The relationship between plastic shrinkage and mold size

When designing a plastic mold, after the mold structure is determined, detailed design of each part of the mold can be performed, that is, the size of each template and part, the cavity and core dimensions, etc. At this time, the main design parameters related to material shrinkage will be involved. Therefore, the size of each part of the cavity can be determined only by specifically grasping the shrinkage rate of the formed plastic. Even if the selected mold structure is correct, but the parameters used are not correct, it is impossible to produce plastic parts of acceptable quality.
 
Plastic shrinkage and its influencing factors
The characteristic of thermoplastics is that they expand after heating and shrink after cooling, and of course their volume will shrink after being pressed. In the injection molding process, the molten plastic is first injected into the mold cavity. After the filling is completed, the melt cools and solidifies. When the plastic part is removed from the mold, shrinkage occurs. This shrinkage is called molding shrinkage. Within a period of time from when the plastic part is taken out from the mold to stabilization, there will still be small changes in size. One kind of change is to continue shrinking. This shrinkage is called post shrinkage. Another variation 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 one is the shrinkage. At present, the methods for determining the shrinkage of various plastics (formation shrinkage + post-shrinkage) are generally recommended in the German national standard DIN16901. That is, the difference between the size of the mold cavity at 23 ° C ± 0.1 ° C and the size of the corresponding plastic part measured under the conditions of 23 ° C and relative humidity of 50 ± 5% after forming for 24 hours is calculated.
The shrinkage ratio S is expressed by the following formula: S = {(D－M) / D} × 100% (1)
Among them: S-shrinkage; D-die size; M-plastic size.
If the mold cavity is calculated according to the known plastic part size and material shrinkage ratio, 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 applied: D = M + MS + MS2 (3)
However, in determining the shrinkage rate, since the actual shrinkage rate is affected by many factors, only approximate values ​​can be used. Therefore, the calculation of the cavity size using equation (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 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 of the same material produced by different factories is not the same, even the shrinkage of the same material produced by different batches in a factory is not the same. Therefore, each plant can only provide users with the shrinkage range of plastic 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. The effects of these factors are described below.
 
Plastic shape
As for the thickness of the formed part, generally due to the longer cooling time of the thick wall, the shrinkage rate is also larger, as shown in FIG. 1. For general plastic parts, when the difference between the size L of the melt flow direction and the size W perpendicular to the melt flow direction is large, the difference in shrinkage is also large. From the perspective of the melt flow distance, the pressure loss at the part far from the gate is large, so the shrinkage rate there is also greater than that near the gate. Shapes such as ribs, holes, bosses, and engraving have shrinkage resistance, so the shrinkage of these parts is small.
 
Mold structure
The gate shape also has an effect on the shrinkage. When using a small gate, the shrinkage of the plastic part is increased because the gate is solidified before the end of the holding pressure. The cooling circuit structure in the injection mold is also a key in mold design. If the cooling circuit is not designed properly, the shrinkage difference will occur due to the uneven temperature of the plastic parts. As a result, the size of the plastic parts will be excessively poor or deformed. In thin-walled parts, the effect of mold temperature distribution on shrinkage is more pronounced.
 
Forming conditions
Barrel temperature: When the barrel temperature (plastic temperature) is high, the pressure transmission is good and the shrinkage force is reduced. However, when a small gate is used, the shrinkage is still large 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.
 
Feeding: In forming conditions, minimize the feeding to keep the size of the plastic parts stable. However, insufficient feeding will not maintain pressure and will increase shrinkage.
Injection pressure: The injection pressure is a factor that has a greater effect on the shrinkage rate, especially the pressure of the holding pressure page 335 after the filling is completed. In general, when the pressure is greater, the shrinkage rate is smaller because the density of the material is higher.
Injection speed: The impact of injection speed on shrinkage is small. However, for thin-walled plastic parts or gates, and when reinforcing materials are used, the injection rate will increase and the shrinkage rate will be small.
Mold temperature: Shrinkage is usually higher when the mold temperature is higher. But for thin-walled plastic parts, the higher the mold temperature is, the smaller the flow resistance of the melt is, and the shrinkage rate is smaller.
Forming cycle: There is no direct relationship between forming cycle and shrinkage. However, it should be noted that when the forming cycle is accelerated, the mold temperature, the melt temperature, etc. will necessarily change, which will also affect the change of the shrinkage rate. In the material test, the forming should be performed in accordance with 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 rotation speed 80rpm forming 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 57cm3 / s Injection time 0.44 ～ 0.52s Holding time 6.0s Cooling time 15.0s
 
Mold size and manufacturing tolerances
In addition to the basic dimensions calculated by the formula D = M (1 + S), the machining dimensions of the mold cavity and core have a problem of machining tolerance. Conventionally, the processing tolerance of the mold is 1/3 of the tolerance of the plastic part. However, due to the differences in the range and stability of plastic shrinkage, the dimensional tolerances of plastic parts formed from different plastics must be rationalized first. That is to say, the dimensional tolerance of plastic molded plastic parts that have a larger range of shrinkage or a poor shrinkage stability should be made larger. Otherwise, a large number of out-of-size waste products may occur. For this reason, countries have formulated national standards or industry standards for the dimensional tolerances of plastic parts. China has also formulated ministerial-level professional standards. But most of them do not have the corresponding dimensional tolerance of the mold cavity. The German national standard specifically formulated the DIN16901 standard for the dimensional tolerance of plastic parts and the DIN16749 standard for the corresponding mold cavity dimensional tolerance. This standard has a greater impact in the world, so it can be used as a reference for the plastic mold industry.
 
About dimensional tolerances and allowable deviations of plastic parts
In order to reasonably determine the dimensional tolerances of plastic parts formed of materials with different shrinkage characteristics, the standard introduces the concept of forming shrinkage difference ΔVS.
△ VS = VSR_VST (4)
In the formula: VS-forming shrinkage difference VSR-forming shrinkage rate in the direction of melt flow VST-forming shrinkage rate in the direction perpendicular to the melt flow.
According to the plastic ΔVS value, the shrinkage characteristics of various plastics are divided into 4 groups. The group with the smallest △ VS value is the high-precision group, and so on, and the group with the largest △ VS value is the low-precision group. The precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups were compiled according to the basic dimensions. It also stipulates that the dimensional tolerances of the plastic formed plastic parts with the most stable shrinkage characteristics can be selected from 110, 120 and 130 groups. The dimensional tolerances of plastic formed plastic parts with moderately stable shrinkage characteristics are selected from 120, 130 and 140. If 110 sets of dimensional tolerances are used for this type of plastic forming plastic parts, a large number of oversized plastic parts may be produced. The dimensional tolerances of plastic formed plastic parts with poor shrinkage characteristics are selected from 130, 140 and 150 groups. The dimensional tolerances of the plastic formed plastic parts with the worst shrinkage characteristics are selected from 140, 150 and 160 groups. When using this tolerance table, note the following points. The general tolerances in the table are used for dimensional tolerances where no tolerances are stated. Tolerances for direct deviations are tolerance bands used to dimension tolerances on plastic parts. The upper and lower deviations can be determined by the designer. For example, if the tolerance zone is 0.8mm, the following various upper and lower deviations can be selected. 0.0; -0.8; ± 0.4; -0.2; -0.5 etc. Each tolerance group has two sets of tolerance values, A and B. Among them, A is the size formed by the combination of mold parts, which increases the error caused by the incompatibility of the mold parts. This increase is 0.2mm. Where B is the size directly determined by the mold parts. Precision technology is a specially set of tolerance values ​​for plastic parts with high precision requirements. Before using plastic part tolerances, you must first know which tolerance groups are suitable for the plastic used.
 
Mold manufacturing tolerances
The German national standard has formulated the corresponding DIN16749 standard for mold manufacturing tolerances for plastic part tolerances. There are 4 tolerances in this table. Regardless of the plastic parts of any material, the tolerances of mold number 1 do not indicate the dimensional tolerances. The specific tolerance value is determined by the basic size range. Regardless of the material, the manufacturing tolerances of molds with medium precision dimensions are the tolerances of No. 2. No matter what kind of material plastic parts, the higher precision size of the mold manufacturing tolerance is the tolerance of No. 3. The corresponding manufacturing tolerances for precision technology are the tolerances of No. 4.
 
It can reasonably determine the reasonable tolerances of various materials and plastic parts and the corresponding mold manufacturing tolerances, which not only brings convenience to mold manufacturing, but also reduces waste and improves economic efficiency.