SMC goes for lighter weight, easier flow.

LOW PRESSURE, LOW DENSITY

The latest formulation advances can trim part weight, cut tool costs, & reduce press-tonnage requirements at no penalty in compound cost, surface finish, or ease of handling.

Among molders of thermoset polyester sheet molding compounds, two phrases are getting a lot of attention today: "low pressure" and "low density." Car and truck manufacturers want low-density SMC to help them shave pounds off. vehicle weight. But until recently, low-density SMC has required a trade-off in lower strength, poorer surface finish, and higher cost. New formulating approaches are said to have overcome those limitations.

Meanwhile, conversion of ever-larger auto and truck parts to compression molding has helped shift low-pressure SMC development into high gear. Easier-flowing materials could help automotive molders stretch existing press capacity to mold larger parts, more cavities, or higher glass loadings with lower tonnage requirements. And the ability to use lower-cost tooling and presses (or no press at all) could make SMC cost-effective for shorter runs than ever before. Some auto companies are considering low-pressure SMC for niche parts with annual volumes of 20,000-25,000.

Low-pressure compounds could also allow smaller molders concerned with styrene emissions or faced with higher-volume production needs to convert from spray-up or RTM to compression molding without huge capital outlays.

All these attractions of low-pres-sure SMC have been apparent for more than two decades (see PT, Aug. '75, p. 8). But only recently have formulators made headway against previous shortcomings in handling, surface appearance, and cost.

SMC LITE LOW-DENSITY COMPOUND

                                SMC Lite       Standard SMC

Tensile Strength, psi             9400             11080

Tensile Modulus, kpsi             1073              1700

Flex. Strength, psi              23900             25000

Flex Modulus, kpsi
 2.5-mm deflection               1066              1300
 0.5-mm deflection               1200              1720

Moisture Absorption, %            0.55              0.59

Specific Gravity                  1.61              1.96

Izod Impact
unnotched, ft-lb/in.              23.5              21.1

% Fiberglass by Weight            30.9              28.3

Source: Ashland Chemical

Reducing Part Density

Low-density SMC got its biggest push forward in recent years from work at General Motors Corp.'s R&D Center in Warren, Mich. GM developed Dbl-Lite SMC, which first saw production use on 1995 vehicles (PT, May '95, p. 14). Dbl-Lite contains glass microspheres, which reduce its density to 1.3-1.5 g/cc, 20-30% below the standard 1.8-1.9 g/cc. Whereas previous low-density SMC required expensive vinyl ester resin to meet automotive specs for stiffness and strength, Dbl-Lite uses polyester resin to remain competitive in performance and cost.

GM has transferred its technology to 11 parts suppliers. "We have made low-density SMC a common material because it is stronger and less expensive than it used to be," says GM staff research engineer Hamid Kia. "It can be used as a direct replacement for SMC at no added cost." He says there are no discernable processing differences to the molder, although Dbl-Lite's reduced mass results in lower energy consumption, faster part cooling, and easier handling.

On the other hand, Dbl-Lite is not suitable for Class A applications. The problem is that glass micro-spheres break when the part is sanded, resulting in more porosity.

Still, commercial applications of Dbl-Lite are on the rise. They include the Corvette wheel-house panel, M-Van and G-Van engine covers, and M-Van lift-gate trim panels. GM's brand-new EV-1 all-electric vehicle uses Dbl-Lite for the hood and deck-lid inner reinforcements and two inner door reinforcements on each side. The weight of one door inner panel was cut from 11.5 lb to 7.5 lb.

ALTERNATIVE APPROACHES

Ashland Chemical's SMC Lite low-density formulation does not contain microspheres. Automotive business manager Jim Jensen says Ashland's low-profile technology permits reducing density by removing some filler while maintaining "true Class A" surface quality. A commercial example is the hood on Chrysler's Dodge Viper sports car. It has a density of 1.65 g/cc and is molded by Cambridge Industries in Centralia, Ill.

Others developing a Class A low-density SMC include GenCorp Automotive, although a company spokesman declined to reveal details. Alpha/Owens Corning is looking for an alternative to using glass microspheres for LD-SMC. One approach that offers limited weight-reduction potential is use of a filler derived from reground SMC. Produced by AOC's partner company, Phoenix Fibreglass, the filler has been used in some automotive SMC parts, where it typically reduces the density by about 10%.

Reducing SMC Molding Pressure

A range of new resins, additives, and molding compounds are expanding the capabilities of low-pressure SMC. Several more materials are in development or in production from various molders and resin suppliers.

Low-pressure SMC technologies have been around since 1975. Some were based on conventional thickening technology, but one that has received the most publicity of late is not. Since around 1988, National Composites Inc. (NCI) has been marketing "LPMC" (low-pressure molding compound) that is based on a physical thickening process. It uses a special crystalline unsaturated polyester resin from Scott Bader Co. Ltd. in England. This Crystic Impreg resin is solid at room temperature but melts at less than 200 F. When blended with a standard liquid polyester and then allowed to cool, the solidified resin causes an immediate viscosity increase. The result is virtually "instant thickening" that does not increase over time.

One disadvantage of this method is the need to modify the SMC impregnation machine to keep the resin hot, which reportedly increases styrene emissions and requires an enclosed environment. The Crystic Impreg resin also may modestly increase compound cost. And this LMPC has not demonstrated the ability to produce Class A surface in production.

Other criticisms leveled against earlier low-pressure compounds include stickiness and problems with mold release and glass carry. But several newer technologies are said to remedy these limitations.

NEW LOW-PRESSURE MATERIALS

Interplastic Corp.'s Commercial Resins Div. is scaling up for commercialization of an automotive low-profile resin that can be molded at 200-500 psi. New COR20-LP-7510 resin uses conventional metal-oxide thickening and can be formulated to provide Class A surfaces, says Lee Hagelee, v.p. of research and technology.

Another polyester for low-pressure SMC may be commercialized in six months by Cook Composites & Polymers. The resin is thickened with magnesium oxide and can be molded at 250-300 psi. It's being evaluated for a non-automotive part, though Cook expects the resin to be able to achieve a Class A finish.

Ashland Chemical's new SMC Lite (see above) can be formulated for low pressure, low density, or both. SMC Lite has demonstrated Class A capability in production of the Viper sports-car hood. That hood was formulated primarily for low density and is molded at standard pressure, but Ashland says SMC Lite can be molded at up to 80% lower pressure than normal SMC.

SMC Lite is based on a proprietary thickening technology that does not use a crystalline resin, allowing the compound to be produced on standard SMC machines. Uniform glass carry is also claimed.

A new one-pack low-profile polyester from Alpha/Owens-Corning is said to be capable of producing Class A surfaces at pressures down to 200-300 psi. Called Ultryl, it uses standard thickeners, according to business manager Reagan Stephens. It allows reducing the filler level to 160 phr, resulting in 6% lower density. Thus, although Ultryl is a little more expensive than AOC's standard Atryl automotive resin, the finished compound costs no more on a cubic-inch basis, Stephens says.

Ultryl's first commercial application will be a Volvo truck hood/fender that was test-molded at less than 300 psi by GenCorp. Because Ultryl can be molded at high or low pressure, Stephens thinks it will replace Atryl in the long run.

Non-automotive potential for Ultryl includes jet-ski hulls, small boats, campers, and bathware. AOC president Gerald Griffith expects that R&D on low-pressure SMC will result in low-cost FRP tooling that will allow end-users to change designs inexpensively. Such tooling may permit radically new technologies such as RF heating, Griffith hints.

AOC supports more than one route to low-pressure SMC. AOC will begin producing Crystic Impreg resin in the U.S. when sales reach a high enough level. AOC will then supply the resin to North American licensees such as NCI.

Stephens says AOC will devote a portion of its R&D to low-pressure SMC made with Crystic Impreg. One goal will be Class A surface finish. Stephens says both Ultryl and Crystic Impreg technologies have merit. Ultryl has the advantage of superior surface quality, while Crystic Impreg offers extended shelf life. The latter is thus a good candidate for molders moving from open molding or RTM to compression molding.

NCI president Sean McMullen notes that Crystic Impreg LPMC has already been proven in a range of applications - including truck wind deflectors, a small windshield cowl on the Dodge Viper, and non-automotive applications such as manhole covers, patio doors, and satellite dishes.

Premix is another compounder with its own low-pressure SMC program. Premix is developing SMC based on a "non-traditional" thickening method that yields a stable viscosity and permits molding at 100 psi. The product, which is expected to become commercial within the next six to 12 months, is being developed for a wide range of applications, according to business manager Warren Owens. At the recent SPI Composites Institute meeting in Cincinnati, Premix demonstrated that its material can be molded in low-cost tooling (nickel shells with ceramic backing) and a self-clamping mold without a press (PT, Jan. '96, p. 20).

Another new low-pressure development is an additive system from Union Carbide Corp. (PT, Jan. '96, p. 22). In a wide range of standard polyesters, Neulon LoBar 101 thickening modifier plus Neulon LoBar 102 chemical thickening agent are claimed to produce dry and easily handled compounds that can be made on traditional SMC machines. The compounds can be molded into car hood at 150 psi with Class A surface and excellent glass distribution, according to Rob Seats, development scientist at Carbide's technical center in South Charleston, W.Va. He adds that the system is very "forgiving" of moisture or thickener concentration.

Besides Class A formulations, the LoBar additives can be used to make low-pressure zero-shrink SMC, as well as pigmentable and even vinyl ester compounds.

MOLDERS WEIGH IN

Several molders have developed low-pressure SMCs or are evaluating them for upcoming jobs. GenCorp Automotive in Shelbyville, Ind., jointly developed a Class A low-pressure compound with Alpha/Owens Corning. GenCorp uses the new 7525 compound to mold automotive and non-automotive parts and also offers it for sale to other molders. Says Vinod Arora, technical manager for reinforced plastics, one of the benefits of low-pressure SMC is that it allows molding larger parts on presses of limited tonnage capacity. Sometimes it also can solve filling problems encountered with traditional SMC.

Arora says the 7525 compound can be molded at pressures as low as 80-90 psi. The chemically thickened compound has a shelf life similar to that of conventional SMC.

GenCorp has molded low-pressure parts of up to 12 sq ft. The only commercial applications so far are Class A parts for the Hyster forklift vehicle. However, 7525 compound has also been approved for R-28 truck programs and is undergoing tests for GM's Corvette and Pontiac Firebird.

Eagle-Picher Industries, Grabill, Ind., has also been active in development of low-pressure SMC. Don Norris, v.p. of engineering services, says his firm is working toward a Class A automotive low-pressure compound.

The Budd Co., Troy, Mich., has also investigated low-pressure compounds, although it presently has no production applications. According Ken Rusch, advanced programs manager, the company has tested low-profile compounds made with a crystalline polyester and observed good glass carry and ability to fill out ribs and bosses. But doubts remain about surface quality. "There are parts like roofs, for example, which are huge pieces and where low pressure would be very significant. But the question is, could we get the same level of surface that we could get with SMC?"

At Cambridge Industries, Dan Allman, SMC operations manager, believes that low-pressure SMC has value, though it remains unproven in some areas. Cambridge uses low-pressure compound to mold an air duct for the Ford Ranger Explorer cross-car beam. The Ranger part contains 40% glass. In such high-glass formulations, low-pressure SMC may be able to cut pressure requirements in half, Allman says. The air duct is made by Cambridge with Ashland's HIPPI resin system, a high-impact structural-grade polyester. Cambridge has also run trials on Ashland's SMC Lite, and Allman says the results were impressive. But Allman is still concerned that the dramatic viscosity drop of the low-pressure systems could affect glass carry.

In Allman's view, low-pressure SMC has yet to prove that it can reduce cost. One reason is in-mold coating: "We feel that you still need a rather significant amount of tonnage to distribute the IMC material and ensure good adhesion on the substrate. So if you mold a low-pressure Class A formula at 400 psi, you are still going to need 800-1000 psi with the coating."

IMPLICATIONS FOR TOOLING

According to Tom Hamstreet, president of Modern Tool, which has built several molds for NCI's LPMC, "One of the benefits LPMC offers is that it allows you to build relatively inexpensive prototype tooling relatively quickly." Epoxy tools offer possibilities for low volumes, he says. For improved surfaces, nickel-shell tooling is also viable for jobs with limited volumes.

Hamstreet adds that there is a significant amount of work being done in thin-wall cast-aluminum tools for low-pressure SMC inner reinforcement pieces of truck and auto hoods. Cast steel or aluminum are suitable for parts that don't require a Class A surface, he says. He notes that some heavy-truck parts are using cast tooling on both the show and non-show sides, and the Viper hood runs in a cast tool for the non-show side.

Vinod Arora confirms that Gen-Corp was able to achieve lower tooling costs on the Hyster forklift mold, resulting from using a smaller steel block, smaller guide pins, and changes in mold shear-edge designs.

Other molders question the degree of potential savings on tooling. Says Don Norris of Eagle-Picher, "I don't think that tooling costs are quite as low as some people had touted initially. But I think there are some things that can be done to reduce the cost of the tool. It might make the difference in a part being placed in SMC or another process."

Some molders are wary of epoxy tools for low-pressure applications. Faber of Nero Plastics (see sidebar) says early claims that low-pressure SMC could run on plastic tools led to failures. Nero uses P-20 steel to mold low-pressure SMC truck parts, but the tool still costs much less than one for standard SMC.

RELATED ARTICLE: RTM Molder Switches to SMC

Nero Plastics, Inc. in Owosso, Mich., is proof that low-pressure SMC can help an RTM molder "graduate" to compression molding when its volume requirements increase. Before going to SMC, Nero found that RTM couldn't keep up. "When the volume of a job went up, we'd lose out," says president Dan Faber. Last year, Faber invested in a 500-ton compression press with an 84 in. x 84 in. platen, which was large enough to encompass 80% of the molder's jobs. Conventional SMC would have required a 1000-1500 ton press for the same parts.

Nero is using Ashland's SMC Lite low-pressure system to mold a Class A air deflector for Kenworth Truck Co. Faber says he has quoted "dozens" of SMC jobs whose volumes are not large enough to interest molders of conventional SMC.