Formation Mechanism of Flash on Plastic Parts and Parting Surface Control in China
1. Core Formation Mechanism of Flash
Flash is excess plastic edge formed by melt overflowing from gaps such as parting surface, insert gaps and ejector pin holes and cooling. Many production personnel simply attribute flash to excessive pressure, but flash is the superposition result of material fluidity, mold fit state, clamping force and molding process. When the pressure inside the cavity exceeds the sealing capacity of the parting surface, the melt will squeeze into the gap to form flash.
The gap is objective caused by mold processing, assembly and wear. As long as the gap is smaller than the critical overflow gap of the material under corresponding process, flash will not occur. Low-viscosity materials such as PP and PE have good fluidity, small critical gap and are easier to produce flash. Glass fiber reinforced materials have high viscosity, but hard fillers will accelerate the wear of parting surface, and the flash risk will rise rapidly in the later stage. Higher melt temperature, faster injection speed and higher packing pressure will improve the fluidity of melt and increase the probability of flash under the same gap. Insufficient clamping force will cause the mold to be pried open during high-pressure filling, which is the most common inducement of batch flash. Local deformation of parting surface and local heat accumulation will also reduce material viscosity and cause local glue overflow.
2. Control Points of Parting Surface in Design Stage
The structural form of parting surface determines the sealing capacity. Plane parting has simple processing and uniform sealing, but has high requirements for template rigidity and uniform clamping force distribution. Inclined and curved parting surfaces have higher requirements for processing and fitting precision. The sealing band of the parting surface shall be reasonably planned, close to the edge of the cavity, with sufficient bearing width to avoid excessive local pressure and collapse of the sealing band. Sharp corners and narrow grooves around the cavity shall be avoided, because stress concentration will occur at sharp corners and form permanent gaps after mold closing. For large-area parting surfaces, support blocks and pressure-bearing blocks shall be added away from the cavity to share the locking pressure and prevent the middle of the template from arching. The parting surface layout shall be symmetrical to ensure that the clamping force can evenly cover the cavity area. The depth of exhaust groove must match the material characteristics. Too deep exhaust groove will directly become an overflow channel and form strip flash along the exhaust direction.
3. Control in Mold Processing and Fitting Stage
The flatness and roughness after processing are the basic indicators of parting surface. Residual tool marks and local depressions will cause poor fitting and dot or linear flash. Fine fitting shall be carried out after CNC processing, and red lead test is used to check the fitting rate. The coloring of the sealing band around the cavity shall be continuous and uniform. The parting surface of inserts, sliders and core pulling positions is the weak point. The fit gap of inserts shall be controlled within the critical overflow gap of the material. The flatness of the parting surface shall be rechecked after assembly. Bumping and scratching during transportation and assembly will form permanent gaps and continuous flash in subsequent production. Steel grade selection shall match the output. High hardness mold steel is selected for mass production molds to reduce deformation and wear speed of parting surface.
4. Parting Surface Verification and Flash Troubleshooting in Trial Mold Stage
During the initial trial mold, the matching between clamping force and product projection area shall be confirmed first. The theoretical value of clamping force calculation is the lower limit, and safety margin shall be reserved in actual production. Conservative processes such as low injection speed and low packing pressure shall be adopted first, and parameters shall be gradually increased to observe the position and timing of flash. If flash still appears under low pressure and low speed, it indicates that there is a gap defect in the mold itself, and the fitting of parting surface and insert gap shall be handled first instead of continuously reducing process parameters. The flash position shall be marked, pressure-bearing blocks shall be added or the sealing band layout shall be adjusted, and the exhaust groove depth shall be rechecked.
5. Parting Surface Maintenance Control in Mass Production
During mass production, the parting surface will be impacted by repeated opening and closing, high temperature and high pressure, and slowly worn by plastic fillers, leading to the increase of gap and gradual rise of flash rate. Regular inspection mechanism shall be established according to production shots. The flash and dust on the parting surface shall be cleaned regularly, because debris trapped between parting surfaces will cause incomplete mold closing and batch flash. The inspection focuses on the edge of cavity, slider parting surface and insert joint. Slight wear can be repaired by polishing and fitting, while collapsed deformation needs welding repair or reprocessing. When sudden flash occurs in production, it is necessary to check whether foreign matters are clamped on the parting surface first, then verify clamping parameters, melt temperature and mold temperature, and judge whether it is mold wear after eliminating process fluctuation.
6. Process Supporting Control to Reduce Bearing Load of Parting Surface
Process control is a supplement to parting surface control, and cannot be used to compensate for inherent gap defects of molds. Multi-stage injection speed is reasonably set to reduce the peak pressure of cavity at the end of filling. Segmented packing pressure is adopted to avoid long-term high-pressure holding opening the parting surface. Raw materials shall be fully dried to reduce local pressure rise caused by trapped gas. For materials with good fluidity, the melt temperature and mold temperature can be appropriately reduced on the premise of meeting filling and appearance requirements to improve melt viscosity and reduce overflow tendency. Flash control is a continuous process from design, processing, trial mold to mass production maintenance. The core of parting surface management is to control gaps, ensure uniform pressure bearing and delay wear. Mold structure and precision are fundamental, and process is auxiliary. The coordinated control of both can stably inhibit melt overflow, reduce flash defects and lower post-processing cost.
