What is plastic material?

Plastic material is a human-made (synthetic material) in the solid-state at normal (room) temperature and does not occur naturally. It is also referred to as polymers and allows temporary and permanent change in the form without deteriorating its coherence with the help of pressure/heat or both. Plastic material is an organic material that has a low weight to strength ratio.

What is the meaning of processing of plastic material?

The term processing of plastic material indicates the process through which the raw material of plastic converts into semi-finished product/output. The processing of plastic materials into an output product can be possible with the help of pressure and heat.

Advantages of plastic materials

There are various advantages of plastic materials with respect to different points of view and requirements given below in the following steps.

• Low ratio of weight to strength
• Strength is weaker than metal
• Easily molded into different shape
• Toughness is good
• Excellent finish
• Economically cheaper
• Electrically insulator
• Good water resistant
• Good adhesiveness

Disadvantages of plastic materials

There are various disadvantages of plastic materials with respect to different points of view given below in the following steps.

• It comes under non-renewable resource category.
• It becomes very soft when heated.
• It can cause very dangerous diseases like cancer.
• Plastics are embrittle at low temperature.
• It deforms under loading.

Plastic types according to processing

Generally, there are two types of plastics, one is thermoplastic, and another one is thermosetting. The basic details about these two types of plastics are given below in the following steps.

Thermoplastic

It is a type of plastic material that shows soft nature under heat and external pressure and becomes hard when it cools. The processing of these types of plastic can be performed a number of times without any deformation or defect. The changes in the original shape can be possible many times

Thermosetting plastics

It is a type of plastic that can process only one time. On the application of heat and pressure, once it becomes soft and after cooling, it becomes hard; after this, the processing of this plastic is not possible. Generally, these types of plastic have a cross-linked structure.

Types of processing process of plastic materials

The processing of plastic materials into the semi-finished output can be possible through different processes that utilize heat and pressure/force. The basic details of different types of processing methods are given below in the following steps.

Compression molding

Compression molding is a type of plastic processing in which charge (thermoset plastic granules) is placed between the upper and lower part of the mold, and by applying external pressure, the output product will obtain. It is simpler than the injection molding process and does not require a runner and sprue. The schematic diagram of compression molding is given below.

Transfer molding

It is also a type of molding process in which charge in the form of molding powder is placed into a chamber and provides heat to become soft, and then external pressure/force applies on the plunger to move the charge into the below cavity. The schematic diagram of transfer molding is given below.

Extrusion molding

It is a molding process of thermoplastic granules into an extruded solid output. In this process, thermoplastic granules enter into a motor and hydraulic pump system and are processed to give a solid output. It is a continuous process in which the solid output can be obtained continuously.

Injection molding

Injection molding is used to heat polymer to a highly plastic state and with the help of external high-pressure force to move into a mold cavity where solidification takes place to produce an output. From this process complex shape of output can be obtained with the help of an injection molding setup. The schematic diagram of injection molding is given below.

Blow molding

It is a type of plastic processing in which air pressure is used to inflate soft plastic material into a specific mold cavity. This process helps to make hollow, thin wall plastic products such as plastic water bottles, etc., in a large quantity. The schematic diagram of blow molding is given below.

Calendering

It is also a plastic processing process in which feedstock input material passes through a series of rotating rolls to reduce the thickness of feedstock to the desired gauge. The calendering process gives the result as pool liners, toys, floor covering, etc. The schematic of the calendering process is given below.

Welding of plastic materials

It is a process of joining two parts of plastic materials with the help of external pressure and heat. Both parts of plastic material fuse together and form joints when it cools below the glass transition temperature/melting points. There are different types of plastic welding processes which are given below:

Hot gas welding

It is a plastic welding process that is similar to oxyacetylene gas welding in metals. In plastic welding, hot gas streams are used in place of open gas flame used in the gas welding. The endpoints of the plastic material should be clean and of the same composition as the filler material required.

Ultrasonic welding

The ultrasonic welding process is used to join two plastic components with the help of high frequency longitudinal mechanical vibration, which generates heat at the joining surface of the plastic material. The heat generated due to surface friction and inter-molecular friction melts the ends surface, and after cooling, the joining surface sticks together.

Friction welding

It is a welding process that is similar to the friction welding of steel. In this welding process, one part of the joining plastic material holds stationary and other parts move at a very high speed. Due to rotation and contact of the surfaces, heat generation takes place, which melts the ends of plastic material.

The quality of the weld is high and simple. This process is suitable where the one part of the material is circular in shape. The solid or hollow circular section components can obtain with the help of this process of welding technique.

Vibration welding

It is a welding process that is similar to the friction welding of steel. In this welding process, one part of the joining plastic material holds stationary and the other part moves at a very high speed. Due to rotation and contact of the surfaces, heat generation takes place, which melts the ends of plastic material. This welding technique is used in automotive, domestic appliances, etc.

Vacuum casting of plastic materials

Vacuum casting is a technique to cast plastic products in small quantities with high quality, accuracy and precision. When a liquid form of plastic resin fills in a specific cavity, vacuum pressure pulls air out of the mold cavity. The mold cavity fills with plastic resins with no air bubbles. The quality of the surface finish is very high and excellent.

Common Mistakes

• Students sometimes get confused regarding why thermosetting plastics can not recycle and remoulded, whereas thermoplastic can be recycled and remoulded. However, thermosetting plastics have a cross-linked structure and, on cooling, form a strong covalent bond.
• Sometimes, students also get confused about how plastic material can weld. However, the plastic materials can weld through different techniques like gas welding of steels by using hot gas streams in place of consumable conducting electrodes.
• The student also gets confused about the elasticity of steel and rubber. However, the elasticity of steel is higher than the elasticity of rubber because the strain produced in rubber is more than the strain produced in steel.

Context and Applications

Processing of plastic materials is very significant in the several professional exams and courses for undergraduate, Diploma level, graduate, postgraduate. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Civil Engineering
• Master of Technology in Mechanical Engineering
• Doctor of Philosophy in Mechanical Engineering
• Diploma in Mechanical and Mechanical Engineering

Related concepts

• Tensile test of plastic materials
• Fluxural properties of plastic materials
• Compressive properties of plastic materials
• Creep properties of plastic materials
• Shear strength
• Hardness test
• Impact properties

Practice problems

Q 1. What is the process used in the manufacturing of plastics pipes?

1. Extrusion moulding
2. Blow molding
3. Vacuum forming
4. Injection molding

Correct option: (a)

Explanation: The extrusion moulding is expressed as the method for producing uniform cross-sections of components. So, the process used in manufacturing plastics pipes is extrusion moulding.

Q 2. Which of the following represents the characteristics of thermosetting plastics?

1. The plastic which can be softened a number of times.
2. The plastic which cannot be recycled and remolded, once set and hardened.
3. Both (a) and (b)
4. None of these

Correct option: (b)

Explanation: Thermosetting plastics cannot be recycled and remoulded once it is set and hardened.

Q 3. The plastic material which contains cross-linking in its molecular structure?

1. Thermosetting
2. Thermoplastics
3. Both
4. None of these

Correct option: (a)

Explanation: Thermosetting plastics are described as a plastic material that contains cross-linking in their molecular structure, which makes them more stronger as compared to thermoplastics.

Q 4. Select the best example of thermoplastic material.

1. Automotive parts
2. Electric plug
3. Electric board
4. Electric insulation

Correct option: (d)

Explanation: Among the given options, electric insulation is the best example of thermoplastic material.

Q 5. Which of the following option is best suited regarding the specific gravity of plastic materials?

1. More than specific gravity of metals
2. Equal to specific gravity of metals
3. Less than specific gravity of metals
4. None of these

Correct option: (c)

Explanation: The specific gravity of plastic materials is smaller as compared to the specific gravity of metals because of the molecular structure.