How to solve the shrinkage problem in plastic injection molding?

 

With the continuous improvement of plastic injection molding processes, minimizing shrinkage and improving product quality is imperative.

 

Shrinkage is more severe in thicker areas of Plastic injection molded parts, such as ribs or protrusions, than in adjacent areas. This is because the cooling rate in thicker areas is much slower than in surrounding areas. This difference in cooling rates causes depressions at the joint surfaces, which are the familiar shrinkage marks. This defect severely limits the design and molding of plastic products, especially large, thick-walled products such as the beveled casings of televisions and monitor housings. In fact, shrinkage marks must be removed for products with strict requirements, such as household appliances, while they are permissible for products with lower surface quality requirements, such as Plastic Toys.

 

Causes of Shrinkage Marks in Plastic Injection Molding

The causes of shrinkage marks in plastic injection molding can be one or more, including processing methods, part geometry, material selection, and plastic mold design. Geometry and material selection are usually determined by the raw material supplier and are not easily changed. However, mold manufacturers have many factors related to mold design that can affect the shrinkage stage in plastic injection molding. Cooling channel design, gate type, and gate size can have various effects. For example, small gates, such as tubular gates, cool much faster than tapered gates. Premature cooling at the gate reduces the filling time in the cavity, thus increasing the likelihood of shrinkage marks. For molding workers, adjusting processing conditions is one way to address shrinkage in plastic injection molding. Filling pressure and time significantly affect shrinkage. After the part is filled, excess material continues to fill the cavity to compensate for shrinkage. Too short a filling phase will lead to increased shrinkage, ultimately resulting in more or larger shrinkage marks.

 

This method of addressing shrinkage in plastic injection molding may not itself reduce shrinkage marks to a satisfactory level, but molding workers can adjust filling conditions to improve them. Another approach is to modify the mold; a simple solution is to modify the conventional core holes, but this method cannot be expected to work for all resins. Additionally, gas-assisted methods can also address shrinkage issues in plastic injection molding.