Microcellular Injection Molding of Gas-Laden Pellets Using Nitrogen (N₂) and Carbon Dioxide (CO₂) as Co-Blowing Agents

A novel combination approach to producing quality foamed injection molded parts has been investigated. By combining extruded, gas-laden pellets with microcellular injection molding, the processing benefits and material characteristics of using both N₂ and CO₂ blowing agents can be realized, thus yielding features superior to that of using either N₂ or CO₂ alone. Using an optimal content ratio for the blowing agents, as well as the proper sequence of introducing the gases, foamed parts with a much better morphology can be produced. In particular, extruding N₂ gas-laden pellets, followed by microcellular injection molding with higher amounts of CO₂, produces a cellular structure that is very fine and dense. In this paper, the theoretical background is discussed and experimental results show that this combined approach leads to significant improvements in foam cell morphology for low density polyethylene (LDPE), polypropylene (PP), and high impact polystyrene (HIPS) using two different mold geometries.