Are you ready for online design? Part 2: Filling a need via the Internet

It was the type of situation custom molders dread: An older mold, built before moldfilling simulation was available, began to produce parts with sink marks in a critical, but not visible, location on the inside diameter of a molded hole. The part itself was insert molded from polyetherimide (Ultem).

Resistance Technology Inc.’s (RTI) Medical Infusion Group (Vadnais Heights, MN) went to work to correct the problem in three ways. To get good parts out quickly, operators removed the core pin, molded parts, and subsequently machined the smooth hole in a secondary operation. Simultaneously, technicians ran processing trials to determine the cause of the sink marks. Later, gate size was enlarged in an attempt to get better filling, and parting line venting was added to eliminate trapped air.

Seeking Answers
Project engineer Melissa Gardner explains, “None of the fixes solved the problem. At volumes of 100,000 per year, machining the holes was not considered a long-term solution. I decided to run a moldfilling simulation using Plasticszone.com’s iMPA product. It was one of the first projects where I used this service.” Previously, Gardner had contracted with other vendors for moldfilling analysis. Rather than purchasing the MPA (Moldflow Plastics Adviser) software outright, Gardner went to the website and paid a one-time fee to perform unlimited simulations on a single CAD model.

Pay-as-you-go software offered by vendors known as application service providers (ASPs) is becoming more popular among designers. Without investing in the software, users can take advantage of its benefits for a nominal fee. Services such as model healing and translation are also offered online by ASPs.

When Gardner ran the iMPA simulation using material properties from the software’s database, the program came back with warnings that the part would have sinks where they had appeared on the actual part. “This gave us a great deal of confidence in the results because the software duplicated what we saw in reality,” she says.

Gating Changes
In addition, the software predicted quality to be poor, and identified gate location as the problem. “We were gating into a thin section,” recalls Gardner. While the nominal wall thickness of the part is .070 to .075 inch, the plastic had to flow through thin sections, or baffles, of .050 to .060 inch to get to an area of the part that required good packing.

“With this software,” she adds, “you can ask for an ideal gate location. We did, but the suggested location wasn’t a practical one. Through more iterations on different gate locations, we found one that would be practical and that also checked out via filling simulation.” Engineers then redesigned the tool and had it rebuilt.

“People were skeptical of building a new tool based solely on the simulation results, so we modified the one we had and it produced sink-free parts from the first shot.” RTI is now in the process of building a new tool based on the redesign.

According to Gardner, it makes more sense to run moldfilling simulations on a pay-per-use basis for now at RTI. “Because the analysis is fairly reasonable over the Internet, we are planning to perform it for most of our new tooling, perhaps 20 to 30 per year. My expectation is that as the engineering and management departments at RTI see how valuable this tool is, they will be more anxious to invest in the software itself.” She adds, “Special projects such as this part may also require filling simulation, but new tooling that is replacement tooling or a very simple part may not."

Editor’s note: Due to proprietary concerns, images of the part described in this article are unavailable.