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Method for improving shrinkage marks in plastic mold processing
In the thicker parts of the plastic mold, the shrinkage formed at the ribs or protrusions is more serious than that of the adjacent parts. This is because the cooling rate of the thicker area is much slower than that of the surrounding area. The difference in cooling rate results in the formation of depressions at the connecting surface, which are known as shrinkage marks. This defect severely restricts the design and molding of precision plastic mold products for household appliances, especially large thick-walled products such as beveled cabinets and display enclosures for televisions.
There may be one or more reasons for the formation of shrinkage marks, including processing methods, part geometry, material selection, and mold design. The geometry and material selection are usually determined by the raw material supplier and are not easy to change. However, there are many factors related to mold design on the mold manufacturer's side that may affect the shrinkage. Cooling runner design, gate type, and gate size may have multiple effects. For example, small gates such as tubular gates cool much faster than tapered gates. Premature cooling at the gate will reduce the filling time in the cavity, thereby increasing the chance of shrinkage marks. For molding workers, adjusting the processing conditions is a way to solve the shrinkage problem. Filling pressure and time significantly affect shrinkage. After the part is filled, the excess material continues to be filled into the cavity to compensate for the shrinkage of the material. Too short filling stage will increase shrinkage and eventually produce more or larger shrinkage marks. This method itself may not reduce the shrinkage mark to a satisfactory level, but the molding worker can adjust the filling conditions to improve the shrinkage mark.
In addition, to modify the mold, the simple solution is to modify the conventional core hole, but it is impossible to make it suitable for all resins. In addition, the gas-assisted method is also worth trying.