Several points of attention when designing plastic molds

When designing a plastic mold， after the mold structure is determined, each part of the mold can be designed in detail, that is, the size of each template and parts, the size of the cavity and the core, etc. At this time, the main design parameters related to material shrinkage will be involved. Therefore, the size of each part of the cavity can be determined only by mastering the shrinkage rate of the molded plastic. Even if the selected mold structure is correct, but the parameters used are not correct, it is impossible to produce qualified plastic parts.

Plastic shrinkage and its influencing factors

The properties of thermoplastics are that they expand upon heating, shrink upon cooling, and of course shrink in volume upon pressurization. In the injection molding process, the molten plastic is first injected into the mold cavity. After filling, the molten material cools and solidifies. When the plastic part is taken out from the mold, shrinkage occurs. This shrinkage 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 a small change in size. 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 dimensional increase is 2%; when the water content of glass fiber-reinforced nylon 66 is 40%, the dimensional increase is 0.3%. But the main role is the forming shrinkage.

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 to say, 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% after being placed for 24 hours after forming is calculated.

The shrinkage rate S is represented by the following formula: S={(D-M)/D}×100%(1)

Among them: S - shrinkage; 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 calculate the mold size:

D=M+MS(2)

If a more precise calculation is required, the following formula is applied: D=M+MS+MS2(3)

However, when determining the shrinkage rate, since the actual shrinkage rate is affected by many factors, only an approximate value 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 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 rates of the same material produced by different factories are not the same, even the shrinkage rates of different batches of the same material produced by a factory are different. Therefore, each factory can only provide users with the shrinkage rate range of the plastic 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, the mold structure and the forming conditions. The influence of these factors is described below.

Plastic shape

For the wall thickness of the 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 difference between the dimension L in the direction of melt flow and the dimension W perpendicular to the direction of melt flow is large, the difference in shrinkage rate is also large. From the point of view of the melt flow distance, the pressure loss in the part far from the gate is large, so the shrinkage rate here is also larger than that near the gate. Because the shapes of ribs, holes, bosses and engravings have shrinkage resistance, the shrinkage rate of these parts is small.

Mold structure

Gate form also has an effect on shrinkage. When a small gate is used, the shrinkage rate of the plastic part increases because the gate is solidified 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 properly designed, the shrinkage difference will occur due to the uneven temperature of the plastic parts, and the result is that the plastic parts are out of tolerance or deformed. In the thin-walled part, the influence of the mold temperature distribution on the shrinkage rate is more obvious.

Forming conditions

Barrel temperature: When the barrel temperature (plastic temperature) is high, the pressure transfer is better and the shrinkage force is reduced. However, when a small gate is used, the shrinkage rate is still large due to the early curing of the gate. For thick-walled plastic parts, even if the barrel temperature is high, the shrinkage is still large.

Feeding: In the forming conditions, the feeding is minimized to keep the dimensions of the plastic part stable. However, insufficient feeding will not be able to maintain the pressure and will also increase the shrinkage rate.

Injection pressure: The injection pressure is a factor that has a great influence on the shrinkage rate, especially the holding pressure after filling. In general, when the pressure is high, the shrinkage rate is small due to the high density of the material.

Injection speed: Injection speed has less effect on shrinkage. However, for thin-walled plastic parts or very small gates, and when using reinforced materials, the shrinkage rate is small when the injection speed is increased.

Mold temperature: Usually, the shrinkage rate is larger when the mold temperature is higher. But for thin-walled plastic parts, when the mold temperature is high, the flow resistance of the melt is small, and the shrinkage rate is small.

Forming cycle: The forming cycle is not directly related to the shrinkage rate. However, it should be noted that when the forming cycle is accelerated, the mold temperature, melt temperature, etc. must also change, which also affects the change in shrinkage. In the material test, it should be formed according to the forming cycle determined by the required output, and the dimensions of the plastic parts should be inspected.

An example of a plastic shrinkage test using this mold is as follows. Injection machine: Clamping force 70t Screw diameter Φ35mm Screw speed 80rpm Molding conditions: Maximum injection pressure 178MPa Barrel temperature 230(225-230-220-210)℃ 240(235-240-230-220)℃ 250(245-250 -240-230)℃ 260(225-260-250-240)℃ Injection speed 1425px3/s Injection time 0.44～0.52s Pressure holding time 6.0s Cooling time 15.0s

Die Dimensions and Manufacturing Tolerances

In addition to the basic dimensions calculated by the D=M(1+S) formula, the machining dimensions of the mold cavity and core also have a machining tolerance problem. By convention, the machining tolerance of the mold is 1/3 of the plastic part tolerance. However, due to the differences in the range and stability of plastic shrinkage, the dimensional tolerance of plastic parts formed by different plastics must be rationalized first. That is to say, the dimensional tolerance of plastic molded parts should be larger due to the larger shrinkage rate range or the poorer shrinkage rate stability. Otherwise, there may be a large number of out-of-tolerance scraps.

To this end, 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 tolerance of the mold cavity. In the German national standard, the DIN16901 standard for the dimensional tolerance of plastic parts and the DIN16749 standard for the corresponding mold cavity dimensional tolerance is specially formulated. 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, let the standard introduce the concept of forming shrinkage difference △VS. △VS=VSR_VST(4)

In the formula: VS-forming shrinkage difference VSR-molding shrinkage in the direction of melt flow VST-forming shrinkage in the direction perpendicular to the flow of melt.

The shrinkage characteristics of various plastics are divided into 4 groups according to the ΔVS value of the plastics. 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 prepared according to the basic dimensions. It is stipulated that the dimensional tolerances of plastic parts with the most stable shrinkage characteristics can be selected from groups 110, 120 and 130. Use 120, 130 and 140 for dimensional tolerances of plastic molded parts with moderately stable shrinkage characteristics.

If the dimensional tolerance of this type of plastic forming plastic parts is selected as 110 groups, a large number of plastic parts with out-of-tolerance may be produced.

The dimensional tolerances of plastic parts with poor shrinkage characteristics are selected from groups 130, 140, and 150. The dimensional tolerances of plastic parts with the worst shrinkage characteristics are selected from groups 140, 150, and 160. The following points should also be noted when using this tolerance table. The general tolerances in the table are used for dimensional tolerances that do not specify tolerances. Tolerances that directly label deviations are tolerance zones used to label the dimensions 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 various 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 non-adherence of the mold parts. This increase is 0.2mm. where B is the dimension directly determined by the mold part. Precision technology is a set of tolerance values specially established for plastic parts with high precision requirements. Before using plastic part tolerances, you must first know which tolerance groups are applicable to the plastic used.

Manufacturing tolerances of molds

The German national standard has formulated the standard DIN16749 of the corresponding mold manufacturing tolerance for the tolerance of plastic parts. There are 4 tolerances in this table. Regardless of the material of the plastic parts, the mold manufacturing tolerances that do not indicate dimensional tolerances use the tolerance of serial number 1. The specific tolerance value is determined by the basic size range. No matter what kind of material, the mold manufacturing tolerance of the medium-precision size of the plastic part is the tolerance of serial number 2. No matter what kind of material, the mold manufacturing tolerance of the higher precision dimension of the plastic part is the tolerance of serial number 3. The corresponding mold manufacturing tolerance of precision technology is the tolerance of serial number 4.

Reasonable tolerances of plastic parts of various materials and corresponding mold manufacturing tolerances can be reasonably determined, which not only brings convenience to mold manufacturing but also reduces waste and improves economic efficiency.