What Are Common Operational Issues in Auto Parts Molding?

The manufacturing of auto parts through injection molding is a complex process where precision and consistency are critical. Despite meticulous setup, operational issues can arise during production, impacting part quality, cycle efficiency, and tooling longevity. Troubleshooting these problems requires a systematic approach that examines the interplay between material, machine, mold, and process parameters. Common operational challenges can be categorized into defects related to flow and filling, issues of dimensional stability and part integrity, surface finish anomalies, and ejection-related failures.

I. Flow and Filling Defects

These issues occur as the molten polymer travels through the sprue, runners, and gates into the mold cavity. They are often the immediate and visible problems on a freshly molded part.

Short Shot: This is a failure of the polymer melt to fill the entire mold cavity, resulting in an incomplete part.

Causes and Solutions: The primary cause is insufficient injection pressure or speed. This can be addressed by increasing the injection pressure, boosting the melt temperature to reduce viscosity, or raising the mold temperature to prevent premature freezing. Other contributing factors include blocked or undersized gates, vent blockages that trap air, or simply an inadequate shot volume set on the machine.

Sink Marks and Voids: Sink marks are surface depressions, while voids are internal hollows, typically occurring in thick sections of a part.

• Causes and Solutions: These defects are caused by inadequate packing pressure or holding time. As the thick section cools and shrinks, there is insufficient material from the gate to compensate for the volumetric reduction. Solutions involve increasing the pack and hold pressure, extending the holding time, or reducing the melt temperature to encourage faster skin formation. Modifying the part design to use uniform wall thickness is a more fundamental, though less immediately adjustable, solution.
• Weld Lines: These are visible lines on the part where two flow fronts meet and fail to knit together seamlessly.
• Causes and Solutions: Weld lines form when the melt flow splits around a core or pin and reunites. They are inevitable in many part designs but can be weakened. To improve weld line strength and appearance, the melt temperature, mold temperature, and injection speed can be increased to allow the polymer chains to better intertwine. Relocating gates or improving venting at the weld line location can also be effective.

II. Dimensional Stability and Part Integrity Issues

These problems affect the structural form of the part, causing it to warp, become brittle, or contain internal stresses.

• Warping: This is a distortion of the part after ejection, where it does not maintain its intended shape.
• Causes and Solutions: Warping is primarily caused by uneven shrinkage throughout the part. This can be due to non-uniform cooling, often from a temperature difference between the two mold halves or from coolant channels that are not placed. Excessive packing pressure or an overly high melt temperature can also contribute. Corrective actions include balancing the mold temperature, increasing the cooling time, and reducing the pack pressure. Ensuring a symmetric gate location can promote more uniform flow and shrinkage.
• Brittleness: The part fractures or cracks easily under minor stress.
• Causes and Solutions: Brittleness is frequently a material-related issue. The polymer may have degraded from excessive heat or a prolonged residence time in the barrel. Contamination with a different material can also cause this. Troubleshooting involves lowering the melt temperature and barrel residence time, ensuring the material is thoroughly dried before processing, and verifying that the correct regrind ratio is being used without contaminating the virgin material.
• Residual Stress: Internal stresses locked into the part may not be visible immediately but can premature failure in the field.
• Causes and Solutions: High injection speeds and premature gate freezing can create significant molecular orientation and residual stress. A lower injection speed and a higher Auto Parts Mould temperature allow the polymer molecules to relax more, reducing internal stress. Annealing the parts after molding is a secondary operation that can relieve these stresses.