What is Environmental Stress Cracking of Plastic Parts: A Comprehensive Guide

What is Environmental Stress Cracking of Plastic Parts: A Comprehensive Guide

Plastic parts are ubiquitous in modern manufacturing, but their susceptibility to environmental stress cracking (ESC) poses a significant challenge.

ESC, accounting for 31% of plastic part failures, is a complex issue influenced by plastic material selection, chemical exposure, and stress endured by plastic components.

In this comprehensive guide, we delve into the mechanisms, characteristics, factors, and countermeasures associated with ESC.

Environmental Stress Cracking: Unraveling the Phenomenon

Environmental Stress Cracking: Unraveling the Phenomenon

ESC occurs when plastic resin degrades due to the synergistic action of chemical substances in the presence of internal stress.

Unlike a chemical reaction, ESC involves chemicals penetrating the molecular structure, accelerating molecular breakage. This phenomenon is responsible for a staggering 40% of plastic part failures when considering chemical attacks.

Steps and Characteristics of Environmental Stress Cracking

  • Steps of ESC: ESC progresses through fluid absorption, plasticization, fine crack generation, crack propagation, and final failure. The rate of fluid uptake is pivotal, determining crack propagation and expansion rates.
  • Characteristics of ESC:
    • Brittle Fracture: ESC damage results in a brittle fracture, typically initiating at high-stress areas in direct contact with active chemicals.
    • Initialization Cracking: ESC begins with multiple single-point cracking, uniting into a unified fracture.
    • Smooth Form: The initial cracked area exhibits a relatively smooth form, with active chemicals accelerating the cracking process.
    • Remaining Fine Cracks: Presence of residual fine cracks signals ESC occurrence.
    • Stretching Fibrils: Plastic fracture is indicated by stretching fibrils in the final fracture zone.
    • Crossbands: ESC involves a progressive cracking propagation mechanism, revealed through characteristic surface tests.

Factors Affecting ESC

ESC correlates with:

  • Plastic Type: Amorphous plastics are more susceptible than semi-crystalline plastics.
  • Molecular Weight: Lower molecular weight diminishes ESC resistance.
  • Lower Crystallinity: Higher crystallinity improves ESC resistance.

Chemicals

Chemicals influencing ESC include those forming hydrogen bonds and chemicals with lower molecular weights. Chemical exposure occurs during production and use.

Stress

ESC is triggered by tensile stress during use and internal residual stress from the injection molding process. Stress during assembly, like ultrasonic welding, can contribute to ESC.

Countermeasures and Suggestions

Countermeasures and Suggestions

Material Strategies

Choosing high-quality, tough, and fluid plastic raw materials improves ESC resistance.

Design Strategies

Rational product design, considering material characteristics, manufacturing processes, and uniform force distribution, helps avoid stress concentration.

Production Process Strategies

Adopting a suitable molding process, ensuring mold quality, and controlling parameters prevent excessive traction and stretching.

Transportation and Handling Strategies

Care during transportation, loading, unloading, and installation prevents stress concentration and marks caused by human factors.

Methods to Prevent Surface Stress Marks

In addition to the above, mold optimization, product quality management, appropriate testing, and timely maintenance contribute to preventing stress marks.

Environmental Stress Cracking: A Case Study

Environmental Stress Cracking: A Case Study

Problem: Environmental Stress Cracking (ESC): What Is It and How Can I Stop It?

Solution: Highly crystalline plastics resistant to chemicals are recommended for parts exposed to various chemicals. Forensic tests, optical microscopy, and chemical analysis aid in identifying and preventing ESC failures.

Conclusion

ESC is a multifaceted challenge, but with informed material selection, thoughtful design, meticulous production processes, and vigilant maintenance, plastic parts can be safeguarded against this prevalent issue.

FAQs

A1: ESC is primarily caused by the synergistic effect of chemical substances and mechanical stresses on stressed plastic components.

A2: Amorphous plastics like PC, ABS, and PMMA are more susceptible than semi-crystalline plastics such as PBT and POM.

A3: Lower molecular weight diminishes ESC resistance, making plastics more susceptible to environmental stress cracking.

A4: Improper handling during transportation can lead to stress concentration and marks on plastic parts. Careful transportation is essential to prevent ESC.

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