Waupaca breaks out of the mold with new gating approach.

An integrated filtration gating system leads Waupaca Foundry to vast quality improvements in its gray and ductile iron castings.

It's like running a marathon that has no finish line, reading a book without a final chapter or riding a train with no destination. Preventing nonmetallic inclusions from forming in high volume castings is a process with no end. For Waupaca Foundry, Inc., Waupaca, Wisconsin, the nation's third largest gray and ductile iron foundry, the adoption of new filtering and gating philosophies proved to be the right defense for winning the battle against nonmetallic defects.

Like all foundries today, Waupaca is concerned about reducing rejects and scrap. The firm's four foundries are equipped with high-volume vertically parted Disamatic molding machines, shipping more than 500,000 tons of castings annually. As a supplier of precision cast components, the foundry places a big emphasis on quality. Castings are inspected for external conformity and examined using radiography and machining tests for subsurface defects.

While these quality tests were effective at identifying inclusions and other defects, Waupaca's management was interested in eliminating their occurrence. Gary Thoe, Waupaca's president, summarized the company's mission as "defect prevention, rather than defect detection." This commitment prompted Waupaca to research new filtration and gating methods designed to control nonmetallic inclusions.

Common Contaminants

Nonmetallic inclusions arise from conditions and/or materials found both outside and inside the mold. Common outside contaminants include dross, slag, slag coagulant and foreign particles. If the gating system doesn't remove these materials, they migrate into the mold and end up in the finished casting. Along with these foreign materials, inclusions can be generated inside the mold from loose sand, mold erosion and turbulence-caused oxidation.

Conventional gating systems, both pressurized and nonpressurized, attempt to remove nonmetallic contaminants through long runners, slag traps, swirl bobs and runner extensions. Flow rates are restricted through a choke, the smallest cross-sectional area in the gating system.

The principle behind conventional gating is that, through adequate cross section and length, along with control of iron flow and direction, nonmetallic contaminants will separate from the molten stream prior to entering the mold cavity. What became apparent to Waupaca engineers, however, is that even the best conventional gating system may not remove enough of the nonmetallic inclusions, resulting in high levels of scrap and rejects.

Filtering the Options

Strategically positioning ceramic filters in a gating system is a proven method for increasing yield and quality. These filters eliminate exogenous inclusions by trapping slag and dross. Inside the mold cavity, filters help control aspiration, turbulence and sand erosion.

While adding filters to conventional gating is beneficial, designing gating systems specifically for filtration can further enhance filter performance and substantially increase yields. Such systems afford simple filter placement, consistent fill times, optimum filtration and minimum gating size. They also furnish prosection against turbulence and secondary oxidation. And, because these gating systems are compact, more parts can be produced on the same pattern plate, or lower pour weights can I be achieved with the same number of parts.

Determined to reduce scrap and rejects, Waupaca completely revamped its casting operations through the implementation of quality control procedures and newly designed filtration gating systems. Metallurgists at the foundry decided to phase in the systems, beginning with Plant 4.

Custom-Designed Gating

Prior to bringing the filtered gating systems on-line, Waupaca metallurgists reviewed gating geometries, discussed filter options and identified problems. In vertically parted molds, casting size and location create unique conditions from plate to plate. Each case, therefore, had to be considered separately.

Prior Foseco research revealed that placing filters as close to the casting cavity as possible traps the highest percentage of inclusions formed within the runner system. While special considerations had to be made for each vertically parted mold, the gating systems for these automatic molds offered maximum effectiveness when the filters were near the bottom of the pattern plate, at the base of the down-sprue and with the choke at the casting ingates. This configuration was shown to generate the most clean and machinable castings. And because the castings would fill from the bottom up, turbulence and oxidation could be minimized.

After completing the proposed filtration designs, Waupaca and Foseco team members took their ideas off the drawing board and down to the foundry floor to conduct on-line tests. Sample lots were created for the various gating systems. During the casting process, iron temperature, casting weights and pour times were monitored. After shakeout, tests determined the level of nonmetallic defects on new castings. They included machining analysis, surface hardness measurements and defect inspection using a scanning electronic microscope.

Elevated to a Higher Cast

Results from the sample lot tests were encouraging. All but a few of the new filtration systems performed as well or better than anticipated. By making minor adjustments to the gating geometries, problems with the remaining molds were rectified. With the success achieved at Plant 4, new gating systems were initiated at the remaining Waupaca foundries.

Today with new gating, filtration and quality systems in place at all four plants, Waupaca has reduced its rejects by 90%, and minimized scrap by more than 75%. The efficient filter gating systems reduced pouring times, improved the machinability of the castings and increased the number of parts per plate or reduced pour weights on the majority of molds. All these improvements have had a desirable impact on yield--convincing Waupaca management that stepping out of the mold of conventional thinking does have its rewards.