Plastic Injection Molds
Filling stage
Filling is the first step in the process of the injection molding cycle, time starts from mold closing injection, into the mold cavity filling to about 95%.In theory, the shorter the filling time, forming efficiency is higher;But in actual production, the molding time (or injection speed) should be restricted by many conditions.
High speed filling.High shear rate during high speed filling, plastic exist due to the effect of shear thinned viscosity drop situation, reduce the overall flow resistance;Local viscous heating effect will also make the solidified layer thickness thinning.So in flow control stage, filling behavior often depends on the size to fill.In flow control stage, because of the high speed filling, the melt shear thinned effect often is large, and thin-walled cooling effect is not obvious, so the rate of utility gained the upper hand.
Low speed filling.Heat transfer control fill at low speed, low shear rate, local higher viscosity, flow resistance is bigger.Because of the hot plastic added rate slower, relatively slow flow, make the heat conduction effect more apparent, heat LengMo wall away quickly.Along with smaller amounts of viscous heating phenomenon, solidified layer thickness is thicker, and further increase the flow resistance of the wall of thin place.
Because of fountain flow, in front of the current wave of plastic polymer chain row almost parallel to the flow of wave front.So both strands of plastic melt in the intersection, the contact surface of polymer chain parallel to each other;With two different melt properties (residence time in the cavity is different, have different temperature, pressure), plastic region on the micro structure strength is poorer.Put the parts in the light of appropriate Angle observation with the naked eye, can be found to have obvious joint line, this is the formation mechanism of weld mark.Weld mark parts surface, not only affect and its microcosmic structure is loose, easy to cause stress concentration, so that that part of the strength and fracture.
In general, in high intensity of fusion seam welding of better.Because of the high temperature circumstances, polymer chain mobility is relatively good, can penetrate each other, in addition to high temperature area of two strands of the melt temperature is relatively close, almost the same thermal properties of the melt, increase the strength of the weld region;Whereas in low temperature area, poor weld strength.