Shrinkage is the enemy of plastic processors, especially for large plastic products with high surface quality. Contraction is a stubborn disease. Therefore, various technologies have been developed to minimize shrinkage and improve product quality. The thicker parts of injection-Injection moulded plastic parts (such as ribs or protrusions) shrink more severely than the adjacent ones because the thicker areas cool much slower than the surrounding areas. The difference in cooling rate will cause the surface of the joint to sink, which is a common sign of shrinkage. These defects severely limit the design and Injection moulding of plastic products, especially large thick-walled products, such as beveled housings for TV sets and display housings.
In fact, for products with strict requirements (such as household appliances), sink marks must be eliminated, while for products with lower surface quality requirements (such as toys), sink marks are allowed. There may be one or more reasons for the formation of sink marks, including processing methods, part geometry, material selection and Injection mould design. Geometry and material selection are usually determined by the raw material supplier and cannot be easily changed. But Injection mould manufacturers still have many factors in Injection mould design that will affect shrinkage. The design of the cooling runner, the type of gate, and the gate size will have many 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 sink marks. For Injection moulding workers, adjusting the processing conditions is a way to solve the shrinkage problem.
Filling pressure and time can significantly affect shrinkage. After filling the part, the excess material will continue to fill the cavity to compensate for the shrinkage of the material. Too short a filling stage will lead to an increase in shrinkage, which will eventually lead to more or greater shrinkage marks. The method itself cannot reduce shrinkage marks to a satisfactory level, but the Injection moulder can adjust the filling conditions to improve the shrinkage marks. Another method is to modify the Injection mould. A simple solution is to modify the conventional core hole, but this method cannot be expected to be applicable to all resins. In addition, the gas-assisted method is also worth a try.
