Is plastic shrinkage important for the internal structure of molds?

      When designing a plastic mold, once the mold design is clear, the overall design of each part of the mold can be carried out, including the specifications of each template and part, as well as the specifications of the concave mold and core. At this point, key design parameters such as the shrinkage rate of related raw materials will be involved. Therefore, only by actually grasping the shrinkage rate of the formed plastic can the specifications of each part of the concave mold be clarified. Even if the selected mold design is appropriate, if the commonly used main parameters are not good, it is unlikely to produce plastic parts that meet quality standards.

       Plastic shrinkage rate and influencing factors

The characteristic of thermosetting plastics is that they swell after heating, shrink after cooling, and their volume will also decrease after natural pressurization. In the entire process of injection molding, molten plastic is first injected into the concave mold of the mold shell. After filling, the wear-resistant material cools and solidifies. When the plastic part is removed from the mold shell, shrinkage occurs, which is called forming shrinkage. During the period from the removal of the plastic part from the mold to stability, there will still be slight changes in the specifications, one of which is shrinkage again, and this shrinkage is called post shrinkage.

Another transformation is that some absorbent plastics experience swelling due to moisture absorption. For example, when the moisture content of polyester 610 is 3%, the specification increase rate is 2%; When glass fiber increases the moisture content of nylon 66 to 40%, the specification increase rate is 0.3%. But the key factor that plays a crucial role in it is forming shrinkage. At present, various methods of plastic shrinkage (forming shrinkage+post shrinkage) are clearly defined, and the requirements of DIN16901 in the French national industry standard are generally strongly recommended. It is calculated based on the difference between the relative plastic part specifications accurately measured at a temperature of 23 ℃ ± 0.1 ℃ and a standard air humidity of 50 ± 5% after forming for 24 hours. The shrinkage rate S is expressed by the above equation: S={(D-M)/D} × 100% (1)

Among them: S-shrinkage rate; D-model shell specification; M-Plastic part specifications.

If the mold shell and concave mold are calculated based on the known specifications of the plastic parts and the shrinkage rate of the raw materials, it is D=M/(1-S). In order to simplify the calculation in stamping molds, the following formula is generally used to calculate the mold shell specifications:

D=M MS(2)

If a more accurate calculation is required, the following equation should be used:

D=M MS MS2 (3) However, when determining the shrinkage rate, since the specific shrinkage rate is affected by many factors, only natural numbers can be used. Therefore, using equation (2) to calculate the specifications of the concave mold mostly meets the requirements. In the production and manufacturing of mold shells, the concave mold is produced and processed according to the lower error, and the core is produced and processed according to the upper error, so that appropriate repairs can be made when necessary.

The key reason for the difficulty in accurately determining the shrinkage rate is that the shrinkage rate of various plastics is not a time constant, but rather a category. Due to the different shrinkage rates of the same raw materials produced and manufactured by different processing plants, even the shrinkage rates of raw materials with the same batch number produced and manufactured by one processing plant are different. Therefore, each manufacturer can only provide customers with the shrinkage range of the plastic produced and manufactured by their own factory. Secondly, the specific shrinkage rate during the entire forming process is also affected by factors such as the shape of the plastic part, mold design, and forming standards. Below is a detailed introduction to the hazards of this element. Plastic appearance

Regarding the wall thickness of formed parts, the shrinkage rate is generally high due to the longer cooling time of thick walled tubes, as shown in Figure 1. For general plastic parts, when there is a significant difference between the flow direction L specification of the wear-resistant material and the flow direction W specification perpendicular to the wear-resistant material, the difference in shrinkage rate is also significant. From the perspective of the flowability spacing of wear-resistant materials, it can be seen that preventing damage from working pressure at a certain part of the inlet is greater, so the shrinkage rate at this point is also greater than that near the inlet. Due to the shrinkage resistance of reinforcement plates, holes, convex molds, and hand carved shapes, the shrinkage rate at this position is relatively small.

Mold Design

The method of entering the glue inlet is also harmful to the shrinkage rate. When using a small inlet, the shrinkage rate of the plastic part increases as the inlet dries and solidifies after the pressure is maintained. The refrigeration control circuit structure in injection molds is also an important part of stamping molds. If the design scheme of the refrigeration control circuit is not appropriate, it will cause shrinkage differences due to temperature imbalance in various parts of the plastic parts, resulting in deviations or deformations in the specifications of the plastic parts. In the thick wall section, the temperature distribution of the mold shell has a more significant impact on the shrinkage rate.

Forming standards

Barrel temperature: When the barrel temperature (plastic temperature) is high, the working pressure transmission is good and the shrinkage force is reduced. But when using a small inlet, the shrinkage rate is still high due to the early drying and solidification of the inlet. For thick walled plastic parts, even if the barrel temperature is high, the shrinkage is still significant.

Feeding: In the forming standards, try to avoid feeding to maintain the specifications of the plastic parts for a long time. But if the replenishment is not enough, it will not be able to maintain the work pressure and will also increase the shrinkage rate.

Injecting working pressure: Injecting working pressure is a factor that poses a significant risk to shrinkage rate, especially for the pressure holding page number 335 after filling. In general, when the work pressure is high, the density of raw materials is high and the shrinkage rate is small.

Injection rate: The harm of injection rate to shrinkage rate is relatively small. But for thick walled plastic parts or those with very small inlet ports, and when using reinforced raw materials, the acceleration of injection rate results in a small shrinkage rate.

Mold shell temperature: Generally, the shrinkage rate is also high when the mold shell temperature is high. But for thick walled plastic parts, the mold temperature is high and the flow characteristic impedance of wear-resistant materials is small, while the shrinkage rate is actually small.

Forming cycle time: There is no immediate correlation between forming cycle time and shrinkage rate. However, it should be noted that when accelerating the forming cycle time, changes in mold temperature, wear-resistant material temperature, etc. will inevitably occur, which will also harm the transformation of shrinkage rate. When conducting raw material experiments, forming should be carried out according to the forming cycle time determined by the required production volume, and the specifications of the plastic parts should be tested. The following is a case study of conducting plastic shrinkage experiments using this mold shell.

Specification and manufacturing dimensional tolerances for mold shells

In addition to the basic specifications calculated based on the formula D=M (1 S), there is also a problem of dimensional tolerances in the production and processing of concave molds and cores. According to international practice, the production and processing dimensional tolerance of mold shells is one-third of the dimensional tolerance of plastic parts. However, due to differences in the range of plastic shrinkage and reliability, it is essential to first clarify the standard tolerances for plastic parts formed from different plastics in a reasonable manner. The standard tolerance for plastic formed parts with a large range of shrinkage rates or a stable and weak shrinkage rate should be increased. Otherwise, there is a high possibility of a lot of waste with specification deviations.

Therefore, countries around the world have formulated national industry standards or national standards for the standard tolerances of plastic parts. Our country has also formulated technical professional standards at the departmental level. But most of them do not have standard tolerances for relative concave molds. The French national industry standard has established the DIN16901 specification for plastic part standard tolerances and the corresponding DIN16749 specification for mold and die standard tolerances. This standard poses significant risks in today's world and can serve as a reference for the plastic machining industry.

Regarding the standard tolerances and allowable errors of plastic parts

In order to better and effectively clarify the standard tolerances for plastic parts formed from raw materials with different shrinkage characteristics, the specification introduced the definition of forming shrinkage difference △ VS. △VS＝VSR_VST(4)

In the formula: VS - forming shrinkage difference

VSR - Forming shrinkage rate of flowability orientation of wear-resistant materials

VST - Forming shrinkage rate in the vertical direction of flowability of wear-resistant materials.

According to the △ VS value of plastics, the shrinkage characteristics of various plastics are divided into four groups. The group with the lowest △ VS value is the high-precision group, and so on. The group with a higher △ VS value is the low precision group. And fine technical, 110, 120, 130, 140, 150, and 160 dimensional tolerance groups were formulated according to the basic specifications. And it is required that the standard tolerances for plastic molded parts with the smoothest shrinkage characteristics can be set to 110, 120, and 130. The standard tolerances for plastic molded parts with moderate and stable shrinkage characteristics are 120, 130, and 140.

If the standard tolerance for forming plastic parts using this type of plastic is 110 sets, there is a high possibility of many plastic parts with specification deviations.

The standard tolerances for forming plastic parts with weak shrinkage characteristics are set at 130, 140, and 150 sets. The standard tolerances for forming plastic parts with the worst shrinkage characteristics are 140, 150, and 160 sets. When applying this tolerance table, special attention should be paid to the following points. The general dimensional tolerances in the table are used for standard tolerances that do not indicate dimensional tolerances. The dimension tolerance immediately indicating the error is a tolerance grade used to mark the dimension tolerance of plastic parts.

The upper and lower errors can be independently determined by the design staff. For example, if the tolerance level is 0.8 millimeters, various upper and lower error compositions can be used. 0.0; -0.8; ±0.4; -0.2; -0.5 level. Each dimension tolerance group has A and B2 dimension tolerance values. Among them, A is a specification generated by the composition of mold parts, which improves the error caused by the lack of sealing at the joint of mold parts. This growth value is 0.2 millimeters. Among them, B is the specification immediately determined by the mold parts. Fine technicality is a set of dimensional tolerance values set up by professionals for the application of high-precision plastic parts. Before using plastic part size tolerances here, it is essential to first understand which size tolerance groups are available for the plastic being used.