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

 

Properties of thermoplastics are that they expand upon heating, shrink upon cooling, and of course shrink in volume upon pressurization. In injection molding process, molten plastic is first injected into mold cavity. After filling, molten material cools and solidifies. When plastic part is taken out from mold, shrinkage occurs. This shrinkage is called forming shrinkage. During period of time when plastic part is taken out of 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 water content of nylon 610 is 3%, dimensional increase is 2%; when water content of glass fiber reinforced nylon 66 is 40%, dimensional increase is 0.3%. But main role is forming shrinkage.
At present, the method of determining the shrinkage rate of various plastics (forming shrinkage + post shrinkage) generally recommends provisions of DIN16901 in German national standard. That is to say, difference between size of mold cavity at 23℃ ± 0.1℃ and size of corresponding plastic part measured at a temperature of 23℃ and a relative humidity of 50±5% after being placed for 24 hours after forming is calculated.
Shrinkage rate S is represented by following formula: S={(D-M)/D}×100%(1)
Among them: S - shrinkage; D - mold size; M - plastic part size.
If mold cavity is calculated according to known plastic part size and material shrinkage rate, it is D=M/(1-S). In order to simplify calculation in mold design, following formula is generally used to calculate mold size:
D=M+MS(2)
If a more precise calculation is required, following formula is applied: D=M+MS+MS2(3)
However, when determining shrinkage rate, since actual shrinkage rate is affected by many factors, only an approximate value can be used, so calculation of cavity size by formula (2) basically meets requirements. When manufacturing mold, cavity is processed according to lower deviation, and core is processed according to upper deviation, so that it can be properly trimmed when necessary.
Main reason why it is difficult to accurately determine shrinkage rate is that shrinkage rate of various plastics is not a fixed value, but a range. Because shrinkage rates of same material produced by different factories are not same, even shrinkage rates of different batches of same material produced by a factory are different.
Therefore, each factory can only provide users with shrinkage rate range of plastic produced by factory. Secondly, actual shrinkage rate during forming process is also affected by factors such as shape of plastic part, mold structure and forming conditions. Influence of these factors is described below.

 

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

Gate form also has an effect on shrinkage. When a small gate is used, shrinkage rate of plastic part increases because gate is solidified before end of holding pressure. Cooling circuit structure in injection mold is also a key in mold design. If cooling circuit is not properly designed, shrinkage difference will occur due to uneven temperature of plastic parts, result is that plastic parts are out of tolerance or deformed. In thin-walled part, influence of mold temperature distribution on shrinkage rate is more obvious.

In addition to basic dimensions calculated by D=M(1+S) formula, machining dimensions of mold cavity and core also have a machining tolerance problem. By convention, machining tolerance of mold is 1/3 of plastic part tolerance. However, due to differences in the range and stability of plastic shrinkage, dimensional tolerance of plastic parts formed by different plastics must be rationalized first. That is to say, dimensional tolerance of plastic molded parts should be larger due to larger shrinkage rate range or 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 dimensional tolerances of plastic parts. China has also formulated ministerial-level professional standards. But most of them do not have corresponding dimensional tolerance of mold cavity. In German national standard, DIN16901 standard for dimensional tolerance of plastic parts and DIN16749 standard for 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 plastic mold industry.

In order to reasonably determine dimensional tolerance of plastic parts formed by materials with different shrinkage characteristics, let standard introduce concept of forming shrinkage difference △VS.​​
△VS=VSR_VST(4)
In formula: VS - forming shrinkage difference VSR - forming shrinkage in direction of melt flow VST - forming shrinkage in direction perpendicular to flow of melt.
Shrinkage characteristics of various plastics are divided into 4 groups according to ΔVS value of plastics. Group with the smallest △VS value is high-precision group, and so on, group with the largest △VS value is low-precision group. Precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups are prepared according to basic dimensions. It is stipulated that 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 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. Dimensional tolerances of plastic parts with poor shrinkage characteristics are selected from groups 130, 140 and 150.
Dimensional tolerances of plastic parts with the worst shrinkage characteristics are selected from groups 140, 150 and 160. Following points should also be noted when using this tolerance table. General tolerances in table are used for dimensional tolerances that do not specify tolerances.
Tolerances that directly label deviations are tolerance zones used to label dimensions of plastic parts. Upper and lower deviations can be determined by designer. For example, if tolerance zone is 0.8mm, 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 size formed by combination of mold parts, which increases error caused by non-adherence of mold parts.
This increase is 0.2mm. where B is dimension directly determined by 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 plastic used.