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PLASTICS

Dr. K. SIVAKUMARDept. of Chemistry, SCSVMV University

Course: B.E (EEE) – Part Time Subject: APPLIED SCIENCE Unit: V

Syllabus: PlasticsThermoplasticsThermosetting plasticsProcessing of plasticsInjection mouldingBlow mouldingExtrusion processes

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THERMOPLASTICS &

THERMOSETTING PLASTICS

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Thermoplastics (resins): Thermoplastics soften when heated and subsequently melt. When cooled they become hard and rigid once again.Example: Polyethylene, PVC, NylonUses: Bags, Mugs, Toys

Thermosetting plastics (resins): Thermosetting resins do not become softy on heating & they never melt once set.Example: BakeliteUses: Electric switches, Telephone parts, Cooker handles

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Moulding constituents of plastic:Higher polymer material is mixed with 4 to 10 ingredients each of which imparts some useful property to the moulded products. Some are,Resins: Holds the different constituents togetherExample: Linear polymers with low molecular weights.Resins form 40-50% of moulding mixturePlasticisers: To increase the plasticity and flexibilityExample: Castor oil, Dibutylphthalate, Alkyl & Aryl Phosphate etc.,

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Fillers: To impart valuable service properties such as strength, resistance, finish and workability.Reducing cost, shrinkage on setting and brittleness.Example: Wood flour, asbestos, china clay, talc, gypsum, saw dust.Fillers form 60% of plasticsLubricants: To facilitate the moulding operations. Gives glossy finishExample: Waxes, soaps, Oleates, stearatesCatalyst/accelerators: To accelerate the polymerisation (added only in thermosetting resins) Example: Hydrogen peroxide, Benzoyl peroxide, Metals (Ag, Cu & Pb)

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Difference between Thermoplastics & Thermosetting plastics

S.No. THERMOPLASTICS THERMOSETTING PLASTICS

1. Formed by addition Polymn. Formed by condensation Polymn.

2. Long chain linear polymers Three dimensional network structure joined by strong covalent bonds

3. Soften on heating and stiffen on cooling

Do not soften on heating

4. Can be remoulded Cannot be remoulded

5. Usually soft, weak & less brittle Usually hard, strong & more brittle

6. Can be reclaimed from wastes Cannot be reclaimed from wastes

7. Soluble in organic solvents Insoluble in organic solvents

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INJECTION MOULDING (for Thermoplastics):•Materials of definite quantity at Hi Temp. is injected into a cold mould under P•In cold mould, the polymer solidifies•Polymer is fed as granules, pellets / powder through hopper•Polymer get softened in horizontal cylinder•Hydraulic piston pushes the molten material into mould •Uniform spreading inside the hot cylinder is ensured by torpedo•Molten plastic is injected through a nozzle into moulded cavity

Mould:•Consists of movable & stationary parts

•Molten plastic is injected at a P of 15000kg/cm2

•Proper flow of material is ensured by preheating the mould to appropriate T

•Mould is cooled by circulating water and then opened to eject the moulded article

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BLOW MOULDING (for thermoplastics); [for hollow plastic articles]•To produce soft drink bottles, containers using Polyethylene, PVC, Polystyrene•PARISON – hot softened thermoplastic made by injection / extrusion moulding•Parison is placed inside a two piece hollow mould (as 2 halves)•Parison kept between two halves encloses a blowing pin at one end •Other end pinches and closes•Parison is blown using pressurized air through the blow pin

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BLOW MOULDING: Continued……•Parison kept under hot condition is inflated and expands until it comes in contact with inner area of the cold hollow mould•Under pressure •Parison acquires the shape of hallow cavity of mould•Mould is cooled and the thermoplastic article is removed from the mould by opening the mould

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EXTRUSION MOULDING: •For producing common plastic materials•eg: films, filaments, hoses, sheets & coating wires and cables with PVC •Plastic material as powder / granules is fed into a cylinder thru hopper•Cylinder is with the provision for heating•Hot plastic is pushed thru cylinder by helically revolving screw•Due to friction the Temperature increases

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EXTRUSION MOULDING: (Continues….) • Plastic material passes through three zones1. Feed zone2. Compression zone3. Metering zone

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EXTRUSION MOULDING: (Continues….) • In compression zone, powder charge is melted by heating element• It is compressed by revolving screw• Polymer flows at a constant rate due to helical movement of screw• Pressure in this zone enables plastic to be in melted stage & enters the die

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EXTRUSION MOULDING: (Continues….) • From die moulded plastic is coming out in desired profile• Extrudated material from die is hot and hence it is cooled rapidly to

avoid any deshaping• Cooling by passing extrudate over a conveyor belt through which

tubes of cold water/cold air are passed

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Good Luck!

Dr. K. SIVAKUMARDepartment of Chemistry

SCSVMV Universitychemshiva@gmail.com

+91 98423 61378

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