What Is The Relationship Between Mold Size And Plastic Shrinkage?

- Jun 27, 2018 -

   When designing the plastic mold, after determining the mold structure, the detailed design of each part of the mold can be carried out, that is, the size of each template and part, the cavity and the core, etc.Major design parameters such as material shrinkage will be involved.Therefore, the size of each part of the cavity can be determined only by knowing the shrinkage rate of the forming plastic.Even if the selected mold structure is correct, but with the wrong parameters, it is impossible to produce qualified plastic parts.

   The shrinkage rate of plastics and its influencing factors

    Thermoplastic is characterized by expansion after heating, shrinkage after cooling, and, of course, shrinkage after pressure.In the process of plastic injection forming, injecting molten plastic into the mold cavity, the first end of the filling liquid cooling solidification, removed from the mold plastic parts is a contraction, this is called forming contraction.During the period from the removal of the mold to the stabilization of the plastic parts, there will still be small changes in the size.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 water content of glass fiber reinforced nylon 66 is 40%, the dimension increase is 0.3%.But the main effect is the forming shrinkage.At present, the determination of various plastic shrinkage rate (forming shrinkage + retraction) generally recommends the German national standard for DIN16901.23 ℃ to + / - 0.1 ℃ when the mold size and shape after 24 hours, at a temperature of 23 ℃, relative humidity for 50-5% measured under the condition of the corresponding parts size to calculate the difference between the.

    The shrinkage rate S is expressed by the following formula: S={(d-m)/D} * 100%(1)

    Where: s-shrinkage;D- mold size;M- dimensions of plastic parts.

    If the mold cavity is calculated according to the known plastic part size and material shrinkage rate, it is D=M/(1-s).

    D = M + MS (2).

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

    However, when determining the shrinkage rate, because the actual shrinkage rate is affected by many factors, the approximate value can only be used, so the calculation of the cavity size by formula (2) also basically meets the requirements.When making the mould, the cavity is processed according to the lower deviation, and the core is processed according to the upper deviation, so as to make proper dressing when necessary.

    The main reason for the difficulty in determining the exact shrinkage rate is, first of all, 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 with different batch Numbers produced by one factory is different.Therefore, each factory can only provide users with the shrinkage of the plastic produced by the factory.Secondly, the actual shrinkage rate in the forming process is also affected by the shape of plastic parts, mold structure and forming conditions.

Shape of plastic parts

     For the wall thickness of the formed parts, generally, the shrinkage rate of the thick walls is large due to the long cooling time, as shown in figure 1.For general plastic parts, when the size of molten material flow direction L is significantly different from the size of W perpendicular to the direction of molten material flow, the shrinkage rate is also significantly different.From the perspective of the flux flow distance, the pressure loss far away from the gate is large, so the shrinkage rate is larger than that near the gate.The shrinkage of these parts is small because of the shrinkage resistance of reinforcement, hole, convex platform and sculpture.

    3. Mold structure

     Gate form also has an effect on shrinkage.When using small gate, the shrinkage rate of plastic parts increases because the gate solidifies before the end of pressure protection.Cooling loop structure in injection mould is also a key part in mould design.If the cooling circuit is not properly designed, it will cause the shrinkage difference due to the temperature imbalance at different parts of the plastic parts. As a result, the size of the plastic parts will be out of tolerance or deformation.In the thin wall, the influence of temperature distribution on shrinkage is more obvious.

     Parting surface and gate

     Mold parting surface, gate form and size directly influence the direction of material flow, density distribution, compression and shrinkage and forming time.

     The direct gate or large section gate can reduce the shrinkage, but the anisotropy is large, and the shrinkage is large along the direction of the material flow.On the contrary, when the thickness of the gate is small, the part of the gate will coagulate and harden prematurely, and the plastic in the cavity will not be replenished in time after shrinking, resulting in greater shrinkage.

     The point gate can be sealed quickly. If the conditions of the parts permit, the multi-point gate can be set up to effectively extend the pressure holding time and increase the pressure of the cavity, so as to reduce the shrinkage rate.