Causes of Cracks in Injection Molded Parts
Cracks are a prevalent defect in injection molded plastic products, often compromising both the aesthetic and structural integrity of the parts. Understanding the underlying causes of cracking is essential for manufacturers to implement effective solutions and improve product quality. The primary causes of cracks in injection molded parts include residual stress, external stress, and environmental factors.
▶ Residual Stress
Residual stress is a significant factor contributing to cracks in molded parts. It arises from various conditions during the molding process:
Overfilling
Overfilling the mold can lead to excessive pressure, resulting in residual stress. This stress can be alleviated by:
Material Selection: Amorphous resins like AS, ABS, and PMMA are more prone to residual stress compared to crystalline resins like PE and POM.
Gate Design: Opting for multi-point, side, or tab gates instead of direct gates can help distribute pressure more evenly.
Resin Temperature: Increasing resin temperature can reduce melt viscosity, improving fluidity and reducing injection pressure.
Mold Temperature: Raising mold temperature can help minimize stress, although high injection speeds may also reduce stress even at lower mold temperatures.
Injection Time: Shortening injection and holding times can prevent excessive stress buildup.
Demolding and Ejection
Stress during demolding and ejection can cause cracks, particularly if:
Ejector Pins: Improperly sized or positioned ejector pins can create localized stress.
Draft Angles: Small draft angles increase ejection force, leading to stress.
Surface Roughness: A rough mold surface can cause sticking and stress during ejection.
Metal Inserts
The use of metal inserts can introduce significant residual stress due to differences in thermal expansion coefficients between metal and resin. Over time, this stress may exceed the strength of the resin, leading to cracking. Preheating metal inserts before molding and using materials like glass fiber reinforced resin can mitigate these issues.
▶ External Stress
External stress is often caused by poor design choices:
Design Flaws
Sharp corners or inadequate support structures in the design can lead to stress concentration points where cracks are likely to form. Ensuring smooth transitions and adequate support in design helps distribute stress more evenly.
▶ Environmental Factors
Environmental conditions can also contribute to cracking:
Chemical Exposure
Exposure to chemicals or moisture absorption can degrade the physical properties of plastics, leading to cracking. Ensuring proper material selection and storage conditions is crucial.
Use of Recycled Materials
Excessive use of recycled materials can degrade material properties. It is advisable to limit regrind use to maintain material integrity.
▶ Conclusion
Cracking in injection molded parts is a multifaceted issue stemming from residual stresses, external stresses due to design flaws, and environmental factors. By addressing these areas—through careful design, appropriate material selection, optimized processing parameters, and environmental controls—manufacturers can significantly reduce the incidence of cracks and enhance product quality.