Causes and Solutions of Internal Stress in Injection Molded Parts

Warpage, cracking, whitening, post-molding deformation and assembly jamming of injection molded parts are mostly caused by internal stress. Internal stress refers to residual stress inside products, formed when molecular chains are stretched and frozen during filling and cooling without sufficient relaxation. This paper analyzes the causes of internal stress from molding process, raw material, mold and post-treatment perspectives, and proposes practical improvement solutions.

1. Main Causes of Internal Stress

Internal stress is divided into flow-induced stress, uneven cooling stress, demolding stress and structural stress. During filling, high speed and pressure stretch molecular chains along the flow direction. If cooling proceeds too fast, molecular chains cannot relax completely, and flow stress remains inside finished parts. Thin-walled products, areas near gates and weld lines are high-risk positions for residual stress.

Uneven cooling is another major cause. Unbalanced water channels lead to different cooling speeds on product surfaces and inner layers. The hardened surface restricts the shrinkage of internal material, generating tensile stress on the surface and compressive stress inside. Products with uneven wall thickness and reinforced ribs will accumulate more internal stress. Low overall mold temperature also accelerates cooling and aggravates residual stress.

Improper process parameters will amplify internal stress. Excessively high injection pressure, holding pressure and long holding time create compressive stress inside products. High injection speed brings severe shear force and molecular orientation. Low barrel temperature leads to incomplete melting, higher filling resistance and increased flow stress.

Demolding and ejection processes may introduce mechanical stress. Insufficient draft angle, rough mold surface and strong ejection force will pull and deform products, resulting in surface whitening and cracking. In addition, amorphous plastics such as PC and PMMA are more likely to retain internal stress due to rigid molecular chains. Sharp corners and abrupt wall thickness changes in product design will form stress concentration points.

2. Methods to Relieve Internal Stress During Production

Optimizing molds and processes is the most effective way to reduce internal stress. Appropriately raise mold temperature for PC, PC/ABS and PMMA to extend molecular relaxation time. Optimize cooling channels to realize uniform heat dissipation and avoid local rapid cooling. For products with uneven wall thickness, separate cooling systems are adopted to narrow temperature differences.

Adjust injection parameters reasonably. Increase barrel and nozzle temperature to reduce melt viscosity and shear force. Lower injection pressure and holding pressure, and shorten holding time to avoid excessive compression. Use segmented injection speed: low speed at the gate, steady speed in the middle stage and slow speed at the end of filling. Optimize gate position and size to reduce filling resistance.

Improve mold structure and demolding system. Set standard draft angles and polish mold surfaces to reduce friction. Distribute ejector pins evenly and control ejection speed to avoid local stress. Use arc transitions for all sharp corners and smooth transitions for wall thickness in product design to eliminate stress concentration.

3. Post-Treatment for Residual Internal Stress

For high-precision transparent parts, electronic connectors and products for spraying, annealing treatment is widely applied to relieve residual stress. The principle is to heat products to a temperature between glass transition temperature and softening temperature, so that frozen molecular chains move and relax slowly. Then cool down to room temperature gradually.

Different materials have matched annealing parameters. PC and PC/ABS are annealed at 100°C–120°C for 1 to 4 hours according to wall thickness. PMMA is treated at 70°C–90°C for 1 to 3 hours. Crystalline plastics like PA and POM use a temperature slightly lower than heat deflection temperature. Slow cooling after heat preservation is required to prevent new stress. Warm water soaking is an alternative method for small thin-walled parts, featuring simple operation and good appearance protection. Polishing can remove surface whitening caused by stress, and professional testing tools are used to verify treatment results.

4. Daily Management and Notes

Strict raw material management is essential. High moisture content will cause bubbles and abnormal fluidity, and indirectly increase internal stress. Raw materials must be fully dried in accordance with requirements. Excessive recycled materials will disrupt stress distribution, so the proportion of recycled materials should be controlled, and processing temperature needs proper adjustment when recycled materials are added.

Internal stress cannot be completely eliminated. Set reasonable control standards based on product application scenarios. Regular sampling inspection and professional testing are carried out in mass production. Continuously optimize molds, processes and post-treatment plans to control defects caused by internal stress in the whole production process.