What is the relationship between mold size and plastic shrinkage?

First, plastic shrinkage rate and its influencing factors

The characteristic of thermoplastics is that they expand after heating, shrink after cooling, and of course the volume will shrink after pressurization. In the injection molding process, the molten plastic is first injected into the mold cavity, and after the filling is completed, the melt is cooled and solidified, and shrinkage occurs when the plastic part is taken out from the mold, and the shrinkage is called forming shrinkage. The size of the plastic part will still change slightly from the time the mold is removed to the stable state. One change is to continue shrinking. This shrinkage is called back shrinkage. Another variation is that some hygroscopic plastics swell due to moisture absorption.

For example, when the water content of the nylon 610 is 3%, the dimensional increase is 2%; and when the water content of the 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 for determining the shrinkage ratio (forming shrinkage + post-shrinkage) of various plastics is generally recommended in the German national standard DIN16901. That is, when the mold cavity size is 23 ° C ± 0.1 ° C and placed after molding for 24 hours, the difference between the corresponding plastic parts measured under the conditions of a temperature of 23 ° C and a relative humidity of 50 ± 5% is calculated.

The shrinkage ratio S is expressed by the following formula: S = {(DM) / D} × 100% (1)

Among them: S-shrinkage; D-die size; M-plastic 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 the mold design, the mold size is generally used to find the mold size:

D = M + MS (2).

If a more accurate 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 the approximate value can be used. Therefore, the calculation of the cavity size by the formula (2) basically satisfies the requirement. When manufacturing the mold, the cavity is machined according to the lower deviation, and the core is machined according to the 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 rates of the same materials produced by different factories are not the same, even the same batch of the same material produced by a factory has different shrinkage rates. Therefore, each factory can only provide users with a range of shrinkage of plastics produced by the plant. 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.

Second, the shape of the plastic parts

For the wall thickness of the formed part, the shrinkage rate is also large due to the long cooling time of the thick wall, as shown in Fig. 1. For a general plastic part, when the difference between the flow direction L dimension of the melt and the dimension W perpendicular to the flow direction of the melt flow is large, the difference in shrinkage ratio is also large. From the flow distance of the melt, the pressure loss away from the gate portion is large, and thus the shrinkage rate at this point is also larger than that near the gate portion. Because of the shrinkage resistance of the ribs, holes, bosses, and engravings, the shrinkage of these parts is small.

Third, the mold structure

The gate form also has an effect on shrinkage. When a small gate is used, the shrinkage of the plastic part is increased because the gate is solidified before the pressure is maintained. 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 is caused by the uneven temperature of the plastic parts, and the result is that the size of the plastic part is excessive or deformed. In the thin-walled part, the influence of the mold temperature distribution on the shrinkage rate is more pronounced.

Parting surface and gate

Factors such as parting surface, gate form and size of the mold directly affect the flow direction, density distribution, pressure-preserving and shrinking action and molding time.

The use of direct gates or large-section gates can reduce shrinkage, but the anisotropy is large, shrinking in the direction of the flow direction is small, and shrinking along the vertical flow direction; conversely, when the gate thickness is small, the gate portion will prematurely condense. After hardening, the plastic in the cavity is not replenished in time, and the shrinkage is large.

The point gate is fast-sealed, and when the condition of the workpiece is allowed, a multi-point gate can be provided, which can effectively extend the dwell time and increase the cavity pressure, so that the shrinkage rate is reduced.

Fourth, forming conditions

Barrel temperature: When the barrel temperature (plastic temperature) is high, the pressure transmission is better and the contraction 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 feed is minimized to keep the size of the plastic part stable. However, if the feed is insufficient, the pressure cannot be maintained, and the shrinkage rate is also increased.

Injection pressure: The injection pressure is a factor that has a large influence on the shrinkage rate, especially the pressure of the pressure-holding page number 335 after the filling is completed. In general, when the pressure is high, the density of the material is large and the shrinkage rate is small.

The pressure during injection molding includes injection pressure, holding pressure, and cavity pressure. These factors have a significant impact on the shrinkage behavior of the plastic parts.

Increasing the injection pressure can reduce the shrinkage of the product. This is because the pressure is increased, the injection speed is increased, and after the filling process is accelerated, on the one hand, the melt temperature is increased due to the shear heat of the plastic melt, and the flow resistance is reduced; on the other hand, the melt temperature can also be obtained. In the state of high temperature and low flow resistance, it enters the pressure-preserving feeding stage earlier. Especially for thin-walled plastic parts and small gate plastic parts, due to the fast cooling rate, the filling process should be shortened as much as possible.

The higher holding pressure and cavity pressure make the products in the cavity dense and shrinkage, especially the pressure in the pressure-holding stage has a greater influence on the shrinkage rate of the product. This can be explained by the fact that the molten resin is compressed under the molding pressure. The higher the pressure, the greater the amount of compression that occurs, and the greater the elastic recovery after the pressure is released, so that the size of the plastic part is closer to the cavity size, so the smaller the shrinkage amount. .

However, even for the same product, the pressure of the resin in the cavity is not uniform in each part; the injection pressure is different in the portion where the injection pressure is difficult to act and the portion where it is easy to act. In addition, the pressure of each cavity of the multi-cavity mold should be designed uniformly, otherwise the shrinkage rate of the products of each cavity will be inconsistent.

Injection speed: The injection speed has little effect on the shrinkage rate. However, when the thin-walled plastic parts or gates are very small, and when the reinforcing material is used, the injection speed is increased and the shrinkage rate is small.

Mold temperature: After the thermoplastic melt is injected into the cavity, it releases a large amount of heat and solidifies. Different plastic varieties require the cavity to be maintained at an appropriate temperature. At this temperature, it will be most beneficial for the molding of plastic parts, with the highest molding efficiency and the minimum internal stress and warpage.

The mold temperature is the main factor controlling the cooling and setting of the product. The influence of the mold temperature on the molding shrinkage is mainly reflected in the process before the product is demolded after the gate is frozen. However, before the gate freezes, although the mold temperature rises, there is a tendency to increase the heat shrinkage, but it is also the higher mold temperature that causes the gate freezing time to prolong, resulting in an increase in the injection pressure and the pressure-preserving effect, and the feeding effect and The amount of negative contraction will increase.

Therefore, total shrinkage is the result of a combination of two types of reverse shrinkage, and the value does not necessarily increase as the mold temperature increases. If the gate freezes, the effects of injection pressure and holding pressure will disappear. As the mold temperature increases, the cooling set-up time will also prolong, so the shrinkage of the product will generally increase after demolding.

Forming cycle: There is no direct relationship between the forming cycle and the shrinkage. However, it should be noted that when the forming cycle is accelerated, the mold temperature, the melt temperature, and the like are also inevitably changed, which also affects the change in the shrinkage rate. In the material test, the forming shall be carried out in accordance with the forming cycle determined by the required output, and the dimensions of the plastic part shall 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.

Forming conditions: maximum injection pressure 178MPa, barrel temperature 230 (225-230-220-210) °C, 240 (235-240-230-220) °C, 250 (245-250-240-230) °C, 260 (225 -260-250-240) ° C, injection speed 57 cm 3 / s, injection time 0.44 ~ 0.52 s, dwell time 6.0 s, cooling time 15.0 s.

Five, mold size 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 have a problem of machining tolerances. By convention, the machining tolerance of the mold is 1/3 of the tolerance of the plastic part. However, due to the differences in the range and stability of plastic shrinkage, it is first necessary to rationalize the dimensional tolerances of the plastic parts formed by different plastics. That is, the dimensional tolerance of the plastic molded part with a large shrinkage ratio or a poor shrinkage ratio should be made larger. Otherwise, there may be a large number of waste products of exceptional size. To this end, countries have developed national or industry standards for dimensional tolerances of plastic parts. China has also developed ministerial professional standards. However, most of them do not have the corresponding dimensional tolerances of the mold cavity. In the German national standard, the DIN16901 standard for dimensional tolerances of plastic parts and the corresponding DIN16749 standard for dimensional tolerances of mold cavities have been developed. This standard has a large impact in the world and is therefore available for reference in the plastic mold industry.

6. Dimensional tolerances and tolerances for plastic parts

In order to reasonably determine the dimensional tolerances of the molded parts of the different shrinkage characteristics, the standard introduces the concept of the forming shrinkage difference ΔVS. △VS=VSR_VST(4)

Where: VS-forming shrinkage difference VSR-forming shrinkage ratio VST of the flow direction of the melt and the forming shrinkage ratio perpendicular to the flow of the melt.

According to the plastic ΔVS value, the shrinkage characteristics of various plastics were divided into four groups. The group with the smallest ΔVS value is a high-precision group, and so on, the group with the largest ΔVS value is a low-precision group. Precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups were developed according to the basic dimensions. It is also stipulated that the dimensional tolerances of plastic molded parts with the most stable shrinkage characteristics can be selected from the group of 110, 120 and 130. Dimensional tolerances for plastic molded parts with moderately stable shrinkage characteristics are 120, 130 and 140. If 110 types of dimensional tolerances for plastic molded parts of this type are used, it is possible to produce a large number of oversized plastic parts. The dimensional tolerances of plastic molded parts with poor shrinkage characteristics are selected from the group of 130, 140 and 150. The dimensional tolerances for plastic molded parts with the worst shrinkage characteristics are selected from the group of 140, 150 and 160. When using this tolerance table, you should 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 labeling deviation is the tolerance band used to dimension tolerances for plastic parts. The upper and lower deviations can be determined by the designer. For example, if the tolerance band is 0.8 mm, the following various upper and lower deviations may be selected, 0.0; -0.8; ±0.4; -0.2; -0.5. There are tolerance values ​​for the two groups A and B in each tolerance group. Where A is the size formed by the combination of the mold parts, which increases the error caused by the incompatibility of the mold parts. This increase is 0.2 mm. Where B is the size directly determined by the mold part. Precision technology is a set of tolerance values ​​specifically set for the use of high precision plastic parts. Before using the tolerances of the plastic parts, you must first know which tolerance groups are suitable for the plastic used.

Seven, mold manufacturing tolerances

The German national standard sets the standard DIN16749 for the tolerance of the moulds for the tolerances of the moulds. There are 4 tolerances in the table. Regardless of the plastic part of the material, the mold manufacturing tolerances in which the dimensional tolerances are not indicated are the tolerances of the serial number 1. The specific tolerance value is determined by the basic size range. Regardless of the material, the mold manufacturing tolerance of medium-precision size is the tolerance of No. 2. Regardless of the material, the mold manufacturing tolerance of the higher precision size of the plastic part is the tolerance of the serial number 3. The corresponding mold manufacturing tolerance for precision technology is the tolerance of No. 4. Reasonable tolerances of various plastic parts 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.