Shrinkage/Warping
Due to the correlation between warping deformation and uneven shrinkage, this analysis begins with studying the shrinkage behavior of different plastics under various processing conditions to understand the relationship between shrinkage and the warping of products. Based on simulations of injection flow, holding pressure, and cooling, a model has been proposed for predicting the shrinkage of injection-molded products through experimental and linear regression methods. Using this shrinkage prediction, the deformation of products is calculated through structural analysis simulation programs.
It is challenging to achieve high dimensional accuracy in products made from materials with high shrinkage rates. To pursue high precision, amorphous resins and resins with consistent shrinkage in all directions should be used as much as possible. Many materials have been tested for shrinkage under varying conditions such as flow rate, holding pressure, holding time, mold temperature, filling time, and product thickness.
Based on test results, the shrinkage of products can be divided into three components: volumetric shrinkage, uneven shrinkage caused by molecular orientation, and uneven shrinkage caused by unbalanced cooling. Methods for predicting shrinkage, considering volumetric shrinkage, crystallinity, mold constraints, and plastic orientation, utilize flow and cooling analysis results to forecast shrinkage strain.
Design of Cooling System
During injection, uneven cooling rates in the plastic components can also lead to uneven shrinkage, which generates bending moments and causes warping in the plastic components.
If there is too great a temperature difference between the cavity and core of the mold used for producing flat plastic parts, the melt near the cold mold cavity surface will cool quickly, while the material layer near the hot mold cavity surface will continue to shrink. This uneven shrinkage will cause the plastic part to warp. Therefore, the cooling of injection molds should ensure that the temperatures of the cavity and core tend toward equilibrium, and the temperature difference between the two should not be too large.
In addition to balancing the temperatures of the inner and outer surfaces of the plastic part, the consistency of temperatures on all sides of the plastic part should also be considered. That is, during mold cooling, efforts should be made to maintain uniform temperatures throughout the cavity and core to ensure balanced cooling rates, which results in more uniform shrinkage and effectively prevents deformation. Thus, the arrangement of cooling water holes in the mold is crucial. Once the distance from the pipe wall to the cavity surface is determined, the distance between cooling water holes should be minimized to ensure uniform temperatures along the cavity walls.
At the same time, because the temperature of the cooling medium increases with the length of the cooling channels, a temperature difference will occur along the cavity and core of the mold. Therefore, the length of each cooling circuit should be less than 2 meters. Multiple cooling circuits should be set up in large molds, with the inlet of one circuit located near the outlet of another. For elongated plastic parts, cooling circuits should be designed to reduce their length and thus minimize the temperature difference in the mold, ensuring uniform cooling of the plastic part.
The design of the ejection system also directly influences the deformation of the plastic part. If the ejection system is unbalanced, it will cause an unbalanced ejection force, resulting in deformation of the plastic part. Therefore, when designing the ejection system, efforts should be made to balance it with the demolding resistance.
Moreover, the cross-sectional area of the ejector rods must not be too small to prevent excessive stress on the plastic part per unit area, especially when the demolding temperature is too high, which could lead to deformation. Ejector rods should be placed as close as possible to areas with high demolding resistance. Without compromising the quality of the plastic part (including usage requirements, dimensional accuracy, and appearance), as many ejector rods as possible should be used to minimize overall deformation of the plastic part.
When using soft plastics to produce large, deep-cavity, thin-walled plastic parts, the high demolding resistance and the soft material can lead to deformation if solely relying on a mechanical ejection method. Instead, using a combination of multi-component ejection or pneumatic (hydraulic) and mechanical ejection methods will yield better results.
Effects of Residual Thermal Stress on Product Warping
During the injection molding process, residual thermal stress is a significant factor in causing warping deformation and has a substantial impact on the quality of injection-molded products. Given the complexity of the effects of residual thermal stress on product warping deformation, mold designers can utilize injection molding CAE software for analysis and prediction.
During the molding process, the orientation and uneven shrinkage of the plastic melt lead to uneven internal stresses, causing warping deformation after the product is demolded due to these uneven internal stresses. Many researchers have analyzed and calculated the internal stresses and warping of products from a mechanical perspective. In some foreign literature, warping is considered to be caused by residual stresses generated by uneven shrinkage.
During the cooling stage of injection molding, when the temperature is above the glass transition temperature, plastics behave as viscoelastic fluids and experience stress relaxation phenomena. When the temperature falls below the glass transition temperature, the plastic transitions into a solid state. This liquid-to-solid phase change and stress relaxation during cooling significantly affect the accurate prediction of residual stresses and residual deformations in products.
The phase transition of the plastic from liquid to solid during cooling and the stress relaxation behavior are analyzed. In uncured regions, the plastic exhibits viscous behavior described by a viscous fluid model; in cured regions, the plastic exhibits viscoelastic behavior described by a standard linear solid model. A viscoelastic transition model and two-dimensional finite element method are used to predict thermal residual stress and the corresponding warping deformation.
Impact of the Plasticization Stage on Product Warping
The plasticization stage refers to the conversion of glassy state resin particles to a viscous flow state, providing the melt required for mold filling. In this process, temperature differences in the axial and radial directions (relative to the screw) can induce stress in the plastic. Additionally, the injection pressure and speed parameters of the injection machine significantly affect the degree of molecular orientation during filling, which in turn causes warping deformation.
Using low speed during the initial injection and high speed during the filling of the mold cavity, then reverting to low-speed injection as filling approaches completion, can prevent and improve various aesthetic defects in the product, such as flash, jetting marks, silver streaks, or scorch marks.
A multi-stage injection control program can be reasonably set based on the structure of the flow channels, the type of gate, and the configuration of the injection molded parts to optimize multi-stage injection pressure, injection speed, holding pressure, and melt feeding methods. This approach helps improve the plasticization effect, enhance product quality, reduce defect rates, and extend the lifespan of molds and machines.
By controlling the oil pressure, screw position, and screw speed of the injection molding machine through a multi-stage program, improvements can be sought in the aesthetic defects of the molded parts and countermeasures against shrinkage, warping, and flash can be implemented to reduce dimensional inconsistencies in molded parts across different molds.
Effects of the Filling and Cooling Stages on Product Warping
The process of injecting molten plastic under pressure into the mold cavity and cooling and solidifying it is a critical stage of injection molding. During this process, the interaction of temperature, pressure, and speed has a significant impact on the quality and production efficiency of the plastic parts.
Higher pressure and flow rates create high shear rates, resulting in differences in molecular orientation parallel and perpendicular to the flow direction and also producing a “freezing effect.” The “freezing effect” generates frozen stress, forming internal stress within the plastic part. The effects of temperature on warping deformation manifest in several ways:
A. The temperature difference between the upper and lower surfaces of the plastic part can induce thermal stress and thermal deformation.
B. Temperature differences between different regions of the plastic part can cause uneven shrinkage between these areas.
C. Different temperature states can affect the shrinkage rates of the plastic parts.
Effects of the Demolding Stage on Product Warping
During the process of the plastic part separating from the mold cavity and cooling to room temperature, it is generally in a glassy state. Imbalances in demolding forces, uneven motion of the ejection mechanism, or improper ejection area can easily lead to deformation of the product. Furthermore, the stresses frozen within the plastic part during the filling and cooling stages will release in the form of deformation once external constraints are removed, leading to warping deformation.
Using a truly three-dimensional method to calculate residual stresses and the final shape (shrinkage and warping) involves considering the effects of the holding pressure stage, dividing the product into three layers, and analyzing residual stress and deformation using a three-dimensional mesh. A numerical simulation model for residual stress and deformation resulting from the holding pressure stage has been proposed.
In calculating residual stresses, a thermal viscoelastic model (including volumetric relaxation) is used. The finite element method employed is based on shell theory formed by a collection of plane elements, which is well-suited for thin-walled injection molded products with complex shapes.
Solutions to the Impact of Shrinkage on Warping Deformation in Injection Molded Products
The direct cause of warping deformation in injection molded products is uneven shrinkage. If the impact of shrinkage during the filling process is not considered during the mold design phase, the geometric shape of the product may deviate significantly from the design requirements, leading to serious deformations that can render the product unusable. Besides deformation during the filling stage, the temperature difference between the upper and lower walls of the mold can also result in differences in shrinkage between the top and bottom surfaces of the plastic part, thus generating warping deformation.
For warping analysis, the shrinkage itself is not significant; what matters is the variation in shrinkage. During the injection molding process, the arrangement of polymer molecules along the flow direction during the filling stage causes the shrinkage rate in the flow direction to differ from that in the thickness direction, resulting in significant differences in final dimensional stability.
The stress relaxation behavior of amorphous plastics during the cooling stage must be considered. The dimension of the plastic part can be divided into the following three groups based on the variability of the shrinkage distribution of the product:
- Dimensionally stable: It is possible to eliminate the effects of residual thermal stress caused by shrinkage variation and ensure sufficient dimensional stability.
- Moderately stable: It may be possible to reduce or prevent some effects of residual thermal stress caused by shrinkage variation, but final dimensional stability may still be compromised.
- Dimensionally unstable: The stresses generated during filling and holding may dominate, and deformation is likely to occur if residual stresses are not alleviated.
In summary, the methodology for analyzing and controlling the shrinkage and warping of injection-molded products requires careful attention to the entire injection process. By optimizing the design of the cooling system, controlling processing parameters, and considering material properties, manufacturers can minimize shrinkage and warping, thereby improving product quality and performance.