Automobile parts

In the production of automotive parts by injection molding, the setting of multi-stage injection and pressure holding processes directly determines whether the parts will have shrinkage marks, warping, fusion marks, or excessive internal stress. Especially for complex and high-requirement automotive parts such as bumpers, instrument panel frameworks, and door panels, mastering these two processes is of vital importance.

Multi-stage Injection: Segmental Control of Filling Rhythm

The core of multi-stage injection lies in "tailoring to the segment". Because the flow behavior of the melt varies greatly in different stages of filling the cavity, it is usually controlled in three to four segments:

• First segment (over gate): The speed must be extremely slow, controlled at around 10%-15%. The purpose is to prevent the melt from spraying 

        into the cavity and causing air marks or spray marks, especially for the leather-textured surfaces of automotive interior parts, this stage is crucial.

• Second segment (main filling): Increase to high speed, approximately 70%-90%. Quickly establish the melt front to avoid rapid cooling during the 

        flow process, which can lead to layering, cold material, or flow marks. For glass fiber reinforced materials, high-speed filling can also reduce the 

        exposure of glass fibers.

• Third segment (filling end segment): Return to medium to low speed, approximately 40%-60%. Allow sufficient time for gas to escape from the 

        exhaust groove in the cavity to prevent end gas entrapment and scorching, and also reduce the obviousness of fusion marks.

• Fourth segment (before V/P switch): Switch to extremely low speed, approximately 10%-20%. Smoothly transition to the pressure holding stage 

        to avoid edge defects or excessive internal stress caused by sudden pressure changes. 

The core of pressure retention is "timely and appropriate shrinkage compensation". When plastic cools, it will contract. The purpose of pressure retention is to continuously push the molten material into the mold to compensate for the volume contraction. If the pressure retention pressure and time are insufficient, shrink marks will appear on the surface of the part, and the size will be smaller; if the pressure retention is too high, the part will warp due to excessive compaction and may even be difficult to demold.

A reasonable approach is to use two-stage pressure retention:

1. The first stage uses a higher pressure to quickly eliminate the shrinkage cavities near the gate;

2. The second stage drops to a lower pressure and continues until the gate is completely frozen to ensure uniform shrinkage compensation throughout the part.

Multiple stages and pressure retention must be coordinated. A common mistake is to only focus on the injection speed and ignore the V/P switching point (the position where injection switches to pressure retention). Switching too early results in insufficient injection; switching too late causes burrs or high internal stress. The correct approach is to first observe the melt filling pattern through a "short shot test" and find the screw position that just fills 95% of the cavity as the switching point. If the part has fusion marks, in addition to increasing the injection speed, the pressure retention time can also be appropriately extended to allow the high-pressure melt to forcibly fill the fusion interface. For automotive parts with significant wall thickness differences, the pressure retention time should prioritize ensuring the shrinkage compensation of the thick-walled areas. In necessary cases, pressure curve optimization can be performed using mold flow analysis software.

In summary, multiple-stage injection and pressure retention are not isolated parameters but a set of interrelated systems. When adjusting the machine, it is recommended to start from the recommended base process and gradually fine-tune the speed and position of each stage based on the results of the short shot test. At the same time, the pressure retention effect can be verified using a weighing method. Only by allowing the melt to "fill smoothly and be compensated evenly" can stable production of automotive parts with qualified dimensions and excellent appearance be achieved.