Plastic injection molding must be based on the premise that the mold can be made according to the required size. However, even if the mold size is certain, the actual size of the product varies with the actual shrinkage. Therefore, in precision injection molding, the control of shrinkage rate is very important.
Whether the injection mold design is suitable or not dominates the shrinkage rate depends on the different resin batches. If the pigment is changed, the shrinkage rate also varies. Because of the different molding machines, the setting of molding conditions, the reproducibility and the action of each molding cycle fluctuate, and the actual shrinkage rate fluctuates, so the control of plastic shrinkage is difficult. In order to know this information, EX Mould will share with you these issues, and how to resolve them or avoid the shrinkage issues for plastic injection mold parts.
Reasons For Plastic Parts Shrinkage Issues
The mold size can be calculated by the product size plus shrinkage rate, so the main factors of shrinkage need to be considered in the injection mold design and there are some reasons for variations in part shrinkage. They are:
Resin pressure
Resin temperature
Mold temperature
Gate sectional area
Injection time
Cooling time
Product wall thickness
Reinforcement material
Directionality
Injection speed
Resin pressure
The resin pressure has a great influence on the shrinkage rate. If the resin pressure is large, the shrinkage rate will be smaller, and the product size will be larger. Even in the first mock exam, resin pressure varies with the shape of the product, resulting in shrinkage. In the case of multi cavity mold, the resin pressure in each cavity is easy to be different, and the shrinkage of each cavity is also different.
Mold temperature
Whether it is non crystalline resin or crystalline resin, if the mold temperature is high, the shrinkage rate will increase. The mold temperature should be maintained at a specific temperature for precision molding. In the mold design, we must pay attention to the cooling circuit design.
Gate sectional area
Generally speaking, when the cross-sectional area of the gate is changed, the shrinkage rates also changes. The shrinkage decreases with the gate size, which is related to the fluidity of the resin.
Wall thickness
For the non crystalline resin, the influence tendency of the resin on the shrinkage rate of the wall thickness is different, the shrinkage rate is also large with the wall thickness, on the contrary, the shrinkage rate is smaller. For the crystalline resin, the change of wall thickness must be avoided. In the case of multicavity, if the thickness of the cavity is different, the shrinkage will also be different.
Reinforcement content
When glass fiber is used to strengthen resin, the more glass fiber is added, the smaller the shrinkage is. The shrinkage in the flow direction is smaller than that in the transverse direction. According to the large difference of resin, in order to prevent twisting and warping, the fluke position of gate shape and the number of gates must be considered.
Directionality
Although the orientation is quite different, there is orientation for all resins. The orientation of crystalline resin is very large, which is different due to the wall thickness and molding conditions.
In addition, there is shrinkage after forming, the main factors are:
Internal stress relief
crystal
temperature
Humidity
How To Fix & Prevent Plastic Injection Mold Shrinkage
How to prevent the shrinkage of plastic injection mold parts? Here are some great measures:
Runner, gate balance
As mentioned above, shrinkage varies with resin pressure. In the case of single cavity mold with multi-point gate and multi cavity mold, gate balance is necessary for the same mold filling. Resin flow is related to the flow resistance in the runner, so it is better to balance the runner before balancing the gate.
Cavity arrangement
In order to make the setting of forming conditions easy, it is necessary to pay attention to the cavity arrangement. The temperature distribution of the mold is concentric with the gate as the center under the condition of general cavity arrangement. Therefore, when choosing the cavity arrangement of multi cavity mold, it is necessary to take not only the balance of runner, but also the concentric arrangement centered on gate.
How To Prevent Forming Variations
The reason of forming variation is that there is internal stress under uneven shrinkage, so it is necessary to prevent uneven shrinkage. In the case of circular products with holes in the center of gears, gates must be set in the center. However, when the shrinkage rate of resin flow direction is quite different from that of vertical direction, it has the disadvantage of producing ellipse. When higher precision roundness is needed, it needs to be set as 3-point gate or 6-point gate.
However, full attention should be paid to the balance of each gate. When the side gate is used, 3-point gate will increase the inner diameter of the cylindrical product. When the outer surface and the end face are not allowed to have gate marks, less use of the inner multi-point gate can get good results.
