If the plastic parts are unevenly shrink-ed, the internal stress distribution will be uneven. If the stress exceeds the rigidity of the plastic part, the warpage and deformation of the plastic part will be found, which will affect the dimensional stability of the plastic part.
But what is warpage?
When internal stress (due to uneven shrinkage) is bigger than material stiffness ,it will lead to warpage.
- Four reasons for uneven shrinkage:
1. The different melting temperature : the high melting temperature will lead to larger shrinkage.
2. Different cooling rates (crystalline materials):
The shrinkage rate of the quick melt cooling is smaller than that of the slow melt cooling (Cause: The molecules with slow cooling rate have more time to cool down, so its amount of shrinkage is large)
3. The different flow direction of the melted (amorphous material): in short, the tensile internal force is large along the flow direction, so the amount of shrinkage is large. During the filling process, the plastics cause alignment of the molecular chains due to the flow alignment. The polymer chains to be aligned are subjected to different stretching conditions in the flow direction and the vertical flow direction, and the shrinkage behavior is also different. It is called directional shrinkage.
In general, the flow direction shrinkage rate is higher than the vertical flow shrinkage rate.
This is because the flow direction of the plastic polymer chain is stretched more seriously, and the trend of restoring the unstretched state is greater. The differential shrinkage caused by the flow alignment often causes warpage of the plastic part. Therefore, if the molecular alignment can be broken, it will contribute to the uniformity of shrinkage and reduce the warpage caused by the direction shrinkage.
4. Different sizes:the bigger size part will have larger shrinkage.
In thicker areas, it need longer cooling time and bigger holding pressure.The cooling time required is longer, and the pressure holding effect is poor.
After the demolding, the local high temperature is maintained and the cooling is continued. Therefore, in the thick wall section , such as ribs, it is easy to have local shrinkage, which causes dents in the plastic parts.
Therefore, for plastic parts with wall thickness changes, the gate position can be set to thick area, which can facilitate the holding pressure.
Even if the solidification occurs at the location, the holding pressure can be smoothly transmitted and the shrinkage dents can be avoided.
- Wall thickness change
Even wall thick plastic parts will have better shrinkage. If the thickness of the plastic parts is unevenly distributed, it should be considered whether the different shrinkage ,caused by different cooling and holding effects, will cause the warpage of the plastic parts .
The sudden transition of the wall thickness could lead to the stress concentration there.
Internal stress concentration in the sudden transition area can cause short-term or long-term warpage problems and reduce the mechanical properties .
Plastic parts can be introduced with ribs to reinforce the structural strength to reduce shrinkage. The contact between the rib and the plastic part wall should be large enough to alleviate the stress concentration problem ,and overcome the flow resistance; However, dent problem would happen at that location.
In general, the size of the dent is also affected by the plastic shrinkage characteristics. Considering the sufficient strength and rigidity,We could use more ribs ,instead of plastic block , to reduce the shrinkage.
Molding conditions and deformation
Special attention must be paid to the molding conditions related to deformation, such as injection and holding time, cooling time, injection speed, and mold temperature.
- injection and pressure holding time
The injection and holding time should be designed to be longer than the gate solidify time. If the sealing time is shorter than the gate, the deformation sometimes increases.
- cooling time
In general, prolonging the cooling time will reduce the deformation.
- injection speed
Depending on the shape of the molded product, sometimes the injection speed is small, and the deformation is small.
Sometimes, the injection speed is slow and the deformation is small. In actual molding, the conditions of the minimum deformation are found by changing the injection speed.
- mold temperature
The molded article with a low mold temperature has small deformation. However, if the temperature of the molded article is high, there may be a problem of post-shrinkage deformation or dimensional stability. The mold temperature is determined by these factors.