Power Distribution Box Covers: Resistant to impact, temperature extremes, and various automotive fuels and fluids

A Very Demanding Application
Power distribution boxes (PDBs) are integral parts of automotive electrical distribution systems that contain fuses, relays, and other components used
to transfer current from the battery to electrical/electronic devices in the vehicle. Previously these boxes were located in the passenger compartment
but are now positioned under the hood, a harsher environment which demands resistance to impact, temperature extremes, and various automotive fuels and fluids.

Ford Motor Company specified a 150°C heat resistance for the PDB covers to be used under-hood in several new vehicle models. In addition to the need for excellent chemical resistance, the upper and lower cases of the boxes were to be secured by snap fits, requiring good elongation. Long-term dimensional stability would also be necessary to ensure a tight fit between the covers to protect the electrical devices inside from contamination. Besides impact resistance, the boxes were expected to endure rough handling that might occur during assembly operations. Other specifications included an excellent appearance in the engine compartment, the capability for sublimation printing, good processability, and the ability to replicate the intricate molded-in diagrams of fuse and relay locations on the inside of one of the covers. In addition, the cover which was located at the battery cable connection required a material having an Underwriters Laboratories (UL) flammability rating of V-0 at 1/32 in. thickness.

Selection Criteria: Many Tests to Pass
Ford worked with Alcoa Fujikura Ltd. (AFL), Dearborn, MI, a producer of electrical distribution systems, to find a thermoplastic material capable
of meeting these demanding requirements. Alcoa Fujikura engineers evaluated a 30% glass-reinforced PBT resin already used in some PDB covers along with two grades of unreinforced Vandar® thermoplastic alloy, one of which is rated UL94 V-0. Initial tests included a one meter drop onto concrete, in which the glass reinforced PBT covers cracked, shattered, or otherwise fractured. The covers molded from both grades of Vandar® alloy, however, survived these punishing conditions. Another test involved applying a 100 lb. force to the cover’s snap-fit latch legs. Again, the glass-reinforced PBT failed while the Vandar® alloys withstood these stresses. An important consideration of the material evaluation was that, because the Vandar® materials did not contain glass fiber reinforcement, a smoother, more attractive surface was obtained for the covers. This in turn facilitated matching the colors, imprinting, and texture specified by Ford. Another advantage was that the flame retardant 8000 grade of Vandar® resin used for the stud cover showed no loss in mechanical properties (see table) compared to the non-flame retardant 2500 grade.

How to explain Vandar®'s success?
Vandar® Grades 2500 and 8000 were selected for the PDB cover because they provided:

• Heat resistance in excess of 150°C.
• Excellent impact strength necessary for assembly and end-use conditions over a range from 0°C to 125°C.
• Outstanding resistance to gasoline, brake fluid, and other under-hood chemicals through the same temperature range.
• The elongation and recovery necessary to endure repetitive snap fits.
• Dependable dimensional stability, even at temperature extremes, to ensure a tight seal of the covers.
• A good surface appearance coupled with the capability for sublimation printing without pretreatment.
• Flame retardance without a reduction in mechanical properties.
• The processability necessary for high-volume production.

The PDB covers produced by AFL for Ford have now been used in thousands of vehicles with excellent results. Vandar® 2500 meets Ford material specification ESL-M4D359-A3, and Vandar® 8000 meets Ford worldwide specification W-SS-M4D935-A1.