Stress Cracking in Polycarbonate

2026-08-20

Stress cracking is one of the common defects found in polycarbonate products. This article will provide a detailed explanation of why stress cracking occurs and how to prevent it.

1. What is environment stress-crack defect (ESC) in polycarbonate?

When PC is under internal stress and comes into contact with solvents or chemicals, the chemicals cause the PC resin to plasticize, resulting in microcracks on the surface of the PC product. These cracks gradually propagate and eventually develop into visible cracks or even fractures.

1.1 Where Do Internal Stresses in Polycarbonate Come From?

1.1.1 Residual Stress in Injection Molding

PC Material ESC common examples include:

Injection speed is too high;
Melt temperature is too low, resulting in insufficient flow rate;
Hold pressure is too high;
Hold time is too long;
The material exhibits significant flow orientation;
Mold temperature is uneven, with insufficient cooling at the center;
Localized high stresses are more likely to occur, especially near thin walls, sharp corners, holes, weld lines, and gates.

1.1.2 Assembly Stress

This is primarily evident in PC structural components that require anchor bolt fastening: once the screws are tightened, the components remain under continuous tensile and compressive stress. If they are then exposed to solvents such as cleaning agents, alcohol, grease, or adhesives, there is a high likelihood of stress cracking.

2. Examples of Typical PC Stress Cracking

2.1 Polycarbonate Cracking Defects in Insert Components

The image on the left shows a stress crack defect in a PC material with an embedded copper component.

This defect occurs because the temperature of the copper component is too low; when the polymer comes into contact with the copper component, the contact surface cools rapidly and contracts first, while the interior does not cool at the same rate. This uneven cooling between the interior and exterior causes the material to crack.

The solution is to heat the copper components during the insert molding process to prevent the polymer at the contact surface from cooling too quickly.

Cracking Defects in Thin-Wall Injection Molding

2.2 Cracking Defects in Thin-Walled PC resin Injection-Molded Products

The image on the left shows a cracking defect in a thin-walled PC resin product caused by the location of the injection gate.

This defect is caused by the small size and thin walls of the gate in thin-walled products, which require high material injection pressure. Excessively high holding pressure and holding time can cause deformation in the finished product, while the short melt flow distance and rapid cooling at the gate area increase the likelihood of stress cracking.

The solution is to adjust the injection gate to avoid placing it at the thinnest point; also, appropriately adjust the holding time and holding pressure.

Cracking in Glass-Fiber-Reinforced Polycarbonate

2.3 Cracks in the PC material screw supports

The image on the left shows cracks in a PC screw support that developed after the screw was tightened.
This defect was caused by the use of glass-fiber-reinforced PC material that did not meet the required strength standards.
The solution is to use a PC/ABS alloy to address the brittleness of PC and improve the material’s toughness.

3. What is the difference between stress cracking and ordinary cracking in PC?

Typical cracking in polycarbonate occurs when external mechanical stresses—such as impact, tension, or bending—exceed the material’s rated strength.

Stress cracking in polycarbonate is a gradual cracking process induced by chemical environments, characterized by a noticeable delay in onset; for example, a part may appear flawless immediately after assembly but develop cracks one week later.

4. How can stress-induced cracking defects in polycarbonate be prevented?

Select a PC resin grade with better ESC resistance, such as Sabic Lexan EXL9330 (which contains silicon and effectively resists cracking in low-temperature environments); if necessary, choose a PC/ABS material to improve the flexibility of the PC material and reduce environmental stress cracking.
Adjust processing conditions. Appropriately increase the melt temperature, reduce the injection speed of the PC resin, and control the holding pressure time, holding pressure, and cooling time.
Adjust the product design. Avoid sharp corners, small radii, significant increases in wall thickness, and designs where screws directly press against structural components.

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