Small Containers

The widespread use of polymers for containers led to many premature failures of the thin walls, often caused by poor design such as sharp internal corners and poor moulding practice. Older battery containers were made from ebonite or hard rubber, and needed thick walls, but failed because of the inherent weakness of the material. A new patent led to much thinner walls and polypropylene is widely used for cases. A low density polyethylene bucket fractured when a weld line failed on a lug, injuring the user. Hydrogen explosions in lead–acid batteries are caused by short-circuits within the battery, but can often be detected before failure by swelling of the case sides caused by internal gas pressure. Fractures in truck cases were caused by poor design and moulding. Failures in polycarbonate miners' battery cases were caused by solvent welding and frozen-in strain from cold moulding and were minimized by changing to transparent polymer with quality control using strain birefringence. A new polishing method was developed to section case parts and moulding conditions closely monitored. Internal stress concentration occurred at corners, lowering drop impact strength. A small lithium cell exploded prematurely causing a serious eye injury. Inspection of the remains showed that a critical plastic seal showed brittle cracking across the radius and sub-critical cracking with melting of some corners, implying that the cracks were present before the explosion. A short-circuit likely caused the explosion and the battery cell was faulty. The electrolyte is based on an organic fluid and is highly inflammable.