What is metal forming and shaping?

Metal forming is a category of metalworking process through which the material can be given the desired shape and size by subjecting it to plastic deformation, where the component undergoes bulk deformation process upon application of loads. Unlike the shearing and machining process, the material does not undergo up to a failure point as indicated in the stress-strain curve. Hence, the stresses induced during the forming process are greater than yield strength but less than the failure strength of the material. The process does not lead to the loss of material, so it can be said to be an economical process. During forming, the material is subjected to tensile forces, compressive forces, bending, and shearing loading conditions. The material should possess the property of ductility to undergo the forming process.

Shaping is a process where the material to be worked on is first liquified in a furnace, then it is poured into a previously prepared mold cavity using sand, clay, additives, and water which bears the shape of the end product. The liquid material upon solidification takes the desired shape of the mold. The conventional name of this process is also called ‘Casting’. One of the main characteristics of shaping is that the component can be given any complex shape with good dimensional accuracy. In the shaping process, both plastic parts and metals are used as raw materials.

Forming process

The different forming processes are grouped into the following two categories:

1. Cold forming
2. Hot forming

Cold forming: When the working temperature of the material is below its recrystallization temperature then the process can be termed cold forming. There is significant strain hardening of the materials after cold working due to reorientation of the grains.

Hot forming: When the working temperature of the material is above its recrystallization temperature then the process is known as hot forming. A large amount of plastic deformation can be imparted to the material without much strain hardening.

Cold forming

Out of the different cold forming processes, the two of the most common cold forming processes are:

1. Drawing
2. Deep drawing

Drawing

The process is used to reduce the cross-section of a wire or a bar. The process involves the material whose diameter is to be reduced, placed at one side, and a conical die (whose end diameter is of the required diameter to be obtained) placed on the other side. The material is pulled out from the die opening by a rotating mechanism, due to compressive forces and significant plastic deformation, the diameter is reduced.

Deep drawing

This process is used to make cups and cans whose height is more than the diameter. The workpiece is a flat sheet of metal known as blank. It is placed firmly on top of a die that has a hollow cavity. The workpiece is forced inside the die by using a mechanical or a hydraulic press. The die is shaped as a hollow cylindrical chamber and the punch of the press has a positive protrusion. Tolerances are provided between the punch and the cylindrical cavity so that the punch can slide inside the die. The workpiece in the form of a sheet, known as a blank, is clamped over the die. Due to the force of the punch, the workpiece gets plastically deformed and takes the shape of the die. The formed material is undrawn from the die with the help of ejector pins.

Hot forming

The most common hot forming processes are extrusion and forging. But forging is mostly used where ever necessary due to less manufacturing cost. In the forging operation, the component is first heated to a suitable temperature and then, is squeezed using two or more dies. The forces induced by the dies are compressive. This process is also known as hot stamping or press hardening. The material usually undergoes recovery and softening after the process. High-strength materials can be hot formed with ease. This process is best suitable for materials that can withstand high forces.

This terminology should not be confused with the warm forming process. In the warm forming process the temperature range usually lies in-between the hot forming and cold forming working temperatures and they require less working forces.

Injection molding process

Injection molding is a shaping process that makes use of raw plastic parts or plastic pellets to give the desired shape and size. The plastic is heated and melted inside an injection molding machine where it is compressed and injected using a reciprocating screw into the mold cavity. Upon cooling, the plastic takes the desired form of the mold. The commonly used plastics in this process are thermoplastics and thermosetting polymers or thermosets. The process is used for mass products having complicated shapes.

The process of injection is divided into six major steps, they are:

1. Clamping
2. Injection
3. Dwelling
4. Cooling
5. Mold opening
6. Removal of products

The mold of the injection molding is a hollow metallic cavity that sustains the hot molten plastic which takes the desired shape of the cavity after solidification. The molds have numerous holed created by drilling for air and water circulation. The solidified plastic is ejected out of the mold cavity by the use of an ejector pin mechanism.

Centrifugal casting process

The process here makes use of metals as the raw material. This process is used to make a wide variety of hollow cylindrical bars having different internal and external diameters. The metal is first converted to liquid form in a furnace and poured into a rotating mold. Due to the centrifugal force generated, the liquid metal is forced outwards creating a hollow cylindrical cavity. The finished parts are sometimes made to undergo a sintering process or heat treatment to provide the desired strength depending on the application where the hollow bars may be used.

One of the major drawbacks in casting processes is the trapping of atmospheric or other gases while the material is undergoing solidification that leads to the various casting process. The main reason behind this is the lack of porosity of the mold.  

Context and Application

This topic finds its major application in the curriculums of

• Bachelors in Technology (Mechanical)
• Master in Technology (Mechanical)
• Bachelors in Production Engineering

Practice Problems

1. Which of the following processes are used to make hollow cylindrical bars?

1. Injection molding
2. Centrifugal casting
3. Forming
4. Deep drawing

Correct option- b

Explanation: Hollow cylindrical bars are produced by the centrifugal casting process because they are produced in a rotating mold, due to the centrifugal force the metal is forced away from the axis of rotation creating a hollow cavity at the center.

2. Which process makes the use of plastic parts as raw materials?

1. Drawing operation
2. Injection molding
3. Investment casting
4. Shell molding

Correct option- b

Explanation: In the injection, the raw plastic parts are first heated and melted in a furnace and are fed into the mold cavity by the use of a reciprocating screw.

3. What causes porosity in the casting process?

1. Moisture present in the mold
2. Less green strength of the sand
3. Air aspiration effects
4. Lack of porosity of the sand

Correct option- d

Explanation: Due to the lack of porosity of the sand the trapped gases produced cannot find a way out of the mold, hence leading to casting defects.

4. Which of the following processes makes use of hydraulic and mechanical press?

1. Drawing process
2. Deep drawing process
3. Forging
4. Cold forming

Correct option- b

Explanation: The raw material or the flat blank used in the deep drawing operation is pressed by cylindrical punches operated either mechanically or hydraulically.

5. Which of the following processes leads to strain hardening?

1. Cold forming
2. Hot forming
3. Machining
4. Blanking operation

Correct option- a

Explanation: In cold forming metals are worked at below their recrystallization temperature. Larger deformations cause more strain hardening due to the re-orientation of the grains.

Related Concepts

• Rolling (hot rolling and cold rolling)
• Lost wax casting
• Shell molding
• Drop forging
• Sand casting