Thermoplastics are characterized by expansion after heating and contraction after cooling. Of course, the volume will shrink after pressurization. In the injection molding process, the molten plastic is first injected into the mold cavity. After the filling is completed, the melt is cooled and solidified. When the plastic part is taken out of the mold, shrinkage occurs. This shrinkage is called forming shrinkage. During the period of time when the plastic part is taken out from the mold to stable, there will still be slight changes in size. 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 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 role is forming shrinkage. At present, the method of determining the shrinkage rate of various plastics (forming shrinkage + post shrinkage) generally recommends the provisions of DIN16901 in the German national standard. That is the difference between the mold cavity size at 23°C±0.1°C. The size of the corresponding plastic part was measured at a temperature of 23°C. And relative humidity of 50±5% after being placed for 24 hours.

The plastic injection molding shrinkage rate 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, it is D=M/(1-S) In order to simplify the calculation in the mold design, the following formula is generally used to find the mold size:

D=M+MS(2)

If a more accurate calculation is required, the following formula is used:

D=M+MS+MS2(3)

However, when determining the shrinkage rate, because the actual shrinkage rate is affected by many factors, only approximate values ​​can be used. Therefore, the calculation of the cavity size by formula (2) basically meets the requirements. When manufacturing the 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 properly trimmed if necessary.

The main reason why it is difficult to accurately determine the shrinkage rate is first because 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 not the same, even the shrinkage rate of the same material with different batch numbers produced in one factory is different. Therefore, each factory can only provide users with the shrinkage rate range of the plastic produced by the factory. Secondly, the actual shrinkage rate during the forming process is also affected by factors such as the shape of the plastic part, mold structure, and forming conditions. The following is an introduction to the influence of these factors.

Plastic part shape

For the wall thickness of the formed part, the shrinkage rate is generally larger due to the longer cooling time of the thick wall. For general plastic parts, when the difference between the L dimension in the melt flow direction and the W dimension perpendicular to the melt flow direction is greater, the difference in shrinkage is also greater. From the point of view of the melt flow distance, the pressure loss of the part far away from the gate is large, so the shrinkage rate there is also greater than the part close to the gate. Because shapes such as ribs, holes, bosses, and carvings have shrinkage resistance, the shrinkage rate of these parts is small.

Mold structure

The gate type also has an effect on shrinkage. When a small gate is used, the shrinkage rate of the plastic part increases because the gate solidifies before the end of the holding pressure. 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 difference in shrinkage will occur due to the uneven temperature of the plastic parts, and the result will be out of tolerance or deformation of the plastic parts. In the thin-walled part, the influence of mold temperature distribution on shrinkage is more obvious.

Forming conditions

Barrel temperature: When the barrel temperature (plastic temperature) is higher, the pressure transmission is better and the contraction force is reduced. However, when using a small gate, the shrinkage rate is still largely due to the early curing of the gate. For thick-walled plastic parts, even if the barrel temperature is high, the shrinkage is still large.

Refill: In the forming conditions, minimize the refill to keep the size of the plastic part stable. However, insufficient feeding will not maintain the pressure and increase the shrinkage rate.

Injection pressure: The injection pressure is a factor that has a greater impact on the shrinkage rate, especially the pressure holding page 335 after filling. In general, when the pressure is higher, the shrinkage rate is smaller due to the higher density of the material.

Injection speed: The injection speed has little effect on the shrinkage rate. But for thin-walled plastic parts or gates are very small, and when using reinforced materials, the shrinkage rate is small when the injection speed increases.

Mold temperature: Generally, the shrinkage rate is larger when the mold temperature is higher. But for thin-walled plastic parts, if the mold temperature is high, the flow resistance of the melt is small, and the shrinkage rate is small.

Forming cycle: The forming cycle is not directly related to shrinkage. But it should be noted that when the forming cycle is accelerated, the mold temperature, melt temperature, etc. will inevitably also change, which will also affect the change of shrinkage. When making material tests, it should be formed according to the forming cycle determined by the required output, and the size of the plastic part should be inspected.

An example of a plastic shrinkage test using this mold is as follows.

Injection machine: clamping force 70t screw diameter Φ35mm screw speed 80rpm. Molding conditions: maximum injection pressure 178MPa. Barrel temperature 230 (225-230-220-210) ℃ ,240 (235-240-230-220) ℃, 250 (245-250 -240-230)℃, 260(225-260-250-240)℃ .Injection speed 1425px3/s .Injection time 0.44～0.52s. Holding pressure time 6.0s Cooling time 15.0s