Reaction-injection molding has become a premier polyurethanemolding process that offers automotive engineers versatility in the materials used to produce high-quality, highly styled parts.
By Harry George
Manager, Specialty RIM Group
Polyurethanes Div., Bayer Corp.
Edited by MD Staff
Reaction-injection molding (RIM) is a relatively new manufacturing technology that produces highquality polyurethane auto parts. One of the earliest commercial applications of RIM was the all-polyurethane bumper fascia on the Monza 2 + 2 for General Motors in the early 1970s. That application demonstrated the process' flexibility and let the auto industry meet government demands for bumpers that would survive the 5-mph crash test. The Monza bumper was so successful that many people still associate RIM with bumper fascia.
But RIM technology has moved well beyond bumpers, thanks to its inherent advantages coupled with new developments and innovations in molding, materials, and equipment.
RIM uses liquids with viscosities ranging from 500 to 1,500 cps, low process temperatures of 90 to 105°F, and relatively low internal molding pressures of between 50 and 150 psi. The low viscosities, temperatures, and pressures give RIM distinct benefits compared to other plastic processes used for automotive parts.
RIM components' low viscosity (flowability) lets them fill molds for large parts. Truckbox outer panels molded with RIM weigh up to 30 lb, yet are 30% lighter than panels made of steel or composite materials. Size limitations on RIM parts depend on the reaction speed of the polyurethane formulation used and the throughput of the metering unit, i.e., how many pounds can be dispensed in a minute.
RIM parts can also include inserts placed during molding. For example, steel, aluminum shapes and frames, window glass, glass preforms, electronic connectors, PC boards, and wiring harnesses have all been encapsulated using RIM.
Molded parts with varying wall thicknesses are a definite problem with plastic processing methods and materials such as thermoplastic injection molding, blow molding, and sheetmolding compound. But RIM can create wall thicknesses ranging from 0.25 to 1.125 in. on the same molded part with minimal or no sink marks.
RIM also handles decorated films, vinyl and fabric preforms, gel coats, and two-component polyurethane paints placed in molds prior to injection. Injected polyurethane bonds to the material, thereby producing decorated parts in the mold. This greatly reduces secondary finishing costs.
The surface finish of parts molded with RIM lets manufacturers produce Class A painted parts. For instance, fenders, spoilers, and fascia parts can match the high-gloss paint finish on metal parts they mount next to in the final assembly.
RIM's low injection pressures let mold builders use a variety of mold materials that are less expensive than p-20 or hardened steel. Alternate materials include machined or cast aluminum, cast kirksite, nickel shell, and even some plastic composites.
Finding the right formula
RIM uses a variety of polyurethane formulations to achieve specific combinations of properties. The diverse range of formulations, however, makes polyurethane RIM more difficult to understand than thermo-plastic injection molding. Therefore, it's helpful to view RIM not as a specific resin with narrowly defined properties but as a process that produces a broad range of parts with a myriad of properties.
RIM parts can be foamed or solid, flexible or rigid, a composite or fiber reinforced. Almost anything from a flexible foam-core part to an extremely rigid, solid part is possible with RIM. For example, rigid, structural-foam RIM is used for parts with molded specific gravities from 0.2 to 0.85.
Moreover, short-glass or mineral reinforcement added to the polyol blend leads to RIM parts with better stiffness and heat performance. Lowviscosity formulations injected into molds containing glass mats or preforms produce stiff, high-strength composite RIM (SRIM) parts.
New materials and processes
Engineers have developed a wide variety of RIM formulations over the past five years. These developments position RIM for growth in the next decade. These formulations include:
Flexible foams. The need for tougher, thicker skins on molded car and truck parts, rather than the traditional water-blown flexible foams with weak skins, is focusing development on finding alternative blowing agents. Pentane, hexane, and cyclopentane are now used as blowing agents, creating new applications in molded flexible foams.
Solid elastomers. Chemical formulations for solid RIM elastomers have either been fast-reacting, highperformance amine-extended systems or slow reacting, low performance, and glycol based. Two recent developments — hybrid amine and high-heat polyurea systems — are expanding the use of solid elastomers.
Hybrid amine elastomers, with flexural moduli between 2,000 and 140,000 psi, offer characteristics similar to the amine systems, but with extended gel times — between 5 and 15 sec — normally associated with glycol-based systems. This has generated new opportunities for processors with smaller throughputmetering units.
High-heat polyurea offers auto engineers performance similar to amine-extended parts but with higher-heat capabilities. Polyurea parts withstand temperatures to 400°F.
Rigid foam. Automotive OEMs demands have pushed development of rigid, structural foam RIM. Companies such as ours have responded by introducing various energy-absorbing foams and formulations that resist heat and can handle modern paint-bake technology.
These energy-absorbing foams range from lightweight and friable to more resilient and recoverable. Friable foams dissipate energy generated in a car crashes and, generally, have a useful life of one collision. Recoverable foams also dissipate energy, but are resilient enough to recoup up to 90% of their initial, molded geometry after a crash. Thus, recoverable foams have service lives covering multiple impacts.
Automakers want rigid structural foam parts that can be mounted on metal assemblies prior to painting the metal components, creating a demand for higher-heat resistant rigid foams. Such parts safely handle bake-paint oven temperatures of 220°F for 1 hr without the damage associated with earlier rigid-foam systems.
Rigid, solid. Market demands have also driven the development of new rigid, solid materials with flexural moduli between 270,000 and 350,000 psi for high-strength applications. The result is a family of materials that meet the demanding specifications for automotive applications, including thin wall thickness and on-line paintability. These systems offer characteristics superior to other composite materials.
Similarly, the construction market has demanded chemical-resistant, rigid polyurethane materials to replace cast iron and concrete. The result: the recent introduction of a family of solid polyurethane RIM materials that meet the demanding requirements of such underground applications as wastewater treatment and grinding pumps.
Reinforced RIM (RRIM). Everincreasing demands for reinforced polyurethane elastomers have driven not only development in new formulations, but also innovation in equipment and filler packages. Filler materials were previously short, glass fibers. New families of fillers — wollastonite, mica and blends — were developed to enhance physical properties, improve processing, and reduce costs. Meanwhile, new methods of placing a sizing or coating on fillers have enhanced the fillers' wet-out, reduced viscosity of the polyol component to which fillers are generally added, and offers better end-use performance of molded parts.
Reinforced high-heat systems let OEMs attach molded-polyurethane parts onto vehicles' metal chassis and then E-coat them along with metal parts during painting. These systems withstand temperatures up to 400°F, making them suitable for most automotive E-coat painting lines.
To meet demand for larger reinforced RIM parts, processing equipment suppliers are producing cylinder-type metering units capable of dispensing 35 lb of polyurethane chemicals/sec. With these metering units, processors can produce molded parts thought to be impossible only a few short years ago.
Composite SRIM. The demands for stiffer, tougher structural polyurethane parts have dramatically affected SRIM materials, processing, and equipment. New regulations on composite technologies like polyester RTM and FRP and innovations in SRIM have created opportunities for polyurethane-based composites. This area of RIM processing will experience the fastest growth in the next decade in both foam and solid SRIM.
Traditionally, SRIM uses either fiberglass preforms, and directional or nondirectional mats as mold inserts through which polyurethane SRIM formulations are injected. Encapsulation of long, glass-fiber inserts gives finished parts strength and other physical properties that make SRIM parts suitable for structural applications.
Foamed polyurethane SRIM will continue growing, as it did in the 1990s. New lower-viscosity, betterflowing chemical systems let auto companies make larger, lightweight, stiff composite parts for applications such as automotive door liners and heavy-duty truck interiors. Vinyl and fabric outer skins are vacuum formed into molds before inserting glass mats and injecting or openpouring SRIM chemicals into the mold. This creates aesthetic, lightweight, structural parts ready the assembly line after some simple trimming.
Solid polyurethane SRIM will grow significantly in the next decade due to recent developments and General Motors' introduction last year of the first composite cargo box on a full-size pickup truck. SRIM materials are injected through a fiberglass preform to form the cargo box in one solid piece. The box not only meets stringent GM performance requirements, it also cuts weight from the vehicle and provides additional ruggedness, long-term durability, low-maintenance costs, and freedom from rust and dents.
Polyurethane RIM was developed in Germany in 1950s when a Bayer scientist injected a polyol resin blend and an isocyanate — the two components of polyurethanes — into a closed mold. The two components mixed and flowed into the mold as a lowviscosity liquid, underwent an exothermic chemical reaction, and formed a polyurethane molded part.
In modern RIM plants, the two reactants are held as liquids in separate, temperature-controlled feed tanks equipped with agitators. From these tanks, polyol and isocyanate feed through supply lines to units that precisely meter them, then let them pass at high pressure to a self-cleaning mixhead.
The liquid reactants enter the mixhead at pressures between 1,500 and 3,000 psi through two small orifices, where they are completely mixed by high-pressure impingement. From the mix chamber, the liquid flows into a mold at about atmospheric pressure and undergoes an exothermic chemical reaction, forming a polyurethane polymer in the mold.
Shot and cycle times vary, depending upon part size and the polyurethane formulation used. Average molds are filled in 1 sec or less and are ready for demolding in 20 to 60 sec. Rigid, structural foam can be injected for 10 to 30 sec and parts removed from the mold between 3 and 10 min later, depending upon the finished part's cross-sectional thickness. The cargo box also has excellent high-heat and cold temperature stability.
To meet growing demand for better composites, processors have developed competing technologies that inject long, glass fibers along with the polyurethane formulations in one step. This replaces the traditional two-step process of inserting preforms or mats into the mold before injecting anything. In these new technologies, a robot-mounted, glass-chopping gun attaches to the mixhead. The preprogrammed robot moves over the mold cavity while dispensing both glass fibers and polyurethane SRIM materials in an open-pour method. At the end of the pour, molds are closed to form the part.
These developments are automating the SRIM process, making parts even more economical to produce. This processing technology uses either a foamed or solid SRIM formulation. The SMART car roof, for example, is produced with long-fiber injection that turns out a complete roof with each molding cycle. In addition, long-fiber injection can produce parts using two-component, in-mold painted composites.
Polyurethane RIM materials bond to a wide variety of materials with excellent adhesion, a characteristic that has been exploited for years in end-use applications. For example, processors have long used foamed polyurethane systems injected behind vacuum-formed vinyl to make automotive interior parts for vehicles. They have also inserted window glass into molds and injected solid polyurethane elastomers on the outer edges to create integral gaskets. Mold inserts currently include aluminum frames and extrusions, steel box beams, wood shapes, and triple-pane glass packs. Inserts provide strength, improve performance and simplify manufacturing.
In addition, the electronics industry has discovered the benefits of encapsulating parts in RIM. Electrical components, PC boards, wiring harnesses, and telecommunication components are being inserted and encapsulated in polyurethane RIM to provide onepiece, water-resistant assemblies.
Another development, in-mold decorating, was limited to a twocomponent in-mold coating process. The finished coat of paint was sprayed onto the mold cavity, the mold was closed, and polyurethane RIM injected into the mold. The RIM material reacts with the in-mold paint, providing a chemical bond between the substrate and coating. The 55-lb polyurethane foam roof made for a combine is a good example of inmold coatings used for decoration. Processors also use polyester gel coats, polyurethane spray elastomers, and water-based paints as in-mold coatings.
Further developments in the use of preprinted thermoplastic films as in-mold decoration are also gaining acceptance and wider use. A clear or pigmented thermoplastic film screenprinted or sublimated with graphics and customer logos can be cut to size and inserted into the mold with vacuum assist. The part emerges from the mold with a fully decorated surface.
Polyurethane RIM used in automobiles and light pickup trucks will continue growing over the next 10 years, in large part due to advances in energy-absorbing foams, RRIM high-heat elastomers, and SRIM composites. Specialty, nonautomotive markets should also see double digit growth in the decade to come.