Plastic shrinkage of plastic molds and its influencing factors.


The characteristic of thermoplastics is that they expan […]

The characteristic of thermoplastics is that they expand after heating and contract after cooling. Of course, the volume will shrink after pressurization. In the injection plastic molding process, the molten plastic is first injected into the cavity of the plastic mold. After the filling is completed, the melt cools and solidifies, and shrinkage occurs when the plastic part is removed from the plastic mold. This shrinkage is called plastic molding shrinkage. During the period when the plastic parts are taken out of the plastic mold and stabilized, there will still be slight changes in the dimensions. One change is to continue to shrink. This shrinkage is called post-shrinkage. Another change is that some hygroscopic plastics swell 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 one of the main functions 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 national standard. That is, the difference between the size of the plastic mold cavity at 23 ° C ± 0.1 ° C and the size of the corresponding plastic part measured under the condition of 23 ° C and relative humidity of 50 ± 5% at 24 ° C after plastic molding.
The shrinkage rate S is expressed by the following formula: S = {(D-M) / D} × 100% (1)

Among them: S-shrinkage rate; D-plastic mold size; M-plastic part size.
If the plastic mold cavity is calculated according to the known plastic part size and material shrinkage rate, it is D = M / (1-S). In the plastic mold design, in order to simplify the calculation, the following formula is generally used to find the plastic mold size:
D = M + MS (2)

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If you need to implement a more accurate calculation, then apply the following formula: D = M + MS + MS2 (3)
But when determining the shrinkage rate, because the actual shrinkage rate is affected by many factors, only approximate values ​​can be used, so the calculation of the cavity size using equation (2) basically meets the requirements. When manufacturing the plastic mold, the cavity is processed according to the lower deviation, and the core is processed according to the upper deviation, so that it can be appropriately trimmed when necessary.
The main reason why it is difficult to accurately determine the shrinkage rate is that the shrinkage rate of various plastics is not a fixed value but a range. Because the shrinkage rate of the same material produced by different factories is different, even the shrinkage rate of the same material produced by a different factory is different. Therefore, each factory can only provide users with the shrinkage range of the plastic produced by the factory. Secondly, the actual shrinkage in the forming process is also affected by factors such as the shape of the plastic part, the plastic mold structure and the forming conditions.