Closing In With Polyurethane RIM

Closing In With

RICHARD MACY
Technical Marketing Representative
Specialty RIM Group, Polyurethanes Div.
Bayer Corp.
Pittsburgh, Pa.

In today's medical-equipment industry, form is quickly becoming as important as function in product design. The trend is particularly strong in large devices, such as lab and diagnostic equipment. Manufacturers and users want their equipment to be attractive, ergonomic, and unique. Many design engineers are meeting this challenge while still watching the bottom line by using the polyurethane reaction-injection-molding (RIM) process to form enclosures for medical equipment. It offers designers an economical way to manufacture attractive, durable, and strong medical enclosure parts in a wide range of sizes.

What Is RIM?
A Bayer chemist invented the use of polyurethane materials in RIM in the 1960s. The chemist injected two liquid components of a polyurethane system — isocyanate and polyol — into a closed mold at low pressure. The components mixed and flowed into the mold as a low-viscosity liquid. When the mold was opened, the liquids had formed a polyurethane structural-foam material with a foam core and a hard, solid outer skin.

Since then, polyurethane systems for RIM have grown into a broad range of formulations developed to produce specific physical properties for specific performance requirements. Polyurethane RIM systems can produce virtually anything from flexible foam-core parts to extremely rigid, solid parts.

In general, the RIM process mixes two low-viscosity, liquid materials and injects them at low pressure (30 to 100 psi) and low temperatures (less than 105°F) into a mold. The process differs from traditional injectionmolding processes that first melt raw materials at high temperature and inject them into molds at high pressures.

In most cases, RIM molds cost less than those used in similar processes. This is because RIM works with lower-cost mold materials, such as aluminum, nickel, and epoxy.

Making the medical grade
Enclosures for medical equipment can be an especially demanding application. They often must be flame resistant, stable at high temperatures, cost effective to manufacture in low volumes, and form large, complex parts.

Special polyurethane RIM formulations have been developed to meet UL94 flammability ratings of V-0 and 5VA. That gives the enclosure the flame resistance a variety of equipment must meet, including X-ray machines and dental appliances.

Polyurethane RIM can take the heat, too. Certain RIM formulations have excellent thermal resistance. In fact, some RIM materials used to mold medicalequipment panels can withstand repeated sterilization in high-temperature, dry-heat ovens. These same formulations can also handle the heat generated from within by electronic components.

Low-volume production is a common characteristic for many medical devices, such as laboratory diagnostic systems. Stamped-metal enclosures may not be an option because of the labor involved. And the high up-front investment to develop and cut molds for injectionmolded thermoplastics may be prohibitive. So in many cases, polyurethane RIM becomes an attractive alternative due to its ability to use lower-cost molds. Some designers, aided by knowledgeable RIM molders, have significantly shortened the time-to-market, which also contributes savings and profits.

Although many medical devices are shrinking from one generation to the next, others remain quite large. Forming large-dimension enclosures is a challenge for any material, but polyurethane RIM often tackles the task with ease. With its low-viscosity liquid-molding process and light weight, polyurethane RIM can produce large panels with a fraction of the weight of metal. Polyurethane structural foam formulations are particularly useful in these applications.

Medical manufacturers also find polyurethane RIM an ally in producing integrated instruments. These multipurpose components must look alike in color, texture, and fit to meet aesthetic requirements. Lab-diagnostic equipment often combines computer workstations with diagnostic components. These components may be enclosed along with the computer in a single housing, or enclosed separately in similarly designed housings.

Polyurethane RIM also lets designers use thick and thin-wall sections in parts. This can be a major advantage with enclosures. Designers can specify thick-wall sections where greater strength is needed and thin-wall sections where it's not. Or they can mold in reinforcing ribs as another way to add strength to thin-walled parts.