How To Resolve Plastic Moulding Parts Stress Mark Issues

In plastic moulding parts products, especially those made from ABS, PP, and PC, it is common to see shiny stress marks on the surface. These include ejector pin stress marks, insert stress marks, and wall thickness difference stress marks.

These shiny or whitish surface marks result from internal stress, which can be categorized into orientation stress and cooling shrinkage stress. Irregular stress marks near the gate are typically caused by orientation stress, while wall thickness differences result from a combination of orientation and shrinkage stress. The formation of ejector pin and angled pin stress marks involves additional factors.

Generally, with an increase in packing pressure and time, the stress marks develop in the following order: wall thickness difference stress marks, ejector pin and angled pin stress marks, and finally, irregular stress marks near the gate (which appear white and shiny from the side). This progression also depends on the specific structure of the product and the gate location.

Therefore, when these plastic moulding parts stress marks appear, reducing the packing pressure and time is crucial. Understanding this, it is more effective to start with modifying the product design and mold design rather than adjusting the molding process.

In Plastic Moulding Parts product design:

• Overall thin wall thickness or thicker wall thickness at the flow end can lead to excessive filling and packing pressure. It may be necessary to add material or reduce thickness at the flow end. When reducing the packing pressure, check if reducing thickness at shrinkage areas improves the situation. If not, adding material to the large area might be required.
• Avoid significant differences in wall thickness; if present, a more substantial differentiation is needed.
• Ribs on the male mold should not be too large to avoid stress marks on the female mold ribs.

In Plastic Moulding Parts mold design:

• Gates that are too small, too few in number, or unevenly distributed.
• Loose fitting of moving parts or improper waterway configuration leading to higher mold temperature.

In Plastic Moulding Parts molding conditions:

• Set packing pressure and time reasonably (reduce them).
• Increase or decrease mold temperature (increasing can improve filling and reduce packing pressure; decreasing can make the textured surface shiny, making the surface and stress marks less noticeable. Generally, lowering mold temperature is the more common choice).

By addressing these factors in design, plastic injection mold design, and process settings, stress marks can be effectively reduced or eliminated.

How Stress Marks on Plastic Moulding Parts Are Formed

Stress marks on injection molded parts mainly arise from factors related to mold structure, molding process, and materials. These factors influence the plastic under injection pressure and flow speed within the mold cavity, resulting in visible flow marks on the surface of the injection molded part. These stress marks not only affect the appearance quality of the injection molded parts product but may also reduce its mechanical performance and lifespan. The primary reasons for the formation of stress marks include:

Uneven Material Flow Filling:

Uneven material flow filling can lead to local overfilling, resulting in significant extrusion shear stress.

Improper Cooling System Design:

Uneven cooling can easily generate local shrinkage stress.

Improper Gate Location:

An improper gate location may cause different pressure transmission effects in various positions, leading to uneven shrinkage and stress warping deformation.

Presence of Sharp Corners:

Sharp corners can easily cause stress concentration, which may lead to stress cracking when subjected to external impact or solvent-induced action.

Uneven Wall Thickness Distribution:

Variations in wall thickness can cause changes in shear speed in those regions, leading to stress formation.

Improper Injection Pressure:

Both too low or too high injection pressure can cause excessive local pressure, resulting in stress.

Low Injection Speed:

Low injection speed can easily generate stress.

Excessive Packing Pressure and Time:

Increased packing pressure and time can also increase stress.

Low Mold Temperature:

Low mold temperature can prevent timely stress release, causing residual stress.

These injection molded parts marks can adversely affect both the aesthetic quality and mechanical performance of the products. Key contributors to stress marks include uneven material flow, improper cooling, incorrect gate location, sharp corners, uneven wall thickness, and inappropriate injection and packing pressures.