Plating Thickness on Screw Threads

Unlike electroless metal plating or chemical conversion coatings in which no electric field from an external source is applied, electroplated metal coating from a bath designed for the purpose coats parts non-uniformly. This may present only a small difficulty if part geometry is not critical and there are no sharp points or edges present, and the area to thickness ratio is not too great. Threads on screws do represent a problem requiring adjustment, especially if screws are lengthy with fine threads.

Physical Characteristics of Electroplating

Consider the nature of the beast. Electroplating involves a number of parameters, including part geometry, current density and plating time. Electricity flows most readily through points and secondly through edges. Contrariwise, the least current flow takes place through recesses and secondly flat surfaces.

Evaluate the geometry. Screws present extremes of high and low current flow regions. The ends of long screws can be compared to a lightning rod. Current flows most readily through points. On the other hand, thread troughs receive the least current flow. Because of their sharpness, thread crests experience the greatest current flow.

Understand the concept of current density. The total electric current that flows through a piece divided by its surface area is called the current density. For example, if 1.2 amperes of current are needed to plate a piece with a surface area of 4 square feet, the current density 0.3 amps-per-square-foot.

Chemical Characteristics and Calculation of Plating Thickness

Recognize the importance of bath chemistry. Electroplating baths vary by chemical composition, which modifies plating characteristics. Some exert a leveling influence on plating thickness, while others exaggerate the problems of geometry. Baths are characterized by the thickness of the metal plate they produce for a given current density for a given time. For instance, a gold-plating bath may produce an average plating thickness of 50 micro-inches (millionths of an inch) in six minutes plating time at 3 ASF (amps-per-square-foot). Although adjusting the current up or down can modify coating thickness, usually it is the plating time that is adjusted.

Consider how most commercial manufacturers have learned to calculate plating thickness on threads. They suggest two solutions to the issue of thread-growth. Some advocate the use of undersized initial thread-pitch diameter. This is costly, and so is usually avoided except in the most critical applications. Instead, most commercial firms choose to reduce plating time by a factor of four. If the desirable average thickness calls for 12 minutes plating time, they will reduce that to 3 minutes.

Case Western Reserve University - Electrochemistry Encyclopedia - Electroplating

National Metal Finishing Resource Center - Nickel Plating from the Sulfamate Bath

Safety Socket LLC - How to Calculate Plating Thickness on Threads

Roton - Plating Consideration for Threads

Greenslade and Company - "Electroplating Thickness Is Not Uniform on Fasteners"