At present, compared with T10 steel dies, the service life of tungsten carbide dies used in standard parts production is increased ten times or even dozens of times. It has been widely applied in cold heading rolling such as small size screws. However, compared with foreign tungsten carbide dies, there is still a clear gap, of which the main reasons may be their short service life and serious die wear.

 

For example, the life span of tungsten carbide dies for cold heading semi round head slot bolts or countersunk cross screws in developed countries (Germany, Holland) can reach 900-1000 million times. However, it is only 200-400 million times in China, and the quality of the die is not stable. So, we analyze the service life and structure of the tungsten carbide dies for the cold heading screw produced by two manufacturers in China and the same mold produced by a German manufacturer.

 

 

1, Analysis of Die Life and Failure Mode

The instruments include vernier caliper, HD-150A type Rockwell hardness tester, analytical balance, N-5400 spectrometer, magnifying glass and scanning electron microscope. And the dies are used for the production of electronic instrument for cold piers, Steel for instrument and equipment, small copper and aluminum screws (below M3) screws.

 

Under the same working conditions, the life span of the German tungsten carbide die is 2. 5-4 times more than that of the domestic die. And the failure form of the German die is the fatigue fracture, the wear and tear and other early failure forms, which lead to a great fluctuation in the service life.

 

2, Analysis of Die Structure

After measuring the basic dimensions of mold core and die sleeve with vernier caliper, it is considered that the design of the product is safe. Main reason for the low service life of tungsten carbide die in China is not the size ratio. The inner hole of the mold sleeve and the outer core of the mold core are all cylindrical in the domestic manufacturers. And the bottom of the mold core and the bottom of the inner hole of the mold sleeve are all flat, of which the structure is simple and easy to process. When the cold setting is used, because of the undirected angle, the surface of the inner hole of the mold sleeve is easily pressed and slanted, and even the mold core will produce micro crack and crack, which makes the pressing quality difficult to be stable. 

 

The contact part between the inner hole and the mold core of the German mould is made of double taper and biconcave structure, of which the shape is complex difficult to progress. However, the guiding angle is conducive to ensuring the quality of the insert. The bottom of die core of German mould and the center of the inner hole of the inner hole of the die sleeve are double convex structure, which is beneficial to the central part of the maximum force to close tightly. This is the reason why German die has long service life and stable quality.