The segmented principle of multistage injection molding

I. Flow Characteristics of Plastic Melt

Screw - Propelling Phase

• Initial Segment:

• Usually starts from the beginning of injection molding. Its main function is to enable the plastic melt to stably enter the main runner of the mold from the barrel. The injection speed is generally slow, mainly to prevent the melt from splashing at high speed and causing air entrainment. For example, for high - viscosity engineering plastics like PC (polycarbonate), the initial speed can be set at 10 - 30% of the maximum injection speed.

• Main Filling Segment:

• Once the melt smoothly enters the main runner, adjust the injection speed according to the complexity of the mold cavity and the wall - thickness distribution. For simple products with uniform wall thickness, a relatively high and stable speed can be used for filling to improve production efficiency. For instance, for a thin - walled simple container made of PP (polypropylene), the injection speed can be set at 60 - 80% of the maximum injection speed.

• For complex products with wall - thickness differences, increase the speed appropriately in thin - walled parts to ensure rapid filling and prevent short - shot. Slow down the speed in thick - walled parts to avoid defects such as gas entrapment and flash. For example, when injection - molding plastic products with ribs, increase the speed when filling the rib areas (thin - walled parts) and slow down in the main thick - walled parts.

• Buffer Segment before Pressure - Holding Switch:

• The speed in this segment gradually decreases to enable the melt to smoothly transition to the pressure - holding phase. The main purpose is to avoid defects caused by sudden pressure changes during the pressure - holding switch. The extent of speed reduction depends on the plastic's properties and product requirements, generally reducing the speed to 10 - 30% of the maximum injection speed.

Pressure - Holding Phase

• First Pressure - Holding Segment:

• At the start of pressure - holding, the pressure is relatively high, mainly to compensate for the volume shrinkage of the plastic during cooling. For crystalline plastics (such as PA - 6 polyamide 6), since their crystallization process leads to significant volume shrinkage, the pressure in the first pressure - holding segment is usually high, which can be set at 70 - 90% of the maximum injection pressure to ensure the product's dimensional accuracy.

• Middle and Final Pressure - Holding Segments:

• As the plastic cools further, the pressure - holding pressure can be gradually reduced. The pressure in the middle segment can be reduced to 40 - 60% of the maximum injection pressure, and in the final segment to 10 - 30%. This can ensure product quality while reducing residual stress and preventing warping and deformation after demolding. Meanwhile, the pressure - holding time should be determined according to the product's wall thickness and the plastic's thermal properties. Generally, thick - walled products require a longer pressure - holding time, while thin - walled products need a shorter one.

II. Characteristics of Product Structure

• Wall - Thickness Differences:

• For products with uneven wall thickness, segment according to the wall - thickness distribution. In thin - walled parts, use a faster injection speed to ensure rapid filling and prevent the melt from becoming too viscous to fill due to excessive cooling. For example, when injection - molding the shell of an electronic product with thin - and thick - walled connections, the injection speed in thin - walled parts can be 1.5 - 2 times that in thick - walled parts. Slow down the speed in thick - walled parts to avoid gas entrapment and sink marks.

• Complexity of Structure:

• For products with complex structures, such as those with side holes, undercuts, ribs, etc. Adjust the injection speed and pressure appropriately when filling areas close to these complex structures. For example, slow down the speed when filling areas near side holes to prevent the melt from breaking through the hole walls; increase the pressure appropriately when filling rib parts to ensure complete filling.

• Appearance Requirements:

• If the product has high appearance - quality requirements, such as no flow marks and weld lines on the surface. Control the injection speed and pressure well during the filling phase of the appearance surface. Generally, use a relatively low speed and appropriate pressure to achieve good surface quality. For example, for automotive interior parts with strict appearance requirements, the injection speed during the filling phase of the appearance surface can be set at 30 - 50% of the maximum injection speed, and cooperate with an appropriate mold temperature to reduce surface defects.