Making a thick-walled part with an internal foam structure isn’t new. For decades, the plastics industry has been using the structural foam and microcellular foam processes to produce skin-core-skin structures with foamed bubbles.
The result is a light, but stiff, part that can be produced with lower tonnage and few, if any, sink marks. The downside is a swirled surface, due to out-gassing trapped at the part/mold interface, which ultimately requires secondary painting for good cosmetics. Several approaches have been employed over the years to improve surface quality, including gas counter pressure, hot/cold mold cycling, expanding molds, Teflon film or other coatings, co-injection molding, vapor polishing, and so on.
A newer approach is expandable thermoplastic microspheres (ETM), commercially known as Expancel, which has been used for some time in the extrusion process but is only now finding its place in injection molding. ETMs are small spherical thermoplastic resin particles comprised of a virtually gas-proof polymer shell that encapsulates hydrocarbons as a blowing agent. The average diameter of the hollow spheres ranges from 6-45 µm. Fully expanded, the volume of the microspheres can increase more than 40 times.
The typical expansion temperatures range from 80º to 190ºC (176º to 374ºF). When heating the microspheres, the pressure of the gas inside the shell increases as the thermoplastic shell softens, resulting in a dramatic volumetric increase. Cooling stiffens the outer shell to its original state, leaving the light, expanded microspheres ready for use. Because little or no gas escapes from the microspheres, the surface quality of the molded part is dramatically improved.
The use of ETMs in injection molding enables part weight reduction while maintaining dimensional stability. Compared with structural foam or microcellular injection molding technology, the expandable microspheres improve surface quality substantially, reducing or eliminating the need for secondary operations. This is accomplished by controlling the size, density and skin layer thickness of the microstructure with the amount of ETM used.
During recent samplings, we have experienced base density reductions up to 50% in ETM-molded parts. This density reduction matches that found in microcellular foam molded parts, and certainly exceeds the 5-25% average density reduction typically achieved with structural foam molding. Part size and wall thickness, as well as the ratio of base resin to ETM, are determining factors in the process and must be optimized on a part-by-part basis.
The above post was written by Michael Hansen, PhD, Senior Technical Development Engineer, MackMedical/Mack Molding