The reflective shine on the back of a DVD is a phenomenon called Birefringence.
What is Birefringence and why does it happen?
Birefringence is the “bending” or refraction of a light wave into two different polarized waves. In plastics, this effects the light transmission, or the clarity of the image being transmitted through the part. This refraction is caused by the residual stresses in the part after injection molding.
When the light strikes a planar surface, the light is divided into two separate components (an “ordinary” and an “extraordinary ray”). This is found in materials having two different indices of refraction in different directions (i.e., when light enters certain transparent materials, such as calcite, the beam splits into two beams which travel at different speeds). Birefringence is also known as double refraction.
What does this have to with DVD vs. Blu-Rays?
When thinking of a laser reading from an optical disc, the importance of this issue with regard to DVD discs is apparent. Due to stresses in the plastic of a DVD disc, this distortion of light can be produced and cause issues. For this reason, birefringence must be kept within specification and is tediously measured by disc manufacturers.
The Blu-ray disc on the other hand, overcomes DVD reading issues by placing the data on top of a 1.1mm thick polycarbonate layer. Having the data on top prevents birefringence and therefore readability problems. In this scenario, the birefringence could be removed, however in many applications that exist today, especially ones that involve plastic parts that are injection molded for optical devices, birefringence cannot be removed altogether. This makes it an important criteria that needs to be tested and accounted for during production. Typically, birefringence can be reduced by modifying the part geometry which in turn would require costly mold reworks.
How do we fix it?
Technically, there is no real way to “fix” birefringence. In today’s injection molding industry, a process is added by using a polarimeter on the actual part. Depending on the results, the part geometry or the cooling lines on the mold are revised, sometimes both. The problem is that this would take a few iterations, since there is no real straight shot approach to get the right injection molded part, with an acceptable level of birefringence. The use of a physical prototype with trial and error works, but it also comes with an increasing number of scraps and multiple mold reworks that can add up in cost. The goal is to find a way to predict birefringence before cutting steel (for the mold)… and the solution is SOLIDWORKS Plastics.
SOLIDWORKS Plastics uses the Stress-Optical law to predict birefringence. This allows users to evaluate two types of Birefringence; Flow Induced Birefringence and Thermal Induced Birefringence.
Flow Induced Birefringence is created by Flow induced residual stress and Thermal Induced Birefringence is created by Thermal induced residual stress. This essentially means that SOLIDWORKS Plastics allows the user to see birefringence at different stages during the injection molding cycle, such as birefringence at filling end, packing end, and after shrinkage or warping.
If you’re interested in learning more about predicting birefringence in SOLIDWORKS Plastics, register for our webinar on April 26th at 12pm to learn how to setup, run and analyze the birefringence results from injection molding plastic parts, all within SOLIDWORKS.