Question

Name the processes used for making of (a) plastic bottles such as those used for storing 1 litre shampoo, (b) plastic top cover of a plain paper copier (photostat machine), and (c) plastic buckets of 20 litre capacity "Explain the various methods (only the principle) available for injection moulding of plastics. Give the comparative advantages and applications for the same "What are the different types of moulds used in injection moulding? Give their relative applications. *What are the two types of polymerisation methods in plastics?

Answer 1

Answer

a), b), "c) for all this applications the manufacturing process used is Injection Moulding. Using this process we can manufacture variety of products.

Principles of Injection Moulding

Injection Molding Machine Heater Mold Cavity Plastic GranulesHopper Mold Moveable Platen Reciprocating Screw Barrel Nozzle lmping Injection

The sequence of events during the injection mould of a plastic part is called the injection moulding cycle. The cycle begins when the mould closes, followed by the injection of the polymer into the mould cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mould opens and the part is ejected.

Advantages of injection moulding process are:-

• Fast production.
• Low labour costs.
• Design flexibility.
• High-output production.
• Multiple materials can be used at the same time.
• Can be used to produce very small parts.
• Leaves little post-production scrap.
• Ability to include inserts.
• Good colour control.
• Good product consistency.
• Reduced requirements for finishing.
• Good dimensional control

Applications of injection moulding are:-

• Aerospace components
• Automotive components
• Avionics components
• Cable assemblies
• Computer electronics
• Electronics components
• Encapsulations
• Engineering prototypes
• Geophysics
• Instrumentation
• Marketing samples
• Material quality testing
• Medical & dental products
• Medical laboratories
• Model shops, toys, hobby
• New product design & development
• R&D labs
• Test specimens

Different types of mould used in injection moulding are :-

Insert Molding
Like overmolding, insert molding is an injection moldingprocess that combines two or more components into a single finished product. Insert injection molding inserts a component into the injection mold cavity and plastic material is filled around the insert. Inserts can add strength to a product and eliminate the need for additional parts to reduce the product weight.

Cold Runner Injection Molding
Cold runner molds use a sprue to fill the runners that inject plastic resin into the mold cavity. In 2 plate molds, the runner system and parts are attached, and an ejection system may be used to separate the pair from the mold.The cold runner can reduce waste by recycling and regrinding the material, but this can also increase the total cycle time. Cold runner systems can be used for a wide variety of polymers and can allow for easy color changes.

Hot Runner Molds
Hot runner molds use a manifold to heat melted plastic resin and then send the material through a gate to fill the mold cavity. The two main types of hot runner molds are externally heated and internally heated. The externally heated molds can be used with polymer that is less sensitive to thermal variations while internally heated hot runner molds allow for better control of material flow.

Because hot runner molding does not require the use of runners, potential waste material is reduced and the recycling and regrind and process of virgin plastic does not impact the total run cycle time.

Types of polymerisation methods are :-

Addition polymerisation, also known as chain growth, requires the activation of monomers so that they may link together.

НН НН pressure _C_Ć catalystH Hn nĢĆ heat H H Ethylene monomer Polyethylene

If we take an ethylene monomer, for example, we know that it has a C=C double bond in its centre. Opening up this double bond means that other monomers can attach to the free electrons on either side of the monomer.

The general reaction for the ethylene monomer to polyethylene is illustrated on the right.

The n is termed the degree of polymerisation, DP, and is equal to the number of monomers reacted to make up the chain.

Addition or chain polymerisation can be divided into 3 steps – initiation, propagation, and termination.

Condensation Polymerisation

In condensation polymerisation, or stepwise growth, two different kinds of molecules react to form a larger molecule. These molecules react further with other molecules. The process is repeated until long polymeric chains are produced. A by-product, which is usually H2O, is always formed from the reactions, hence the term condensation.

The resulting molecular chains are regular and symmetrical and therefore tend to take on a crystalline structure.