Plastic sheets are an inevitable part of every vertical. Be it automotive, electronics, furnishing, aerospace etc. For them, maybe the sky is the limit. Their products are manufactured in our world at an increasing rate and are used in all possible applications. From domestic to industrial, they are the most sustainable and economical option. Like very best things in the world, plastics delivering so many products must contain quality in its processing methods. And maybe many of plastic manufacturers seems to be operating the advanced level of quality testing upon them. But not every time, the flaws are those which can be seen or which deforms the look and feel of the product. Some defects are hidden and come out under special conditions. We are talking about the residual stress here! In this blog section, we will talk about the generation and effects of stress distribution in plastic sheets and methods of detecting them using Polariscope.
Residual Stress and Its detection
The manufacturing processes of Plastic itself is the reason behind the formation of residual stress. It occurs when hot molten polymer substance is cooled down and shaped. This happens due to differential cooling rates, non-uniform temperature or flow rate, material contaminants, mechanical loading, thermal cycling during transport and storage, inadequate or non-uniform annealing, and sharp corners or protrusions that cause localized stress concentrations.
Sometimes, these stresses are intentional, the manufacturers want to add some stresses to impart desirable characteristics, such as in oriented PET films and bottles. But, in most of the cases, is a nob desirable problem occurred in manufacturing unit. These stress in the product can lead to its failure in quality and performance. High levels of stress also lower a product’s impact strength and can contribute to environmental stress cracking and diminished performance at high or cycling temperatures.
To prevent this issue and improve the production quality, a very precisely designed testing instrument such as Polariscope is required. It is a highly effective testing device which is used to measure the residual stresses and strains that are introduced during the manufacturing process of the products that severely affects the strength of the product. It is a type of PET preform testing equipment and is widely used by all forms plastic sheet manufacturers.
It works on the principle of photoelasticity. In the test, a special source of light is used and it is allowed to pass the product. The change in the refractive index of those material having non-uniform strain distribution will appear as multi-coloured patterns. These patterns then can be compared with a standard defect chart and analysed.