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Industry News
Inside look: Closed molding course couples talks with concept demos

CT sits in on a closed molding course and reports back on how this fast-changing process is evolving.

Author:
Posted on: 8/1/2012
Source: Composites Technology

Billed as a hands-on seminar, the recent Advances in Closed Molding Tool Design and Production Materials event had the feel of a full-blown tech conference. Held May 2-3, 2012, in Cape Coral, Fla., the seminar highlighted the benefits of closed molding and provided attendees with an up-close look at materials and processes used in a number of closed molding methods. The course was sponsored by the Closed Mold Alliance, a collaboration of distributor Composites One (Arlington Hts., IIl.), resins manufacturer CCP Composites (Kansas City, Mo.), infusion equipment supplier Magnum Venus Plastech (MVP, Clearwater, Fla.) and two Canada-based companies, the FormaShape div. of Whitewater Composites (Kelowna, British Columbia) and RTM North Ltd. (Vanastra, Ontario). The event was hosted by JRL Ventures Inc./Marine Concepts at the companies’ Cape Coral facility.

The course presentations were prefaced by a tour of the host companies’ 50,000-ft2 (4,645m2) facility. JRL Ventures/Marine Concepts specialize in the design and build of plugs and tools used to mold fiberglass parts for a variety of industries.

The event featured a number of workshops, but a key feature was an accompanying series of technology and molding demonstrations. For example, Doug Smith, technical specialist with RTM North, reported to attendees that more molders are showing interest in using light RTM. Unlike standard RTM, which uses two rigid tools to make a part, light RTM features a rigid A-side tool and a semirigid or flexible B-side tool. Smith contended that one of the main benefits of both light RTM and standard RTM, compared to open-mold linear laminating, is significantly less postmold repair and finishing, especially on complex parts. Further, RTM and other closed molding processes in general are capable of producing parts with glass loadings as high as 45 to 60 percent, compared to glass loadings of 35 to 37 percent that are typical of parts produced by open molding.

RTM North complemented Smith’s talk with a light RTM demonstration, molding a miniature catamaran hull. The mold cavity was treated with CCP Composite’s IMEDGE PCT600 in-mold coating and then lined with a single layer of Rovicore, from Chomarat North America LLC (Anderson, S.C.), a mat reinforcement that comprises a stitched nonwoven that, in turn, combines a polypropylene (PP) core and chopped glass fibers. The resulting mat is designed to conform to deep pockets and tight radii in the mold cavity. RTM North built the mold, which consisted of a rigid A-side tool coated with a vinyl ester gel coat and vinyl ester laminating resin, both supplied by CCP, and a semirigid B-side tool made entirely of vinyl ester. The part was infused with CCP’s injection-grade CCP-040-8085 unsaturated polyester resin, at approximately 18 psi/1.24 bar, using full vacuum on the tool flange and half-vacuum in the cavity. The part was fully cured in approximately 30 minutes.

RTM North also demonstrated its Flex Molding process on a boat hatch, using a reusable vacuum bag produced from Wacker Chemical Corp.’s (Adrian, Mich.) ELASTOCIL C platinum-curing, one-part sprayable silicone rubber. The processing conditions, reinforcement and resin were identical to those used in the light RTM molding of the catamaran hull. RTM North has developed a novel method of forming the bag. It involves spraying silicone over plugs made of strips of wax laid down and shaped to the dimensions of the part. The company also uses wax to shape ridges and other textures in the flange areas of the bag, thus maximizing the surface area available on which to build and hold a vacuum. Smith says the wax plugs have better short-term dimensional stability than fiberglass plugs, which inevitably shrink after curing. The shrinkage translates to a loose-fitting bag, which, in turn, can produce wrinkles and folds in the bag surface. “When you create any kind of texture inside the mold surface of the bag the resin will grab onto it and every time you pull a bag off a part you’re going to take a piece of silicone with it,” Smith notes.

Rick Pauer, CCP’s marketing manager, addressed the issue of mold shrinkage, pointing out that one of the problems with traditional vinyl ester tooling resins is postcure shrinkage that can lead to orange peel, fiber print or mold cracking. CCP Composites offers high-temperature-resistant OptiPLUS polyester resins as a substitute. Used for the fabrication of composite tooling via vacuum infusion, OptiPLUS resin chemistry is designed to counteract postcure shrinkage. Styrene monomers are reacted in the presence of a catalyst, releasing a large amount of exothermic heat. This heat drives up the rate of the initial cure, thereby reducing the amount of what CCP calls residual cure that remains in the mold. Residual cure, if substantial, can cause shrinkage as the mold is reheated during subsequent production runs. Pauer reported that OptiPLUS resins contain a thermoplastic filler that has a positive coefficient of thermal expansion, which helps to offset shrinkage in the polyester as it cures. Vacuum-infused OptiPLUS materials accommodate glass loadings up to 65 percent. Pauer claims that tooling made with OptiPLUS has better dimensional stability and longer service life than tooling made with conventional methods.

CCP demonstrated the vacuum infusion of a mold for a typical ski/wakeboard boat hull section. The vacuum-bag configuration on the approximately 3-ft./0.9m-wide plug was intended to be representative of sequential infusion of a production-scale 25-ft to 30-ft (7.6m to 9.1m) hull mold. A four-layer laminate was assembled on the plug. First down was a layer of Finishmat D7760, a nonwoven veil produced by Lantor Composites (Veenendaal, The Netherlands) and distributed in the U.S. by Baltek Inc., a division of 3A Composites (Colfax, N.C.). The Finishmat, which would typically go directly behind the gel coat, forces glass fibers away from the surface of the mold to provide a smooth finish. Three layers of Vectorply glass mat, supplied by Vectorply Inc. (Phenix City, Ala.), were then applied in the following order: Vectorply E-BXM 1715-6VC, a high-flow stitched glass mat; Vectorply E-TLYA 3612, a conformable PP media that is designed to fill tight mold radii and chines, minimizing bridging in these areas; and Vectorply E-3LTi 10800, a 36-oz triaxial glass mat, used to provide a tight, flat B-side surface. Airtech International (Huntington Beach, Calif.) supplied the nylon bag. “The three Vectorply materials were used to maximize flow [and] surface quality and for rapid bulking,” Pauer says.

Although the room-temperature viscosity of OptiPLUS at 75°F/24°C is approximately 100 cps, shop temperatures exceeded 90°F/32°C on this particular day, so Pauer estimated the viscosity at 75 to 80 cps, which shaved about eight minutes off the normal 24-minute mold-fill time. The mold reached its maximum exothermic temperature in about 65 minutes. The part became fully white, or translucent, at 75 minutes, indicating cure.

SR Composites LLC (Henderson, Nev.) demonstrated an alternative to silicone for closed molding, its trademarked Sprayomer elastomeric materials. Sprayable silicones tend to be fairly thick, from 0.080 inch to 0.25 inch (2.03 mm to 6.35 mm), and have high viscosity, typically in the range of 8,000 cps. When silicone is sprayed, the resulting surface roughness can affect the uniformity of the film. In sprayable form, SR’s prevulcanized, modified natural rubber has a much lower viscosity (1,500 to 3,000 cps), which enables the manufacture of a thin, lightweight bag. SR used an 0.040-inch/1.01-mm thick premade bag to mold a marine radar enclosure (see photo on p. 13). SR partner Rich Rydin claims that Sprayomer materials have half the thermal conductivity of silicone at the same thickness. Rydin also says the bag’s net-shaped design and flexibility deters bridging in mold corners. The investment in a Sprayomar bag is recouped in about 20 cycles, according to Rydin.

Course moderator and Composites One technical support manager Corbett Leach summed up the two-day event for attendees by lauding the diversity of closed molding, pointing out that a molder’s choice of closed molding process ultimately depends on the design and production objectives. Although closed-molding materials still cost slightly more than those used in open molding, he observed that the performance and environmental benefits of closed molding can more than offset this upfront expense. 


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