One of the industry’s first uses of nanocomposite foams for strong, lightweight composites was introduced at April’s 2007 Plastics Parts Innovation Conference of the SPI Alliance of Plastics Processors (APP) in Memphis, Tenn. (The APP was formerly SPI’s Structural Plastics Div.) Other news highlights of the meeting included one of the first uses of external gas-assist molding in the U.S., a new use of direct in-line compounding of long-fiber thermoplastics (D-LFT) to make glass-mat thermoplastic (GMT) sheet, and a unique one-piece automotive grille guard with a molded-in chrome tube.
Lighter, stronger foams
Nanoclays and carbon nanotubes from 5 to 10 nanometers thick have been incorporated into foams of PVC, polystyrene, and polypropylene in a collaborative project called the Center for Multifunctional Polymer Nanomaterials and Devices (CMPND) at Ohio State University. The development effort involves six university partners and 60 industry participants. Performance results to date have been impressive, according to the Center’s co-director, Sharell Mikesell. The nanoclay particles serve as a nucleating site to raise cell density 10 to 1000 times higher than in conventional foams, while reducing cell size by a factor of 100 to 10,000. As a result, foams boast 20% greater modulus than standard versions and are 20% to 40% lighter.
The enhanced foam is used as a core layer 1 to 3 in. thick in a sandwich structure with glass- or carbon-fiber reinforcements on either side. Carbon nanotubes can be added at 2% to 5% by volume to the reinforcement layers, which reportedly creates a stronger bond between the skins and the foam core. The composite structure is produced and sold by WebCore Technologies Inc. It is intended to be placed in a mold where the skins are infused with liquid resin.
The composite structures range in size from 2 ft square up to 5 ft wide x 12 ft long. Targeted applications include wind turbine blades and structural panels for truck trailers and aerospace applications. Recreational vehicle maker Fabwell in Elkhart, Ind., is working with the consortium to develop composite flooring and panels that are expected to be commercial by 2008. Increased strength and modulus of the lightweight nanocomposite structures reportedly could allow wind blades to be extended from the current 35-meter length to 50 or 75 meters.
The consortium is working with a material supplier and a subsystems integrator to develop a full-scale manufacturing process. A pilot operation is expected in about 18 months.
External gas arrives
Mack Molding Co., Arlington, Vt., showed off two of the first commercial applications in the U.S. for external gas-assist technology. Mack is one of the first U.S. firms to license the process from Cinpres Gas Injection Inc. Gas is injected on the core side of the mold, where it presses uniformly against the plastic in the cavity, gently squeezing out any sink marks. As a result, the gas exerts the packing pressure, not the molding machine. This permits use of a lower tonnage press.
The front fascia of the Triton 8100 Series ATM machine, measuring 11.5 x 17 x 4 in., was previously molded in structural foam to obtain the required impact strength, stiffness, and aesthetics. External gas allowed the use of conventional injection molding with heavy reinforcing ribs and bosses on the inside of the part while still maintaining a sink-free exterior. The fascia has six different types of mounting features and about 30 internal ribs. With internal gas assist, all the ribs and bosses would have been linked to a single gas channel or to multiple channels. External gas is said to make material management easier and tool engineering less extensive, according to Ken Kincaid, Mack’s technical engineering manager. A critical feature is the design of appropriate sealing features into the mold to contain the gas and prevent it from seeping around to the cavity side or through parting lines, he notes.
The 195.5-in.2 PC/ABS fascia was molded on a 300-ton press. Without gas assist, a 600- to 800-ton press would have been required, Kincaid says. The enclosure is very flat and straight and has less molded-in stress than typical injection molded parts. Pressurizing the core with gas means less chance for ribs to stick and easier part ejection, claims Kincaid. Overall costs are reduced due to the lower tonnage press and more efficient material usage. A typical 8-lb part uses only about 7.5 lb of plastic due to the gas packing pressure. Tooling modifications add about 10% to the cost of the mold, according to Kincaid.
Mack also used external gas-assist for a cosmetic cover for a Trane residential air conditioner. That part won top honors in the parts competition’s building and construction category. (In addition, Mack recently molded a base for an external air-conditioner on truck trailers using a combination of both external and internal gas assist. See Learn More above.)
Mix & mold GMT in-line
PlastiComp’s D-LFT technology, previously used to produce long-fiber thermoplastic composites—is now capable of producing GMT sheet. The company’s Pushtrusion system is being adapted so it can make GMT from raw resin and reinforcement and deliver the hot sheet directly to a compression molding press, thus eliminating the expense of buying premade sheet. Material cost reduction of 20% to 30% could be realized, according to Eric Wollan, manager of PlastiComp’s technical development center.
The company’s patented cutting technology produces fiber lengths up to 2 in. A molten shot of material is robotically transferred to a press where it is flattened into a sheet blank, and the hot sheet is then delivered directly to the compression molding tool. The system can reportedly produce 20% to 40% glass-reinforced PP sheet with properties similar to those of traditional premade GMT. PlastiComp is currently manufacturing PP-based sheet for evaluation and has produced GMT automotive underbody panels. It plans to sell machinery and also license the technology. Commercialization is expected later this year.
Among the most innovative applications in the parts competition was an automotive brush grille guard that was produced for the first time in a one-step operation. A 17-lb chrome-plated tube is molded directly into the 80-in.-wide composite structure for General Motors’ Chevrolet Avalanche, Suburban, and Tahoe vehicles. One previous manufacturing method was to weld an assortment of tubes and then either paint or powder coat the entire assembly. Another method was to weld or mechanically fasten a tube assembly to a steel substructure and cover it with a rubber bumper or a plastic “beauty cover.”
A direct in-line compounding process developed by Composite Products Inc. (CPI) was combined with compression molding to produce the 35-lb part in about 2 min, compared with hours for a traditionally made grille guard. Unique tooling was developed to accept the large chrome tube. A special shut-off mechanism was placed at six different locations in the mold, resulting in flashless parts that require no trimming. CPI developed a special 40% long-glass PP compound with superior uv resistance to prevent fading. A cost savings of up to 50% was achieved versus traditional brush grille guard assemblies.