It can occur in just about any injection-molded part or product—yet the experienced injection molder knows how to eliminate warpage from the production cycle and maintain a steady throughput of high-quality product that meets all customer specifications, including dimensional accuracy and tight tolerances.
Warpage occurs when the shape of the final product has deviated from the shape of the mold cavity; sometimes warpage is so slight it is still “within spec” and not a functional or aesthetic problem. In other cases function and fit are compromised and the part or product must be rejected. Warpage can result from one or several variances that may occur during the injection-molding process. However, application of scientific molding principles, combined with a deep understanding of material science and plastic flow behavior, can eliminate this problem.
Temperature differences are the main cause of warpage—whatever variances contribute to these unwanted temperature differences are the production parameters that need to be addressed. These include moisture in the pellets, improperly designed molds and tooling, incorrect melt or mold temperatures, mold contamination, and inaccurate feeding systems.
The cooling system is always critical. A precisely calculated cooling rate must be maintained for uniform cooling and shrinkage; if not, uneven cooling may result in deformation or bending. It is critical to have plenty of cooling channels that are properly located to facilitate even cooling (this can be more challenging in products with complex shapes). The gating system must be properly designed with sufficient gates at the right locations. Wall thickness can also be a problem; even slight variations in wall thickness can result in uneven cooling and warpage.
Plastics that are reinforced with glass fiber or other reinforcement materials have greater potential for warping because of inconsistent orientation of the glass fibers. These types of hybrid materials will not flow exactly as the homogenous plastics do—therefore injection molders must be highly vigilant when monitoring this process, especially temperature, pressure, rate of flow, and fiber orientation. Higher pressure and flow rate will also produce higher shear rates which can also impact molecular orientation, resulting in internal stress, uneven cooling, and warpage.
Molten plastic is a liquid that must be carefully monitored through every phase of the injection-molding process. It “wants” to follow the path of least resistance; any variances in cooling that have created harder sections have also created channels that can affect the flow and cooling of the remaining molten plastic. When cooling rates are inconsistent this remaining material may not even fully harden, creating soft spots.
This is where an injection molder’s experience really comes into play. Warpage must be considered during the earliest design phases so the proper materials, mold design, settings, and process can be designed and tested to make sure warpage is eliminated from the final production process.