The shrinkage that develops in thicker parts of a plastic mold part, such as ribs or protrusions, is more severe than in adjacent locations because the thicker areas cool much more slowly than the surrounding areas. Different cooling rates lead to the formation of depressions at the joints, known as shrink marks.

This defect seriously limits the design and molding of precision plastic mold products for home appliances, especially for large thick-walled products such as beveled cabinets for TV sets and monitor casings. In fact, shrinkage marks must be eliminated on products with strict requirements such as household appliances, while the existence of shrinkage marks is allowed for some products with low surface quality requirements such as toys.

There may be one or more reasons for the formation of shrinkage marks, including processing method, part geometry, material selection, and mold design. Among them, the geometry and material selection are usually determined by the raw material supplier and are not easily changed. But there are many other factors on the mold maker's side about mold design that can affect shrinkage. Cooling runner design, gate type, gate size can have a variety of effects.

What are the ways to reduce shrinkage in plastic mold processing?

For example, small gates such as tubular gates cool much faster than tapered gates. Premature cooling at the gate reduces fill time in the cavity, increasing the chance of shrinkage marks. For molding workers, adjusting processing conditions is one way to solve shrinkage problems.

Filling pressure and time significantly affect shrinkage. After the part is filled, excess material continues to fill the cavity to compensate for the shrinkage of the material. A filling stage that is too short will result in increased shrinkage and eventually more or larger shrinkage marks. This method by itself may not reduce the shrinkage marks to a satisfactory level, but the molder can adjust the filling conditions to improve the shrinkage marks.

In addition to the above methods, there is also modification of the mold, the simple solution is to modify the regular core hole, but it is impossible to make it suitable for all resins. In addition, the gas-assisted method is also worth a try.