In the thicker parts of the plastic mold factory, such as the protrusions of the ribs (commonly known as bones), the contraction is more serious than the adjacent positions, because the cooling rate of the thicker area is much slower than the surrounding area. Different cooling speeds lead to the formation of depressions at the connection surface, which are familiar shrinkage marks. There may be one or more reasons for the formation of shrink 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 easily changed
There are many other factors in the mold manufacturer’s mold design that may affect shrinkage. Cooling runner design, gate type, and gate size may produce multiple effects. For example, a small gate such as a tube gate cools much faster than a tapered gate. Premature cooling at the gate will reduce the filling time in the cavity, thereby increasing the chance of shrinkage marks.
For molding and injection workers, adjusting the injection molding process is a way to solve the shrinkage problem. Injection pressure and time also affect the shrinkage. After the part is filled, excess material continues to be filled into the cavity to compensate for the shrinkage of the material. If the injection time is too short, the shrinkage will increase, and eventually more or larger shrink marks will be produced. This method may not reduce shrinkage marks to a satisfactory level by itself, but molding workers can adjust the filling conditions to improve shrinkage marks.
In addition to the above methods, modifying the mold is the simplest solution. You can try to modify the conventional core hole. This method may not be suitable for all resins. The gas-assisted method also improves this situation to a certain extent. In addition, it is also a method to replace the plastic shrinkage rate of the composite mold of the material. Glass fiber reinforced, mineral filled and other materials are helpful to reduce molding shrinkage. Replacing plastics of different materials has different shrinkage rates.
