Do you know the relationship between injection mold cavity size and plastic shrinkage?

Do you know the relationship between injection mold cavity size and plastic shrinkage?

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 molding shrinkage.

At present, the method of determining the shrinkage rate of various plastics (molding 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 molding is calculated.

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 different, 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 injection molding process is also affected by factors such as the shape of the plastic part, the mold structure and the injection molding conditions.

The shape of plastic parts

For the wall thickness of the molded plastic part, the shrinkage rate is generally larger due to the longer cooling time of the thick wall. For general plastic parts, when the difference between the dimension L in the flow direction of the melt and the dimension W perpendicular to the direction of the melt flow is large, the difference in shrinkage is also large.

 From the point of view of the melt flow distance, the pressure loss at the part far from the gate is large, so the shrinkage rate here is also larger than that at the nearest gate. Because the shapes of ribs, holes, bosses, and engravings have shrinkage resistance, the shrinkage rate of these parts is small.

Mold Structure

Gate type 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.

Mold 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.

 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 are 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 molding shrinkage difference △VS.

△VS＝VSR_VST(4)

In the formula: VS - molding shrinkage difference VSR - molding shrinkage in the direction of melt flow VST - molding 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 ΔVS value is the high-precision group, and so on, the group with the largest ΔVS 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 the dimensional tolerances of plastic parts with moderately stable shrinkage characteristics. If the dimensional tolerance of this type of resin molded the 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 indicate deviations are tolerance zones used to label 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. Each tolerance group has two sets of tolerance values, A and B. 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.