Precision injection molded parts frequently suffer from warpage, dimensional deviation, stress cracking, and cracking after spraying and electroplating, which are mostly caused by residual internal stress. Internal stress forms when molecular chains are forcibly stretched and squeezed during melt flow and cooling shrinkage, then frozen and solidified without sufficient relaxation. It poses the most prominent impact on precision matching parts, transparent appearance parts, and thin-walled structural parts. To effectively control and eliminate internal stress, it is not feasible to rely solely on post-remediation. A full-process collaborative optimization covering product structure, mold design, injection molding process, post-molding treatment, and daily management is required to systematically reduce residual stress and ensure the dimensional stability and service life of precision injection parts.
1. Core Causes of Internal Stress in Molded PartsThe formation of internal stress in precision injection parts results from the superposition of flow shear, shrinkage difference, and external force. When melt passes through narrow gates, the flow velocity increases sharply, forcing molecular chains to stretch and orient. If the cooling speed is too fast, the molecules cannot rebound and relax, forming orientation stress. Uneven wall thickness of product structure leads to inconsistent cooling and shrinkage rhythms in adjacent positions, generating tensile shrinkage stress.
Unreasonable layout of mold cooling water channels causes inconsistent heat dissipation between cavity and core, further enlarging the shrinkage difference between the surface and inner layers of the molded parts. Meanwhile, the thermal expansion characteristics of metal inserts differ greatly from plastics. Without pretreatment, concentrated stress is easily formed around inserts. Insufficient demolding draft angle and uneven ejection force will also introduce mechanical forced internal stress at the moment of ejection. The superposition of multiple factors eventually forms permanent residual internal stress that cannot dissipate spontaneously.
2. Optimization of Product Structure DesignOptimizing structural design at the source is the most cost-effective way to reduce internal stress. The conventional wall thickness of precision molded parts is controlled within 1.2～3.0mm, and the wall thickness difference at adjacent positions is strictly limited to within 0.5mm to avoid unbalanced cooling and shrinkage caused by sudden thickness changes. Arc transition is adopted at all corner positions to abandon right-angle and sharp-angle shapes, fundamentally avoiding stress concentration. When adding reinforcing ribs to products, the thickness of ribs is set to about 60% of the main wall thickness, which not only meets the structural rigidity requirements but also prevents shrinkage marks and internal stress accumulation caused by excessively thick ribs. For precision molded parts with metal inserts, the thickness of plastic coating around inserts should not be less than 1.5mm to weaken tensile stress caused by the difference in thermal expansion coefficient between metal and plastic. The structural layout should be symmetrical and balanced as much as possible, avoiding slender cantilevers, large-area flat plates and other shapes that easily accumulate stress and induce warpage.
3. Rational Optimization of Mold StructureThe mold is the key to controlling melt flow and cooling rhythm, directly determining the basic level of internal stress of molded parts. The main runner and sub-runner of the mold adopt the conventional specification of 6～8mm, and the gate thickness is set to 50% of the product wall thickness. Appropriately enlarging the injection section slows down the melt flow velocity and reduces orientation stress generated by high-speed shearing. The spacing between cooling water channels and the cavity surface is maintained at 20～30mm, and the temperature difference between front and rear molds during production is controlled within 2℃ to ensure synchronous overall heat dissipation and cooling of molded parts and avoid shrinkage stress caused by inconsistent local cooling speed. The demolding draft angle of precision appearance parts is set above 1°, the cavity surface is polished to high-gloss mirror finish, and the ejection structure is arranged evenly to ensure smooth demolding of molded parts, eliminating whitening and deformation caused by forced demolding and artificially induced mechanical internal stress.
4. Precise Debugging of Injection Molding Process
The injection molding process is the core means to quickly regulate internal stress on site. Reasonable matching of parameters can greatly reduce residual stress during molding. Taking the common ABS material as an example, the production mold temperature is stably controlled at 60～80℃. Appropriately increasing the mold temperature can slow down the cooling speed of the part surface, leaving sufficient relaxation time for molecular chains and preventing the oriented structure from being frozen quickly. The injection speed of precision thin-walled parts is maintained at 30～60mm/s, adopting a medium-low speed stable injection mode to weaken shear friction during melt flow. The cooling time is reserved for 8～12 seconds per millimeter of wall thickness to ensure sufficient shaping of the thick-walled core of the molded part and complete the shrinkage process slowly, restraining the generation of internal stress to the greatest extent from the molding link. Meanwhile, the holding pressure and holding time are reasonably regulated, adopting a segmented decreasing holding mode to prevent excessive holding pressure from excessive extrusion of molecular chains and stress accumulation.
5. Stress Relief Treatment After MoldingFor molded parts with potential stress hazards, stress relief can be carried out by annealing and moisture conditioning. Conventional ABS parts can be placed in a constant temperature oven at 70～75℃ for 1.5～2 hours, allowing plastic molecules to relax slowly at a suitable temperature. After heat preservation, cool down slowly with the oven instead of taking them out for sudden cooling, preventing new stress generated by temperature difference. Hygroscopic plastic parts such as nylon are suitable for hot water moisture conditioning treatment. Moisture plays a plasticizing role to weaken intermolecular forces, which can not only effectively eliminate residual internal stress but also stabilize the subsequent moisture absorption deformation of parts and ensure long-term dimensional stability. Avoid secondary processing such as forced assembly and violent polishing of finished products to prevent external forces from destroying the molecular structure and inducing new hidden stress hazards.
6. SummaryThe control and elimination of internal stress in precision injection molded parts adhere to the core idea of source prevention supplemented by post-remediation. It is impossible to rely solely on adjustment of a single link. Full cooperation of product structure, mold design, injection molding process and post-treatment process is necessary. Reasonable wall thickness design fundamentally avoids stress concentration; optimized mold water channel and gating system balance melt flow and cooling rhythm; accurate injection process parameters reduce molecular orientation and shrinkage difference during molding; standardized annealing and moisture conditioning can effectively release residual stress of formed parts. Strictly following various practical standards and key parameters in production and controlling every production link can keep the internal stress of precision injection parts within the qualified range, thoroughly solve common quality problems such as warpage and deformation, dimensional out-of-tolerance and stress cracking, and improve the qualified rate of products and long-term service stability.
