Engineer's Notebook

By: 
June 23, 1997

Co-locating one of our engineers with our customer's engineers allowed us
to cut design time for a four-piece Kenworth T2000 truck instrument panel and
glovebox assembly from the industry-standard 36 to 48 weeks to just 26 weeks.
The reason: Our project manager, Dave Wix, could provide molding guidance to the
customer immediately on a feature-by-feature basis, rather than waiting weeks to
review multiple design changes.

Among the major challenges of the project were the panel's complex geometry--including ribs, bosses, and variable cross sections--and strict material-performance requirements. The truck's windshield, with the steepest slope in the industry, made quantification of the thermal conditions and environment difficult.

In solving these problems, we developed a new molding process called TRIMTM that we believe provides more design flexibility and thermal-mechanical performance.

Software aids design. For the vehicle's interior components, Kenworth required that we minimize instrument-panel weight, maximize strength, and create the internal volume for HVAC ducting, fuse boxes, wires and harnesses, and gauges. We realized that connections between the four molded parts would have to be designed to optimize the upper and lower tab/flange thickness on the middle of the instrument-panel part. The most conservative assumption was that the required 6g load in the aft section would act completely on the middle instrument panel and glovebox assembly, since the horizontal nature of the loading would translate directly through the upper and lower instrument-panel parts.

We used ANSYS finite element analysis software to determine the appropriate material thickness.

Material development. Kenworth required that the panel achieve a heat deflection temperature (HDT) of &160F at 66-psi loading. Additionally, the coefficient of linear thermal expansion (CLTE) had to be &3.0x10-5 in/in/degree F.

To meet those requirements, Tempress and BASF jointly developed a design of experiments (DOE) using a Taguchi Lorthogonal array. Actual test results exceeded our expectations.

Results from another test led us to change material formulations to improve impact properties and nearly double elongation before failure with no appreciable loss of other properties.

Perhaps the biggest benefit to Kenworth was the addition of multifunctional ribs and bosses inside the instrument panel. They aid in registration and assembly of HVAC ducting, sheet-metal bracketry, fuse boxes, and routing of electrical wires and harnesses. Including all those features on the molded product eliminated components and minimized processing cost.

Design objective

Design and develop a vinyl-clad reinforced urethane instrument panel and glovebox assembly. Heat deflection temperature: &160F at 66-psi loading. Coefficient of linear thermal expansion: &3.0x10-5 in/in/degree F.

Challenges

  • Windshield slope, steepest in the market, made quantification of thermal conditions difficult.

  • Abusive environment for trucks, and Kenworth's 1,000,000-mile reliability requirement.

  • Parts for the 6-ft-long, 2-ft-high, 2-ft-deep assembly had to be lightweight, as follows: Urethane surface top, 7.71 lb; middle, 3.0 lb; lower, 7.1 lb; glovebox, 0.8 lb. Vinyl surface top, 12.5 lb; middle 5.4 lb, lower, 11.3 lb, glovebox, 1.7 lb.

  • Accommodating growth and shrinkage among engineering materials from RIM thermosets to steel, aluminum, thermoplastics, and others.

  • Kenworth's desire for soft yet durable vinyl exterior on urethane RIM parts.

  • Uniform density and reinforcement additive distribution across molded parts.

  • Complex geometry.

  • QS 9000/ISO 9000 requirements.

  • Integration of new CAD system at Kenworth.

  • Two-month deadline.

Design tools/ suppliers

  • Pro/ENGINEER CAD software from Parametric Technology Inc.

  • ANVIL 5000 CAD software from MCS Inc.

  • Silicon Graphics workstations

  • IBM RS6000 workstations

  • ANSYS finite element analysis software from ANSYS Inc.

  • CAM software from CAMAX

  • BASF

  • Tooling supplier EPW

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