PROCESSING OF PLASTICS

PRESENTED BY: -

AMIT KUMAR

ME – A1

MECHANICAL DEPARTMENT

PLASTICS : - Plastic is a material consisting of any of a wide range of synthetic or semi-synthetic organic that are malleable and can be moulded into solid objects of diverse shapes.

Plastics are typically organic polymers of high molecular mass, but they often contain other substances.

They are usually synthetic, most commonly derived from petrochemicals.

The world's first fully synthetic plastic was Bakelite, invented in New York in 1907 by Leo Baekeland who coined the term 'plastics‘.

TYPES OF PLASTIC : -

1.THERMOSETTING PLASTIC: - The molecules of thermosetting plastics

are heavily cross-linked. They form a rigid molecular structure.

Although they soften when heated the first time, which allows them to be shaped they become permanently stiff and solid and cannot be reshaped.

Ex. Polyester resin and urea formaldehyde

Cross-linked molecules

Contd.

2. THERMOPLASTIC: -

Thermoplastics can be heated and reshaped because of the ways in which the molecules are joined together.

The molecules of thermoplastics are in lines or long chains with very few entanglements.

When heat is applied the molecules move apart, which increases the distance between them, causing them to become untangled. This allows them to become soft when heated so that they can be bent into all sorts of shapes.

Ex. Polyvinyl chlorides(PVC), Nylons, etc.

Contd.

individual monomer molecule

PROCESSING OF PLASTICS: -

1.COMPRESSION MOULDING

2.TRANSFER MOULDING3.INJECTION MOULDING4.EXTRUSION

MOULDING5.BLOW MOULDING

6.CALENDARING7.THERMOFORMING8.ROTATIONAL

MOULDING9.LAMINATING

1. COMPRESSION MOULDING : -Compression moulding is the forming process.

Compression moulding process is followed by two step first one preheating and pressurizing.

Measured charge of compound placed between halves of split mold

Pressure is applied so that compound flows to fill mold cavity

Compression moulding is generally used for thermosetting plastic.

Example:- urea formaldehyde and phenol formaldehyde.

PROCESS DESCRIPTION

PARAMETERS : -The quantity of charge ( moulding material ) put into the mould.

Pressure of the moulding process:- Range of pressure 2000-3000 psi ( 13.8-20.7 mpa ).

Mould temperature:- Temperature range 300ºf to 375ºf ( 149⁰C- 191⁰C )

Cure time variables:- The period required to harden thermosetting material to partial and

complete polymerization is called cure time. Cure times typically range from 1 - 5 min

2. TRANSFER MOLDINGS : - It is similar to compression moulding.

Thermoset polymers can be formed when a preset amount of material is placed in a separate cavity and heated.

A plunger moves the material into the shaped mould with high pressure.

3. INJECTION MOULDING : - It is a manufacturing technique for making parts

from thermoplastic and thermoset materials . Injection moulding is a highly automated production

process for producing large quantities of identical items.

A measured amount of molten thermoplastic is driven by a ram past a heating system into the mould.

The mould is split to allow finished object to be removed after cooling.

Example : - Polystyrene, Nylon, Polypropylene, PVC, Acrylonitrile Butadiene Styrene

PROCESS : - Plastic powder are fed from a hopper into a hollow steel barrel

 which contains a rotating screw. The barrel is surrounded by a jacket of heaters which melt the

plastic material. The screw is forced back as the melted plastic collects at the end

of the barrel. Once a sufficient charge of melted plastic has accumulated a

hydraulic ram forces the screw forward injecting the thermoplastic through a sprue into the mould cavity.

This one is capable of exerting forces of up to 250 tonnes. Pressure is kept on the mould until the plastic has cooled

sufficiently for the mould to be opened and the component ejected.

4. EXTRUSION MOULDING: - Continuous process  used to produce both solid and hollow

products that have a constant cross-section. Ex: - Window frames, hose pipe, curtain track, garden trellis. Thermoplastic granules are fed from a hopper by a rotating screw

through a heated cylinder. The tapered screw compacts the plastic as it becomes elasticised.

The die which is fitted to the end of the extruder barrel  determines the cross-section  of the extrusion.

Thicker cross-sections are extruded more slowly as more time is required  for the initial heating and subsequent cooling of the larger quantities of material which are involved.

As the extrusion leaves the die it is cooled  by passing through a cooling trough containing cold water.

5. BLOW MOULDING: - Blow Moulding is an automated  process that is used extensively to make

bottles and other lightweight, hollow parts from thermoplastic materials. A hollow length of plastic, called a parison, is extruded down between the

two halves of the mould. The mould closes Compressed air is blown into the inside of the parison which inflates it,

pushing the soft plastic hard against the cold surfaces of the mould. The mould is then opened the moulding ejected and the waste (called

flash) is trimmed off with a knife. Example : - high density polythene and low density polythene are used.

1. 2.

3. 4.

Blow Moulding Process

Extrusion Blow

Moulding

Injection Blow

Moulding

Stretch Blow

Moulding

6. CALENDARING : - It is a intermediate process where the extruded plastic sections are

reduced to sheet.

Only used for thermoplastics.

Involves rolling out a mass of premixed plastics material between large rollers to form a continuous and  accurately sized film.

The process begins with the ingredients being blended and fluxed in a mixing mill at approx. 100°c.

Nip rollers control the thickness of the sheet material can be gradually reduced in thickness.

Rolls of semi-rigid PVC which will be used to manufacture transparent A4 folder 'pockets'.

Example: - 1. Flexible PVC :- Rainwear, Shower curtains,

Tapes, etc.2. Rigid PVC: - Credit cards, Lamination

7. THERMOFORMING: - Process used to shape thermoplastic sheet into discrete parts.

Basic principles: -

i. Heat a thermoplastic sheet until it softens.

ii. Force the hot and pliable material against the contours of a mold by using either mechanical, air or vacuum pressure

iii. Held against mold and allow to cool, and plastics retains its shape

This can be done by: -vacuum and pressure forming.

Lower pressures are required to thermoform.

8. ROTATIONAL MOULDING : - Rotational moulding is a process used for producing hollow plastic

products.Such as road cones and storage tanks up to 3m³ capacity.

A measured weight of thermoplastic is placed inside a cold mould.

Heated to a temperature of 230-400 c whilst being rotated around both vertical and horizontal axes.

The mould moves into a cooling area  or chamber where it is cooled by air or water jets.

The hollow moulding can be removed as soon as it is cool enough to hold its shape.

The rotational molding (roto-molding or roto-casting) process. Trash cans, buckets, and plastic footballs can be made by this process.90% of rotational mouldings are made from polyethylene

9. LAMINATING: -

Layers of materials are bonded with a resin into a strong solid structure, often with heat and pressure.

This can be done in following ways: -

Gravure coating

Wet bond laminating

Dry bond laminating

Solvent less laminating

Extrusion laminating

Example: -Foil, Snack food bag, Saran- coated nylon, Metallized propylene, etc.

Extrusion d ie

Ch ill ro ll

Substra te A

Substrate B

Strip -o ff ro ll

P ressure ro ll

N ip ro ll

Extrudate Three p ly lam ina te

Extrusion laminating Dry bond laminating

Gravure coating