Investment casting yields juicy gains

Keeping things fresh is key to Evergreen Packaging Equipment's beverage packaging systems. So it's no surprise that the Cedar Rapids, IA-based company adopted a fresh approach when upgrading the oven heads that heat the carton tops on its gable-top carton filling and sealing equipment. Switching from sheet metal fabrications to investment castings has yielded juicy quality gains and production savings.

A key component in every Evergreen gable-top carton packaging machine is an oven head-extremely thin-walled and box-shaped-that heats the paper in preparation for carton sealing. To create the triangular carton top, the paper is formed into a partial carton shape, heated with hot air under the stainless steel oven head, and then pressed into the gable configuration. As the carton cools, the seal forms.

Since proper oven heating is critical for a good carton seal, the oven heads must meet strict specifications. Each oven head must fit closely around the partially formed carton to provide uniform heat delivery to the paper. The oven heads vary in size, in order to fit around carton sizes ranging from a half-pint to a half-gallon. Specifications for the oven heads call for sharp interior corners, with no rounded edges, and walls with a mere 0.0625 plus or minus 0.003-inch thickness. And no porosity can be present in the finished part.

Fabrication complications. Given the thin walls of the oven heads, Evergreen initially used sheet metal fabrications for these components. But with this method of production Evergreen had difficulty maintaining proportional consistency from one oven head to the next-a requirement to ensure that each carton-sealing machine produced consistent high-quality sealing. Fabrications were also costly and labor intensive, involving considerable machining and polishing to meet the dimensional specifications and surface finish requirements.

The opportunity to resolve the oven head issue came in the form of another long-time Evergreen supplier, who at first seemed an unlikely source for producing the oven heads: Lake Geneva, WI-based Northern Precision Casting Co. (NPC). "Upon hearing of the oven head issue, NPC looked at the fabrications to see if they could investment-cast the parts-even though this wouldn't be a typical casting job," said Alan Baumgartel, senior buyer at Evergreen.

Two features of the oven head stood out as atypical for an investment casting: the extremely narrow walls and the right-angle corners. Thin walls can be challenging to fill with molten metal since the passageways are so tight that the metal can solidify before complete filling occurs. And tight corners require care to make sure metal doesn't pool and create unwanted fillets. Nonetheless, NPC determined that with careful attention to metal filling techniques, they could meet the specifications-at less cost and in less time than the fabrications required. And because investment casting uses one precise part pattern for each oven head size, consistency between same-sized oven heads would improve.

"Knowing NPC's history of quality and their familiarity with the requirements of the food processing equipment industry, we were confident that the NPC parts would not only meet the specifications, they would be far superior to anything we could have had fabricated," said Jim Bulechek, manager of purchasing for Evergreen.

To create the investment cast oven head designs, NPC engineers worked from finished part prints for fabricated oven heads. They devised a unique tool design to prevent fillets in the corners. And to ensure that the walls would fill completely, they developed a sophisticated design for the runners and gates-the channels by which the molten metal fills the mold during investment casting.

Increase quality, reduce costs. The investment-cast oven heads are a success in a variety of ways. Their exact and uniform proportions, for example, have greatly reduced secondary machining. Additionally, the surface finish of an investment casting is one of the finest among all metalworking processes. And NPC specializes in producing tight finishes, using a finer size of abrasive in the surface cleaning process as well as finer sands for the prime coat in the mold building process. "NPC's process yields a 125 RMS (Root Mean Square) surface finish, a smooth finish by food industry standards," says Baumgartel.

This fine surface finish is important to Evergreen in more ways than one. First, it precludes the presence of porosities, helping to comply with FDA standards. Second, it reduces the polishing needed to bring to the oven heads to spec, resulting in lower costs.

Cost has been a major benefit. "Due to the large reductions in welding, machining, and polishing, the conversion to investment casting has resulted in a 60% savings on oven heads," reports Bulechek.

Seven steps to quality metal parts

Although investment casting-or the "lost wax" process as it is known-has roots in ancient history, it has evolved into a cost effective metal-forming process for producing high-quality metal parts when performed by experienced and knowledgeable casting specialists.

1. DIE CONSTRUCTION. Create a wax duplicate pattern that matches the part's configuration and finish. A simple hand-operated, single-cavity die can be easily modified for prototypes, or multiple-cavity tooling is fully automated for volume production runs.

2. WAX-PATTERN INJECTION AND ASSEMBLY. Inject dies with wax to produce heat-expandable patterns for each part. Disposable patterns are slightly larger to accommodate shrinkage. Multiple wax patterns, connected by "gates" to wax sprues, form a cluster.

3. CERAMIC INVESTMENT. Clusters are dipped into a ceramic slurry, drained, and coated with fused silica sand. After drying, repeated dipping with progressively coarser ceramic material eventually forms a self-supporting shell ranging from 3/16- to 5/8-inch thick.

4. DE-WAXING AND CURING. Once dried, shells are heated in a steam autoclave to melt and evacuate wax from the mold. Then the ceramic mold is fire cured at temperatures between 1,600 and 2,000F.

5. ALLOYING AND CASTING. At pouring temperature levels, add elements to achieve proper chemistry and improve castability. Depending on the application, a vacuum- or centrifugal-force pour can be used.

6. SHELL RELEASE AND CASTING CLEANING. Once sufficiently cooled, mechanical vibration and chemicals remove the ceramic shell. Technicians separate parts from the cluster and prepare the casting for secondary operations such as heat treating, machining, or applied finishes.

7. QUALIFICATION. After visual and dimensional inspection, parts are x-rayed to ensure component integrity.