Understanding Common Defects in Plastic Injection Molding Parts
Plastic injection molding is a widely used manufacturing process for producing high-volume, precision components. However, defects such as shrinkage marks and bubbles can compromise the structural integrity, surface finish, and dimensional accuracy of injection molding parts. Identifying the root causes and implementing corrective measures is essential for maintaining product quality and minimizing waste. This article examines the technical causes and practical solutions for two prevalent defects: shrinkage marks and bubbles.
Shrinkage Marks in Injection Molding: Causes and Solutions
Shrinkage marks, also known as sink marks, are depressions or indentations on the surface of a molded part, typically occurring over thick ribs, bosses, or internal features. These defects arise due to uneven cooling and differential shrinkage within the material.
Material-Related Causes
Amorphous and semi-crystalline materials exhibit different shrinkage behaviors. Semi-crystalline plastics (e.g., PE, PP) undergo higher volumetric shrinkage due to crystallization, making them more prone to sink marks. Inadequate material drying can also exacerbate shrinkage by introducing internal voids. Selecting materials with lower shrinkage rates and consistent thermal properties is critical for reducing sink marks in injection molding parts.
Process Parameter Optimization
Improper holding pressure, short hold time, and low injection pressure are primary process-related causes. Insufficient pressure during the packing phase fails to compensate for material contraction as the part cools. Increasing holding pressure (typically 80–100% of injection pressure) and extending hold time until gate freeze ensures adequate material replenishment. Additionally, optimizing mold temperature (within 10–20°C above the material’s recommended range) promotes uniform cooling and minimizes differential shrinkage.
Design and Tooling Adjustments
Part geometry plays a significant role. Thick sections cool slower, creating localized shrinkage. Design modifications such as rib thickness reduction (to 50–60% of adjacent wall thickness), use of coring, and uniform wall sections help balance cooling rates. Mold design improvements include increasing gate size or adding multiple gates to improve melt flow and packing efficiency.
Bubbles in Injection Molded Parts: Root Causes and Mitigation
Bubbles, or voids, are air pockets trapped within the molded component, leading to weak spots and potential failure points. They are classified as gas porosity (from trapped air, moisture, or volatiles) and vacuum voids (from localized under-packing).
Moisture and Contamination
Hygroscopic materials like ABS, PC, and PA absorb moisture from the atmosphere. If not properly dried (typically 2–4 hours at 80–120°C, depending on material), water vaporizes during injection, forming bubbles. Implementing a consistent drying protocol with dehumidified dryers is essential. Pre-drying verification via moisture analysis ensures compliance.
Venting and Mold Design
Inadequate mold venting prevents air evacuation during cavity filling. Poorly designed vents (typically 0.025–0.05 mm deep) can trap air, especially in deep ribs or corners. Optimizing vent placement at the end of flow paths and using vacuum-assisted venting systems improves air escape. Additionally, ensuring proper mold alignment and avoiding sharp corners reduces turbulence and air entrapment.
Injection and Packing Parameters
High injection speed can trap air, while low melt temperature reduces flowability, leading to incomplete cavity filling. Use of a two-stage injection profile—high speed for filling, followed by lower speed for packing—reduces shear heating and air entrapment. Adjusting back pressure (8–15 bar) enhances melt homogenization and removes entrapped gases. For critical applications requiring high clarity or strength, such as refrigerator plastic profiles, process consistency is paramount.
Preventive Quality Assurance and Process Control
Implementing statistical process control (SPC) and real-time monitoring (e.g., cavity pressure sensors, temperature probes) enables early detection of deviations. Routine mold maintenance, including vent cleaning and gate inspection, prevents long-term defect accumulation. Material traceability and supplier quality audits further support consistent production of high-quality injection molding parts.
Conclusion and Next Steps
Addressing shrinkage marks and bubbles in plastic injection molding requires a systematic approach involving material selection, process optimization, and tooling design. By focusing on root causes—thermal gradients, pressure profiles, and gas management—manufacturers can achieve defect-free components with reliable performance. For applications demanding precision and durability, such as custom plastic profiles for refrigerators and appliances, partnering with an experienced extrusion specialist ensures optimal material performance and dimensional accuracy. Dalang specializes in custom extrusion services for PVC, UPVC, ABS, PE, and other plastic materials, widely used in windows, doors, refrigerators, and various applications requiring precision plastic components.