If the plastic part shrinks unevenly, it often causes uneven distribution of internal stress. If this stress exceeds the rigidity of the plastic part, it will cause warping and deformation of the plastic part, affecting the dimensional stability of the plastic part.
1, What is warping for injection mold parts?
When internal stress (due to uneven shrinkage) > material rigidity => warping
Four major reasons for uneven shrinkage:
If the injection mold plastic part shrinks unevenly, it often causes uneven distribution of internal stress. If this stress exceeds the rigidity of the plastic part, it will cause warping and deformation of the plastic part, affecting the dimensional stability of the plastic part.
- Different melt temperatures: The higher the melt temperature, the greater the shrinkage
- Different cooling speeds (crystalline materials):
Reason: Fast cooling speed < Slow cooling speed (Reason: The molecules of the slow cooling speed have time to arrange, so the shrinkage is large) - Different melt flow directions (amorphous materials): In short, the tensile internal stress along the flow direction is large, so the shrinkage is large. During the filling process of plastic, the molecular chains are oriented due to the flow orientation. The oriented polymer chains are stretched differently in the flow direction and perpendicular to the flow direction, which makes the shrinkage behavior different. This is called directional shrinkage.
Generally speaking, the shrinkage rate in the flow direction is higher than the shrinkage rate in the perpendicular flow direction. This is because the plastic polymer chains in the flow direction are stretched more seriously and tend to recover to the unstretched state. The differential shrinkage caused by the flow orientation often causes the warping of plastic parts. Therefore, if the molecular orientation can be broken up, it will help the uniformity of shrinkage and reduce the warping caused by directional shrinkage.
- Different sizes: Simply put, the larger the size, the greater the shrinkage.
2, injection mold part warp area caused by design:
Thickness
In areas with thicker thickness, cooling and holding pressure are more difficult, the cooling time required is longer, and the holding pressure effect is poor. After demolding, the local high temperature is still maintained and cooling continues. Therefore, in areas with thicker thickness, such as ribs, local shrinkage is prone to cause dents in plastic parts. Therefore, for plastic parts with workpiece changes, choosing the pouring position at a thicker part can be beneficial for pressure holding. Even if solidification occurs at the workpiece, the pressure holding pressure can still be smoothly transmitted to improve shrinkage.
Thickness change
Uniform thickness of plastic parts will improve shrinkage. If the thickness of plastic parts is unevenly distributed, it should be considered whether the shrinkage difference caused by different cooling and pressure holding effects will cause warping and deformation of plastic parts, as well as the stress concentration problem caused in the thickness transition area
The internal stress concentration phenomenon in the thickness transition area (buffer area) will cause short-term or long-term warping problems and reduce the mechanical properties of plastic parts. Plastic parts can introduce reinforcing ribs to strengthen the structural strength to reduce shrinkage. The contact part between the rib and the plastic wall should be large enough to alleviate the stress concentration problem and overcome the flow resistance; but attention should also be paid to the dent problem that may be caused. Generally speaking, the size of the dent is also affected by the shrinkage characteristics of the plastic. Considering the strength and stiffness, if the hollowing method can be used to reduce the thickness of the plastic part, it will help reduce shrinkage.
Injection mold working conditions and deformation
As molding conditions related to deformation, special attention must be paid to injection and holding time, cooling time, injection speed, and mold temperature.
- Injection and holding time
The total time of injection and holding should be designed to be longer than the gate closing time. If it is shorter than the gate closing time, the deformation may increase. - Cooling time
Generally speaking, extending the cooling time will reduce the deformation. - Injection speed
Depending on the shape of the molded product, sometimes the deformation is small when the injection speed is fast, and sometimes the opposite is true, the deformation is small when the injection speed is slow. In actual molding, the condition with the smallest deformation should be found by changing the injection speed. - Mold temperature
Molded products with low mold temperature have small deformation. However, if the use temperature of the molded product is high, it may sometimes cause post-shrinkage deformation or dimensional changes. The mold temperature should be determined based on these factors.
