Precision 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 fixed, the actual size of the product varies with the actual shrinkage. Therefore, the control of shrinkage rate is very important in precision injection molding.
Whether the mold design is appropriate or not determines the shrinkage rate, which also varies with different resin batches. If the pigment is changed, the shrinkage rate will also be different. Because of the different molding machines, the setting of molding conditions, reproducibility and the action of each molding cycle fluctuate, and the actual shrinkage rate fluctuates, so it is difficult to control the shrinkage.
1. The main factors affecting the shrinkage
The mold size can be obtained by the product size plus the shrinkage rate, so the main factors of shrinkage rate should be considered in mold design
Gate sectional area
Wall thickness of products
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 mold cavity is easy to be different, and the shrinkage rate of each mold cavity is also different.
Whether it is amorphous resin or crystalline resin, if the mold temperature is high, the shrinkage rate will increase. Precision molding should maintain the mold temperature at a specific temperature. In mold design, attention must be paid to the design of cooling circuit.
Gate sectional area
Generally speaking, the shrinkage rate also changes when the gate cross-section area is changed. The shrinkage rate decreases with the increase of gate size, which is related to the fluidity of resin.
Wall thickness of products
The wall thickness of the product also affects the yield. For the amorphous resin, the shrinkage rate increases with the increase of the wall thickness due to the different influence tendency of the resin on the wall thickness. For crystalline resin, it is necessary to avoid large wall thickness variation. In the case of multi cavity mode, if the cavity wall thickness is different, the shrinkage will also be different
When the resin is reinforced with glass fiber, the more glass fiber is added, the smaller the shrinkage rate is. The shrinkage rate in the flow direction is smaller than that in the transverse direction. According to the resin, the difference is large. In order to prevent the distortion and warpage, the shape of gate, the location of fly away gate and the number of gate must be considered.
Although there is a great difference in the orientation, it exists for all resins. Due to the special difference in crystallinity and wall thickness of the resin.
In addition, there is shrinkage after molding, the main factors are:
Internal stress relief
2. Possible measures
Runner, gate balance
As mentioned above, shrinkage varies with resin pressure. In the case of single cavity mold with multiple gate and multi cavity mold,
To do the same, gate balancing is required. The resin flow is related to the flow resistance in the runner, so it is better to take the runner balance before taking the gate balance.
In order to make the setting of forming conditions easy, it is necessary to pay attention to the arrangement of mold cavities. Because the molten resin will be hot into the mold, the mold temperature distribution takes the gate as the center in the case of general mold cavity arrangement. Therefore, when selecting the cavity arrangement of the multi cavity die, it is necessary to take the balance of the runner and the concentric circular arrangement centered on the gate
3. Prevent forming deformation
The reason of forming deformation 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 the gear, a gate must be set in the center. However, when there is a big difference in the shrinkage rate between the flow direction and the vertical direction of the resin, there is a defect of ellipse. When a higher precision roundness is needed, a 3-point or 6-point gate should be set.
However, it is necessary to pay attention to the balance of each gate. When using side gate, 3-point gate will increase the inner diameter of cylindrical products. In the case of no gate mark on the surface and end face, less use of inner multi-point uniform gate can get good results.
About PTJMould Industrial Company Limited
PTJmould is a China mold maker of plastic molds- injection mold, die casting moulds, plastic blow molding, rotational molding, medical plastic injection molding, two shot plastic injection molding, insert molding, overmolding, metal injection molding, micro injection molding, powder injection molding, ceramic injection molding, liquid injection molding, husky injection molding, household mold, casting mold, die mold tool, custom molds, china moulds, rapid prototyping tooling, plastic prototyping tooling, punch press tooling, die and tooling for mobile/ cell phone parts, automotive parts, vacuum cleaners, rechargeable tools, telephones, copiers, computers, multimedia speakers, and many other electronic products and household appliances. And also a plastic product manufacturer, mold manufacturer China– plastic parts, plastic water tank, plastic balls, plastic containers, plastic buckle, plastic anchor, plastic hanger, plastic spoon, plastic pipe fitting, plastic tumble, plastic tableware, plastic cups, plastic bottles, plastic tray, plastic cosmetic container, plastic case, plastic food container, plastic chairs, plastic caps, plastic cap closure, plastic tubes, plastic water pipes, plastic knobs, plastic tubing, plastic utility boxes, plastic racks and so on.
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