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How to solve shrinkage and warping?
2025-07-24
Plastic Injection Molding inherently shrinks because the density of the polymer changes when the process temperature drops to room temperature, causing shrinkage. The difference in shrinkage of the entire plastic part and the cross section will cause internal residual stress, which has the same effect as external force. If the residual stress is higher than the strength of the plastic part structure during injection molding, the plastic part will warp after demolding or crack due to external force.
Residual stress
Residual stress is the stress that is caused by the flow of the molten plastic or the thermal effect when the plastic part is formed, and is frozen in the plastic part. If the residual stress is higher than the structural strength of the plastic part, the plastic part may warp during injection molding, or break under load later.
Residual stress is the main cause of shrinkage and warpage of plastic parts. Good molding conditions and design that can reduce the shear stress caused by filling the mold cavity can reduce the residual stress caused by melt flow. Similarly, sufficient pressure holding and uniform cooling can reduce the residual stress caused by thermal effects. For materials with added fibers, improving molding conditions with uniform mechanical properties can reduce the residual stress caused by thermal effects.
1. Residual stress caused by melt flow
Under no stress, long-chain polymers are in a state of equilibrium with arbitrary curling at a temperature higher than the melting point. During the forming process, the polymers are sheared and stretched, and the molecular chains are aligned along the flow direction.
If the molecular chain solidifies before it is completely relaxed and balanced, the molecular chain orientation is frozen in the plastic part. This stress freezing state is called flow-induced residual stress, which will cause uneven mechanical properties and shrinkage in the flow direction and perpendicular to the flow direction. Generally speaking, the flow-induced residual stress is one power smaller than the residual stress caused by thermal effects.
The high orientation of the surface of the plastic part near the mold wall will freeze immediately due to the interaction of high shear stress and high cooling rate, as shown in Figure 1. If the plastic part is stored in a high temperature environment, the plastic part will release some of the stress, resulting in shrinkage and warping.
The heat insulation effect of the solidified layer keeps the polymer core layer at a higher temperature, which can release more stress, so the molecular chains in the core layer have a lower orientation. Forming conditions that can reduce the shear stress of the melt will also reduce the residual stress caused by flow, including:
•High melt temperature.
•High mold wall temperature.
•Long filling time (slow melt speed).
•Reduce the holding pressure.
•Short flow path.
Fig. 1 The molecular chain orientation frozen during the filling and packing stages leads to flow-induced residual stresses.
(1) indicates high cooling rate, high shear stress or high orientation; (2) indicates low cooling rate, low shear stress or low orientation.
2. Residual stress caused by thermal effect
The causes of residual stress caused by thermal effects include the following:
(1) Plastic shrinks when it drops from the set process temperature to room temperature.
(2) When the plastic solidifies, the plastic part undergoes different thermal and mechanical processes from the surface to the center layer, such as different cooling times and different holding pressures.
(3) Changes in pressure, temperature, molecular chain orientation, and fiber orientation due to changes in density and mechanical properties.
(4) The design of the mold limits the shrinkage of the plastic part in certain directions.
The shrinkage of plastic during injection molding can be explained by the example of free cooling. If a plastic part with uniform temperature is suddenly clamped by the cold mold walls on both sides, in the early stage of cooling, when the surface of the plastic part cools and begins to shrink, the polymer inside the plastic part is still in a high-temperature molten state and can shrink freely.
However, when the center temperature of the plastic part drops, the local thermal shrinkage is limited by the solidified surface layer, resulting in a typical stress distribution with tensile stress in the center layer and compressive stress in the surface layer, as shown in Figure 2.
The difference in cooling rate from the surface to the center of the plastic part will cause thermal residual stress. What's more, if the cooling rate of the mold wall on both sides of the mold is different, it will also cause asymmetric thermal residual stress. The asymmetric distribution of tensile stress and compressive stress in the cross section of the plastic part will cause bending moment, causing the plastic part to warp, as shown in Figure 3.
Plastic parts with uneven thickness and areas with poor cooling will cause this unbalanced cooling, resulting in residual stress. Complex plastic parts have more complex residual stress distribution caused by thermal effects due to factors such as uneven thickness, uneven mold cooling, and mold restrictions on free shrinkage.
Figure 3 Uneven cooling of the cross-section of the plastic part causes residual stress caused by asymmetric thermal effects, causing the plastic part to warp.
Figure 4 illustrates the change in the specific volume of the solidified layer caused by the pressure history of the pressure holding. The left figure is the temperature distribution curve of a section of the plastic part. For the convenience of explanation, the plastic part is divided into 8 layers along the thickness direction, and the solidification time of each layer is shown on the curve as t1~t8.
Note that the plastic part solidifies from the outermost layer, and the longer the solidification time is, the closer to the center layer. The middle graph shows the typical pressure history of each layer solidification, P1~P8. The pressure in the filling stage usually rises gradually, reaching the highest pressure at the beginning of the pressure holding, and then gradually decreases due to cooling and gate solidification.
As a result, the surface and center layers of the plastic part solidify at low pressure, and the other intermediate layers solidify at high holding pressure. The right figure illustrates the specific volume history of the 5th layer on the PvT diagram, as well as the specific volume of each layer at the final solidification, and is marked with solid dots.
Given the solidification specific volume of each layer, the shrinkage behavior of each layer of the plastic part will shrink differently according to the PvT curve. Assuming that the layers are separated as shown in Figure 5, the result is shrinkage to the middle figure. The middle layers 2, 5, 6, 7 shrink less because of the low solidification specific volume (or high solidification density). In fact, the layers are connected together, resulting in a compromise shrinkage distribution, the middle layer is compressed, and the outer layer and the center layer are stretched.
Figure 5 The differences in specific volumes of each solidified layer interact with each other, resulting in different residual stresses and deformations of the plastic part.
3. Process-induced residual stress and cavity residual stress
In terms of injection molding simulation, process-induced residual stress is more important than in-cavity residual stress. The following introduces the definitions of these two terms and provides an example to illustrate their difference.
After the plastic part is ejected, the restraint imposed on the plastic part by the mold cavity is released, and the plastic part can shrink and deform freely until it reaches a balanced state.
At this time, the stress remaining in the plastic part is the residual stress caused by the process, or simply referred to as residual stress. It includes the residual stress caused by flow and the residual stress caused by thermal effect, with the thermal effect being the main influence.
When the plastic part is still constrained by the mold cavity, the internal stress stored by the solidification of the plastic part is called the mold cavity residual stress. This residual stress will drive the plastic part to shrink and warp after ejection.
The upper left figure in Figure 6 shows the residual stress in the mold cavity (usually the tensile stress shown in the figure) of the molded plastic part before ejection, which is still constrained by the mold.
Once ejected, the mold is freed from the restraint on the plastic part, and the plastic part will release the residual stress in the mold cavity and shrink and warp. The residual stress distribution curve of the thermal effect caused by the shrinkage distribution of the ejected plastic part is shown in the lower left figure of Figure 6. In the absence of external force, the tensile stress of the plastic part section is equal to the compressive stress and reaches a balanced state.
The lower right figure of Figure 6 shows that the thickness of the plastic part is subjected to uneven cooling, resulting in asymmetric residual stress and warping.
Fig. 6 (top) Distribution curve of residual stress in the mold cavity and (bottom) distribution curve of residual stress induced by the process and shape of the plastic part after ejection.
Conditions that can result in adequate holding pressure and uniform mold wall temperature can reduce residual stress caused by thermal effects. These conditions include:
(1) Appropriate holding pressure and holding time.
(2) All surfaces of the plastic part are cooled evenly.
(3) The plastic part has uniform cross-sectional thickness.
Contraction
When the injection molded plastic parts are cooled from the process temperature to room temperature, the volume shrinkage can be as high as 20%. When crystalline and semi-crystalline materials are cooled below the glass transition temperature, the molecules are arranged in a more regular manner and form crystals, which are particularly prone to thermal shrinkage;
The amorphous material does not undergo microstructural changes during phase changes, and its thermal shrinkage is relatively small. Therefore, the volume difference between the molten phase and the solid phase (crystallization) of crystalline and semi-crystalline materials is larger than that of amorphous materials, as shown in Figure 7. In addition, the cooling rate also affects the PvT behavior of crystalline and semi-crystalline materials.
Fig. 7 PvT curves of amorphous and crystalline polymers.
The change from the process state (point A) to the normal pressure and room temperature state causes a specific volume change △υ.
NOTE: As pressure increases, specific volume decreases.
The reasons for excessive shrinkage of plastic parts include too low injection pressure, insufficient holding time or cooling time, too high melt temperature, too high mold temperature, too low holding pressure, and the relationship between shrinkage, process parameters, and wall thickness is shown in Figure 8:
During injection molding, if the volume shrinkage of the plastic part is not compensated, it will cause depression on the surface of the plastic part or air holes inside. Therefore, the shrinkage of the plastic part must be considered when designing the mold. Controlling the shrinkage rate of the plastic part is very important for plastic part design, mold design, and process condition setting, especially for combined plastic parts.
Holding the mold cavity after filling can reduce/eliminate sink marks and pores to determine the size of the plastic part. Mold flow analysis software can predict the shrinkage of the plastic part and provide guidelines for the correct design of the mold.
