When designing a plastic mold, after the mold structure is determined, the detailed design of each part of the mold can be carried out, that is, the size of each template and parts, the size of the cavity and the core, etc. are determined. This will involve key design parameters such as material shrinkage. Therefore, the size of each part of the cavity can only be determined by knowing the shrinkage rate of the formed plastic. Even if the selected mold structure is correct, but the parameters used are not appropriate, it is impossible to produce qualified plastic parts.
The characteristic of thermoplastics is that they expand after heating and shrink after cooling, and of course the volume will also shrink after pressurization. In the injection molding process, the molten plastic is first injected into the mold cavity, and after filling, the molten material cools and solidifies, and shrinks when the plastic part is taken out of the mold, which is called molding shrinkage. During the period of time when the plastic part is taken out of the mold and stabilized, there will still be slight changes in size. One kind of change is to continue to shrink, and 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 it is the forming shrinkage that plays a major role.
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 and the corresponding plastic part size measured at 23°C and relative humidity of 50±5% after forming for 24 hours is calculated.
The 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 mold design, the following formula is generally used to find the mold size:
D=M+MS(2)
If a more precise calculation is required, the following formula should be applied: D=M+MS+MS2(3)
However, when determining the shrinkage rate, since the actual shrinkage rate is affected by many factors, only approximate values can be used, so 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 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 in a factory 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 during the forming process is also affected by factors such as the shape of the plastic part, mold structure and forming conditions. The influence of these factors is introduced below.
Plastic shape
For the wall thickness of the formed part, generally due to the longer cooling time of the thick wall, the shrinkage rate is also larger. For general plastic parts, when the difference between the dimension L in the flow direction of the molten material and the dimension W perpendicular to the direction of the molten material flow is large, the difference in shrinkage rate is also large. From the point of view of the flow distance of the melt, the pressure loss at the part far away from the gate is large, so the shrinkage at this place is also larger than that near the gate. Shapes such as ribs, holes, bosses, and engravings are shrink-resistant, so these areas will shrink less.
Mold structure
Gate form also has an effect on shrinkage. When a small gate is used, the shrinkage 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 point 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 parts, and the result will be that the size of the plastic part is out of tolerance or deformed. In thin-walled parts, the influence of mold temperature distribution on shrinkage is more obvious.
Mold Dimensions and Manufacturing Tolerances
In addition to calculating the basic dimensions through the D=M(1+S) formula, the machining dimensions of the mold cavity and core also have a machining tolerance problem. By convention, the processing tolerance of the mold is 1/3 of the tolerance of the plastic part. However, since the shrinkage range and stability of plastics are different, it is first necessary to rationally determine the dimensional tolerances of plastic parts formed by different plastics. That is to say, the dimensional tolerance of plastic molded parts should be larger if the shrinkage range is large or the shrinkage stability is poor. Otherwise, there may be a large number of waste products with out-of-tolerance sizes.
For this reason, various countries have specially 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 tolerances of the mold cavity. In the German national standard, the DIN16901 standard for the dimensional tolerance of plastic parts and the corresponding DIN16749 standard for the dimensional tolerance of the 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.
Dimensional tolerance and allowable deviation of plastic parts
In order to reasonably determine the dimensional tolerances of plastic parts formed by materials with different shrinkage characteristics, the standard introduces the concept of forming shrinkage difference △VS. the
△VS＝VSR_VST(4)
In the formula: 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 plastic △ VS 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 by analogy, the group with the largest △VS value is the low-precision group. And according to the basic size, precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups are compiled. It is also stipulated that the dimensional tolerances of plastic parts with the most stable shrinkage properties can be selected from 110, 120 and 130 groups.
120, 130 and 140 are used for dimensional tolerances of plastic molded parts with moderate and stable shrinkage properties. If 110 sets of dimensional tolerances are used for forming plastic parts of this type of plastic, a large number of out-of-tolerance plastic parts may be produced. 130, 140 and 150 groups are selected for the dimensional tolerances of plastic parts with poor shrinkage properties.
The dimensional tolerance of the plastic molded parts with the worst shrinkage properties is selected from 140, 150 and 160 groups. When using this tolerance table, also pay attention to the following points. The general tolerances in the table are for dimensional tolerances where no tolerances are specified.
The tolerance that directly marks the deviation is the tolerance zone used to mark the tolerance of the plastic part. 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. There are two sets 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 mismatch of mold parts.
This increase is 0.2mm. Where 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 the tolerances of plastic parts, you must first know which tolerance groups are applicable to the plastics used.
