Continual Improvement of Mold Design
Sometimes we get the opportunity to take a step back and see how far our industry has advanced over the last ten years. Since Natech first made the commitment to embrace the newest technical advances including Scientific Molding and Scientific Processing, the decision has proven itself to be valuable. Improved mold designs and processing have contributed to better control over quality and more efficient use of resources. A strong talent development program drives this organizational learning. Clients have realized the tangible benefits associated with an improved control over quality.
A diagnostics client recently asked Natech to build a replacement 8-cavity mold for a reagent vial. The previous mold had completed two million cycles over the span of ten years. The differences between the old mold and the new mold serve as a good indicator of the internal competency development of the team as well as the advancements in the industry.
Over the course of the life of the tool, the improvement opportunities of the current mold design relative to the part geometries became apparent with experience. This was true especially with the observations toward the end of the tool’s useful life, which was what the Engineers sought to extend in the new tool. The original mold design included only a single gate per cavity, and flow lines would occasionally present themselves where the diverting flow fronts would meet. These flow lines would sometimes result in short shots during startup, which increased the risk of QC rejections and the potential for end-user leaks.
Over time, the size of the process window was shrinking, and the risk of seeing short shots and flash on different cavities within the same shot began increasing. This introduced the risk of the client running defective parts within their automated assembly equipment which would lead to parts either getting ejected in-process or passing through and jamming their equipment.
To be fair, these issues of disrupted process control emerged over time and increased after the mold had far exceeded its official useful life. However, these issues provided valuable insight that the Natech Engineers could use to feed back into the design of the replacement mold to increase the useful life of that mold.
The original mold employed a single sub-gate on one side of the cylindrical part, which produced an asymmetric fill pattern so one side of the part filled before the other. Also, across the eight cavities the runner design resulted in slight fill imbalances across cavities.
Based on a full mold flow analysis, the Natech engineers designed a new 8-cavity mold which included a dual sub-gate approach with symmetric gate placement within each cavity. The short shot analysis performed on this mold during mold qualification later confirmed the accuracy and symmetry predicted during the simulated fill progression.
The Natech Engineers then incorporated MeltFlipper® technology into the runner design in order to minimize the risk of cavity-to-cavity output variation. MeltFlipper® is a fill pattern optimization tool which improves rheological control in the mold by creating a uniform cross-sectional melt temperature profile throughout the runner system. This in turn minimizes shear imbalances inherent to all injection mold runner systems, and results in each cavity within a multi-cavity mold filling at a uniform rate. This is a critical detail to achieve a controlled injection molding process.
The Natech Engineers also performed an IQ/OQ/PQ Equipment Qualification using the principles of Scientific Processing and Scientific Molding. They completed a Viscosity Curve, a Cavity Balance Study, a Pressure Window Characterization, a Gate Seal Time Study, and a Cooling Time Study. With the results of these studies, they were able to define high and low values for a full factorial Design of Experiments using hold pressure, melt temperature, and mold temperature as the variables, and the critical control dimensions as the monitored responses.
During qualification, the Natech Engineers confirmed that the dual sub-gate achieved a balanced fill within each cavity and the updated runner design with MeltFlipper® achieved a balanced fill across all cavities. Along with the Scientific Processing the new mold design provided the needed inputs to define a wide aesthetic window and processing window to achieve a more controlled process with a 40% reduction in the cycle time.
Replacement molds provide a unique opportunity to tap into the long-term experience with a mold to improve the mold design. Not only can issues be more efficiently identified and resolved, they can sometimes be altogether avoided. As the injection molding body of knowledge advances, the ability to control quality tightens. Engineers must stay current on the latest industry developments and keep close connections with those who are advancing the body of knowledge. Knowledge means more than gathering information. It requires skills development over the long term through expert guidance. Sometimes this can take years or even decades.