Polyurethane Reaction Injection Molding System Enables Complex Design Geometries, Features In Medical System Enclosure Panels

Advanced polyurethane Reaction Injection Molding technology from Exothermic Molding, Inc. enabled cutting-edge design in an automated diagnostic system that is as attractive as it is functional.

The PHOENIX Automated Microbiology System is under development by the Biosciences division of Becton, Dickinson and Company, Sparks, MD, for use in large commercial and hospital laboratories. The system performs automated analysis of infectious agents and has the capacity to perform from one to 100 determinations simultaneously. The manufacturer selected Exothermic's PRISM® CM-200 solid polyurethane Reaction Injection Molding system from Bayer's Polyurethane's Division to mold the access door, door frame, and right and left front panels on the front of the unit's carousel bay, which is the largest part of the system. The vertically mounted parts are a materials challenge of complex geometries, varying wall thickness, deep reinforcing ribs, and molded in-bosses.

The PRISM® CM-200 solid polyurethane Reaction Injection Molding system, with a UL94 flammability rating of V-0 and 5VA, is well suited for applications in medical, electronic, and other markets. The material is excellent for thin-wall molding and offers improved strength, surface finish, and large-part capability.

"We looked at a number of processes as we began designing the PHOENIX system," said David Lentz, Project Engineer for Becton, Dickinson. "We've used the gamut of processes, from sheet metal to structural foam to thermoforming to Reaction Injection Molding." He said the selection of Reaction Injection Molding over other processes was based on a low production volume of approximately 500 units per year and lower tooling costs. "With Reaction Injection Molding, the per-part cost was very competitive," he said.

Bayer's PRISM® Reaction Injection Molding system provides design flexibility, cost-competitiveness, good strength-to-weight ratio, and an attractive surface finish for the PHOENIX System's parts, Lentz said.

Complex Curved Surface

The four Reaction Injection Molding parts feature a complex, convex-curved surface. For example, at more than 32" tall and 21" wide, the door and its frame feature an arc length of 19.55" and arc radius of 18.115". The door has raised surface features, too. The closed door conceals the frame. At 8.51 pounds, the door is the heaviest RIM part in this application. The door frame weighs 3.40 pounds.

The right front panel is more styled. The door's handle area interlocks with the right front panel via a male/female connection. The shape of the part also suddenly changes near its midpoint to form a transition into the separately connected electronic display unit. The panel is 34" high by 4.12" wide, and is the lightest of the Reaction Injection Molding parts involved - just 1.68 pounds. The curved left front panel is approximately 34" high by 13" wide, with a weight of 5.31 pounds. "We couldn't achieve such complex geometries with sheet metal, and the door frame was quite a challenge due to the radius," Lentz said.

Compounding the surface complexities are variations in wall thickness from as little as 0.090" to as much as 0.40", as well as numerous reinforcing ribs that are up to 3" deep, according to Lentz. "The PRISM® Reaction Injection Molding system gave us a great deal of design flexibility. We could vary the wall thickness where needed, creating thick sections and raised features," Lentz said. "And even with the extensive reinforcing rib structure, we are achieving molded parts free of any sink marks."

The parts are molded with aluminum tooling cut directly from electronic design files. Three different mold tools are used, with the right and left front panels produced simultaneously from a family mold. Exothermic Molding applies a polyurethane paint to all four parts, followed by a textured matte finish. The Becton, Dickinson logo is screen-printed on the door. Bayer provided finite element analysis and design assistance to optimize part strength and improve part functionality for both the molder and the OEM.

Offers Fastener Flexibility

In addition to producing quality parts, Exothermic Molding's PRISM® Reaction Injection Molding system also offers the flexibility to use different fasteners during assembly operations. For instance, pre-formed, threaded metal inserts are pressed into load-bearing components such as the door handle and the door locking mechanism after they are molded.

As a thermal seal, Becton, Dickinson applies die-cut, closed-cell rubber thermal insulation between the reinforcing ribs of the molded panels. A sheet metal liner, attached with self-tapping screws, covers the insulation. An extruded silicon rubber seal attached to the door frame with adhesive provides a thermal and light seal. "The finished panels provide a good thermal and light seal for the PHOENIX system," Lentz said.

Becton, Dickinson attaches the fully assembled panels to the carousel bay's aluminum frame with machine screws, which pass through holes molded into the panels.

 Reaction Injection Molding