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.
It is also important to establish the production process prior to making steel adjustments for the mold. Many processing parameters can influence the size of the part and its geometric characteristics—which can then impact functionality, longevity, and performance. For example, O-ring groove dimensions—diameter, roundness, depth, surface finish—can be negatively affected by process variations, resulting in diminished performance or even failure.Process changes can have an adverse effect on part assemblies, as well as how parts fit and work in assembly fixtures, inspection fixtures, and other ancillary equipment.
Therefore, having the process “dialed in” before making steel adjustments allows you to make accurate steel changes for future part processing, which is extremely important for consistency—especially when your process calls for work-in-progress parts that will be overmolded at a later time.
Other critical processing parameters to consider in the pre-mold design stage include the location of vents, cooling channels, and transducers. Venting is a very important part of tool function/process development because vents can change flow patterns in the tool. Venting can be easily added later if needed, after steel changes to a developed process. Cooling channels are important to the cooling rate of developing a process; infrared cameras or temperature sensors can identify the need for changing steel or adding cooling to these hot spots. Strategically placed transducers help develop a robust process that gives shot-to-shot consistency for making the determination through part measurement and applying it to steel change.
Therefore total tool functionality is a must before “setting” the process and making the necessary tool changes. This is determined by initial mold sampling. For example, if the part sticks in a certain location in the mold, more polishing or ejection is needed. If the cycle is prolonged, or the part comes out deformed, there may be a hot spot in the tool that requires additional cooling.
Putting process before mold design is an important part of design for manufacturability—determining the best production process by thoroughly evaluating the impact of part design, materials, tooling, and other production variables early in the design process.
For example, if a part does not have uniform wall thicknesses, or has thick areas, there may be a cooling issue that needs to be addressed. This can be evaluated with mold flow and cooling analysis to pinpoint these spots and make the necessary processing and tooling adjustments, resulting in a smoother, more-efficient production process. At that point, the actual specific dimensions of the mold can be calculated to fit the process, not the other way around.