How to Fix Sink Marks on Injection‑Molded Parts: Dual Optimization of Process and Mold
Sink marks and local depressions are common defects of injection‑molded products, mostly appearing at thick‑wall positions, bosses and rib junctions. Light sink marks affect surface appearance, while serious sink marks bring insufficient wall thickness, abnormal assembly clearance and decline of local structural strength. Many production personnel only adjust injection process parameters to improve sink problems, but the improvement effect of parameter adjustment is limited. It is necessary to cooperate with mold structure optimization to adopt two‑way improvement ideas of process and mold, so as to take account of production efficiency and product quality and realize stable mass production.
1. Process adjustment points for sink mark improvement
Process adjustment is a quick emergency improvement method, suitable for trial mold and small‑batch temporary production. The combination adjustment of packing pressure, material temperature, mold temperature, cooling time and injection speed compensates the volume loss caused by plastic shrinkage. Increasing packing pressure is the most direct way to improve sink marks. Appropriately increasing packing pressure and extending packing time make melt continuously fill into the cavity during cooling to make up for the volume vacancy caused by shrinkage. However, packing pressure cannot be increased without limit. Excessive pressure will lead to flash, large internal stress and ejection deformation. The packing switch position should be set reasonably. Too early switch will lead to incomplete cavity filling, and packing pressure cannot be transmitted to thick‑wall positions. Barrel temperature should be controlled within the recommended range of raw materials. Too high temperature will increase plastic shrinkage rate, while too low temperature will reduce melt fluidity and cause early solidification of feeding channels. Mold temperature also needs reasonable control. Too high mold temperature increases shrinkage, and too low mold temperature will make the surface solidify rapidly, and internal thick‑wall positions still produce depressions due to insufficient feeding. Appropriately extending cooling time promotes full setting of thick‑wall areas and reduces post‑shrinkage after demolding. Moderately reducing injection speed avoids early surface sealing and blocking packing feeding channels.
2. Mold end optimization and rectification scheme for sink marks
When the process parameters have been adjusted to a reasonable limit and sink marks still cannot be eliminated, the root cause lies in mold design defects, and mold modification must be carried out. Gate and runner system determine whether packing pressure can be effectively transmitted to sink‑prone areas. Too small gate section will cause early condensation and sealing of the gate. Even high packing pressure cannot continue to feed materials into the product. It is necessary to appropriately increase gate and runner size, delay gate freezing time and ensure smooth feeding channels. Thick‑wall positions are high‑risk areas for sink marks. Try to reduce local thick rubber in product design and realize smooth transition of wall thickness. If the product structure cannot be modified, optimize cooling layout, add cooling measures for bosses and thick bosses, and accelerate cooling speed of thick‑wall areas. Poor mold venting will cause trapped gas and form pit defects similar to sink marks. It is necessary to widen and deepen the venting at the melt end and rib root, and add venting inserts. For unchangeable thick‑wall structures, material‑removing treatment can be carried out on the non‑appearance inner side to reduce local rubber volume and shrinkage, and attention should be paid not to cause new appearance indentation.
3. Auxiliary control of raw materials and production environment
Different grades of raw materials have different shrinkage rates. Raw material drying process cannot be ignored. Excessive moisture will produce surface pits similar to sink marks. Too high proportion of regrind will change material fluidity and shrinkage characteristics and aggravate sink defects. The mixing proportion of regrind should be stabilized in mass production, and raw material batches should be checked regularly. Excessive fluctuation of workshop ambient temperature will cause unstable mold temperature and intermittent sink marks. Keep the temperature of molding area stable and reduce the interference of external environment on mold temperature.
Conclusion
The improvement of sink marks of injection‑molded products cannot only rely on molding adjustment. Process adjustment can alleviate sink marks, but there are parameter upper limits. Excessive adjustment will induce flash, warpage, stress cracking and other secondary defects. In actual production, process debugging should be carried out first. If sink marks still exist, mold rectification should be carried out from gate runner, wall thickness, cooling water channel and mold venting. Cooperate with raw material and production environment management, adopt two‑way optimization of process and mold, fundamentally solve sink defects, and ensure that product appearance, size and structural strength meet mass‑production standards.
