PLASTIC FORMING PROCRSSES

Introduction Plastic forming processes are those group of manufacturing processes by which the part shape, dimensions, or both are changed by applying pressure forces till it reaches the plastic state. Then allowing the metal to flow or deform in special equipment to have the required shape and dimensions.

The two principal types of metalworking in which material can undergo plastic deformation are hot working and cold working. Hot working of metals is done by preheating the metal before working to a temperature above that of recrystallization. For example, steel recrystallization starts around 500-700 Co. During all hot working operations the metal is in a plastic state and is readily formed by pressure. Cold working of metals is done when metals are rolled, extruded, pressed, or drawn at a temperature below the recrystallization temperature. Usually most metals are cold-worked at room temperature, although the action of forming them will cause temperature increase.

Hot working The main advantages of hot working are: 1- Working properties are generally improved, mainly due to the grain refinement during the process. Ductility and resistance to impact are improved, strength is increased, and greater homogeneity is developed in the metal. 2- Porosity in the metal is largely eliminated and impurities in the form of inclusions are broken up and distributed through the metal. 3- A pressure needed to change the shape of steel in the plastic state is far less than that required when the steel is cold. The main disadvantage of hot working results from the high temperature of metal which causes rapid oxidation or scaling of the surface. This will make hot-worked parts have poor surface finish and low dimensional accuracy.

Cold working (Advantages) Cold working will improve strength, machinability, dimensional accuracy, and surface finish of metals and, because oxidation is less for cold working, thinner sheets and foils can be rolled than by hot working. (Disadvantage) Cold working will need higher pressures and heavier equipment than needed for hot working.

1- Rolling Rolling Principles Rolling is a plastic forming process by which the metal is squeezed during its pass through a gap between rotating rolls. Squeezing is performed because the gap thickness is less than the thickness of metal before rolling. Hot rolling will cause grain refinement as illustrated diagrammatically in Fig.

Blooms and billets are further hot rolled into different shapes as rods, bars, rails, or sections. Slabs are rolled into plates, sheets, or thin strips. All of these products find wide application in building and engineering industries.

Rolling Mills Various arrangements of rolls used in rolling mills

Successive rolls of a round bar and a T section

Pipe and tube rolling

2- Forging Forging principles Forging is a process by which a malleable material is formed into specific shapes by heating and applying pressure forces on it till takes the required shape. There are two main types of forging operations: a- Open-die forging or smith forging: where the heated metal is hammered either by hand tools or between flat dies in a hammer. b- closed-die forging where, the heated metal is forced to take the shape of an impression in a two-halves die. The pressure force applied on the metal may be in the form of either sudden blows, as in drop forging, or a steady squeeze action, as in press forging.

The main advantages of forging are: 1-The grain flow follows the shape of the part and the grains are refined. This makes the part stronger, tougher, and more ductile. 2-Forged parts have good uniformity in size and shape. 3-Metals can be machined and welded after forging. 4-The forging process can, in many cases, be automated 5-Production rates are good.   The main disadvantages of forging are: 1-Dimensional accuracy of forged parts are low, though tolerances as close as  0.4 mm can be achieved. 2-Forging presses for large work are very expensive. 3-Heating furnaces will also increase production costs of forged parts.

Coining. Connecting rod.

3- Extrusion Extrusion is the process by which metal is formed into specific shapes by forcing it out through specially formed dies. The principle of extrusion is similar to the act of squeezing toothpaste out from a tube. Some metals such as lead, tin, and aluminum, may be extruded cold, whereas others require preheating before extrusion.

There are mainly two ways by which the process can be done: Direct and indirect extrusion

4- Wire, bar, and tube drawing Drawing is a plastic forming process by which wire, bar stock, or tubing is tension-drawn through the hole of a special tool, the drawing die, the hole is smaller in size than the initial material. Section through a wire drawing die

Tube drawing

Plastic Forming Processes 1- What are the main advantages and disadvantages of hot and cold working 2- What is the principles of rolling, sketch how to produce T section by rolling, sketch how to produce 40 mm bar from a 100 mm x 100 mm billet by rolling 3- What is the principles of forging, state the main types of it 4- What are the main advantages and disadvantages of forging 5- What is the principal of extrusion 6- With the aid of sketch compare between open die and closed die forging 7- Using sketch, illustrate the principles of direct and indirect extrusion 8- Draw a section through a wire drawing die 9- What is the principal of wire drawing 10- What is the principal of tube drawing