Optical Lens Residual Stress Analysis

During the assembly process of the optical lens, when the lens is fixed by the fixing component, poor fixing often occurs, resulting in residual stress and poor optical imaging and changes in optical characteristics. Therefore, how to observe the internal residual stress of the optical lens is a very important and urgent problem in the optoelectronics industry.

In 1853, Maxwell proposed the law of stress optics, proposing that when a material is stressed, its optical refractive index will change. This law establishes the theoretical basis for photoelastic stress analysis. Transparent materials are widely used, such as panels, optical lens industries. In its production, assembly and operation process, it will inevitably cause residual stress, thereby reducing the quality of the product. To change this phenomenon, we must first use a stress meter to measure its stress. The photoelastic experiment method is the most suitable method to measure the global stress. When the finished optical lens is cracked or damaged in use, from the viewpoint of material science, this phenomenon means that the total stress value of the optical lens in the damaged area exceeds the physical strength value of the material itself.

The residual stress in injection molded products is mainly caused by two reasons: one is the molecular alignment caused by the flow residual stress in the filling stage; the other is the thermal residual stress caused by uneven shrinkage in the cooling stage. The flow residual stress is mainly caused by the high shear rate during the plastic filling flow, and the cooling and demolding void point after filling is continuously released or frozen. Thermal residual stress is generated by uneven shrinkage and density changes after the high temperature plastic material is cooled to the glass transition temperature.

To solve the problem of product damage in use, we should start from how to increase the physical strength of the material and how to reduce the stress value of the finished product. The stress on optical lenses can usually be divided into internal stress and external stress according to the source.

Birefringence occurs when transparent plastic and glass optical lenses are stressed. At this time, the incident polarized light will be divided into a fast beam and a slow beam, and the relative distance of the speed difference is called the phase difference or retardation. In a monochromatic photoelastic fringe, the thick line represents the point where the principal stress direction is parallel to the polarized light. Therefore, the phase difference between the two beams of light is an integer wavelength, resulting in bright and dark fringes in the light field, and the fringes in the light field can be observed. The denser the fringes, the greater the stress, that is, the place where the stress is concentrated, and where the failure of the material first begins. The sparser the fringes, the smaller the residual stress. Because the composition of plastics is a long-chain polymer, generally there will be some residual stress left in the product during the molding process, especially in the injection molding process. During injection, the high shear rate, fast cooling, small feed gate and other factors will cause the residual stress of the injection product to be relatively serious. Therefore, it is inevitable that plastics will be stressed in the processing process (different types of plastics have different stress levels). The direction of the effort is how to reduce the residual stress of plastic products.

In the following figures (a) and (b), a qualitative Polariscope is used to observe the transparent plastic optical lens, from which the sensitivity of different plastics to molding stress can be known. (c)(d)(e) are the stress observation images of the glass material optical lens. (c) is the image taken by the optical lens without any external force, which is evenly distributed; (d) is the image taken after the optical lens is equipped with a fixing ring. When irradiated by polarized light, the stress distribution presents a circular arc distribution; (e) is the stress distribution diagram obtained by fixing the four-point screw of the optical lens.

After research, we found that using a Polariscope to observe the residual stress of optical lenses is a relatively simple qualitative observation method, which enables optical lenses to be assembled on the production line to quickly obtain stress distribution information, and can quickly adjust the setting of molding parameters and reduce the number of finished products. residual stress and reduce the possibility of product damage.