Resin transfer molding (RTM) is frequently defined as a composite molding process well-suited for annual production volumes of 500 to 50,000 parts. In reality, most commercial RTM programs do not operate on the high end of that range. Much more common are annual production volumes of 5,000 to 15,000 — below the threshold where more durable steel and nickel-shell tooling becomes economical.
When VEC Technology LLC (Greenville, Pa.) developed its patented VEC (Virtually Engineered Composite) Floating Mold technology in the 1990s, the goal was to cut the cost of RTM tooling in the 5000 to 15,000 parts-per-year range by replacing steel molds and the expensive support structures they require with thin-shell composite tooling supported by noncompressible fluids (usually water) in two pressure vessels. One is floor-mounted and the other is suspended from a gantry. Described by VEC as universal vessels, these structures can support upper and lower shells of matched composite molds in a wide range of sizes and shapes. The relatively thin mold shells are designed to form a tight seal at the vessel edges, permitting mold pressure to be applied hydraulically and precisely by pressurizing the fluid. The vessels are integrated into a computer-controlled resin injection manufacturing cell, which monitors and manages more than 500 process variables to precisely engineered specifications.
Although VEC’s original intent was to develop and sell molding equipment, the company quickly moved into parts fabrication. “We realized we had to demonstrate that the process actually works and how to use it in order to convince customers to buy the manufacturing cells,” says Bob Hearns, VEC’s sales and marketing manager. The move provided an unanticipated benefit: “Our technology is based on a complete system: part design, pattern- and moldmaking, materials and process control and equipment and parts production,” he explains, contending that the company’s expertise in each of these areas has enabled it to develop and make a composite part that can meet customer quality demands at production volumes of 40,000 to 50,000 parts per year.
VEC now maintains production volumes in the 40,000 to 50,000 parts-per-year range on eight fiberglass fan housings that the company manufactures for a supplier of pork and poultry farm ventilation systems. The housings vary in size from 3 ft/0.9m to 6 ft/1.8m square. Because the fan housings have neither high cosmetic nor abrasion-resistance requirements, the expense and additional processing steps involved in applying a gel coat or other finish are unnecessary. VEC’s composite housings also benefit from UV inhibitors added to all of VEC’s corrosion-resistant resins to provide outdoor durability, and the two-sided molding delivers a smooth surface finish on the housing’s interior and exterior — one that is easier to keep clean, eliminating the surface contaminants that pose animal health problems.
By contrast, metal fan housings offered by competitors require expensive coatings or simply do not hold up well under the extremes of weather and corrosion that are common to the farm environment. Therefore, the equipment supplier’s fan customers now prefer the composite housings for their greater performance efficiency and longer service life. Composites also are well suited, from a design standpoint, to the complex geometry of these parts, which transition from a square fan enclosure to a cylindrically shaped venturi duct.
Making the Mold
VEC designed the housings in its own pattern and mold fabrication facility, where tooling is produced to support its manufacturing cells and those owned by customers. Fabrication began with a 3-D computer aided design (CAD) model of the part to be produced, supplied by the customer. (When a CAD model is not available, VEC can develop it using reverse engineering.) The tool design was adjusted to account for draft, mat layouts and shrinkage, which have an impact on processing. The resulting CAD information was converted to machine code to drive VEC’s 5-axis CNC gantry mill, supplied by PaR Systems (Shoreview, Minn.) as it cut female and male plugs for the upper and lower matched molds. Various materials can be used during plug construction depending on the customer’s dimensional and budget requirements.
Molds then were pulled from the plugs. There is a wide range of materials that VEC uses in construction of its floating mold shells. Prototype, preproduction and small-production volumes (500 to 1,000 pulls) use chopped fiberglass spray technology, which has a cost comparable to open-molding tools. Small- to medium-volume molds (2,000 to 6,000 pulls) are fabricated using hand-laminated and vacuum-infused glass fiber, carbon fiber and/or steel fiber reinforcements.All composite tooling is produced by VEC. Nickel-shell or steel typically would be selected for higher volumes (50,000 and greater). Regardless of the mold material, the resulting mold skins are fairly thin, typically less than 1 inch (25.4 mm). Although the mold will be supported by the noncompressible fluid in the molding cell, minimal steel support structure is used for handling. Hearns explains, “We allow the mold skin to flex and bend. We use finite element analysis (FEA) to detail exactly where we need additional reinforcement so that we obtain the optimum performance without the cost of extra thickness across the mold.”
For the fan housings, the decision was made to go with composite tooling layed up with glass fiber. The cost of these composite mold skins, which typically have a life of 2,000 to 6,000 cycles (depending on mold complexity, finished part surface requirements and part design), is roughly one-quarter to one-half that of traditional steel or nickel-shell RTM tooling. This reduced tooling cost and labor allow for a faster, more economical production start-up. Tools are replaced periodically, but are refurbished and recycled as back-ups to the production tooling and can be used, if the need arises, to supplement high volumes.
The matching upper and lower composite mold skins were then integrated into their respective pressure vessels (each vessel holds two mold skins) with a steel flange that is bolted to the vessel, creating an airtight seal between the vessel and skin. Fabricated from steel, each vessel is designed to handle the maximum potential pressure generated during the resin injection process. Hearns notes that the vessels are reusable and, for VEC equipment customers, supplied with the initial molding equipment package. Built to a predetermined standard size that will accommodate the customer’s anticipated part configurations, they help minimize the capital investment in new tooling. Mold changes reportedly are swift and easy, accomplished by unbolting the old mold flanges from the vessel and bolting on the new mold.
There really isn’t a physical size limit of VEC manufacturing cells. Currently, VEC cells are used to produce boat hulls up to 24 ft/7.3m long and industrial parts as large as 8 ft by 4 ft by 30 inches deep (2.4m by 1.2m by 0.762m). The limiting factor, according to VEC, is economics — parts with one dimension of 25 ft or greater are typically produced in smaller volumes, where RTM is less attractive than either vacuum infusion or hand layup.
Producing The Part
The first step in fan housing production is to kit the reinforcement materials from fabric stock supplied on rolls. While the fan housings require E-glass fabrics, Hearns notes that S-glass, aramid, carbon, natural and steel fibers also can be incorporated into the VEC molding process. To optimize material use while cutting the fabrics, VEC uses the Lectra Design Concept 3-D software package developed by Missler Software (Evry, France), which digitally flattens the three-dimensional part from the CAD design file into 2-D ply patterns. These patterns are then used as inputs to cut the reinforcements using either a CNC machine or a mat cutter. The kitted materials are arranged on shelves in the mold-prep area for easy access by operators during layup.
The resin system is a hybrid polyester from Interplastic Corp. (St. Paul, Minn.), formulated for the specific gel and cure time required by the application. Because Interplastic is a VEC stakeholder, along with VEC parent company Genmar Holdings (Minneapolis, Minn.), VEC’s resin is supplied in bulk, via tankers, and compounded onsite at VEC’s mix plant. Hearns points out that mixing at the point of use “results in little or no change in the intended properties of viscosity and gel time and eliminates the issue of shelf life. We have found that even slight changes in viscosity over a resin’s shelf life can cause serious molding problems with RTM.” For the fan housings, VEC mixes two to three batches of the hybrid polyester — enough for that day’s production.
Prior to layup, the molds are uniformly heated over their entire surface by the liquid supporting them in the universal vessel. Molding temperatures range from 120°F/49°C to 180°F/82°C and are maintained on the upper and lower halves during the molding cycle via the control system. Kitted reinforcements are placed into each cavity in the mold, and the operator then closes the mold and begins injecting the resin. Injection times vary, depending upon the size, thickness, and number of cavities in the mold, but usually average 3 minutes when using a single-injection-point process. Multiple injection points were incorporated into the fan housing mold to accommodate the multi-cavity arrangement. Once the molds have been closed the computer-controlled resin injection equipment goes into action, operating according to a production recipe developed during part prototyping and preprogrammed into the system’s software.
The time from injection to peak exotherm is approximately 20 minutes. The VEC computer control system monitors resin temperature in the part throughout the cure and signals that the mold is ready to be opened once peak exotherm has been reached and the resin begins to cool. Total cycle time for VEC’s fan housings averages 30 to 45 minutes.
In the VEC system, the number of mold cavities can be varied, depending on the desired production volume, which Hearns says can vary during periods of peak demand each year. For the fan housings, it was important to take a “load-level” approach, he explains, balancing the number of mold cavities with the total cycle time in order to meet the daily output required by the customer’s production schedule. After molding is complete, the parts are removed and edge-finished using a 6-axis robotic trimmer supplied by FANUC Robotics America Inc. (Rochester Hills, Mich.). Parts are packaged as requested and shipped to the customer.
Although the fan housings do not require a Class-A finish, VEC’s molding technology can produce smooth, mirror-like part surfaces once possible only with metal tooling, using a companion technology called VEC Shield. This technology uses a separately thermoformed and then comolded high-gloss thermoplastic outer shell in place of gel coat, providing Class-A looks at a significantly smaller capital investment than would be required with steel or aluminum tooling (see “Learn More”). Thus, VEC’s process technology offers the traditional benefits of RTM, including low-emission closed molding, two-sided finish and a high degree of dimensional tolerance, with tooling costs affordable in quantities of less than 15,000. Yet it also is capable of pushing the theoretical limits of RTM in terms of production volumes, part repeatability, process efficiency and cosmetics.
Hearns says that with almost 20 years of processing experience, VEC is pursuing new materials and composites solutions. In addition to fan housings, VEC production cells at the company’s Greenville facility also turn out products as diverse as golf cart hoods, commercial truck bumpers, institutional furniture and in-ground floor trench systems. Further, the company reports that Trim Systems, a VEC customer in Concord, N.C., is installing three VEC molding stations to manufacture parts for Class 6, 7, and 8 trucks.
In the future, VEC anticipates wider application of its RTM process, particularly in the aerospace industry where the company believes its molding system’s part precision, repeatability and the reduced capital investment it requires can be easily justified.
“We see real benefits in using our process not just for fiberglass and polyester, but for carbon and quartz fiber structures,” says Hearns. “We have a lot of flexibility in the process that we have yet to utilize in production parts, from incorporating tailored inserts and urethane and foam layers to possibly even hybridizing with thermoplastics.”