Analyzing snapfit pays off for nozzle design

If you build a better subsystem, will automakers beat a path to your door? That's what happened to Bowles Fluidics Corp., a Columbia, MD-based custom molder specializing in automotive systems. The company's patented windshield washer nozzle has captured 90 percent of the domestic auto market and a good share from Japanese and European automakers as well. Designers attribute the nozzle's success, in part, to finite-element analysis early in the design cycle.

Unlike its counterparts, the Bowles nozzle has one orifice, not two, and no metal is used in the part, which eliminates the need for insert molding. As a result, it costs less to produce. But what really sets this product apart is not apparent until you wash the windshield of your car. Interior geometry causes the fluid to oscillate, spraying the windshield in a fan-shaped pattern. Patterns are customized to accommodate specific automobile models, so there are many variations. Rather than two separate streams of fluid, the result is consistent coverage that hits even those annoying squashed bugs in the corner.

Design challenges for the nozzle were more complex than its size would indicate, according to product developers. Bowles employed Pro/E for modeling, Moldflow for filling analysis, and Ansys for structural analysis and assembly simulations.

In addition to windshield washer nozzles, Bowles Fluidics also makes defroster components and air conditioner outlets. Many of its products, including the nozzles, are snapfit parts. Installation force, the amount of force required to snap the nozzle onto the mounting hole as the car travels down the assembly line, is directly related to snapfit design.

Automakers want installation force to be as low as possible. Explains Qin Zou, computer analysis and process engineering manager at Bowles, "During assembly, the people on the line have about 15 seconds to insert the nozzles. They do this 8 hours a day, so the parts must be easy to snap in. If it's too hard, the snap feature may possibly crack."

Although installation force needs to be low, the automakers also don't want the nozzle to dislodge too easily. "They don't want someone shoveling snow off the car hood to accidentally remove the nozzle, too," says Zou.

Getting the perfect level of firmness in the nozzle snap feature is a constant balancing act for engineers. In the past, problems in this area were found after a nozzle was being installed on cars. The solution meant modifying the part and the molds, an expensive and time-consuming process.

To address this while a nozzle design was still in progress, Bowles chose a CAE solution. Ansys software was selected, says Zou, because the snapfit problem is nonlinear, both in terms of geometry (large deflection) and material, and the program supports nonlinear analysis.

Now when the company gets an order for a new nozzle, designers model it in Pro/E, then transfer it to Zou for simulation. His goal is to determine the correct shape and thickness for the nozzle's cantilever snap feature so that it deflects the correct distance when a customer-specified installation force is applied. Deflection is usually in the neighborhood of 2 mm; maximum installation force is around 10 lb.

"If you think of the snap feature as a cantilever beam, what we need to do is push the beam down 2 mm without breaking it," explains Zou. "We also want the force that pushes the beam 2 mm to be no more than 10 lb. After seeing the results of simulation, I adjust the thickness or length of the beam until the snap feature performs to those specifications."

Typically this process takes Zou two or three iterations, and requires between 8 and 20 hours. If the company were to create a prototype part to evaluate installation force issues, building the prototype tool would take six to eight weeks and cost between $7000 and $18,000. Now, unless a customer wants a prototype, the company skips this step and goes directly to production tooling.