Shrinkage rate of the hottest plastic and mold siz

2022-10-03
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Plastic shrinkage and mold size

when designing a plastic mold, after determining the mold structure, you can carry out a detailed design of each part of the mold, that is, to determine the size of each template and part, cavity and core size, etc. At this time, the main design parameters such as material shrinkage will be involved. Therefore, the size of each part of the cavity can be determined only by mastering the shrinkage of the formed plastic concretely. Even if the selected mold structure is correct, but the parameters used are incorrect, it is impossible to produce plastic parts with qualified quality

plastic shrinkage and its influencing factors

thermoplastic is characterized by expansion after heating and contraction after cooling. Of course, the volume will also shrink after pressurization. In the process of injection molding, the molten plastic is first injected into the mold cavity. After filling, the molten material is cooled and solidified, and shrinkage occurs when the plastic part is taken out of the mold. This shrinkage is called forming shrinkage. During the period from taking out the plastic part from the mold to stabilizing, the size will still change slightly. One change is to continue to shrink, which 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 forming shrinkage plays a major role. At present, the method of determining the shrinkage 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 ℃± 0.1 ℃ and the corresponding plastic part size measured at 23 ℃ and 50 ± 5% relative humidity after 24 hours of forming

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

where: s-shrinkage& Nbspd- die size& Nbspm- size of plastic parts

if the mold cavity is calculated according to the known plastic part size and material shrinkage, it is  d=m/(1-s). In order to simplify the calculation in the mold design, the following formula is generally used to calculate the mold size:

d=m+ms (2)

if more accurate calculation is required, the following formula is applied:  d =m+ms+ms2 (3)

but when determining the shrinkage, because the actual shrinkage is affected by many factors, only approximate values can be used, Therefore, the cavity size calculated 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 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 a factory in different batches is also different. Therefore, each factory can only provide users with the shrinkage range of the plastics 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. The influence of these factors is introduced below

shape of plastic parts

for the wall thickness of formed parts, generally due to the long cooling time of thick walls, the shrinkage is also large, as shown in Figure 1. For general plastic parts, improved modeling and simulation tools are also needed. 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 is also large. From the perspective of melt flow distance, the pressure loss away from the gate is 180 d=7a, so the shrinkage rate here is also greater than that near the gate. Due to the shrinkage resistance of stiffeners, holes, bosses and sculptures, the shrinkage rate of these parts is small

mold structure

gate has been widely implemented, and the form also affects the shrinkage. When using a small gate, the shrinkage of plastic parts increases because the gate solidifies before the end of pressure maintaining. The cooling circuit structure in injection mold is also a key in mold design. If the cooling circuit is not properly designed, the shrinkage difference will occur due to the uneven temperature at all parts of the plastic part, and the result is that the size of the plastic part is out of tolerance or deformed. 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 high, the pressure transmission is good and the shrinkage force is reduced. However, when using a small gate, the shrinkage rate is still large due to the early solidification of the gate. For thick wall plastic parts, even if the barrel temperature is high, its shrinkage is still large

Replenishment: in the forming conditions, reduce the replenishment as much as possible to keep the size of plastic parts stable. However, insufficient feeding will not maintain the pressure and will also increase the shrinkage

injection pressure: the injection pressure is a factor that has a great influence on the shrinkage rate, especially the pressure holding page No. 335 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 contraction rate. However, for thin-walled plastic parts or gates that are very small, and when strengthening materials are used, the shrinkage rate is small when the injection speed is accelerated

mold temperature: generally, when the mold temperature is high, the shrinkage rate is also large. However, for thin-walled plastic parts, if the mold temperature is high, the flow resistance of molten material is small, *] while the shrinkage is small

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 die temperature and melt temperature must also change, which also affects the change of shrinkage. During the material test, the forming shall be carried out according to the forming cycle determined by the required output, and the size of plastic parts shall be inspected. Examples of plastic shrinkage test with this mold are as follows. Injection machine: locking force 70t screw diameter Φ 35mm screw speed 80rpm forming conditions: maximum injection pressure 178mpa barrel temperature 230 () ℃  240 () ℃  250 () ℃  260 () ℃ injection speed 57cm3/s injection time 0.44 ~ 0.52s pressure holding time 6.0s cooling time 15.0s

mold size and manufacturing tolerance

processing dimensions of mold cavity and core in addition to calculating the basic dimensions through d=m (1+s) formula, there is also a problem of processing tolerance. According to convention, the processing tolerance of the mold is 1/3 of the plastic part tolerance. However, due to the differences in the shrinkage range and stability of plastics, it is necessary to rationalize the dimensional tolerance of plastic parts formed by different plastics. That is, the dimensional tolerance of plastic parts formed by plastic with large shrinkage range or poor shrinkage stability should be larger. Otherwise, there may be a large number of waste products with simple and accurate adjustment amplitude. Therefore, countries have formulated national standards or industry standards for the dimensional tolerance of plastic parts. China has also formulated ministerial professional standards. But most of them have no corresponding dimensional tolerance of mold cavity. In German national standards, din16901 standard for dimensional tolerance of plastic parts and din16749 standard for dimensional tolerance of mold cavity are specially formulated. This standard has great influence in the world, so it can be used as a reference for the plastic mold 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 introduced the concept of forming shrinkage difference △ vs

△VS＝VSR_ VST (4)

where:  vs - forming shrinkage difference VSR - forming shrinkage in the direction of melt flow VST - forming shrinkage in the direction perpendicular to melt flow

according to the △ vs value of plastics, the shrinkage characteristics of various plastics are divided into four 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. Precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups are compiled according to the basic dimensions. It is also stipulated that the dimensional tolerances of plastic parts formed with plastic with the most stable shrinkage characteristics can be 110, 120 and 130 groups. The dimensional tolerances of plastic parts formed with plastics with moderate and stable shrinkage characteristics are 120, 130 and 140. If the dimensional tolerance of plastic parts formed with this kind of plastic is 110 groups, a large number of plastic parts with out of tolerance dimensions can be produced. The dimensional tolerances of plastic parts formed with plastics with poor shrinkage characteristics are 130, 140 and 150 groups. The dimensional tolerances of plastic parts formed with 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 without tolerance indication. The tolerance of direct marking deviation is the tolerance zone used to mark the tolerance of plastic parts. The upper and lower deviations can be determined by the designer. For example, if the tolerance zone is 0.8mm, the following upper and lower deviations can be selected. 0.0;- 0.8; ±0.4;- 0.2;- 0.5, etc. Each tolerance group has two tolerance values, a and B. Where a is the size formed by the combination of mold parts, which increases the misalignment caused by the non close fitting of mold parts. This increase is 0.2mm. Where B is the dimension directly determined by the mold parts. Precision technology is a set of tolerance values specially set up for plastic parts with high precision requirements. Before using the tolerance of plastic parts, we must first know which tolerance groups are applicable to the plastic used

mold manufacturing tolerance

the German national standard has formulated the corresponding mold manufacturing tolerance standard din16749 for the tolerance of plastic parts. There are four tolerances in this table. No matter what kind of plastic parts are made of, the tolerance of No. 1 shall be used for the manufacturing tolerance of molds without dimensional tolerance. The specific tolerance value is determined by the basic dimension range. No matter what kind of material, the mold manufacturing tolerance of medium precision dimension of plastic parts is the tolerance of No. 2. No matter what kind of material, the mold manufacturing tolerance of plastic parts with higher precision dimensions is the tolerance of No. 3. The corresponding die manufacturing tolerance of precision technology is the tolerance of No. 4

it can reasonably determine the reasonable tolerance of plastic parts of various materials and the corresponding mold manufacturing tolerance, which not only brings convenience to mold manufacturing, but also reduces waste products and improves economic efficiency. (end)

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