1. The cycle time is short, that is to say, the residual time of molten material in the barrel is short and the cycle time is prolonged.
2. The barrel temperature in the feeding area is too high, which reduces the barrel temperature in the feeding area.
3. Screw speed is too fast, reduce screw speed.
4. Screw back pressure is too low, increase screw back pressure.
In addition, the reason related to the design of the two-color mold is that the unreasonable screw geometry selects the screw with a long feeding section, and the screw groove of the feeding section is deep.
If the measurement process of the two-color mold injection process starts too early, and the air wrapped in the particles in the screw feeding area does not overflow the feeding port, the air will be squeezed into the melt. However, the pressure in the feeding area is too low to move the air to the rear. The air squeezed into the molten material in the barrel will cause grey black spots in the products. Just like what happens in the compression ignition diesel engine, the coking phenomenon caused by the air squeezed into the barrel is sometimes called “diesel engine effect”. The coking phenomenon can explain the high temperature produced by compression at the junction of the molten material and the extruded bubble, and the fracture of the molten material caused by the oxidation of oxygen in the air. The melting process should be started in the middle of the feeding area when the injection process of the two-color mold is debugged, where the melting pressure is high, forcing the air between the particles to move backward and overflow the material mouth.
The preferred apparent gray black markings may occur near the gate, in the middle of the runner and away from the gate. This phenomenon can only be seen in transparent parts, and is often found in two-color mold injection products made of PMMA, PC and PS materials.
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