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Tips to avoid sink mark in plastic injection molding
Avoiding sink marks entirely might be impractical in some cases, but here are some strategies to minimize their appearance and impact:
Remember:
The most effective approach often involves a combination of these strategies tailored to your specific part design, material, and production requirements.
Consult with experienced engineers and moldmakers to determine the best strategies for your project.
By implementing these strategies and considering advanced techniques when necessary, you can significantly minimize the appearance and impact of sink marks in your plastic injection molded parts.
Part Design:
1Minimize wall thickness variations:
Uneven cooling due to thick sections leads to sink marks. Aim for uniform wall thickness or use gradual transitions between thick and thin sections.
2Eliminate sharp corners:
Sharp corners create stress concentrations and encourage sink marks. Use rounded corners whenever possible.
3Design ribs and bosses strategically:
Place ribs and bosses effectively to add structural support and reduce sink marks in critical areas. Use the "seven degree rule" for rib angles to facilitate proper packing and avoid surface blemishes.
4Incorporate gates and vents:
Place gates strategically to ensure even material flow and pressure throughout the part. Use vents to allow trapped air to escape, preventing sink marks caused by air pockets.
Processing Parameters:
1Optimize injection pressure and packing pressure:
Higher pressures improve packing and reduce voids that lead to sink marks. However, balance pressure with potential gate freeze-off or warpage.
2Control melt temperature:
Overheating the melt can increase shrinkage and sink marks. Use the recommended temperature range for your chosen material.
3Adjust mold temperature:
Warmer mold temperatures can slow cooling and potentially reduce sink marks, but maintain temperatures within acceptable limits for your material and cycle time considerations.
4Control cooling time:
Longer cooling allows for better packing and reduces sink marks, but balance it with cycle time requirements. Consider implementing sequential or two-stage cooling.
Material Selection:
1Choose materials with low shrinkage:
Some materials naturally shrink less upon cooling, minimizing the risk of sink marks. Consider alternative materials if feasible.
2Use additives:
Certain additives can be incorporated into the material to reduce shrinkage and improve flow properties, potentially minimizing sink marks.
Advanced Techniques:
1Moldflow analysis:
Simulate the injection process to identify potential sink marks and optimize design and processing parameters before tooling.
2Vacuum-assisted molding:
This technique removes air trapped in the mold, reducing potential sink marks and improving part quality.
3Coinjection molding:
Injecting a core with a material less prone to shrinkage can minimize sink marks on the surface layer.
