Reaction Injection Molding Polyurethane's Low-Pressure Processing Fosters Design Of Low-Cost Modular Tooling For Molding Enclosure Doors.

Low-pressure Reaction Injection Molding (RIM Molding) of a polyurethane system, allowing a novel tooling design, significantly contributed to tooling cost savings of $159,000 in the production of two large doors for a process control equipment enclosure.

The doors are designed to meet National Electronic Manufacturing Association (NEMA) 1 and 12 standards for enclosures of electronic equipment. They are being used with the Advanced Process and Control System's (APACS) process controllers, which are manufactured by Moore Products Co. The controller monitors and controls inputs from field sensors at plants manufacturing products including chemicals, pharmaceuticals, and paper.

The radically reduced tooling cost was made possible by a modular aluminum tooling design based on the low, 100 psi (pounds per square inch) pressure possible with the Reaction Injection Molding process and the minimal flash inherent in processing the Baydur® 726 IBS (interactive blowing system) polyurethane structural foam system.

In addition to good performance and processing characteristics, the Baydur® 726 IBS polyurethane structural foam system does not contain any cholorofluorocarbons (CFC's) or hydrogenated chloroflurocarbons.

The molds have a modular design, using interchangeable cores for the production of four different left- and right-hand enclosure doors using only two molds. Left- and right-hand hinge attachments, and left-hand latch attachments simply are bolted to either core section to create the four tooling options.

"Although the NEMA 1 and NEMA 12 enclosures each contain only a single door, it was necessary to provide the option of hinging the doors either on the left or right, depending on specific access and 'door swing' requirements," stated Randy Smith, Senior Mechanical Development Engineer, Moore Products Co. "The cost-effective solution was to build two molds with interchangeable cores to produce all four doors."

Reaction Injection Molding Process Allows Intricate Design Features

The NEMA 1 and NEMA 12 enclosure doors (39 1/2" high by 35" wide by 2 1/2" deep with a cross sectional wall thickness ranging from 1/4" to 5/8" thick) are differentiated by their provisions for heat dissipation. The left- and right-hand doors for the NEMA 1 enclosure contain bottom vents for forced air circulation, as well as cosmetic vertical grooves. By contrast, the NEMA 12 enclosures have two large vertical parallel openings (35 1/2" high by 10" wide) in the doors to allow the mounting of solid, finned aluminum heat sinks.

Side panels of the NEMA 1 and NEMA 12 enclosures, RIM-molded in a separate tool with Baydur® 726 IBS polyurethane structural foam system (39 1/2" high by 15 1/2" wide), each contain a similar-sized aluminum heat sink. The other parts of the enclosures are fabricated steel framing and screw-attached sheet metal.

Stackable Enclosures Designed With Tight Tolerances

"The enclosures were designed to be standard rack-mounted and to stack, thereby minimizing the space they require within the plant," said Langsdon Palmer, Systems Manager for Moore Products Co.

Towards that end, the tolerances for the dimensions of the doors were held within 0.001" per inch, according to Paul Steck, President of Exothermic Molding, Kenilworth, N.J, the doors' molder.

Edward Varga, D&D Technology Inc., Union, NJ, a design consultant on the project, said that the doors, with variable wall thickness from 1/4" to 5/8", are RIM-molded with only 75 tons clamping force. "The Baydur® polyurethane material molds with a hard skin and very rigid foam core, resulting in a strong, lightweight structure," he commented.

The Baydur® 726 IBS polyurethane structural foam system has a density of 55 pounds per cubic foot, flexural modulus of 240,00 psi, elongation of 8 percent, and Charpy impact strength of 15 ft-LB/inch for a 0.25" thick specimen. (These items are provided as general information only. They are approximate values and are not part of the product specification) The application also required the material's UL (Underwriters' Laboratories) 94 V-O 5V flammability rating. (References to flammability characteristics are based on small-scale test for purposes of relative comparison and are not intended to reflect hazards presented by this or any other material under actual fire conditions).

Modular Tooling Simplifies Multiple Mold Requirements

Modular tooling helped to simplify the multiple-mold requirements of the processors' enclosures, in which using either of two core sections accommodated the different door-front designs, Steck noted.

The RIM-molded doors contain integral hinge stiffeners with encapsulated studs, provided for in the two basic molds, adding structural support to the doors. "In conjunction with Reaction Injection Molding processing," Steck continued, "our system has several inherent advantages over other fabrication methods for applications requiring encapsulations, because of the low pressure. These include the high strength and adhesive properties of the material in its liquid state as well as its capacity for variable wall thickness without sink marks, which is due to the polyurethane's self-expanding and thermosetting properties."

Steck added that, because of these features, much design freedom is possible with the Reaction Injection Molding process. The inherent freedom of the Reaction Injection Molding process is complemented by the modular aluminum tooling, which also eases engineering design changes, he said.

For example, for the RIM-molded side panels of the NEMA enclosures, the Baydur® 726 IBS polyurethane structural foam provides the appearance advantages of 1/16" deep by 1/10" wide vertical grooves and other cosmetic features. A removable tool insert for imprinting logos on the doors also is available.

Short Cycles With Internal Mold Release Material

Cycles times for the 18 1/2-pound NEMA 1 doors and the 8 1/2-pound NEMA 12 doors are five to six minutes, thanks to the internal mold release properties of the material, according to Steck.

"When molding these parts of Baydur® 726 IBS polyurethane structural foam, which contains an internal mold release, additional spraying of a release agent to the mold is necessary on only 30 percent of the shots," he declared. "Conventional polyurethane systems require release agent between each shot. The material's mold release properties shorten the average molding cycle by as many as three minutes."

Polyurethane Coating Produces Textured Finish

Exothermic Molding's Steck commented that the Baydur® material flows well, producing excellent surface characteristics. Following molding, the doors are lightly sanded and deflashed, washed, and then primed with a Sherwin-Williams polyurethane spray fill primer to promote adhesion. They are then painted with POLANE T-Plus paint polyurethane raw materials supplied by Miles. A final spatter coating with the same paint formulation is applied to produce a textured effect.

Reaction Injection Molding
Reaction Injection Molding
Reaction Injection Molding