Plastic

UNIT - 5

MANUFACTURE OF PLASTIC COMPONENTS

Types and characteristics of plastics - Moulding of thermoplastics working principles and typical applications injection moulding - Plunger and screw machines Compression moulding, Transfer Moulding - Typical industrial applications - introduction to blow moulding - Rotational moulding - Film blowing -Extrusion - Thermoforming - Bonding of Thermoplastics.

1. INTRODUCTION

In general, many organic materials are used in engineering industry. The plastics are attained a firm place in numerous industries today. It is due to the characteristics of plastics such as inexpensive, high strength, durable, less complications in designing plastic products etc.

Plastics are belonged to the family of organic materials. Organic materials are those materials obtained directly from carbon and chemically combined with oxygen, hydrogen and other non-metallic compounds.

These organic materials are classified into two types. They are:

1. Natural organic materials

2. Synthetic organic materials.

1. Natural organic materials:

The wood, coal, petroleum and natural rubber are under the categories of natural organic.

2. Synthetic organic materials:

The plastics, synthetic rubbers and ceramics glass are under the categories of synthetic organic. Technically, these organic materials are called polymers.

1. Polymers

The term polymer has its base in Greek terminology, where 'Poly' means 'Many' and 'Mers' means 'Parts'. The term polymer stands to represent a substance built up of several repeating ‘units'.

A single unit is called 'monomer'. The monomers are small molecules. A polymer is made up of thousands of monomer joined together to form a large molecule. The characteristics of a polymer are that the molecule is either a long chain or a network of repeating units.

Plastics are one kind of polymer. It is defined as an organic polymer. It can be moulded into any required shape with the help of pressure or heat or both heat and pressure.

The liquid form of plastic is called resin and it contains carbon as a central element. Oxygen, nitrogen and chlorine are linked to the carbon atoms to form the molecules.

The main raw material for making plastics is resin. The different types of resin are acrylic resin, polyethylene resin and amino resin. These resins are produced by different types of polymerization process. Catalysts, binders, dyes and lubricants are added with the resin to form the plastics

2. Polymerization Process

A polymer is made up of linking thousands of monomer. The process of obtaining large molecule by linking many monomers is called polymerization process. It is achieved by one of the two processing techniques. They are:

1. Addition polymerization.

2. Condensation polymerization.

1. Addition polymerization:

In addition polymerization, similar monomers of large numbers are chemically added one by one. These monomers form a long chain molecule. The basic principle which is used in this bonding is Van der Waals force. The polyethylene is produced by addition polymerization.

In addition polymerization process, no catalyst is used. Polymerization takes place by applying energy in the form of pressure and heat. The addition of two or more different monomers is called co-polymerization process.

2. Condensation polymerization:

In condensation polymerization, two or more unlike monomers are linked and there is a repetitive elimination of smaller molecules to form a by-product. During this process, by- product such as water or ammonia is formed. This by-product formation is known as condensation.

The condensation polymerization requires high pressure and it requires hours or days to complete the process.

3. Materials Used for Processing of Plastics

The properties of polymers are modified by the addition of agents such as ‘additives' and 'fillers'.

a) Additives:

1. Plasticizers:

To improve the plastic behaviour of the polymer, the plasticizers are added: The plasticizers are in the form of liquids with high boiling point. It acts as an internal lubricant for increasing the toughness and flexibility. The main role of a plasticiner is to separate the macromolecules. Thus, the inter-molecular force decreases and facilitates the relative movement between molecules of the polymer. Thus, it makes the deformation easier.

Examples:

Water, organic solvents and resins.

2. Catalyst:

Catalysts are usually added to promote faster and complete polymerization. The catalysts are also called 'accelerators' and hardeners.

3. Dyes and pigments:

Dyes and pigments are added to impart a desired colour to the material.

4. Initiators:

The initiators are used to initiate the reaction i.e. it allows to begin polymerization. They stabilize the end reaction of the molecular chains.

Example:

H2O2 is a common initiator.

5. Modifiers:

It is used to improve the mechanical properties of plastics such as strength, damping capacity, toughness, ductility, plasticity etc.

6. Lubricants:

It is used to reduce friction during processing, prevent parts from sticking to mould walls and prevent polymer films from sticking to each other. 

Examples: Oils, soaps and waxes. 

7. Flame retardants:

The flame retardants are added to the plastics to enhance the non-inflammability of the plastics. 

Examples: Compounds of chlorine, bromine and phosphorous.

8. Solvents:

It is useful for dissolving certain fillers or plasticizers and helps to allow the processing in the fluid state. 

Example: Alcohol

9. Elastomers:

It is added to the plastics to enhance their elastic properties.

10. Stabilizers:

It is added to the plastics to retard the degradation of polymers.

11. Fillers:

It is used to economize the quantity of polymer required and to vary the properties to some extent. The fillers are used to improve the strength and stability of the plastics. The type of fillers used in plastics is mica and cloth fiber. The mica and asbestos are used to improve the heat resistance capacity of the plastics.

4. Characteristics of Plastics

The plastics possess various characteristics or properties which help the end users to move for lightweight materials. The properties are listed below:

(i) Elongation

(ii) Heat resistance

(iii) Resistance to corrosion

(iv) Insensitive to tension cracks

(v) High rigidity

(vi) High surface hardness

(vii) High viscosity

(viii) Short term maximum usage temperature

(ix) Resistance to shock

(x) Good durability

(xi) Low density

(xii) High strength-to-weight ratio

(xiii) Low humidity absorption

(xiv) Availability of wide variety of colours and transparent

(xv) General chemical corrosion resistance

(xvi) Ease of fabrication

(xvii) Low electrical and thermal conductivity

(xviii) Low unit cost for mass-production