Filling stage
Filling is the first step in the process of injection molding; the filling time begins from the close of the plastic mold until the 95% filling of mold cavity. In theory, the shorter the filling time is, the higher the molding efficiency will be. But in practice, the molding time or injection speed are restricted by many factors.

High speed filling. High speed filling will lead to high shear rate, plastic viscosity will decrease due to the dilute effect of shear, which would reduce the flow resistance of the plastic; local viscous heating effect also will make the solidified layer thickness thinner. Therefore, in the flow control stage, the filling is often determined by the size of the volume. Due to the high speed filling, the shear dilute effect of the melt is very obvious, but the cooling effect for the thin wall item is not obvious.

Low speed filling. When the heat conduction controls the low speed filling, the shear rate is low, but the local viscosity and the flow resistance are high. Due to the slow rate of heat compensation, the flow is slow and the heat conduction effect is more obvious. The heat quickly is taken away by the cold wall. With a small amount of viscous heating, the thickness of the solidified layer gets more, and the flow resistance is further increased.

Due to the fountain flow, the plastic polymer chains in the front of the flow wave are parallel to the wave front. Therefore when two plastic melt meet, contact surface polymer chains are parallel to each other; and the heterogeneous nature of the two melt in cavity (different stay time, pressure and temperature) results in the poor structure strength on the joint location.

In the light of the appropriate angle of the observation with the naked eye, there is a joint line can be found clearly, which is the generating mechanism of the weld line. Weld lines will not only affect the appearance of plastic parts, but also because of the loose structure of the micro structure, it is easy to have the stress concentration, which makes the strength of the part poor.

Generally speaking, the high temperature will produce better strength weld lines items, because of the high temperature circumstances, polymer chain activities are better and can penetrate each other mutually. In addition, on the area of high temperature, the two melt temperature is close, so the thermal properties of the melt is almost the same, which could increase the weld strength of the region; whereas in the low-temperature injection molding, welding strength is poor.