Due to the small size of the Roofing Bolts, the shape is irregular, and some need thread fitting, so there are some obvious defects in hot-dip galvanizing
1. It is difficult to tighten the thread after plating. After hot-dip galvanizing, the remaining zinc sticks in the thread is not easy to remove cleanly, and the thickness of the zinc layer is uneven, which affects the fit of the threaded parts. The external threads of fasteners should be processed or rolled according to the GB196 standard before hot-dip plating, while the internal threads can be processed before or after hot-dip plating. However, in practical applications, customers often require galvanized layers on both internal and external threads. Therefore, people have adopted various measures to solve the hot dip galvanization of threaded mating parts. Such as screwback after plating; reserve a large fit clearance; centrifugal shake and other methods. Back tapping can easily damage the plating of the threaded portion, and even expose the steel substrate, causing the fastener to rust. Intentionally exceeding the range to increase the nut hole diameter or leaving a clearance for the fit, it is easy to reduce the strength of the fit, which is not allowed for high-strength fits.
2. The high operating temperature of hot-dip galvanizing will reduce the mechanical strength of high-strength Round Head Bolt. The strength of some threads of class 8.8 bolts after hot-dip galvanizing is lower than the standard requirements; the strength of bolts above class 9.8 after hot-dip galvanizing cannot basically meet the requirements.
3. Poor working environment and serious pollution. The hot dip galvanizing process of fasteners is performed at high temperature. When the solvent is dried and the workpiece to be plated is dipped into the pool, a strong irritating hydrogen fluoride gas will be precipitated. The zinc pool is exposed to zinc at the high temperature for a long time Steam, the atmosphere of the entire working environment is harsh.
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