Preventive Measures for Stress Cracking in PC Plastic Injection Molding
Polycarbonate (PC) features outstanding rigidity and high light transmittance, yet its rigid molecular chains make molded components highly susceptible to residual internal stress. Under external mechanical force, solvent contact or drastic temperature shifts, visible whitening, micro-cracks and complete fracture frequently occur, especially at thin-walled sections, sharp corners, metal insert junctions and wall thickness transition zones. Residual stress originates from five core dimensions: mold structural design, injection molding parameters, raw material control, post-molding treatment and service environment. This paper systematically elaborates targeted preventive measures for each root cause.
Mold Structure Optimization to Eliminate Stress Concentration at Source
Remove sharp angles and implement fillet transition above R0.5
All internal and external product corners, rib roots, snap fit bases and screw boss roots must avoid right-angle structures, as sharp edges form severe stress concentration points that generate micro-cracks immediately after cooling. Continuous arc transitions shall be adopted for all turning sections, with enlarged fillets at snap fits and screw column roots to disperse shrinkage stress evenly.
Maintain consistent wall thickness and eliminate abrupt thickness disparity
Wall thickness differences exceeding twice the nominal value trigger uneven shrinkage, with thick sections retaining massive tensile internal stress due to slow cooling. Nominal wall thickness shall be controlled between 1.5mm and 4mm; material removal hollowing shall be applied to thick zones, and slope gradient transition shall replace step-like thickness changes.
Optimize gate layout to reduce shear-induced residual stress
Narrow side gates and pinpoint gates directly facing thin walls or cosmetic surfaces are forbidden, as high-speed and high-pressure melt filling generates intense shear stress around gate positions. Fan gates and large direct sprue gates are prioritized, with enlarged gate thickness and width to lower melt shear rate; gate locations shall avoid mechanically stressed areas including snap fits and screw columns.
Refine ejection and core-pulling systems to prevent demolding stress
Insufficient ejector pins, concentrated ejection force and undersized ejector pins lead to ejection whitening and cracking. Ejector pin diameters shall be enlarged with uniform distribution; ejector sleeves and stripper plates shall be configured for synchronized ejection. Clearance between sliders and core-pulling components shall be precisely calibrated to reduce surface stress caused by tensile demolding.
Distribute cooling channels rationally to minimize mold temperature gradient
Localized temperature discrepancy across mold cavities creates inconsistent shrinkage rates and residual stress. Separate temperature control shall be configured for cavities and cores; cooling channels shall be laid close to molded surfaces, with independent cooling circuits added at screw bosses and thick-wall areas to achieve uniform mold temperature distribution across the entire tool.
Raw Material Drying Control to Avoid Hydrolysis-Induced Brittleness and Cracking
Strict dehumidifying drying with standardized moisture content limits
PC resin readily absorbs atmospheric moisture; water molecules trigger molecular chain hydrolysis under high melt temperature, drastically reducing material toughness so minor stress can induce cracking. Standard drying parameters are 120°C to 130°C constant temperature for 4 to 6 hours, with dehumidifier dew point maintained below -40°C and melt moisture content limited within 0.02%.
Stabilize regrind proportion and prohibit excessive recycling
Multiple crushing and reprocessing degrade PC molecular weight and anti-cracking performance. Virgin resin shall serve as the primary raw material, with regrind proportion capped at 20%; uniformly sized crushed pellets are required to prevent localized overheating and degradation from fine powder. Yellowed and carbonized regrind shall be fully screened out and excluded from production batches.
Injection Molding Parameter Adjustment to Reduce Formed Residual Stress
Elevate barrel temperature to lower melt viscosity and shear intensity
Low melt temperature produces high-viscosity fluid that demands excessive injection pressure and speed for cavity filling, generating irreversible shear stress. Barrel temperature shall be set within 270°C to 310°C, with gradual temperature increase at low-temperature ranges to achieve homogeneous melt flow and reduce required injection pressure.
Raise mold temperature to extend molecular relaxation duration
Cold mold surfaces rapidly freeze the outer melt layer while internal resin continues contracting, creating severe tensile stress on component surfaces. Mold temperature shall be controlled at 80°C to 110°C, with a maximum of 120°C for thick-wall products. Elevated mold temperature prolongs molecular relaxation time and substantially releases residual forming stress.
Reduce injection pressure and adopt moderate filling speed
High-pressure, high-speed filling forces polymer chains into compressed orientation and accumulates persistent residual stress. Segmented medium-low injection speed is implemented with minimized filling pressure; only mild packing pressure is applied in the final filling stage to avoid over-compaction and internal stress accumulation.
Shorten holding duration and lower packing pressure
Prolonged high-pressure packing continuously replenishes melt into the cavity and locks compressive stress inside molded parts. Packing shall terminate immediately upon gate solidification; multi-stage gradient packing is adopted for thick components to mitigate internal compressive stress.
Extend cooling cycle for complete uniform shaping
Insufficient cooling leaves semi-molten inner sections that sustain secondary shrinkage and cracking after ejection. Cooling time shall be moderately prolonged without sacrificing throughput to guarantee full molecular relaxation and shaping, eliminating post-demolding shrinkage stress.
Post-Molding Annealing Treatment: The Most Effective Stress Relief Remedy
Standardized oven annealing parameters
Constant-temperature oven annealing shall follow the rule of 30 minutes holding time per 1mm wall thickness at 110°C to 120°C. Thin miniature parts require 30 to 60 minutes insulation, while thick screw structural components demand over 2 hours of heat preservation.
Regulated heating and cooling curves to avoid thermal shock
Temperature rise rate shall not exceed 15°C per hour; finished components shall cool down naturally inside the furnace to ambient temperature after annealing. Direct exposure of hot molded parts to cold air is prohibited, as drastic temperature variation generates additional stress micro-cracks.
Mandatory application scope
Annealing procedures are compulsory for snap-fit components, threaded housings, transparent casings, parts with metal inserts, and workpieces requiring subsequent spraying, bonding or chemical contact. Ordinary non-load-bearing cosmetic parts may selectively skip annealing according to production specifications.
Protection Measures for Inserts, Post-Processing and Service Environment
Preheat metal inserts to eliminate interfacial stress
Metal substrates transfer heat far faster than PC resin; massive interfacial tensile stress forms as molten PC shrinks around cold inserts during cooling. Metal inserts shall be preheated to approximately 100°C before mold loading to narrow temperature disparity between metal and plastic and prevent cracking surrounding embedded hardware.
Avoid secondary stress during post-processing operations
High-speed cutting and forced extrusion shall be avoided during drilling, tapping and trimming. Sharp cutting tools reduce tensile surface micro-cracks generated by machining; short secondary annealing shall be implemented after cutting to release processing-induced stress.
Isolate components from corrosive solvent environments
PC parts with residual stress rapidly crack upon contact with ethanol, acetone, industrial detergents and lubricating oil. Only neutral water-based mold release agents are permitted in production; finished products shall be isolated from organic solvents, and cleaning operations shall utilize pure water or dedicated neutral detergents only.
Stabilize service environment to prevent extreme temperature cycling
Rapid alternation between high and low ambient temperatures amplifies latent residual stress and gradually propagates invisible micro-cracks into visible fractures. Long-term exposure of PC components to drastic thermal fluctuation shall be avoided.
Rapid Troubleshooting Workflow for Stress Cracking Defects
When stress whitening or cracking occurs on molded PC parts, raw material drying status shall be verified first, followed by mold temperature elevation and reduction of injection and packing pressure for short-term defect mitigation. Permanent batch resolution requires synchronous optimization of mold fillet design, wall thickness consistency and gate layout. Comprehensive control covering raw materials, mold structure, molding parameters and post-treatment eliminates stress cracking defects fundamentally and reduces scrap rates.
