Methods to Reduce Internal Stress in Injection Molded Products
In the injection molding process, internal stress in products is a key hidden hazard affecting part quality and a common technical challenge in the industry. Internal stress is essentially residual stress caused by forced orientation and freezing of molecular chains, uneven thermal contraction, and external extrusion during melt filling, packing, and cooling. Although invisible to the naked eye, such stress directly leads to cracking, warpage, whitening, deformation, and dimensional instability, severely reducing mechanical properties, appearance quality, and service life, and even causing mass rejection and increased production costs. To fundamentally reduce internal stress, adjustments to a single process are insufficient. A systematic approach of “prevention, process control, and post-elimination” must be adopted, covering raw material pretreatment, mold optimization, injection parameter tuning, and post-molding treatment. Through refined full-process control, internal stress can be effectively suppressed and relieved, ensuring the stability and qualification rate of injection molded parts.
1. Raw Material Selection and Pretreatment: Reducing Internal Stress at the Source
Raw materials form the foundation of injection molding, and their properties and pretreatment directly determine the likelihood of internal stress. High-rigidity, low-flow resins tend to create greater molecular orientation stress. Manufacturers should prioritize grades with good flow, moderate toughness, and uniform shrinkage, and avoid mixing incompatible materials or recycled resin with excessive degradation. For hygroscopic materials such as PA, ABS, and PC, sufficient drying is essential to prevent hydrolysis, uneven flow, and localized stress concentration caused by moisture. Strictly controlling the proportion of recycled material and ensuring uniform mixing also helps avoid additional stress from inconsistent shrinkage.
2. Mold Design Optimization: Eliminating Stress Concentration
The mold is the primary carrier of molding, and its structure directly influences stress formation. Gate location and size should avoid sharp corners, thin sections, and stress-concentrated areas to reduce excessive shear. Runner systems should be short, thick, and smoothly radiused to minimize flow resistance. Uniform wall thickness and rounded transitions prevent uneven cooling and stress peaks. A well-balanced cooling system ensures consistent temperature across the cavity and core, avoiding thermal stress from rapid local cooling. Proper draft angles and polished surfaces reduce friction during ejection, preventing deformation and additional stress from forced demolding.
3. Precision Injection Parameter Adjustment: Controlling Stress During Molding
Process parameters are the most direct way to control internal stress. Reducing injection speed and pressure minimizes shear-induced molecular orientation. Moderately lowering packing pressure and time avoids over-compression and residual pressure. Increasing barrel and mold temperatures improves melt flow and allows molecular chains to relax before freezing, especially for crystalline materials. Controlling screw speed and backpressure prevents excessive shearing, thermal degradation, and intensified molecular orientation. Stable cooling and cycle times further reduce warpage and residual stress.
4. Post-Molding Treatment: Fully Releasing Residual Stress
For complex, thick, or stress-prone parts, annealing effectively releases frozen orientation and thermal stress. Parts are heated to slightly below their heat distortion temperature, held for a period, then slowly cooled. Hygroscopic materials such as nylon benefit from moisture conditioning to balance water content, relax molecular chains, and reduce internal stress. These treatments significantly improve dimensional stability and resistance to cracking.
Conclusion
Controlling internal stress in injection molded products requires systematic coordination across material preparation, mold design, processing parameters, and post-treatment. By implementing standardized controls based on material properties, product structure, and equipment performance, manufacturers can effectively eliminate defects such as cracking, warping, and whitening, improve qualification rates, reduce costs, and support stable and efficient production.
