PPO (polyphenylene oxide), also known as MPPO in its modified form, is a high-performance engineering plastic widely used in automotive components, electronic housings, precision parts, and electrical insulation applications. It offers excellent dimensional stability, heat resistance, mechanical strength, and electrical properties. However, PPO and MPPO products are highly prone to stress cracking during or after injection molding, which severely affects appearance, strength, and service life. Stress cracking in PPO is mainly caused by internal residual stress, structural stress concentration, external force, and chemical corrosion. To effectively prevent such defects, manufacturers must implement systematic control over material preparation, product design, mold structure, injection parameters, post-treatment, and application environment.
Material Drying and Compatibility ControlPPO has low moisture absorption, but even trace moisture can cause micro-bubbles, degradation, and internal stress during high-temperature molding. Therefore, adequate drying is essential. Raw materials should be dried in a hot-air circulating dryer at 100–120°C for 3–4 hours, ensuring the moisture content is below 0.05%. Over-drying must be avoided to prevent yellowing or material degradation. During production, materials from different suppliers or grades should not be mixed arbitrarily, as inconsistent flowability and shrinkage can increase internal stress. Recycled materials should be used with caution, and their proportion should be strictly limited to maintain uniformity and stability.
Product Structure Design to Reduce Stress ConcentrationStress concentration is the primary cause of cracking in PPO parts. Sharp corners, abrupt wall thickness changes, and excessive assembly interference should be eliminated. All internal angles must be rounded, with a radius of at least 0.5 mm; for stressed areas, the radius should exceed 1 mm. Wall thickness should be as uniform as possible, with differences limited to 30%. Transitional sections should be used where thickness changes occur. The thickness of reinforcing ribs should not exceed 60% of the main wall thickness, and rounded corners should be added at the root. Fasteners, screw posts, and inserts should leave appropriate gaps to avoid excessive interference. Metal inserts must be preheated to reduce internal stress caused by thermal contraction differences.
Mold Design for Uniform Flow and CoolingMold structure directly affects the formation of residual stress. The gate position and type are critical; fan gates, submarine gates, or multiple pin-point gates are recommended to reduce jetting and shear stress. Gates should be placed at thicker sections to ensure balanced filling. The runner system should be short, smooth, and free of sudden changes to minimize pressure loss. Venting must be sufficient to prevent burning, trapped gas, and internal stress. The cooling system should be evenly distributed to ensure consistent temperature across the cavity, reducing warping and shrinkage stress. The ejection system should use balanced ejection to avoid deformation or stress during demolding.
Injection Molding Parameters to Minimize Internal StressPPO is sensitive to shear and pressure, so parameters should be set to reduce internal stress. Barrel temperature should be moderate: 260–280°C in the front zone, 270–290°C in the middle, and 280–300°C at the nozzle. Higher temperature reduces viscosity and stress, but overheating causes degradation. Injection pressure and speed should be moderate; high pressure increases molecular orientation and residual stress. A slow-fast-slow injection profile is suitable. Packing pressure and time should be minimized, stopping once the gate freezes. Mold temperature should be controlled at 80–100°C to improve crystallization and reduce stress.
Post-Mold Annealing and Stress ReliefFor high-precision or high-stability PPO products, annealing is highly effective. Parts should be treated in an oven at 100–120°C for 30–60 minutes per mm of wall thickness, then cooled slowly to room temperature. This process relaxes molecular orientation and eliminates internal stress. After molding, parts should be handled gently to avoid external stress. Products should not be exposed to chemicals, oils, or solvents during storage or assembly, as these can accelerate stress cracking.
ConclusionStress cracking in PPO injection molding can be effectively prevented through strict material control, reasonable structural design, optimized mold structure, moderate injection parameters, and proper post-treatment. By minimizing internal stress and stress concentration, manufacturers can significantly improve product quality, yield, and reliability.
