Injection molding is a dynamic, complex process that, simply by the nature of its many variables, requires some testing and adjustments to get it just right before you can start production. Some manufacturers, however, choose to focus on completing the mold first and then build the process around the finalized mold, thinking this saves time and money. However, this approach typically results in production problems that slow the whole process down, and reduces quality and repeatability.
The best approach to streamline a plastic part project utilizing injection molding is to develop a consistent, efficient process first, and then fine-tune the mold to fit that process. As a guideline, the main steps of the injection molding 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
- Ejecting the part after the proper amount of cooling
By designing your mold around these steps — rather than trying to figure out how to complete certain steps with your finished mold — your production will be much more efficient and a faster time to market will be realized. To help you streamline these processes and avoid running into bigger problems later on in production, here are several tips for optimizing the injection molding and tooling process:
Maximize Your Processing Window
It’s important to develop a process that has the largest possible processing window. 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, 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.
Likewise, the final part will have differences in shrink due to direction of material flow. Final shrink is influenced by many molding parameters and molding within the range of acceptable parameters, cross-checked with final dimensions, allows the final processing “window” to be established. In general, if you want a high-quality part that meets performance specifications and looks good, the larger the processing window is, the lower the risk for problematic start-ups and inconsistent product quality.
Utilize Scientific Molding
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 monitored 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. There are many different ways scientific molding can benefit your process development, however, and our scientific molding whitepaper can help you learn more.
Establish Production Process Prior to Mold Adjustments
As mentioned earlier, it's crucial that you establish the production process prior to making any 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 also have an adverse effect on part assemblies, including 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.
Pay Attention to Mold Temperatures and Venting
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. This can easily be added later if needed, after steel changes to a developed process. Cooling channels are also important to the cooling rate, where infrared cameras or temperature sensors identify the need for changing steel or adding cooling to these hot spots. Strategically placed transducers also help develop a robust process that gives shot-to-shot consistency for making the determination through part measurement and applying it to steel change.
Finally, before you set the process and make the necessary changes to your tool, it's crucial that you consider the tool's functionality during the initial mold sampling. If the part sticks in a certain location in the mold, for example, 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. Likewise, 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. At that point, the actual specific dimensions of the mold can be calculated to fit the process, not the other way around.
Want to learn more about it what it takes to optimize your injection molding processes, design and tooling? Download our free Mold Design & Tooling whitepaper below.