Nozzle mold shrinkage and its influencing factors

The characteristic of thermoplastics is that they expand after heating and shrink after cooling. Of course, the volume will shrink after pressurization. In the injection molding process, the molten plastic is first injected into the mold cavity. After the filling is completed, the melt is cooled and solidified, and shrinkage occurs when the plastic part is removed from the mold. This shrinkage is called molding shrinkage. During the period when the plastic part is taken out of the mold and stabilized, there will still be a slight change in the size. One change is to continue to shrink. This shrinkage is called post-shrinkage.

Another change is that some hygroscopic plastics swell 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%. But one of the main functions is forming shrinkage. At present, the methods for determining the shrinkage of various plastics (forming shrinkage + post-shrinkage) generally recommend the provisions of DIN16901 in the German national standard. 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 a temperature of 23 ° C and a relative humidity of 50 ± 5% when placed for 24 hours after forming.

· 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 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 find the mold size:

· D = M + MS (2)

· If you need to implement a more accurate calculation, apply the following formula: D = M + MS + MS2 (3) However, when determining the shrinkage rate, because the actual shrinkage rate is affected by many factors, only approximate values ​​can be used, so use Equation (2) calculated cavity size 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 appropriately 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 rate of the same material produced by different factories is different, even the shrinkage rate of the same material produced by a different factory is different. Therefore, each factory can only provide users with the shrinkage range of the plastic produced by the factory. 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 influence of these factors is introduced below.

Shape of plastic parts

For the wall thickness of molded parts, generally the cooling time of the thick wall is longer, so the shrinkage rate is also larger. For general plastic parts, when the difference between the dimension L of the melt flow direction and the dimension W perpendicular to the direction of the melt flow is large, the difference in shrinkage ratio is also large. From the point of view of the flow distance of the melt, the pressure loss away from the gate part is large, so the shrinkage rate at this part is also larger than that at the part near the gate. Because the shapes of ribs, holes, bosses, and carvings have shrinkage resistance, the shrinkage rate of these parts is small.

Mould 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 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. The result is that the plastic parts are out of tolerance or deformed. In the thin-walled part, the influence of mold temperature distribution on shrinkage is more obvious.

Mold size and manufacturing tolerance

In addition to calculating the basic dimensions through the formula of D = M (1 + S), the machining dimensions of the mold cavity and core also have a problem of machining tolerance. By convention, the processing tolerance of the mold is 1/3 of the tolerance of the plastic parts. However, due to differences in plastic shrinkage range and stability, first of all, the dimensional tolerances of plastic parts formed by different plastics must be rationalized. That is, the dimensional tolerances of plastic molded plastic parts should be made larger due to the larger shrinkage range or the poor shrinkage stability. Otherwise, a large amount of waste products with excessive size may appear.

For this reason, various countries have specifically 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 dimensional tolerances of plastic parts and the corresponding DIN16749 standard for dimensional tolerances of mold cavities. This standard has a 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-difference in forming shrinkage VSR-forming shrinkage in the direction of melt flow VST-forming shrinkage in the direction perpendicular to the flow of melt.

According to the plastic △ VS value, the shrinkage characteristics of various plastics are divided into 4 groups. The group with the smallest ΔVS value is the high-precision group, and so on, the group with the largest △ VS value is the low-precision group. And according to the basic size, we have compiled precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups. It also stipulates that 110, 120 and 130 groups can be used for the dimensional tolerance of plastic molded plastic parts with stable shrinkage characteristics.

Molding plastic parts with medium and stable shrinkage characteristics, the dimensional tolerances are 120, 130 and 140. If the dimensional tolerance of such plastic forming plastic parts is selected as 110 groups, a large number of out-of-tolerance plastic parts may be produced. Use 130, 140 and 150 groups for the dimensional tolerance of plastic parts with poor shrinkage characteristics. Use 140, 150 and 160 groups for the dimensional tolerance of plastic parts with poor shrinkage characteristics. When using this tolerance table, also pay attention to the following points. The general tolerances in the table are for dimensional tolerances that do not indicate tolerances.

The tolerance for direct marking deviation is the tolerance band used for marking the tolerance of plastic parts. The upper and lower deviations can be determined by the designer. For example, the tolerance zone is 0.8mm, you can use the following various upper and lower deviations. 0.0; -0.8; ± 0.4; -0.2; -0.5 etc. Each tolerance group has two tolerance values ​​of A and B. Among them, A is the size formed by the combination of mold parts, which increases the error caused by the non-closed joint of the 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, we must first know which tolerance groups are suitable for the plastics used.