Author: MULAN -Plastic Molding Manufacturer
Shrinkage formed in thicker parts of the plastic mold part, such as ribs or protrusions, is more severe than adjacent positions, because the cooling rate of thicker areas is much slower than that of surrounding areas. The difference in cooling rate leads to the formation of depressions at the joining surfaces, which are known as shrinkage marks. This defect severely limits the design and molding of precision plastic mold products for home appliances, especially large thick-walled products such as inclined plane casings of televisions and display casings.
In fact, shrinkage marks must be eliminated for products with strict requirements such as household appliances, while shrinkage marks are allowed for some products such as toys that do not require high surface quality. There may be one or more reasons for the formation of shrinkage marks, including processing methods, 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 easy to change.
However, mold manufacturers still have many factors about mold design that may affect shrinkage. Cooling runner design, gate type, gate size can have various effects. For example, small gates such as tube gates cool much faster than tapered gates.
Premature cooling at the gate will reduce the fill time in the cavity, thereby increasing the chance of shrinkage marks. For molding workers, adjusting processing conditions is one way to solve the shrinkage problem. Fill pressure and time significantly affect shrinkage.
After the part is filled, excess material continues to fill the cavity to compensate for material shrinkage. Too short a filling phase 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 the modification of the stamping die. The simple solution is to modify the regular core hole, but it is impossible to apply it to all resins. In addition, the gas-assisted method is also worth a try.