Plastic mold size and shrinkage of the relationship（2）
The mould structure
Gate type also has an effect on shrinkage. When using a small gate, the shrinkage of the plastic part will be increased because the gate is solidified before the end of pressure maintenance. The structure of cooling loop in injection mould is also a key in the design of mould. The improper design of the cooling circuit will lead to the differential shrinkage of the plastic parts due to the uneven temperature at different places, which will result in the out-of-tolerance or deformation of the plastic parts. In the thin-wall section, the influence of mold temperature distribution on shrinkage rate is more obvious.
Mold dimensions and manufacturing tolerances
In addition to calculating the basic dimensions of mold cavity and core through D=M(1+S) formula, there is also the problem of machining tolerance. However, due to the difference of shrinkage rate and stability of plastics, the dimensional tolerance of plastic parts formed by different plastics must be reasonably determined first.Otherwise, there may be a large number of out-of-size waste products. China has also formulated professional standards at the ministerial level. However, most of them do not have corresponding cavity dimensional tolerances. The German national standard specifies the tolerance DIN16901 for plastic part dimensions and the corresponding tolerance DIN16749 for cavity dimensions.This standard has a great influence in the world, so it can be used as a reference for the plastic mold industry.
Dimensional tolerance and allowable deviation of plastic parts
According to the value of plastic delta VS, the shrinkage characteristics of various plastics were divided into four groups. The group with the smallest VS value is the high-precision group, and so on, the group with the maximum VS value is the low-precision group. Precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups were prepared according to the basic dimensions. It is stipulated that the dimensional tolerances of plastic parts formed with the most stable shrinkage characteristics can be selected from groups 110, 120 and 130.
Groups 140, 150, and 160 were selected for dimensional tolerances for molded parts with the worst shrinkage characteristics. The following points should also be noted when using this tolerance chart.The general tolerances in the table are used for dimensional tolerances that do not indicate tolerances.
The tolerance of direct marking deviation is the tolerance band used to mark the tolerance of plastic parts. The upper and lower deviations can be determined by the designer himself. For example, if the tolerance band is 0.8mm, the following upper and lower deviations can be used.0.0; 0.8;+ / - 0.4;0.2;- 0.5, etc. Each tolerance group has tolerance values of group A and group B. A is the size formed by the combination of mold parts, which increases the error caused by the misalignment of mold parts.
This increment is 0.2mm. Where B is directly determined by the mold parts size. Precision technology is a set of tolerance values specially established for the use of plastic parts with high precision requirements. Before using plastic tolerances, it is necessary to know which tolerance groups are applicable to the plastic mold used.