Plastic-metal hybrids' momentum is building.

* Prospects for metal replacement with plastic-metal hybrid (PMH) structures continue to improve, one advance being the launch of an unconventional PMH process by the British-based Corus Group. A Dutch R&D unit of this steel and aluminum sheet supplier has developed what it calls Plastics Injection Forming (PIF), which is aimed initially at decorative housings for cell phones and computer mice. PIF uses regular injection molding machines to make deep-draw parts from flat sheets of a plastic-metal sandwich. Forming takes place entirely within the mold.

Meanwhile, Bayer MaterialScience in Germany is pushing the established PMH injection molding process into new automotive uses. This method involves overmolding glass-reinforced nylon 6 or 66 over stamped metal inserts so that the melt penetrates holes in the inserts and mechanically interlocks the two materials.

Conventional PMH methods have made inroads mostly in front-end modules. There is also activity in bicycles, hand tools, and even blow molded seating. Now, Bayer is reporting the first PMH successes in automotive electrical control modules and structural truck components.

Plastic-steel sandwich

Corus's PIF technology is a novel method for making high-strength plastic-steel hybrid shapes in a single step using existing molding machines and special molds. Corus offers PIF for license to OEMs and processors.

"PIF allows molders to marry the strength of a thin polymer-coated steel sheet to the freedom of form only a thermoplastic can provide," declares Jac Sandberg, Corus' PIF business development manager. The thin (0.2 to 0.8 mm) low-carbon steel sheet is encapsulated with 0.04-mm layers of thermoplastic. The coating offers corrosion resistance and corner protection, while the sheet imparts the stiffness and feel of metal. Proprietary technology is said to achieve good polymer-metal adhesion.

PIF enables Corus' polymer-encapsulated sheet to be shaped, drawn, and backed up with additional plastic in one step. First, coated sheet is cut into blanks that are inserted in the mold. Then the tool is partially closed, which shapes and draws the sheet and trims it as well. Finally, the tool is fully dosed to allow the final injection stop to over-mold the insert. PP, PET, PC, and PC/PBT have been used successfully both as coating and back-up materials in PIF. Since the same material is usually used as both the steel coating and injected backing, an indestructible chemical bond is said to result.

PIF structures offer high stiffness and strength relative to part thickness and weight, Sandberg says. He cites a 1.2-mm plastic-steel part with stiffness equal to a 3.8-mm all-plastic structure.

In PIF, rough shaping and drawing are done as the mold closes, but final shaping is accomplished by the pressure of the injected melt. The result is said to be excellent replication of mold detail. The injection step allows one side to incorporate hinges, snaps, ribs, bosses, and other features.

Sandberg says PIF offers potential in cell phones, kitchen appliances, light fixtures, cutlery, tools, gift boxes, automotive switches and antennas, and furniture. Furthermore, there is also potential in structural parts.

Hugo de Wolf, Corus' product engineer for PIE says that the thickness ceiling of polymer-coated sheet is currently 0.8 mm, but thicker and stronger+ versions are in development. Versions based on aluminum and stainless steel are being investigated, along with a broader menu of coating polymers, including PE and ABS.

Successes in automotive

Bayer has broken new ground by applying PMH overmolding to an integrated antenna module for Volvo's XC90, a new off-road SUV. The design incorporates formerly separate antennas for radio, TV, and GFS into a 4-1b module that is positioned in the rear roof area to improve reception. The antennas are embedded in a film, but their effectiveness depends on a vibration-resistant PMH support housing of Bayer's Durethan BM 130H2.0 nylon 6.

Another recent success is the first use of Bayer's technology in a truck part: side supports that brace a 12-1b, 79 x 10 in. radiator grille on Mercedes-Benz's Actros truck. The part provides major weight savings by replacing metal. In addition, the PMH struts improve force distribution when the hood is opened or closed, improving warp resistance and stability of the assembly. The grille is molded in a high-flow nylon supplied by Bayer.

Meanwhile, in front-end modules, Bayer has extended the scope of PMH design with Ford's C-Amax compact van. It incorporates the air filter, radiator bleeder valves, cables, and rotation devices.

NEED TO KNOW MORE? For more information, enter PT Direct code at www.plasticstechnology.com

Bayer Polymers, Pittsburgh [phone], FT Direct 468PT Corus Research Development & Technology, Ijmuiden, Netherlands

[phone], www.corusgroup.com