Some flow phenomena in the injection molding of polystyrene

Abstract

The steps in an injection molding cycle comprise dead time, filling, packing, discharge sealing, sealed cooling, and mold open time. Those steps involving polymer flow are discussed in this paper. The filling time is directly proportional to the polymer melt viscosity, and inversely proportional to a power of the ram pressure. Raising the mold wall temperature lowers the filling time. Reducing friction between the solid polymer and metal lowers the filling time, but only if the cylinder ahead of the heating chamber is cool. Studies with a glass-window mold show that hot polymer flows only in a central region, there being stationary layers next to the mold walls. When flowing polymer reaches the advancing front it contacts the mold wall, cools, and ceases flowing. In a typical case the thickness of the central region was 40% of the total thickness. An approximate equation is presented for calculating maximum pressure in the mold during packing. Agreement with experimental values is good. Approximate treatment of sealing by “freezing” in the gate leads to a linear relationship between temperature and pressure in the mold at the sealing point.