When designing a plastic mold, after determining the mold structure, the various parts of the mold can be designed in detail, that is, the size of each template and part, the size of the cavity and core, etc. will be determined. At this time, the main design parameters such as the shrinkage rate of the material will be involved. Therefore, only by specifically mastering the shrinkage rate of the molded plastic can the size of each part of the cavity be determined. Even if the selected mold structure is correct, it is impossible to produce plastic parts of qualified quality if the parameters used are inappropriate.
Plastic shrinkage rate and its influencing factors
The characteristics of thermoplastics are that they expand after heating and shrink after cooling. Of course, the volume will also shrink after pressurization. In the injection molding process, the molten plastic is first injected into the mold cavity. After the filling is completed, the molten material cools and solidifies. When the plastic part is taken out of the mold, it shrinks. This shrinkage is called molding shrinkage. During the period from the removal of the plastic part from the mold to the stabilization, the size will still change slightly. One change is continued shrinkage, which is called post-shrinkage. 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 moisture content of glass fiber reinforced nylon 66 is 40%, the size increase is 0.3%. However, the main factor is the molding shrinkage. At present, the method for determining the shrinkage rate of various plastics (molding shrinkage + post-shrinkage) generally recommends the provisions of DIN16901 in the German national standard. That is, it is calculated by the difference between the mold cavity size at 23℃±0.1℃ and the corresponding plastic part size measured at 23℃ and 50±5% relative humidity after 24 hours of molding.
The shrinkage rate S is expressed by the following formula: S={(D-M)/D}×100%(1)
Where: S-shrinkage rate; D-mold size; M-plastic part size.
If the mold cavity is calculated based on the known plastic part size and material shrinkage rate, then D=M/(1-S). In order to simplify the calculation in mold design, the following formula is generally used to calculate 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, since the actual shrinkage rate is affected by many factors, only an approximate value can be used. Therefore, using formula (2) to calculate the cavity size can basically meet 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 appropriate trimming can be made 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 different batches of the same factory is different. Mold Master WeChat: mojuren Therefore, each factory can only provide users with the shrinkage rate range of the plastics 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, the mold structure and the forming conditions. The following is an introduction to the influence of these factors.
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Plastic part shape
For the wall thickness of the formed part, the shrinkage rate is generally larger because the cooling time of the thick wall is longer, as shown in Figure 1. 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 large, the shrinkage rate difference is also large. From the perspective 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 larger than that of the part close to the gate. Because the shapes such as ribs, holes, bosses and carvings have shrinkage resistance, the shrinkage rate of these parts is small.
Mold structure
The gate form also affects the shrinkage rate. When a small gate is used, the shrinkage rate of the plastic part increases because the gate solidifies before the end of the pressure holding. The cooling circuit structure in the injection mold is also a key in the mold design. If the cooling circuit is not designed properly, the shrinkage difference will occur due to the uneven temperature of the plastic part, resulting in the size of the plastic part being out of tolerance or deformed. In thin-walled parts, the influence of mold temperature distribution on shrinkage is more obvious.
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Molding conditions
Barrel temperature: When the barrel temperature (plastic temperature) is high, the pressure transmission is better and the shrinkage force is reduced. However, when a small gate is used, the shrinkage is still large because the gate solidifies early. For thick-walled plastic parts, even if the barrel temperature is high, the shrinkage is still large.
Feeding: In the molding conditions, try to reduce the feeding to keep the size of the plastic part stable. However, insufficient feeding will not be able to maintain the pressure, which will also increase the shrinkage.
Injection pressure: Injection pressure is a factor that has a greater impact on the shrinkage, especially the holding pressure after filling. In general, when the pressure is high, the shrinkage is small due to the high density of the material.
Injection speed: The injection speed has little effect on the shrinkage. However, for thin-walled plastic parts or very small gates, and when reinforced materials are used, the shrinkage is small when the injection speed is increased.
Mold temperature: Generally, the shrinkage is larger when the mold temperature is higher. However, for thin-walled plastic parts, the higher the mold temperature, the smaller the flow resistance of the melt, and the smaller the shrinkage rate.
Molding cycle: There is no direct relationship between the molding cycle and the shrinkage rate. However, it should be noted that when the molding cycle is accelerated, the mold temperature, melt temperature, etc. will inevitably change, which will also affect the change of the shrinkage rate. When testing materials, molding should be carried out according to the molding cycle determined by the required output, and the size of the plastic parts should be inspected. The following is an example of using this mold to test the plastic shrinkage rate. 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 57cm3/s injection time 0.44～0.52s holding time 6.0s cooling time 15.0s
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Mold size and manufacturing tolerance
In addition to calculating the basic size through the D=M(1+S) formula, the processing size of the mold cavity and core also has a processing tolerance problem. According to convention, the processing tolerance of the mold is 1/3 of the tolerance of the plastic part. However, due to the differences in the shrinkage range and stability of plastics, the dimensional tolerance of plastic parts formed by different plastics must first be rationally determined. That is, the dimensional tolerance of plastic parts formed by plastics with a larger shrinkage range or poor shrinkage stability should be larger. Otherwise, a large number of waste products with excessive dimensions may appear. For this reason, countries have specially formulated national standards or industry standards for the dimensional tolerance of plastic parts. China has also formulated ministerial professional standards. However, most of them do not have corresponding dimensional tolerances for mold cavities. The German national standard specifically formulates the DIN16901 standard for the dimensional tolerance of plastic parts and the corresponding DIN16749 standard for the dimensional tolerance of mold cavities. This standard has a great influence in the world and can be used as a reference for the plastic mold industry. Mold People Magazine WeChat, the first in the industry!
About the dimensional tolerance and allowable deviation of plastic parts
In order to reasonably determine the dimensional tolerance of plastic parts formed by materials with different shrinkage characteristics, the standard introduces the concept of molding shrinkage difference △VS.
△VS＝VSR_VST(4)
Where: VS－Molding shrinkage difference VSR－Molding shrinkage rate in the direction of melt flow VST－Molding shrinkage rate in the direction perpendicular to the melt flow.
According to the △VS value of plastic, 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, the group with the largest △VS value is the low-precision group. According to the basic dimensions, precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups are compiled. It is also stipulated that the dimensional tolerances of plastic parts formed by plastics with the most stable shrinkage characteristics can be selected from groups 110, 120 and 130.
The dimensional tolerances of plastic parts formed by plastics with moderately stable shrinkage characteristics are selected from groups 120, 130 and 140. If the dimensional tolerances of plastic parts formed by this type of plastic are selected from group 110, a large number of plastic parts with out-of-tolerance dimensions may be produced. The dimensional tolerances of plastic parts formed by plastics with poor shrinkage characteristics are 130, 140 and 150 groups.
The dimensional tolerances of plastic parts formed by plastics with the worst shrinkage characteristics are 140, 150 and 160 groups. When using this tolerance table, you should also pay attention to the following points. The general tolerances in the table are used for dimensional tolerances that do not indicate tolerances.
The tolerances with direct deviations are used to mark the tolerance bands for plastic part dimensions. The upper and lower deviations can be determined by the designer. For example, if the tolerance band is 0.8mm, the following upper and lower deviations can be selected. 0.0;-0.8;±0.4;-0.2;-0.5, etc. There are two groups of tolerance values, A and B, in each tolerance group. Among them, A is the size formed by the combination of mold parts, which increases the error caused by the lack of tightness at the joint of mold parts.
This added value is 0.2mm. Among them, B is the size directly determined by the mold parts. Precision technology is a set of tolerance values specially established for plastic parts with high precision requirements. Before using plastic part tolerances, you must first know which tolerance groups are applicable to the plastics used.
