How can multi-stage injection speed be effectively utilized to adjust the injection molding process?

Multi-Stage Injection Speed Control and Its Application in Injection Molding

Proportional control of injection speed has been widely adopted by injection molding machine manufacturers. This article systematically explains the advantages of applying multi-stage injection speed control and provides a general overview of its applications in eliminating defects such as short shots, air traps, and sink marks.

The close relationship between injection speed and product quality makes injection speed a critical parameter in injection molding. By determining the beginning, middle, and end stages of the filling-speed profile and achieving a smooth transition from one set point to another, a stable melt-front velocity can be maintained to obtain the desired molecular orientation and minimize internal stress.

We recommend the following principles for injection speed segmentation:

1. The velocity of the melt front should remain constant.
2. High injection speed should be used to prevent the melt from freezing during injection.
3. Injection speed settings should allow rapid filling through critical areas such as runners while reducing the speed at the gate.
4. The injection speed should be set so that filling stops immediately after the mold cavity is completely filled, thereby preventing overpacking, flash, and residual stress.

The basis for setting injection speed segments must take into account mold geometry, other flow restrictions, and potential instability factors. Proper speed settings require a clear understanding of both the injection molding process and material characteristics; otherwise, product quality will be difficult to control. Since melt flow velocity is difficult to measure directly, it can be estimated indirectly by measuring screw forward velocity or cavity pressure, provided that the non-return valve is confirmed to be free of leakage.

Material characteristics are extremely important because polymers may degrade under different stress conditions. Increasing the molding temperature may cause severe oxidation and chemical degradation, but at the same time, shear-induced degradation may decrease because higher temperatures reduce material viscosity and therefore reduce shear stress. Undoubtedly, multi-stage injection speed control is particularly beneficial when molding heat-sensitive materials such as PC, POM, and UPVC, as well as their compounds.

Mold geometry is also a determining factor. Thin-wall sections require the highest injection speeds. Thick-wall parts require a slow-fast-slow speed profile to avoid defects. To ensure that part quality meets specifications, injection speed settings should maintain a consistent melt-front velocity.

Melt flow velocity is extremely important because it affects molecular orientation and surface appearance in the molded part. When the melt front reaches a cross-sectional transition or intersection area, the speed should be reduced. For complex molds involving radial spreading, the melt throughput should increase evenly. Long runners should be filled rapidly to reduce cooling of the melt front. An exception is the injection of high-viscosity materials such as PC, because excessive injection speed may carry cold material through the gate and into the cavity.

Adjusting injection speed can help eliminate defects caused by flow slowing at the gate. When the melt passes through the nozzle and runner and reaches the gate, the surface of the melt front may already have cooled and solidified. Alternatively, the melt may stagnate because the runner suddenly becomes narrower, until sufficient pressure is established to force the melt through the gate. This can result in a pressure peak at the gate.

High pressure can damage the material and cause surface defects such as flow marks and gate burn. These problems can be overcome by reducing the injection speed immediately before the gate. This reduction prevents excessive shear at the gate, after which the injection speed can be increased back to its original value. Because precise control of injection speed at the gate is very difficult, reducing the speed near the end of the runner is a better practical solution.

Controlling the final-stage injection speed can prevent or reduce defects such as flash, burn marks, and air traps. Reducing the speed during the final filling stage prevents excessive cavity packing, thereby avoiding flash and reducing residual stress. Air traps caused by inadequate venting or filling problems at the end of the mold flow path can also be addressed by reducing the filling speed, particularly during the final stage of injection.

Short shots are caused by factors such as excessively low speed at the gate or localized flow restrictions resulting from premature melt solidification. Increasing injection speed immediately after the melt passes through the gate or encounters a local flow obstruction can solve this problem. Flow marks, gate burn, molecular fracture, delamination, and peeling in heat-sensitive materials are caused by excessive shear as the material passes through the gate.

A smooth part surface depends strongly on injection speed. Glass-fiber-reinforced materials are particularly sensitive, especially nylon. Dark spots or wavy patterns can result from flow instability caused by viscosity variations. Distorted flow can produce wavy patterns or an uneven hazy appearance; the specific defect depends on the degree of flow instability.

When the melt passes through the gate at excessively high injection speed, high shear is generated. Heat-sensitive plastics may then burn, and the degraded material can be carried through the cavity to the flow front, eventually appearing on the surface of the molded part.

To prevent jetting, the injection speed profile should provide rapid filling through the runner area followed by slower filling through the gate. Identifying the correct speed-transition point is the key issue. If the transition occurs too early, the filling time will become excessively long. If it occurs too late, excessive flow inertia will cause jetting. The lower the melt viscosity and the higher the barrel temperature, the greater the tendency for jetting to occur. Small gates require high-speed, high-pressure injection and are therefore also an important source of flow defects.

Sink marks can be improved through more effective pressure transmission and a smaller pressure drop. Low mold temperature and excessively slow screw forward speed greatly shorten the effective flow length and must therefore be compensated for by higher injection speed. High-speed flow reduces heat loss and, because shear heating generates frictional heat, increases the melt temperature. This slows the thickening of the outer layer of the molded part. Cross-sectional transition areas in the cavity must have sufficient thickness to avoid excessive pressure drops; otherwise, sink marks may occur.

In conclusion, most injection molding defects can be addressed by adjusting injection speed. Therefore, the key to optimizing the injection molding process is to set the appropriate injection speed and divide it into appropriate stages.