Injection molding is a dynamic, complex process that, simply by the nature of its many variables, requires some testing and some adjustments to get it just right, prior to production. Some manufacturers, however, choose to focus on the specifications of the mold first and then build the process around the finalized mold, thinking this saves time and money. This approach, however, typically results in production problems that slow the whole process down and reduce quality and repeatability. The best approach is developing a consistent, efficient process first, followed by fine-tuning the mold to the process.
The main steps of the process are proper melting of the plastic resin, injection at the correct rate, packing at the proper pressures, cooling with the correct temperature mold surface, and ejecting the part after the proper amount of cooling.
It’s important to get a process worked out that has the largest possible processing window—this gives the engineering team more flexibility and range in designing the mold. If the team strictly processes for dimensions only, the process may be insufficient to create the molding conditions that will yield the most consistent part.
Building the mold with critical dimensions up front is risky because there are subtleties in the process that cannot be fully predicted without testing. For example, injection speed may need to be adjusted to counteract splay created by shear stress, which can be caused by the shape of the part. Injection speed can also influence dimensional results.
The final part will have differences in shrink due to direction of material flow. Final shrink is influenced by many molding parameters. Molding within a range of acceptable parameters, cross-checked with final dimensions, allows the final processing “window” to be established. In general, the larger the processing window is, the lower the risk for problematic start-ups and inconsistent product quality.
The bottom line is that the processing window must be large enough to create a high-quality part that meets performance specifications and looks good.
This is where scientific molding comes into play. Following the principles of scientific molding is typically the best way to factor in the many variables that come into play and determine the best process. For example, with scientific molding, the proper viscosity of the material can be determined by pressure curves. When lot-to-lot material variations occur, the molding process can be adjusted to produce the same pressure curves that were generated during the initial process development—ensuring repeatability and saving time. Take a look at our Scientific Molding Whitepaper to understand more about this.