Moulding and forging are production processes in which the material is deformed plastically, in order to change its initial form by applying external compression forces on special machines called presses and hammers. The molding is by far the most important process, for the variety of shapes of the pieces that you can get: crankshafts for engines, turbine discs, sprocket wheels and a wide variety of components.
Forging and molding are made on machines that are called hammers and presses. In these machines there is a mobile organ able to exert on the piece, placed on the fixed part, the necessary strength to its deformation. Hammers and presses have certain characteristics in common:
- Available energy (Nm or joules): is the energy supplied by the machine’s moving part, to carry out the work of deformation;
- Available force: is the force that the machine’s moving part can exert on material to be deformed;
- Efficiency: is the ratio of the available energy available to that provided to the machine. The efficiency is of course influenced by multiple factors such as the losses in the electric motor, friction in the moving parts, the elastic deformation of machine;
- Number of strokes per minute: determines the productivity of the machine;
- Contact time (under load): is an important parameter in hot-working. During this time, in fact, there is heat transfer between the workpiece and the mold, faster when the piece is under load. A high contact time does increase the wear of the mold; the consequent greater cooling of the workpiece causes an undesirable increase in the force required to deform.
Power Hammers consist of a moving part (Mace) and a fixed component (Anvil). There is also a mount that hosts the supporting elements, the mace guide and the engine block. Plastic deformation of the workpiece to be moulded or forged is achieved using the kinetic energy of the mace; this can be acquired or as the mace is free falling (simple-effect hammers) or under the further acceleration provided by hydraulic pressure (double effect hammers).
In both types of machines, the mass of the anvil must be significantly higher than the mace, to ensure a good efficiency. To note also that the considerable vibration transmitted to the environment from these machines can create installation and localization problems in the layout of the factory. In hammers, the rush of mace proceeds until its total energy is not dissipated in work.
This work is spent on the plastic deformation of the workpiece, in the elastic deformation of the machine itself and moulds and in friction. For this reason, it is of great importance in these machines the available energy.
In presses a mobile piece (slide), which during its active run exerts a force on the material to be deformed, performs an alternate motion. The presses are divided into mechanical press, in which the slide is put in motion by mechanisms such as crank-connecting rod or screw-nut, and hydraulic presses, in which the movement is achieved with a fluid (hydraulic oil) under pressure.
Crank mechanical presses use the crank-connecting rod mechanism, which is implemented by means of an crankshaft. A flywheel, located in the upper part of the machine, is kept in continuous rotation by an electric motor. When you must make an operation, a clutch connects the flywheel shaft with the crankshaft. The latter, making a full turn, drives the sled up to bottom dead centre and then at top dead centre: here it is braked by disengaging the clutch and a brake intervention, in order to avoid a dangerous repeated stroke pattern, if it works single-stroke.
The deforming operation can be completed if the force required by the process is less than what is available in the press. From the energy point of view, the operation may be performed if the flywheel by means of a slowdown, can provide enough energy compared to the necessary deformation work. Then, after each operation, it is up to the electric motor to bring back the flywheel to the original rotation speed. If the press works continuously, the motor must have high power, in order to reduce the time between a stroke and the next one.
Screw mechanical presses use a friction wheel drive system to accelerate the group flywheel-screw-sled and to bring back the slide to the top dead centre: the rotatory kinetic energy, this way, is converted into linear kinetic energy of the sled. Any energy in excess compared to the work of deformation and friction, can cause unnecessary stress to the machine and molds: for this reason this type of presses have control and regulation systems of the energy available, by acting on the flywheel speed and the duration of contact between the flywheel and clutch disc. In this kind of press, the bottom dead centre varies and depends on the energy balance.
Hydraulic presses are based on the movement of one or more cylinders obtained through oil under pressure. Unlike eccentric presses, in these machines the maximum force is available anywhere along the slide run. Therefore, the available energy is always very high. Another characteristic is the relative ease with which one can limit the maximum force through hydraulic pressure relief valves. In addition, you can vary the speed of the slide during the race, even as a function of the required force or temperature of the workpiece.
Hammers and presses have clearly defined and distinct areas of application. In the molding and forging of small to medium sized pieces it is convenient to use the hammer, while for large parts, requiring strength generally greater, the press works better.
Further considerations need to be made. Not all materials can bear the higher strain rates obtainable with hammers. From a structural point of view of the molded part, the hammer would be preferable to press for its more uniform deformation; however, large parts would require huge hammers, no longer used due to the difficulty of regulating their power and for the excessive dimensions that they should have to equal the power of modern hydraulic presses (up to 50,000 tons). Also, if you require a high degree of surface finishing with respect to the type of work, it may be convenient to execute a finishing operation on the preform molding using a hammer light and fast, which provides high surface deformations with small depths of penetration.