Plastic mold shrinkage can not be ignored

Plastic mold shrinkage can not be ignored

Shrinkage is the enemy of plastic processors, especially for large plastic products with high surface quality. Shrinkage is a stubborn disease. Therefore, various technologies have been developed to minimize shrinkage and improve product quality.

Shrinkage at the thicker locations of injection molded plastic parts, such as ribs or protrusions, is more severe than adjacent locations because the thicker areas are cooled much more slowly than the surrounding area. The difference in cooling rate causes a depression at the joint surface, which is a familiar shrinkage mark. Such defects severely limit the design and molding of plastic products, especially large thick-walled products such as the beveled casing and display casing of television sets. In fact, for the demanding products such as household electrical appliances, the shrinkage marks must be eliminated, and for products such as toys that have low surface quality requirements, shrinkage marks are allowed.

Plastic mold shrinkage
    
There may be one or more reasons for the formation of shrink marks, including processing methods, component geometries, material selection, and mold design. The geometry and material selection are usually determined by the raw material supplier and are not easily changed. But moldmakers still have a lot of factors about mold design that can affect shrinkage. Cooling runner design, gate type, gate size can have multiple effects. For example, small gates such as tubular gates cool much faster than tapered gates. Premature cooling at the gate reduces the fill time in the cavity, which increases the chance of shrink marks.

For molding workers, adjusting the processing conditions is one way to solve the shrinkage problem. Filling pressure and time significantly affect shrinkage. After the part is filled, the excess material continues to fill the cavity to compensate for the shrinkage of the material. Too short a filling phase will result in increased shrinkage and eventually more or larger shrinkage marks. This method may not reduce the shrinkage marks to a satisfactory level by itself, but the molder can adjust the filling conditions to improve the shrinkage marks.
 
Another method is to modify the mold. A simple solution is to modify the conventional core hole, but this method cannot be expected to be suitable for all resins. In addition, the gas-assisted method is also worth a try.

Plastic mold shrinkage

Plastic mold shrinkage
In the injection molding process, the molten plastic is first injected into the mold cavity, and after the filling is completed, the melt is cooled and solidified, and shrinkage occurs when the plastic part is taken out from the mold, and the shrinkage is called forming shrinkage. The plastic part is taken out from the mold for a period of time, and the size still changes slightly. One change is to continue shrinking. This shrinkage is called back shrinkage. Another variation is that some hygroscopic plastics swell due to moisture absorption. For example, when the water content of the nylon 610 is 3%, the dimensional increase is 2%; and when the water content of the glass fiber reinforced nylon 66 is 40%, the dimensional increase is 0.3%. But the main role is the forming shrinkage.

At present, the method for determining the shrinkage ratio (forming shrinkage + post-shrinkage) of various plastics is generally recommended in the German national standard DIN 16901. That is, when the mold cavity size is 23 ° C ± 0.1 ° C and placed after molding for 24 hours, the difference between the corresponding plastic parts measured at a temperature of 23 ° C and a relative humidity of 50 ± 5% is calculated.
So Plastic mold shrinkage can not be ignored.

Plastic mold shrinkage

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