Canning and Fish Packaging Technology

Unit 14 - Glass containers and Closures

14.1.8.Physical properties of Glass Bottles

• Mechanical strength increases with the thickness of the wall whereas thermal resistance decreases. It is always better to make the bottom about twice the thickness of the sides and the thickness is slowly tapered off. Sudden changes in the wall thickness increase the stress concentration and reduce the strength. Limiting value of tensile strength is about 5000 psi.
• Surface damage or scratching weakens the strength.
• Resistance to internal pressure up to 1000 psi bursting pressure in new bottles.

This reduces with use and handling, as given below.

Bursting pressure	New Untouched	New Delivers to user	After use
	1000 psi	600 psi	400 psi