How Cooling Design Prevents Warpage in Injection Molding

Uneven cooling creates internal stresses that cause warpage. Better mold cooling improves dimensional stability and part quality.

Warpage is one of the most common problems in injection molding. It is essentially a deformation or deviation from the part’s intended shape, compromising the component’s quality and usability. In a cylindrical part, warpage can make the component oval-shaped. In a flat molded component, the part may curve.

You can also read: Read Your Material Datasheet to Cut Molding Costs and Defects

Warpage originates within the part itself. It indicates imbalanced in-mold stresses, and the resulting deformation reflects that imbalance. To prevent warpage, examine why molded parts develop these internal stresses.

Stress in Injection Molding

As an injection-molded part cools inside the mold, it develops a bell-shaped temperature distribution. The temperature is lower near the mold wall, where the plastic solidifies first, and higher toward the center of the part.

Because plastics have very low thermal conductivity, the solidified outer layers inhibit heat transfer toward the mold. This effect can last for minutes, depending on the wall thickness.

The heat accumulated inside the component provides energy for molecular movement and arrangement. While the cooler regions near the mold walls have little to no molecular motion because of limited available energy, the inner regions remain hotter. As a result, molecules in these inner regions continue to move and shrink.

Consequently, the inner regions pull on the outer regions, creating compressive stress near the surface. Meanwhile, the inner regions experience tensile stress. This stress state is inevitable and generally harmless as long as the stresses balance in both directions.

These stresses are thermally induced. The part remains balanced only when the same amount of heat transfer occurs in every direction within the component. In other words, the heat transfer rate must be homogeneous.

Consequences of Unbalanced Cooling

To achieve a balanced state of in-mold stress, heat must leave the part at the same rate in all directions. In real molds, however, some regions are often cooled more efficiently than others. For example, cooling problems are common in deeper sections of the mold core. As a result, plastic components can cool unevenly.

As shown in the figure, when one mold half is cooler, more material solidifies on that side. This shifts the neutral stress axis. As a result, compressive stresses become higher on one side of the part than on the other. To balance this stress, the part bends.

Notably, the part can indicate which side of the mold is hotter because it bends toward the hotter side.

In this case, the solution is to balance the cooling rates on both sides of the mold. One option is to cool the hotter side, although this may not always be feasible. Another option is to temper the cooler side by raising its temperature to match the hotter mold half. This allows the part to experience similar boundary conditions on both sides of the mold.

Crystallization can intensify this problem. In semi-crystalline materials, higher mold temperatures increase the crystallization rate. Higher crystallization leads to greater shrinkage. Therefore, the part bends not only because of imbalanced in-mold stresses, but also because the crystalline structure near the surface pulls on the surrounding material as the volume shrinks.

The Importance of Cooling Design

Warpage highlights the importance of cooling both sides of the mold properly. Conformal cooling is one of the best alternatives for achieving homogeneous heat transfer in all directions.

As the name suggests, conformal cooling uses cooling channels that closely follow the shape of the mold wall. By maintaining a consistent distance from the mold wall and applying similar cooling conditions throughout the cavity, conformal cooling promotes more uniform heat transfer and reduces stress imbalances.

Conformal cooling not only improves productivity by shortening cycle time, but also increases part quality by helping prevent warpage. This solution has proved especially valuable for assembly parts.