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Filling stage of plastic injection molding


Filling is the first step in the entire injection cycle process. The time starts from when the mold is closed and the injection is started until the mold cavity is filled to about 95%. In theory, the shorter the filling time, the higher the efficiency of plastic injection molding; but in actual production, the molding time (or injection speed) is restricted by many conditions.

High-speed filling

The shear rate is high during high-speed filling, and the viscosity of the plastic decreases due to the effect of shear thinning, which reduces the overall flow resistance; the local viscous heating effect will also make the thickness of the solidified layer thinner. Therefore, in the flow control stage, the filling behavior often depends on the volume to be filled. That is, in the flow control stage, due to the high-speed filling, the shear thinning effect of the melt is often great, and the cooling effect of the thin wall is not obvious, so the effect of the speed prevails.

Low speed filling

When the heat conduction controls the low-speed filling, the shear rate is lower, the local viscosity is higher, and the flow resistance is higher. Due to the slower replenishment rate and slower flow of the hot plastic, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. Coupled with a smaller amount of viscous heating, the thickness of the cured layer is thicker, which further increases the flow resistance at the thinner part of the wall. Due to the fountain flow, the plastic polymer chains in front of the flow wave are aligned almost parallel to the flow wave front. Therefore, when the two strands of plastic melt meet, the polymer chains on the contact surface are parallel to each other; in addition, the properties of the two strands of melt are different (the residence time in the mold cavity is different, and the temperature and pressure are different), resulting in the convergence area of the melt. The structural strength is poor on the microscopic level. Place the parts at an appropriate angle under light and observe with the naked eye, you can find that there are obvious joint lines, which is the formation mechanism of weld marks. The weld mark not only affects the appearance of the plastic part, but also has a loose microstructure, which is easy to cause stress concentration, which reduces the strength of the part and breaks.
Generally speaking, the strength of the weld line that produces the weld in the high temperature zone is better. Because under high temperature conditions, the polymer chains are relatively mobile and can penetrate each other. In addition, the temperature of the two melts in the high temperature area is relatively close, and the thermal properties of the melt are almost the same, which increases the strength of the welded area; In the low temperature area, the welding strength is poor.

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