Causes and Rectification Methods of Flash on Mold Parting Surface

Flash refers to excess thin plastic edge formed on the edge of plastic parts when molten plastic overflows from gaps on mold parting surface during injection molding. It is a common molding defect. Flash on parting surface increases trimming workload in post-processing and affects product assembly precision. Severe flash may be stuck between parting surfaces, causing mold indentation and wear of parting surface, shortening mold service life. The causes of flash cover raw materials, mold machining, assembly and injection molding processes. Systematic troubleshooting of root causes and corresponding rectification measures can control flash generation and ensure stable mass production of molds.

1. Manufacturing and Assembly Problems of Mold Parting Surface

Insufficient mold machining precision acts as the basic inducement of flash on parting surface. Machining deformation of cavity area and flatness deviation of parting surface lead to incomplete fitting after mold closing and form tiny gaps for melt overflow. In mold assembly, uneven clamping force of templates or excessive fitting clearance of positioning pins, guide pins and guide sleeves cause cavity dislocation during mold closing and partial gaps. After long-term mass production, repeated extrusion and friction on parting surface bring wear and pits, reducing fitting performance and resulting in continuous flash. Residual plastic scraps and metal impurities on parting surface will be pressed to form permanent overflow channels. Control flatness of parting surface in machining stage, lock templates evenly during assembly and verify fitting precision of guide and positioning components. Clean foreign matters on parting surface before production. Repair worn parting surface by grinding, surfacing and re-polishing.

2. Flash Caused by Insufficient Mold Clamping Force and Rigidity

Insufficient clamping force is a frequent reason for flash on parting surface. Melt generates huge cavity pressure in filling stage. When clamping force cannot resist cavity opening force, templates will be slightly pried open and melt overflows through gaps on parting surface. Insufficient template thickness and thin mold frame structure cause elastic deformation of templates under high injection pressure, partial opening of parting surface and flash. Unbalanced filling of each cavity in multi-cavity mold leads to excessive pressure of individual cavity and local parting surface opening. Calculate required clamping force and select injection molding machine meeting requirements. Appropriately increase clamping force, optimize mold frame structure by thickening templates or adding support pillars to improve overall mold rigidity. Adjust runner and gate layout to balance filling pressure of each cavity and avoid overload pressure in single cavity.

3. Influence of Unreasonable Injection Molding Process Parameters

Excessive injection speed creates instantaneous peak pressure in cavity and easily pries open parting surface. Too high injection pressure and packing pressure continuously feed melt into cavity and raise risk of melt overflow. High barrel temperature and mold temperature reduce melt viscosity and improve fluidity, making melt easier to seep into tiny parting gaps. Late packing switch time keeps cavity under high pressure and aggravates flash. Reduce injection speed to lower instantaneous filling impact, cut excessive packing pressure and switch packing position in advance. Appropriately reduce barrel and mold temperature within allowable material range to raise melt viscosity and reduce overflow tendency, while avoiding other defects such as short shot and sink mark brought by parameter adjustment.

4. Product, Gate and Runner Structural Factors

Large product wall thickness requires greater packing feeding volume, higher internal cavity pressure and higher possibility of flash on parting surface. Gates close to parting surface let high-pressure melt directly impact parting area and increase overflow risk. Excessively large runner section brings too much melt into cavity and lifts overall cavity pressure. Reduce local thick rubber positions in product structural design and add glue reduction structure for thick walls when necessary. Adjust gate position away from parting surface to reduce melt impact on parting area. Optimize runner size to limit melt entering cavity and control overall filling pressure.

5. Influence of Raw Material Characteristics

Raw materials with low viscosity have good fluidity and are easier to seep and form flash under the same parting gap. Soft plastics added with tougheners feature improved melt fluidity and higher overflow tendency compared with hard materials. Degraded raw materials show decreased melt viscosity and higher flash probability. Evaluate fluidity difference when replacing raw material grades and select raw materials with matched viscosity. Control material residence time in barrel to prevent high-temperature degradation. For high-fluidity materials, synchronously lower injection pressure and clamping pressure to match molding characteristics.

6. Daily Production Maintenance Control Points

Clean parting surface regularly during mass production to remove plastic scraps and dust and prevent foreign matters from damaging parting surface. Inspect wear condition of guide pins, guide sleeves and positioning pins regularly and replace accessories when clearance exceeds standard. Check fitting state of parting surface after mold trial or product replacement and confirm no foreign matters before mold closing. When continuous flash occurs, reduce process pressure for troubleshooting first. If flash is not caused by process problems, shut down machine to inspect fitting state of parting surface. Prefer low-cost process adjustment before mold repair to cut unnecessary mold maintenance cost.