Excessive internal stress in injection molding and its solution
Injection molding internal stress is generated during the molding process due to molecular orientation and uneven cooling. Excessive internal stress can lead to defects such as cracking, warping, and dimensional instability in the part, and can even cause failure due to stress release during use. Internal stress is primarily categorized as orientation stress, cooling stress, and shrinkage stress. Orientation stress originates from the shear flow of the melt in the runners and cavity, which aligns the molecular chains along the flow direction. Cooling stress is caused by differences in cooling rates across the part, leading to uneven shrinkage. Shrinkage stress arises from the constraints of the part and the mold, and is particularly pronounced in parts with complex structures. Accurately identifying the type and cause of internal stress is a prerequisite for effective solutions.
Optimizing process parameters is the primary measure for reducing internal stress. By adjusting injection pressure, holding pressure, melt temperature, and cooling time, molecular orientation and cooling uniformity can be significantly improved. Excessively high injection pressure can exacerbate molecular orientation and increase orientation stress. Therefore, the injection pressure should be appropriately reduced (e.g., from 150 MPa to 120 MPa), while the injection speed should be increased to shorten the filling time and minimize shear. The holding pressure and holding time must match the shrinkage characteristics of the plastic part. For crystalline plastics (such as PP), a “high pressure, short time” holding strategy (e.g., 80 MPa, 5 seconds) can be used to avoid excessive shrinkage and the accumulation of internal stress. For amorphous plastics (such as PC), a “low pressure, long time” holding strategy (e.g., 50 MPa, 10 seconds) is required to allow sufficient time for molecular chains to relax. Increasing the melt temperature (e.g., from 280°C to 300°C for PC) can reduce melt viscosity, reduce shear stress, and thus reduce orientation stress. However, excessive temperature should be avoided, as it may cause material degradation.
Improving mold structure plays a crucial role in reducing internal stress, with a focus on optimizing gate design, cooling systems, and demolding mechanisms. The gate location should ensure a uniform melt flow path within the mold cavity, preventing melt impact on the cavity wall and localized stress. For example, for large, flat parts, a center gate can reduce flow stress more effectively than an edge gate. The gate size should match the part thickness. For a 3mm thick part, a 1mm diameter point gate can be used to ensure melt filling at a low shear rate. The cooling system must ensure consistent cooling rates across the part. For parts with uneven wall thickness, denser cooling channels (e.g., 10mm spacing) should be installed in thicker areas, and the channel spacing should be increased (e.g., 20mm) in thinner areas to keep the overall temperature difference within 5°C. The demolding mechanism should avoid forced demolding. The ejector should be located in a rigid area of the part, with an ejection speed controlled at 5-10mm/s to prevent additional stress during demolding.
Pretreatment and formulation adjustments of plastic raw materials can improve material flow and crystallization properties, thereby reducing internal stress. Highly hygroscopic plastics (such as PA6 and PC) require thorough drying (PA6: dry at 120°C for 4 hours) to keep the moisture content below 0.05% to prevent moisture from causing bubbles and stress concentration during the molding process. Adding an appropriate amount of plasticizer (such as DOP for PVC) to the raw materials can reduce intermolecular forces and orientation stress, but the dosage should be controlled within 5%-10%. Excessive amounts can affect part strength. For crystalline plastics, nucleating agents (such as talc for PP) can be added to promote uniform crystallization and reduce shrinkage stress caused by uneven crystallization. Furthermore, plastic grades with a broad molecular weight distribution (such as HDPE with a molecular weight distribution index of 10-20) exhibit better flowability than those with a narrower distribution, reducing shear stress during molding.
Post-processing of plastic parts is an effective supplemental method for eliminating internal stress. Heating relaxes the molecular chains and releases internal stress. Annealing is the most common method. The plastic part is placed in an oven at a temperature 10-20°C below its heat deformation temperature for 2-4 hours, then slowly cooled to room temperature. For example, a PC plastic part with a heat deformation temperature of 130°C can be annealed at 110°C for 3 hours, achieving a 60%-80% internal stress elimination rate. For parts requiring high dimensional accuracy, a stepwise annealing process can be used, gradually reducing the temperature (e.g., 20°C per hour) to avoid the generation of new stresses due to rapid cooling. Alternatively, boiling water bath annealing (100°C) can be used for small parts. This is simple and cost-effective, but the processing time is extended to 4-6 hours. After post-processing, the plastic part should be tested for internal stress (e.g., using a polarimeter to observe stress striations) to ensure that internal stress is reduced to an acceptable level (striation level ≤ Level 2) to ensure the stability of the part during use.
