What Is The Relationship Between Mold Size And Plastic Shrinkage?

- Aug 17, 2018-

   When designing the plastic mold, after the mold structure is determined, the various parts of the mold can be designed in detail, that is, the size of each template and part, the cavity and the core size are determined. This will involve major design parameters such as material shrinkage. Therefore, the size of each part of the cavity can be determined only by specifically grasping the shrinkage rate of the shaped plastic. Even if the selected mold structure is correct, if the parameters used are not correct, it is impossible to produce a qualified plastic part.

First, plastic shrinkage rate and its influencing factors

    The characteristic of thermoplastics is that they expand after heating, shrink after cooling, and of course the volume will shrink after pressurization. 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 size of the plastic part will still change slightly from the time the mold is removed to the stable state. 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 DIN16901. 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 under the conditions of a temperature of 23 ° C and a relative humidity of 50 ± 5% is calculated.

    The shrinkage ratio S is expressed by the following formula: S = {(D - M) / D} × 100% (1)

    Among them: S-shrinkage rate; D-mold size; M-plastic part size.

    If the mold cavity is calculated according to the known plastic part size and material shrinkage rate, then D=M/(1-S). In order to simplify the calculation in the mold design, the mold size is generally used to find the mold size:

    D = M + MS (2).

    If a more accurate calculation is required, the following formula is applied: D=M+MS+MS2(3)

    However, when determining the shrinkage rate, since the actual shrinkage rate is affected by many factors, only the approximate value can be used. Therefore, the calculation of the cavity size by the formula (2) basically satisfies the requirement. When manufacturing the mold, the cavity is machined according to the lower deviation, and the core is machined according to the deviation, so that it can be properly trimmed if 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 rates of the same materials produced by different factories are not the same, even the same batch of the same material produced by a factory has different shrinkage rates. Therefore, each factory can only provide users with a range of shrinkage of plastics produced by the plant. Secondly, the actual shrinkage during the forming process is also affected by factors such as the shape of the plastic part, the mold structure and the forming conditions.

Second, the shape of the plastic parts

    For the wall thickness of the formed part, the shrinkage rate is also large due to the long cooling time of the thick wall, as shown in Fig. 1. For a general plastic part, when the difference between the flow direction L dimension of the melt and the dimension W perpendicular to the flow direction of the melt flow is large, the difference in shrinkage ratio is also large. From the flow distance of the melt, the pressure loss away from the gate portion is large, and thus the shrinkage rate at this point is also larger than that near the gate portion. Because of the shrinkage resistance of the ribs, holes, bosses, and engravings, the shrinkage of these parts is small.

Third, the mold structure

    The gate form also has an effect on shrinkage. When a small gate is used, the shrinkage of the plastic part is increased because the gate is solidified before the pressure is maintained. The cooling circuit structure in the injection mold is also a key in the mold design. If the cooling circuit is not properly designed, the shrinkage is caused by the uneven temperature of the plastic parts, and the result is that the size of the plastic part is excessive or deformed. In the thin-walled part, the influence of the mold temperature distribution on the shrinkage rate is more pronounced.

    Parting surface and gate

    Factors such as parting surface, gate form and size of the mold directly affect the flow direction, density distribution, pressure-preserving and shrinking action and molding time.

    The use of direct gates or large-section gates can reduce shrinkage, but the anisotropy is large, shrinking in the direction of the flow direction is small, and shrinking along the vertical flow direction; conversely, when the gate thickness is small, the gate portion will prematurely condense. After hardening, the plastic in the cavity is not replenished in time, and the shrinkage is large.

    The point gate is fast-sealed, and when the condition of the workpiece is allowed, a multi-point gate can be provided, which can effectively extend the dwell time and increase the cavity pressure, so that the shrinkage rate is reduced.