Press Forging – Process , advantages and Disadvantages
Introduction to Press Forging
Press forging may be defined as the process of shaping a metal that is placed between two dies by applying mechanical or hydraulic pressure. Press forging is usually done on a forge press – a machine that applies gradual pressure on the forging dies. The shape of the metal is usually accomplished by a single stroke of the press for each die station.
The key difference between impact forging and press forging is that in the former method a sudden impact force is applied on the die, while in the latter method a gradually increasing pressure is applied on the die.
Press forging may be undertaken as a hot or cold forging process. Press forging is suitable for high volume productivity of forgings. The various forging operations in a press forge and its applications are described below.
Read More: What is Forging and Its types ,Advantages
The press forging is similar to drop forging but uses a single, continuous, slow squeezing action instead of a series of impact blows. Because of slow ram travel and continuous action of the hydraulic presses, the deformation penetrates deeper so that the entire volume of the work piece simultaneously and uniformly undergoes plastic deformation. While in case of drop forging, the energy is only transmitted into the surface layers of the work part.
A good industrial practice is to use shaped blanks or preforms so that it can be forged in a single continuous stroke. The press forging may be of flash type of fleshless. In flash-press-forging, the surplus metal is flashed out in the gutter, provided at the parting line, while in flashless-press-forging, the entire material is utilized to fill the die cavity.
Technique of Press Forging
Press forging is a technique that applies gradual pressure on a die holding a work piece. This can be done as an open die or closed die forging process. In an open die press forging process, the metal is surrounded by the die on one side only. In the case of impression die or closed die forging methods, the metal work piece is enclosed in a die and pressure is applied on the die. Plastic deformation of the work piece occurs and it fills the die cavities.
Types of Forging Presses
The three main types of forging presses used for press forging are listed below:
- Mechanical Press – converts the rotation of the motor into linear motion of the ram.
- Hydraulic Press – hydraulic motion of the piston moves the ram.
- Screw press – screw mechanism actuates the ram movement.
Advantages of Press Forging:
1. Higher productivity than drop forging.
2. Greater accuracy in terms of tolerances within 0.01 to 0.02 inch.
3. Dies used in press forging have less draft, therefore more complicated shapes can be forged with better dimensional accuracy.
4. The speed, pressure and travel of the die are automatically controlled, in press forging.
5. Possibility of process automation through mechanism of blank feeding and forging removal.
6. The operation is completed in a single squeezing action, saves the time.
7. The plastic deformation goes deep into the centre of the work piece, provides uniform and simultaneous deformation throughout the metal.
8. The capacity of presses ranging from 500 to 9000 tons and the number of working strokes per minute can be high as 40 or even 50. Means 40 to 50 parts produced per minute.
9. The press forging, therefore, is suitable for mass production of nuts, bolts, rivets, screws, break levers, bearing races, valves, etc.
Limitations of Press Forging:
Nevertheless, the press forging has certain limitations, these are:
1. The initial capital cost is higher compared with drop forging, because the cost of crank press is always higher than that of an equivalent hammer.
2. The difficulty of descaling the blank is another short coming of this process.
3. The process is economically suitable only when the equipment is efficiently utilized.
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what would happen If a 750T Mechanical Crank press strokes per minute is reduced ., for example 18 SPM rated press is run at 12 SPM what would happen to the press mechanism. Will the Crank and other mechanical parts undergo strain? Please clarify