By Harry George
Specialty RIM and Composites
Bayer MaterialScience, LLC
Consumers of home, industrial, and business goods are increasingly demanding high qualityproducts with robust design and aesthetic qualities. Although most buyers take the technology used in their products for granted, not so obvious are the cutting-edge materials and processing technologies behind it, which make this revolution in art and design possible.
One major contributor is the polyurethane reaction injection molding (RIM) process that has evolved continuously since its introduction in 1969. In addition to its ability to form parts for a wide variety of applications, further innovations in polyurethane RIM formulations, fillers, and processing methods have increased the capabilities of RIM technology to include uses that are more demanding. Long fiber technology (LFT), for example, has helped change the way products are designed and produced, which also has increased sales and profits for OEMs and processors.
The two major liquid reactants that comprise the RIM process move from temperature-
controlled tanks to a high-pressure chamber and finally into the mold at 50 to 150 psi.
RIM Molding Process Advantages
RIM should be viewed as a process capable of molding parts with a broad range of properties. Depending on how the polyurethane RIM process is formulated, the parts can range from those with flexible foam-cores to rigid solids. In addition, part density can vary widely with specific gravities from 0.2 to 1.6. Because of this versatility, RIM can produce components for a broad range of industries, including automotive, appliance, construction, furniture, sports, and recreation.
At the heart of the polyurethane RIM process is a chemical reaction. Two liquid reactants, an isocyanate component and a polyol resin blend, are held in separate, temperature-controlled feed tanks. The polyol and isocyanate liquids feed through supply lines to control units that contain metering pistons and recirculation pumps. The control units increase the reactant pressure andprecisely meter the liquids to the self-cleaning mixhead.
When injection begins, the liquid reactants enter a chamber in the mixhead at 1,500 to 3,000 psi through two extremely small orifices where the high-pressure impingement mixes them further. When the liquid leaves the mix chamber, however, the pressure drops to near atmospheric as it flows into the mold and undergoes an exothermic chemical reaction. This forms the polymer in the mold. Pressures inside the mold range from only 50 to 150 psi. Low viscosity, low temperatures, and low pressures are beneficial properties of the RIM process compared with other plastic process methods. The advantages include:
• Large parts: The excellent flowability of RIM material systems makes them ideal for large parts. The process has produced parts measuring more than 6 ft x 6 ft and weighing more than 100 lbs. The size of the part that can be molded depends on the speed of the reactivity profile of the material system and the throughput of the metering pumps. • Encapsulation of inserts: Many types of inserts can be put in a mold before injecting the RIM material. Steel and aluminum shapes and frames, window glass, glass preforms, electronic connectors, PC boards, and wiring harnesses are some of the materials that have been successfully encapsulated. This often reduces or eliminates secondary manufacturing steps, which can boost productivity and cut costs, as well as consolidate parts.
• Thick and thin walls: Variable wall sections within the same molded part are a definite problem with many plastic processing methods and materials, such as thermoplastic injection molding, blow molding, sheet molding compound (SMC), and other polymers. However, thickness variations are common with many types of RIM parts. Wall thicknesses from 0.25 inches to 1.125 inches can be molded with minimal to no sink marks.
• Class-A surfaces: The surface finish of RIM molded parts is ideal for Class A painting. For example, automotive manufacturers can produce fenders, spoilers, and fascia parts that match the high-gloss painted metal parts adjacent to them in the final assembly.
• In-mold coating and decorating: Polyurethane in-mold paints, polyester gel coats, and pre-decorated films can be used with polyurethane RIM materials to produce a finished part immediately out of the mold. In-mold coating and decorating can reduce the secondary costs for painting, which might be a significant portion of the total part cost. In-mold coating is a unique advantage for polyurethane RIM materials based on achieving a urethane chemical bond between the polyurethane paint and the polyurethane RIM substrate.
• Low-cost tooling options: The low injection pressures of the RIM process let mold builders use a variety of materials less expensive than P-20 or hardened steel. Alternate materials range from machined or cast aluminum to cast kirksite, nickel shell, and some plastic composite materials.
RIM Technology Comes a Long Way
LFT is a new technique for making structural RIM components. A fiberglass chopper attaches to the polyurethane dispensing mixhead, which in turn, attaches to a robot. The robot moves over the open mold cavity while simultaneously dispensing both the long glass fibers and the polyurethane resin in an open-pour method. At the end of the pour, the mold is closed to form the part.
LFT has significant advantages for automobiles, agricultural and heavy-duty construction equipment, and heavy trucks. As composite parts, they are an increasingly attractive alternative to steel and other polymers typically used in these products. LFT improves the economics of molding by automating and streamlining what had been a traditional two-step process of inserting preforms and mats into molds. Furthermore, this technology is attractive for users of traditional composite technologies, such as polyester resin transfer molding (RTM) and fiberglass-reinforced plastics (FRP), because it does not generate styrene emissions.
Robots deposit long-glass fibers (LFT process) and resin as they move over open molds.
This is an economical method to make large parts such as vehicle components and
Unique benefits of LFT include:
• It can use lower-cost fiberglass rovings rather than mats, which also reduce troublesome worker handling of the mats. Also, the amount of glass fiber reinforcement in the part can be varied. More reinforcing fiber can be applied to areas of the part that require the greatest strength, while other areas receive less reinforcement.
• It can use either a foamed or a solid polyurethane formulation.
• It can produce parts using two-component, in-mold paints. LFT lets designers decorate parts inside the mold using polyurethane elastomer skins, two-component polyurethane paints, films, fabrics, or vinyl skins. High-quality decorated parts directly out of the mold can be made with these more automated techniques. In addition, LFT techniques often significantly reduce costs compared to secondary painting and decorating operations after the part leaves the mold.
LFT also opens a wider range of physical properties for RIM parts. Flexural moduli can range from 250,000 psi to 1.1 million psi. This property varies according to the amount and the length of the glass fiber used, and whether the polyurethane RIM system is foamed or a solid. When weight is critical, a foamed polyurethane RIM system with LFT may weigh significantly less than other composite materials. For all these reasons, LFT continues to gain popularity among composite fabricators and their customers.
Open door to RIM technology
One example that uses LFT is an exterior door, made by JELD-WEN Premium Fiberglass Doors using what is called PuR-Fiber Technology. The doors come in more than 30 different styles and three grain patterns; oak, mahogany, and alder. The skins of these doors are molded using the LFT process and a customized formulation of polyurethane materials. To complete the door manufacturing process, JELD-WEN laminates the composite door skins to a polystyrene core and wood edge frame.
Bayer worked with JELD-WEN to develop a Baytec polyurethane and Baydur STR polyurethane formulation specifically for this application. These polyurethane materials increase the doors’ dimensional stability and improve impact resistance and strength.
A clear view around the RIM
In another example,the outer cabinets for Clearwater Spas’ entire 2007 line are formed from the Baydur STR RIM system, which contributes to the spas’ attractive appearance and offers significant production and processing advantages over the previously used synthetic material. Clearwater Spas offers four series of spas, and each series comes in four or five different models. Previously, each enclosure was assembled using extruded tongue-and-groove synthetic material that had to be cut to size and attached piece by piece to a structural frame. It was essentially stick building, which was more time consuming and labor intensive. Using the Baydur system and LFT, molder Creative Urethanes is able to offer an engineered solution that removes the inefficiencies of the previous process.
Each complete cabinet in this modular system consists of eight pieces, four corner pieces, and four side panels. It comprises five different lengths and three different heights of side panels, with the largest measuring 34 in. high and 72 in. long. Molded-in flanges offer two benefits; they add strength, which eliminates the need for the structural frame, and they align the components for quick and easy assembly.
Molding the components in various sizes offers Clearwater Spas the flexibility for modular construction. The LF cabinets feature a wood grain and tongue-and-groove appearance, without the assembly problems of previous methods. In addition, the durability and impact resistance of the LF enclosures are other key benefits. Because of their size and weight, spa shipping and handling is a costly problem in the industry. However, the LF cabinets are extremely durable, ensuring that the spas will be delivered to the homes of customers in the same condition as when they left the production facility.
These applications indicate how RIM, in general, and LFT, in particular, can produce products not possible with other manufacturing methods. The growing demand for more economically produced parts with complex geometries is driving further technological advances and innovations in RIM materials and manufacturing methods.
:: Design World ::
Filed Under: Factory automation, Materials • advanced, Molding • injection molding components