Shrinkage refers to the percentage of error between the size of plastic products and the size of the original mold after cooling and curing and demoulding, which can be measured in accordance with ASTM D955. In the design of plastic mold, the shrinkage must be considered first, so as not to cause the error of finished product size, resulting in poor finished product.
The characteristic of thermoplastic is that it expands after heating, shrinks after cooling, and its volume will shrink after pressurizing. In the process of injection molding, the molten plastic is first injected into the mold cavity. After filling, the molten material is cooled and solidified. When the plastic part is taken out of the mold, it shrinks. This shrinkage is called forming shrinkage. During the period from the removal of the plastic part to the stabilization of the mold, there will still be small changes in the size. One change is to continue to contract, which is called post shrinkage. Another change is that some hygroscopic plastics expand due to moisture absorption. For example, when the water content of nylon 610 is 3%, the size increase is 2%; when the water content of glass fiber reinforced nylon 66 is 40%, the size increase is 0.3%. But the main function is forming shrinkage. At present, the methods to determine the shrinkage rate (forming shrinkage + Post shrinkage) of various plastics generally recommend the provisions of DIN 16901 in German national standard. That is to say, the difference between the mold cavity size at 23 ℃ ± 0.1 ℃ and the corresponding plastic part size measured at 23 ℃ and 50 ± 5% relative humidity is calculated.
The shrinkage s is expressed as follows: S = {(D-M) / D} × 100% (1)
Where: s-shrinkage; d-die size; m-plastic part size.
If the mold cavity is calculated according to the known plastic part size and material shrinkage, it is d = m / (1-s). In order to simplify the calculation in the mold design, the following formula is generally used to calculate the mold size:
D=M+MS(2)
If a more accurate calculation is required, the following formula shall be applied: D = m + MS + MS2 (3)
However, in determining the shrinkage rate, because the actual shrinkage rate is influenced by many factors, only approximate values can be used, so the calculation of cavity size with equation (2) basically meets the requirements. When making a mold, the cavity is processed according to the lower deviation, and the core is processed according to the upper deviation, so that proper trimming can be made when necessary.