In the design of plastic mold, the mold structure can be determined after the detailed design of each part of the mold, that is, to determine the size of each template and parts, cavity and core size. At this time will be involved in the material shrinkage and other major design parameters. Therefore, only the specific grasp of plastic forming shrinkage can determine the size of each part of the cavity. Even if the selected mold structure is correct, but the parameters used are improper, it is impossible to produce qualified plastic parts.
Plastic shrinkage and its influencing factors
Thermoplastics expand when heated, contract when cooled and, of course, shrink when pressurized. 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. When the plastic parts are removed from the mold, shrinkage occurs. When the plastic part is removed from the mold and stabilized, there will still be a small change in size. One change is the continuous shrinkage, which is called post shrinkage. Another change is that some hygroscopic plastics expand as a result of 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%. But the main effect is shaping shrinkage. At present, a variety of methods to determine the plastic shrinkage rate (forming shrinkage + post shrinkage) are generally recommended in the German national standard 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.
S={(d-m)/D} 100%(1)
Where: s-shrinkage rate; 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). 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)
D=M+MS+MS2(3)
However, when determining the shrinkage rate, as the actual shrinkage rate is affected by many factors, the approximate value can only be used. Therefore, the calculation of cavity size with formula (2) basically meets the requirements. In the manufacture of molds, the cavity is processed according to the lower deviation, and the core is processed according to the upper deviation, so as to make appropriate dressing when necessary.
The main reason that decides shrinkage rate accurately hard, it is because of all sorts of plastic above all shrinkage rate is not a fixed value, however a limits. Because the shrinkage rate of the same material produced by different factories is not the same, even the shrinkage rate of the same material produced by different batches produced by one factory is not the same. Therefore, each factory can only provide users with the shrinkage rate range of the plastic produced by the factory. Secondly, the actual shrinkage rate in the forming process is also affected by the shape of the plastic parts, mold structure and forming conditions. The impact of these factors is described below.
Shape of plastic parts
For the wall thickness of formed parts, generally due to the longer cooling time of the thick wall, the shrinkage rate is also larger, as shown in figure 1. For general plastic parts, when the size of molten material flow direction L is greatly different from the size of W perpendicular to the molten material flow direction, the shrinkage rate is also greatly different. From the perspective of melt flow distance, the pressure loss away from the sprue is large, so the shrinkage rate there is larger than that near the sprue. Because the shape such as reinforcing rib, hole, boss and sculpture has shrinkage resistance, the shrinkage rate of these parts is smaller.
The mould structure
Gate form also has an effect on shrinkage. When using small sprue, the shrinkage rate of the plastic parts increases because the sprue solidifies before the end of holding pressure. The cooling circuit structure in injection mold is also a key point in mold design. Improper cooling circuit design will result in shrinkage difference due to uneven temperature of plastic parts, which will result in oversize or deformation of plastic parts. In the thin wall part, the effect of mould temperature distribution on shrinkage is more obvious.
The forming conditions
Cylinder temperature: when the cylinder temperature (plastic temperature) is high, the pressure transfer is good and the contraction force is reduced. However, when using small sprue, the shrinkage rate is still high due to early solidification of sprue. For thick wall plastic parts, even if the cylinder temperature is higher, its shrinkage is still larger.
Feeding: in the forming condition, the feeding should be reduced as much as possible to keep the size of plastic parts stable. However, insufficient material can not maintain the pressure, but also increase the shrinkage rate.
Injection pressure: injection pressure is a major factor affecting the shrinkage rate, especially the pressure of holding page 335 after filling. In general, when the pressure is large because of the density of the material, the shrinkage rate is small.
Injection speed: the injection speed has little effect on the shrinkage rate. However, for thin-walled plastic parts or gate is very small, and the use of reinforced materials, injection speed faster shrinkage rate is small.
Mold temperature: usually mold temperature higher shrinkage is also larger. However, for thin-wall plastic parts, the flow impedance of molten material is small when the mold temperature is high *], while the shrinkage rate is small.
Forming period: there is no direct relation between forming period and shrinkage rate. However, it should be noted that when the forming cycle is accelerated, the mold temperature and molten metal temperature will inevitably change, which will also affect the shrinkage rate. When material test is carried out, it shall be formed according to the forming cycle determined by the required output, and the size of the plastic parts shall be inspected. The example of plastic shrinkage test with this mould is as follows. Injection machine, clamping force 70 t screw diameter of 35 mm screw rotation speed 80 RPM Φ forming conditions: high injection pressure 178 mpa cylinder temperature 230 ℃ (225-230-220-230) (235-240-230-240) of 240 ℃ to 250 ℃ (245-250-240-250), 260 (225-260-250-260), 57 ℃ injection speed cm3 / s injection time is 0.44 ~ 0.52 s the holding time of 6.0 s cooling time of 15.0 s
Mold dimensions and manufacturing tolerances
In addition to calculating the basic dimensions by D=M(1+S) formula, there is also the problem of machining tolerance. According to the convention, the processing tolerance of the mold is 1/3 of the tolerance of the plastic parts. But because the plastic shrinkage rate range and the stability have the difference, must first rationalize the determination different plastic shapes the size tolerance. That is to say, the dimensional tolerance of plastic forming parts should be larger due to larger shrinkage range or worse shrinkage stability. Otherwise, there may be a large number of out-of-tolerance waste products. For this reason, each country has formulated the national standard or the industry standard specially to the plastic part size tolerance. China has also set professional standards at the ministerial level. But most of them do not have corresponding dimensional tolerance of mold cavity. The German national standard has specifically formulated the DIN16901 standard for dimensional tolerance of plastic parts and the corresponding DIN16749 standard for dimensional tolerance of mold cavity. This standard has great influence in the world, so it can be used as a reference for the plastic mold industry.
Dimensional tolerances and tolerances for plastic parts
In order to reasonably determine the dimensional tolerances of molded parts of materials with different shrinkage characteristics, the standard introduces the concept of forming shrinkage difference (VS).
Delta v = VSR_VST (4)
Where: vs-forming shrinkage difference VSR- forming shrinkage rate in the direction of molten melt flow VST- forming shrinkage rate in the direction perpendicular to molten melt flow.
Shrinkage characteristics of various plastics can be divided into four groups according to the VS values. The smallest VS group is the high-precision group, and so on; the largest VS group is the low-precision group. Precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups have been prepared according to basic dimensions. It is also stipulated that 110, 120 and 130 sets of dimensional tolerances can be selected for plastic parts with the most stable shrinkage characteristics. The dimensional tolerances of plastic parts formed with moderately stable shrinkage properties are selected as 120, 130 and 140. If 110 sets of dimensional tolerances of plastic parts formed by such plastics are selected, a large number of out-of-tolerance plastic parts may be produced. Sets of 130, 140 and 150 are selected for dimensional tolerances of plastic parts formed by plastics with poor shrinkage characteristics. Sets of 140, 150 and 160 are selected for dimensional tolerances of molded parts made of plastics with the worst shrinkage characteristics. The following points need to be noted when using this tolerance table. The general tolerances shown in the table are used for dimensional tolerances without indication. The tolerance of direct marking deviation is the tolerance band used to mark the dimensional tolerance of plastic parts. 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 tolerance values of A and B in each tolerance group. Where A is the size formed by the combination of mold parts, which increases the error caused by the misalignment of mold parts. This added value is 0.2mm. Where B is the size directly determined by the mold parts. Precision technology is a special set of tolerances for plastic parts with high precision requirements. Before this is done, it is necessary to know which tolerance groups are applicable to the plastic used.
Mold manufacturing tolerances
According to the German national standard, the corresponding mold manufacturing tolerance standard DIN16749 has been formulated. There are 4 kinds of tolerances in this table. No matter what kind of material plastic parts, which does not indicate the dimensional tolerance of the mold manufacturing tolerance using the serial number 1 tolerance. The specific tolerance value is determined by the basic size range. No matter what kind of material plastic parts of medium precision size mold manufacturing tolerance for no. 2 tolerance. No matter what kind of material plastic parts with high precision size of the mold manufacturing tolerance for no. 3 tolerance. Precision technology corresponding to the mold manufacturing tolerance for no. 4 tolerance.
It can reasonably determine the reasonable tolerance of all kinds of plastic parts and the corresponding mold manufacturing tolerance, which not only brings convenience to mold manufacturing, but also can reduce the waste and improve the economic benefit.
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