The Relationship Between Plastic Mold Size and Shrinkage Rate

When designing plastic molds, after determining the mold structure, detailed design of each part of the mold can be carried out, including determining the dimensions of each template and part, cavity and core dimensions, etc. At this point, major design parameters such as material shrinkage rate will be involved. Therefore, only by specifically understanding the shrinkage rate of the formed plastic can the dimensions of each part of the mold cavity be determined. Even if the selected mold structure is correct, if the parameters used are inappropriate, it is impossible to produce qualified plastic parts.

Plastic shrinkage rate and its influencing factors

The characteristic of thermoplastic is that it expands after heating, shrinks after cooling, and of course, its volume will also decrease after pressurization. In the injection molding process, molten plastic is first injected into the mold cavity, and after filling is completed, the molten material cools and solidifies. When the plastic part is removed from the mold, shrinkage occurs, which is called forming shrinkage. During the period from the removal of the plastic part from the mold to stability, there will still be slight changes in size, one of which is continued shrinkage, known as post shrinkage.

Another change is that some hygroscopic plastics expand due to moisture absorption. For example, when the moisture content of nylon 610 is 3%, the size increase is 2%; When the moisture content of glass fiber reinforced nylon 66 is 40%, the size increase is 0.3%. But the main factor playing a role is forming shrinkage. At present, the methods for determining various plastic shrinkage rates (forming shrinkage+post shrinkage) generally recommend the provisions of DIN16901 in the German standard. Calculate the difference between the mold cavity size at 23 ℃± 0.1 ℃ and the corresponding plastic part size measured after being placed for 24 hours at a temperature of 23 ℃ and a relative humidity of 50 ± 5%.