Many times we hear that improving speed, quality, or cost involves a corresponding tradeoff with one or more of the others. But once in a while the right approach can offer improvements to all three. The injection molding process balances hundreds of details. Decisions in one mold design variable can adversely impact production often in unknown ways. Rather than live with unnecessarily longer cycle times which translate into longer lead times and higher costs per part, more companies are now starting to take advantage of Mold Flow Analysis during the mold design phase.

You can think of Mold Flow Analysis as an increasingly important Design for Manufactuability (DFM) tool. Advancements in the simulation software have strengthened the justification for the upfront investment in engineering time to run the models, analyze the results, make optimization decisions, and implement the changes into the design. As quantities increase over time, the economic impact of making the right or wrong choice intensifies. A multi-cavitation project illustrates the point.

The Application

A medical device client was developing a collection mechanism for a reactive material in a diagnostics application. The component included a Tipped end which was to be inserted into a separate Handle. A Ledge in the center reduced the sample material drip risk. A Chamfered section would allow break off once the Ridged collector was inserted into the test container. The Ridges were designed to increase the surface area contact and thus the adherence with the sample material. A Through Hole served as a collection port to firm the mechanical engagement and interlock with the sample material.

Quality Risks

Mold Flow Analysis was conducted for fill/flow progression, gating, cooling, and warpage. The Mold Flow Analysis revealed three primary quality concerns: Gate Freeze, Volumetric Shrinkage, and Warpage.

Gate Freeze poses an issue because if the part has cooled near the gate too early in the pack and hold stage, short shots or weak regions can result. Some level of Volumetric Shrinkage is a given in any thermoplastics molding, but 17% Volumetric Shrinkage is on the high side. This can impact the consistency of the part geometries pushing features out of tolerance. However, Volumetric Shrinkage can be managed in part through the design of the part and mold. Warpage in this case is the deflection of the part due to the differential in the preliminary shrinkage. 0.010” of Warpage on a 1.5” part with tight-tolerance features poses too high of a risk to just blindly accept.

Economic Risks

The analysis of the Fill Progression showed that over half the cycle time was consumed by the cooling stage. At first glance, this would not necessarily jump out as an issue, but the simulation showed that this was higher than necessary and could be reduced while also improving the part quality.

Optimization

The Natech Engineering Team decided to core out the cylindrical features to create length-wise ribs of the optimized wall thickness. They lengthened the Through Hole to maintain a more consistent wall thickness. They moved the gate from the tip of the part to the Drip Guard closer to the middle of the part.

Results

The mold flow analysis on the optimized design confirmed that these optimization adjustments yielded a 25% reduction in cycle time. Time and costs were reduced enough that the Four-Cavity Mold scheduled to produce quantities of 500,000 in the first year with 100,000 parts per order would essentially pay for the investment by the second order.