Canning and Fish Packaging Technology

Unit 14 - Glass containers and Closures

14.1.5.Manufacture of glass

• Raw materials are kept in the furnace and fused at 1500 to 1600^oC. In the furnace, temperature, glass level, chemical composition, homogeneity and colour are to be maintained. From the furnace mass of glass goes to the feeder.
• Hot glass emerging from the orifice is cut into gobs of correct mass (required for 1 bottle) by a power driver sheers. In this operation, control of temperature, viscosity and glass mass are important.
• The gob is then taken up by the forming machines, neck or finish is pressed or moulded and it is blown to an approximate shape of the bottle (Parison). Then it is transferred to a second mould and blown to final shape.
• Control of manufacturing process and inspection of moulds are important.
• After this the glass bottles are annealed by heating to 600^oC and by cooling at very slow controlled rate to avoid stresses which might otherwise lead to shattering.
• During the annealing process usually hot end and cold end surface treatments are given, At the hot end, vapour or atomized spray of SnO₂ (Stannic oxide) or other metallic oxides like SiO₂ is given to reduce defects or flaws.
• At the cold end it is sprayed with synthetic waxes such as polyoxyethylene stearates which reduce the friction (impact) between the bottles.
• Sometimes glass containers are annealed in the atmosphere of SO₂; SO₂ which reacts with glass to form sodium sulphate and thereby improving the strength.