Vibration-welded manifolds coming on strong

Cost is the watchword of the automotive world, and to keep costs down, designers must consider manufacturing processes during the product development phase. A recently introduced manifold for Ford's 5.4-liter truck engine illustrates why more OEM engineers are considering whether air-intake manifold (AIM) designs can take advantage of lower-cost vibration welding rather than lost-core molding.

Nylon manifolds have been a reality since Porsche commercialized one, the world's first, for its 911 model using glass-reinforced nylon 6/6 (DuPont Zytel) in the early 1970s. Today, the conversion of manifolds from cast aluminum to nylon is one of the great success stories for automotive plastics. Globally, OEMs continue to convert to plastics for weight and cost reduction with improved engine performance.

Among the two top processes for making air-intake manifolds worldwide-lost core and vibration welding-a shift is taking place, according to Ken Nelson of DuPont Automotive. "There is increased interest in vibration welding from OEMs and Tier Ones now," he says, "based on the fact that manufacturing costs are lower. This interest in the process comes both from European carmakers, who have been using the process for five years, and from the U.S. auto industry, which recently produced its first vibration-welded AIM."

A Domestic First
Nelson is referring to North America's first multi-shell, vibration-welded manifold, a result of a 14-month development effort by Ford, molder Montaplast, and DuPont Automotive. The new AIM (Figure 1) will be unveiled on 1999 Ford F-series 150 and 250 trucks, Ford Expedition, and Lincoln Navigator models. Originally designed for the lost-core molding process, the team identified cost savings possible with vibration welding and verified them together.

In the past, U.S. OEMs were hesitant to try vibration welding because of concerns over rupture at the weld joint. Over time, however, test results have shown that a properly designed weld joint can actually be stronger than the rest of the manifold and welded AIMs meet burst strength and backfire requirements.

"The design of this particular manifold," says Michael Ellenbeck of Montaplast, "lent itself readily to vibration welding." Ford engineers designed the part for performance and function while Montaplast fine-tuned the process and part layout. In addition to supplying a welding-enhanced grade of Zytel nylon 6/6, engineers from DuPont Automotive performed testing and moldfilling analysis and created several prototypes.

Vibration welding was invented at DuPont in the early '70s, and involves rubbing two thermoplastic parts together under pressure at a specific frequency and amplitude to generate enough heat to melt the polymer. When they stop vibrating, parts are aligned and the molten resin solidifies to create a weld. Branson supplied the equipment for the Ford project, which included both the welders and fixtures.

Design Caveats
Certain types of manifolds are better suited to vibration welding, which boasts lower capital investment and reduced cost of productivity. "From a cost-effectiveness standpoint, simpler, less complex geometries that require welding of two parts make the most sense," Nelson explains.

The manifold must have a design that can be split along one or more weld joints. If one weld joint is impossible, designers can use other weld joints and shells. He also recommends allowing adequate space for relative movement of the shells during the welding process. Designs should include internal flash traps to catch any overflow into the plenum and runners as well.

Part design must also take into account the need for support fixtures during the welding process. "Moving supports must be placed in the bottom of the fixture," he notes, "and the fixturing design should allow dimensions to trap all flash."

Vibration welding offers several other benefits for these types of designs. One advantage is greater dimensional stability. Without a tin-bismuth core and associated core shift to worry about, wall thicknesses and features stay where they belong, according to Nelson. Secondly, overall component weight is lower because wall thicknesses can be optimized with the shell halves. "With lost core, there are some areas you cannot access to make them thinner," he says. "As a result, wall thicknesses, which are normally about 3 mm throughout the part, can be as high as 12 mm. With molded shells, designers can fine-tune wall thicknesses in certain areas to take out weight and save material."

Nelson explains that not all manifold designs lend themselves to vibration welding. Lost-core AIMs can be inherently stronger although welded parts can be strengthened by design and material selection. Tightly packed air runners may not offer enough room to include a weld joint, which adds 8 to 10 mm width to each side of the manifold. Siamese runners sharing a wall, such as the type used on the manifold for GM's Northstar engine, cannot be accommodated with a shell design. Other complex geometries that require all the details in one tool will still require the lost-core process, he adds. "Lost-core is not obsolete by any means," says Nelson, "but designers now have another option for certain designs. What's important is knowing ahead of time what the requirements are for packaging, backfire strength, burst strength, and other performance criteria."

Material Decisions
To specify materials at the development phase for an AIM, designers may choose between several types of nylon 6 (PA6) and nylon 6/6 (PA66). Lisa Roessler and Jordan Lee of DuPont Automotive created a decision tree (Figure 2) to simplify the process, including considerations for EGR (exhaust gas recirculation) and coolant crossover applications.

As for material comparisons (Table 1), PA66 has higher thermal and chemical resistance and also supplies higher tensile strength and better dimensional stability. However, PA6 has better weldability and slightly higher elongation, which improves burst strength. The welding-enhanced grade of PA66 contains all of the base polymer's properties with added burst strength up to 11 bar and a 35 percent improvement in burst pressure at the weld joint.

Finally, moldfilling analysis should always be performed for either process. For lost core, designers need to also conduct a structural analysis to predict core shift.