The polariscope is an optical inspection equipment used to detect the internal stress of glass and other transparent materials (such as plastics, etc.). It is mainly composed of a light source and two crossed polarized lenses. The polariscope light source is installed under a lens and powered by an external power source. The material to be inspected is placed between two polariscope lenses and observed through the lens opposite to the light source lens. Polariscopes are manufactured in a wide range of configurations, from standard portable devices that are usually in stock to instruments customized for specific applications.
Some materials, such as glass and plastic, are usually not birefringent, but they become birefringent when they are subjected to external forces or internal stresses in the manufacturing process. These materials with internal stress will fail due to cracks or deformation. Improperly annealed glass may be subject to a certain degree of internal stress, so that a small external force (such as the external force applied when the glass lens are mounted in eyeglass frames) can cause the lens to shatter. Heat sealing and sonic sealing of transparent packaging (such as plastic packaging for pharmaceuticals and sterile medical products) can create stress areas in the seal. When sandwiched between the lenses of the poariscope, these stress areas can be seen as color bands or patches. The continuous color band on the seal indicates that the seal may be good. Discontinuities in the color band indicates that there may be an incomplete seal through which contaminants can enter the package. When viewing the material through a polarizer, the presence or absence of stress areas should not be interpreted as a final indication of improper or proper heat treatment or sealing. In these fields, the polariscope has not been established as a decisive analytical testing instrument.
Stress in glass products
In the process of glass production, the cooling of the glass after it is taken out of the tin bath is very important. If the glass ribbon is cooled too fast in the annealing furnace, or the temperature difference in the width direction is not uniform, the temperature difference will cause the glass to produce a lot of stress. Large stress will cause the glass to burst and difficult to cut in hot and cold conditions. In severe cases, the glass will break by itself during storage and mechanical processing. Therefore, the annealing of glass is to control the stress generated by the glass during the cooling process within the allowable value range.
The stress generated in the cooling process of glass is divided into: end stress (thickness direction) and plane stress (width direction); these stresses include temporary stress and permanent stress.
The end stress refers to the stress caused by the unavoidable temperature gradient in the thickness direction during the cooling process.
Plane stress refers to the stress caused by uneven temperature distribution in the width direction during cooling.
Temporary stress refers to the stress that exists in the glass with the existence of a temperature gradient and disappears with the disappearance of the temperature gradient.
Permanent stress refers to the residual stress after the temperature gradient in the glass disappears. The stress caused by the uneven chemical composition of the glass leading to the uneven structure is also a permanent stress. Obviously, this kind of stress cannot be eliminated by annealing.
The mechanism of plastic internal stress
Plastic internal stress refers to a kind of internal stress produced by factors such as the orientation of macromolecular chains and cooling shrinkage during plastic melting processing. The essence of internal stress is the unbalanced phenomenon formed during the melt processing of macromolecular chains. This unbalanced conformation cannot immediately return to an equilibrium conformation compatible with environmental conditions when it is cooled and solidified. The essence of this unbalanced conformation is one A reversible high-elastic deformation, while the frozen high-elastic deformation is stored in plastic products in the form of potential energy. Under suitable conditions, this forced unstable conformation will transform into a free and stable conformation. The change is released for kinetic energy. When the force and entanglement between the macromolecular chains cannot withstand this kinetic energy, the internal stress balance will be destroyed, and the plastic products will have stress cracking and warping deformation.