GLass Expert Discusses Chill Cracks

Glass failure analysis expert witness discusses a common glass bottle manufacturing defect. After the bottle has been formed it is removed hot from the clam shell tool. If it encounters any localized cooling at this stage, it can form a manufacturing defect called a "chill crack". Thermal shock cause the cooled area to shrink, and this causes localized tensile stresses. This event can create a small crack that can cause later failure of the bottle. Above are two photos of the same chill crack. Upper left is a 20X photomicrograph, and upper right is a 40X photomicrograph. To start, this crack is less than 0.1" long, and it has grown to be approximately 0.7" long. The bottling line has had numerous failures from this lot of bottles, and the cause has been traced back to this type of chill crack.