Causes and Elimination Methods of Internal Stress in Chinese Injection Molded Products
During injection molding, plastic melt is cooled and solidified rapidly from a high-temperature molten state. Molecular chains are frozen inside the product without sufficient relaxation, forming internal stress. Internal stress may cause deformation and cracking of Chinese injection molded products during storage, assembly or high-temperature environment, and reduce solvent resistance. It has a prominent impact on precision appearance parts, transparent parts and assembly buckles widely manufactured in Chinese injection molding factories. Internal stress is divided into orientation stress, cooling stress and ejection stress. Different types of stress have different root causes, which need to be investigated and improved from multiple dimensions including product design, mold, process and post-treatment.
1. Internal stress induced by product structural design
Uneven product wall thickness, sharp corners and abrupt wall thickness changes are common sources of internal stress. Thick-wall areas cool slowly while thin-wall parts solidify quickly. The inconsistent shrinkage rate in different regions produces uneven shrinkage stress. Without sufficient fillets at product corners, molecular chains are forcibly stretched at sharp corners and retain high orientation stress after cooling. Structures such as buckles and long thin ribs make the melt stretched during molding, and molecular chains are arranged directionally, which easily accumulates oriented internal stress. Stress concentration is more obvious around product holes and inserts. The shrinkage rate difference between metal inserts and plastics causes asynchronous shrinkage of the two materials in the cooling stage and continuously generates stress around inserts. Rectification ideas: Optimize product wall thickness to keep uniform thickness and reduce wall thickness mutation. Add fillet transitions at all corners to avoid sharp corners. Preheat inserts to narrow the temperature difference between plastics and metal parts. Reduce the design of excessively long and thin ribs to lower the stretching degree of melt.
2. Internal stress inducement related to mold design
Gate type, gate position and runner layout directly affect melt flow orientation and become an important source of orientation stress. Small-section gates subject melt to strong shear when passing through gates. Elongated and oriented molecular chains retain a large amount of shear stress after cooling. If the gate is set at the stressed area of the product, high orientation stress concentrates here, and the product is prone to cracking during use. Uneven distribution of mold cooling channels leads to large mold temperature differences in different cavity positions. The cooling speed of each part of the product varies and forms uneven cooling stress. Insufficient mold draft angle and poor mold surface polishing cause forced pulling of products during ejection and generate ejection stress. Poor exhaust leads to local short shot and high-pressure feeding, further amplifying internal stress accumulation. Rectification ideas: Enlarge the gate section, adopt low-shear gates such as fan gates and film gates, and avoid single-point small gates. Adjust the gate position to stay away from stressed and bent areas of products. Optimize cooling channels to balance the temperature of cavity and core. Design reasonable draft angle and polish the molding surface to reduce ejection friction resistance. Optimize exhaust to reduce stress accumulation caused by local high pressure.
3. Internal stress caused by injection molding process parameters
Process parameters are the most direct link for on-site commissioning to control internal stress. If the melt temperature is too low, poor fluidity of plastics requires higher injection pressure to push melt flow. Strong shear brings plenty of orientation stress. Excessively high melt temperature degrades molecular chains, reduces material toughness and weakens stress release capacity. Excessively high injection pressure and packing pressure compact the cavity. The melt shrinkage in the cooling stage is restricted, and huge internal stress is frozen. Too fast injection speed leads to violent melt shear and rapid orientation of molecular chains. Too slow speed causes cooling of the melt front, requiring higher pressure to complete filling and raising stress level. Low mold temperature makes the melt contact the mold wall and cool instantly. Molecular chains have no enough time to relax and are fixed, resulting in significantly increased internal stress. Unreasonable cooling time also brings risks. Insufficient cooling leads to ejection before the product core cures and introduces new stress by ejection deformation. Overlong cooling makes the product keep pressed inside the mold and stress cannot be released. Rectification ideas: Appropriately raise barrel temperature to improve melt fluidity and reduce injection pressure demand. Reduce excessive packing pressure and shorten packing time to cut compression stress inside the cavity. Adopt segmented injection speed to lower shear strength near gates. Raise mold temperature to extend the relaxation time of molecular chains. Match reasonable cooling cycle to avoid premature ejection.
4. Influence of raw materials on internal stress
Different plastic raw materials have distinct molecular structures and different sensitivity to stress. Amorphous materials such as PC, PS and PMMA are more prone to residual internal stress, while crystalline materials such as PP and POM have relatively low risk of stress cracking. Inadequate drying of raw materials causes hydrolysis of materials under high temperature, breaks molecular chains and reduces material toughness. Products are more likely to suffer stress cracking under the same molding conditions. Improper proportion of tougheners and fillers leads to obvious anisotropic stress for glass fiber reinforced materials because fibers and molecules orient along the flow direction. Batch fluctuation of raw materials and difference in fluidity cause unstable internal stress of products produced by one set of mold. Rectification ideas: Strictly follow raw material drying specifications to remove moisture. Select appropriate material grades according to product application scenarios. Optimize processes for glass fiber reinforced products to reduce shear orientation. Fix raw material grades and suppliers to reduce stress difference caused by batch fluctuation.
5. Post-molding methods to eliminate internal stress
Even with optimized front-end design and processes, post-treatment is still required for some precision Chinese injection molded products to release residual internal stress. Annealing treatment is the most commonly used method. Place products in a constant temperature environment below the material heat distortion temperature for heat preservation, then cool down slowly. Frozen molecular chains relax gradually to release residual internal stress. The annealing temperature is generally controlled 5 to 20℃ below the heat distortion temperature. Too high temperature deforms products, while too low temperature leads to poor stress release efficiency. The holding time is adjusted according to product thickness. Thicker products require longer holding time. Rapid cooling is forbidden in the cooling process to prevent new stress. Solvent fumigation can quickly detect internal stress and serve as a stress elimination method for small-batch products, but it carries the risk of solvent residue and is not applicable to food and medical products. Natural aging at room temperature means products are placed at room temperature for a long time for slow stress release, which is only suitable for low-demand products with low efficiency.
To sum up, the control of internal stress for Chinese injection molded products follows the principle of prevention first and post-treatment as supplement. Prioritize reducing stress generation from the source of product structure and mold design, then lower shear stress and cooling stress generated during molding by optimizing injection processes, and finally eliminate residual stress through annealing and other post-treatment means. Solvent immersion and polariscope inspection can be adopted in production to test internal stress and evaluate improvement effect, reducing bad problems such as assembly cracking, environmental stress cracking and dimensional drift, and improving the long-term service stability of injection molded products manufactured in China.
