic parts. The underlying rationale is to create enough voids smaller than the pre-existing flaw in polymers so that the amount of plastic used could be reduced without compromising the mechanical properties. While realizing a part weight reduction of 5-95% by replacing plastics with gas, the microcells also serve as crack arrestors by blunting crack tips, thereby, greatly enhancing part toughness. When properly prepared, microcellular polystyrene (PS) has five times the impact strength of its unfoamed counterpart. The fatigue life of microcellular polycarbonate (PC) with a relative foam density of 0.97 is four times that of its solid counterpart. Furthermore, since the gas fills the interstitial sites between polymer molecules, it effectively reduces the viscosity and the glass transition temperature of the polymer melt. Therefore, the material can be processed at much lower pressures and temperatures. Owing to their low viscosity and transition temperature, MCPs have been successfully molded with wall thickness of less than 0.5 mm.

Anticipated benefits:

• A possibly viable process for producing complex plastic parts with class "A" surfaces, variable wall thicknesses, and dimensional stability.
  

• Additional design freedom, reduced material cost and cycle time, low injection pressure, high strength-to-weight ratios, and a means to recycle of post-consumer plastics.