Cold headed parts are, in reality, a special form of upset forging in which cold drawn wire is used as the raw material and in which the forming is done cold.
The process by which cold heading is done is highly automatic and is capable of extremely high production rates. The wire is fed from reels, automatically cat to proper length, fed to the dies and then shaped.
The stock is held in dies and pressure is exerted on the end of the stock to force the metal into proper shape. Because of the extremely high production rate which often reaches, or even exceeds, 400 pieces per minute, cold heading is a low cost process despite relatively high tool costs.
The intricacy of design of parts made by the cold heading process is limited to cylindrical shapes as is the case with upset forging. The amount of upset is less than in hot forging. Collars as well as heads can be formed, but shapes of heads other than round are difficult to form.
When hexagonal shapes, for example, are needed for bolts, a trimming operation is usually necessary to cut the hex from a circular head. Shapes produced by cold heading are limited in size to those which can be formed from a 1-in. dia wire.
Tolerances as low as ±0.002 in. are possible in the upset areas of cold headed parts, although those in the order of ±0.003 in. are more common. Tolerances on lengths, however, are seldom less than 1/32 in.
The surface finish of cold headed parts is superior to that produced by the forging processes, primarily because of the quality of the stock used and because the method does not require the use of heat. Cold heading is best suited to the making of simple flanged or headed products such as bolts, screws and rivets or mechanical parts having similar contours.
Wide, thin heads are not recommended nor are recessed heads. When fasteners are made by cold heading, threads are usually provided through a simple secondary roll threading operation.
Materials for cold heading must be quite ductile and available in cold drawn wire form. Metals that are too hard are likely to be brittle and crack during forming.
At the other extreme, soft materials will become mushy and not form readily. In addition, the metals should have low work hardening rates. Included in the group that meet these specifications and which are most frequently used for cold headed parts are low and medium carbon and alloy steels, stainless steels, aluminum alloys, brasses and bronzes and many of the nickel alloys.
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