CHAPTER 6 PLASTIC

An organic polymer material ( with carbon and hydrogen base ) with the ability to flow into a desired shape when heat and pressure are applied to it and retain the shape when they are withdrawn.

In more technical terms, a plastic is a material that can be heated and molded so that it keeps its molded shape after it cools.

DEFINITION

POLYMERS

Polymer molecules are very large macromolecules consisting of long flexible chains with carbon atoms as a backbone.

POLYMERS

Polymer is composed of a large number of repetitive structures called mer.

A single mer is called monomers.

Thus, a polymer is made up of thousands of monomers joined chemically together to form a large molecule.

POLYMERS

POLYMER STRUCTURE

MOLECULAR STRUCTURE

Linear polymers are those in which the mer units are joined together end to end in single chains. These long chains are flexible and may be thought of as a mass of spaghetti where each circle represents a mer unit.

Some of the common polymers that form with linear structures are polyethylene, polyvinyl chloride, polystyrene, polymethyl methacrylate, nylon, and the fluorocarbons.

LINEAR POLYMERS

Polymers may be synthesized in which side-branch chains are connected to the main ones, these are fittingly called branched polymers.

The branches, considered to be part of the main-chain molecule, result from side reactions that occur during the synthesis of the polymer.

BRANCHED POLYMERS

In crosslinked polymers, adjacent linear chains are joined one to another at various positions by covalent bonds.

The process of crosslinking is achieved either during synthesis or by a nonreversible chemical reaction that is usually carried out at an elevated temperature.

CROSSLINKED POLYMERS

Trifunctional mer units, having three active covalent bonds, form three-dimensional

networks and are termed network polymers.

These materials have distinctive mechanical and thermal properties; the epoxies and phenol-formaldehyde belong to this group.

NETWORK POLYMERS

Polymerisation is the process of forming a polymer.

This process can be classified as addition polymerisation and condensation polymerisation.

POLYMERISATION

The process chemically combines similar monomer to form a long chain of molecule (linear molecule).

ADDITION POLYMERISATION

Two or more chemically different monomers are polymerised to form a cross linked or linear polymer, along with a by-product such as water or ammonia.

CONDENSATION POLYMERISATION

EXAMPLE OF PLASTIC PRODUCT

There are two types of plastics:

1)Thermoplastics

2)Thermosetting

TYPES OF PLASTICS

Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be moulded again and again.

THERMOPLASTICS

Deformable at higher temperatures and becomes hard again after cooling.

Can be softened and hardened by heating and cooling any number of times.

Since the flow easily by heating, they are suitable for processing by extrusion or injection moulding.

Easy to be machined on.

THERMOPLASTICS POLYMER

Thermoplastics Long chain molecules

Polyethylene

Polypropylene

Polystyrene

Polyvinyl Chloride (PVC)

Polymethyl Methacrylate (Perspex)

Polyamide (Nylon)

Example of thermoplastics

EXAMPLE THERMOPLASTICS

DENTAL THERMOPLASTICS

SOCKET MADE OF THERMOPLASTIC

A thermosetting plastic, also known as a thermoset, is polymeric material that irreversibly cures. The cure may be done through heat (generally above 200 °C (392 °F)), and involve a chemical reaction.

THERMOSETTING

Are soft or soften upon heating.

Cannot be re-softened once they have set and hardened.

Due to cross-linking of molecule chains, thermosetting materials are hard, tough, non-swelling and brittle.

Ideal for moulding and casting into components.

THERMOSETTING POLYMER

Thermosetting plastics

Cross-linked molecules

Phenol fomaldehyde

Expoxide

Polyester

Urea formaldehyde

Melamine formaldehyde

Examples of thermosetting plastics

THERMOSET TOILET SEAT PRODUCT

MOULDED PART

No. Thermoplastics Thermosetting

1 Composed of chain molecules Cross linked molecules

2 Addition polymerisation Condensation polymerisation

3 Deforms at high temperature Cannot be deformed at high temperature

4 Plasticity increases with temperature

Plasticity does not increase with temperature

5 Easily moulded and remoulded into any shapes

Cannot remoulded into new shapes

Comparison between thermoplastics and thermosetting plastics

1) Injection Moulding

2) Extrusion Moulding

3) Compression Moulding

4) Blow Moulding

Plastic Production Process

INJECTION MOULDING PROCESS

Step 1: Plastic granules are melted down and with screw injected into

the mold.

Step 2: The mold is held under pressure (hold pressure) until the

plastic material cools and hardens.

Step 3: Once the material ( plastic ) hardens, the mold is opened and the

part is removed from mould.

Step 4 : Mould again closed and the process can be repeated.

EXTRUSION MOULDING PROCESS

Process that supplies a continuous stream of thermoplastic material with equally cross section where it is directly produced through a shaping tool or to some other subsequent shaping process(dies) placed directly on the end of the extrusion machine.

Extrusion Moulding Process

Extrusion Moulding Process

Working principles-

1.The pellets or resins are inserted into the hopper

2.The material then fed into the heated cylinder by rotating screw

3.When the material become softer,it will be forced continously by the rotating screw ram.

4.The products or outcomes are formed into continous shape.

5.After the product exiting from the die,it will be cooled by air,water or become cold by interact with cold roll surface where then it will be solidifies while rolling

6.Extruded product such as pipe,rod,etc,extrude this way because it can be curved or bended after extruded with hotwater by sinking it in it.

Extrusion Moulding Process

Extrusion Moulding Product

COMPRESSION MOULDING PROCESS

Compression Moulding

Compression Moulding

Compression Moulding

Compression moulding is principally used for thermosetting plastics.

Preheated resin is placed into a hot mould cavity. The upper section of the mould is subsequently forced down onto the resin to create the desired product shape. The applied pressure and heat forces the liquified polymer to fill the cavity.

Compression Moulding

Following the compression, a period of heating is required to force cross-linking of the thermosetting polymer.

Throughout the process heat and pressure are maintained until the polymer has cured.

Compression Moulding Process

Compression moulding is a high-volume, high-pressure plastic moulding method that is suitable

for moulding complex, high-strength objects.

And with its short cycle time and high production rate, many organizations in the automotive

industry have chosen compression moulding to produce parts.

BLOW MOULDING PROCESS

Working principle for extrusion blow moulding method :

The resins or pellets are inserted into the hopper,

With gravity manipulating, the materials enter the injection or extrusion chamber or cylinder which surrounded with heater,

When the materials become softened ( plasticizers ), it will be injected or extruded using screw rotator or torpedo plunger,

The hot and softened material is called a parison.

The materials will be flow straight to the nozzle and enter the mould cavity.

Top of the parison will be cut off using a blade,

The air then blowed into the parison through a cavity at the top of the mould,

The plastic will be cooled, then solidified in the mould under water system cycling in the channels in the mould,

After that the mould and the product will be retrieved,

The connecting opener will retrieved the product using ejector pin to eject the product out the mould.

The resins or pellets are inserted into the hopper.

The materials are heated and become softened. Then it will be injected by using screw rotator.

The materials will be flow straight to the nozzle and enter the mould cavity.

Top of the parison will be cut off using a blade. The air then blowed into the parison through a

cavity at the top of the mould.

The plastic will be cooled, then solidified in the mould under water system cycling in the

channels in the mould. After that the mould and the product will be

retrieved,

Parison (blue) Blown bottle (blue)

Plastic engineering application

Advantages

1. Excellent electrical insulation properties

2. Transparent and translucent

3. Easy coloured

4. Ease of fabrication

5. Good wear and scratch resistance

Disadvantages

1. Plastics produces toxic fumes when it is burnt

2. Low heat resistant

3. Softer and less elastic than metal

4. Brittle at low temperature