Enhanced Corrosion Resistance
Fasteners exposed to the atmosphere and prone to corrosion-such as those on the four-door outer belt moldings, underbody shields, and bumpers-must withstand 480 hours without exhibiting red rust. The required surface treatment is a black or gray non-electrolytic zinc-aluminum coating, commonly known as "Dacromet" treatment. Fasteners treated with this zinc-aluminum coating method offer excellent corrosion resistance and carry no risk of hydrogen embrittlement.
Product Appeal Optimization
Fasteners that remain visible-such as those for the liftgate latch, liftgate side bumpers, fuel filler door stopper, and seats-typically undergo black zinc plating or a black non-electrolytic zinc-aluminum coating treatment. Fasteners with a black finish create less visual contrast against surrounding components, thereby enhancing the overall aesthetic appeal of the product.
Insufficient Fastener Engagement Length
After tightening, fasteners are required to expose 2 to 3 threads; failure to meet this requirement carries a risk of thread stripping or the fastener failing to seat flush. Design modifications to address this issue typically involve replacing the fastener with a longer one or adding a counterbore to the mating component to reduce its effective wall thickness.
Small Contact Area of Fastener Flanges
The mounting holes for the dashboard assembly bolts are often oversized to accommodate vehicle assembly tolerances, creating a risk that the underlying sheet metal hole may become exposed; therefore, bolts with large washers must be used to cover these openings. Similarly, injection-molded components-such as underbody shields-feature oversized mounting holes to absorb body assembly tolerances. If the flange area of the fastener is too small, tightening the fastener can result in stress concentration, causing the plastic shield to exhibit whitening, crushing, or cracking. It is necessary to consider using fasteners with large flange faces or integrated large washers; generally, the flange face is required to provide a single-sided coverage margin of ≥4 mm, with a tightening torque of ≤5 N·m.
Fastener Torque Decay
During durability testing, fasteners used in components such as side door check straps, sliding door rails, and door latches have exhibited torque decay. To address this, an anti-loosening adhesive treatment must be applied. A common anti-loosening method for automotive fasteners involves the pre-application of engineered plastic patches; fasteners treated in this manner are typically referred to as "Nylok" screws. During the locking process of fasteners with internal and external threads, the compression of the engineering plastic generates a powerful reaction force; this significantly increases the friction between the threads, resulting in full-pitch contact and thereby achieving an anti-loosening function.
