How Can Internal Strain of Glass Be Test?

 The glass is widely used as packaging material in industries where non-reactive properties of the materials are required. In the pharmaceutical industry, in chemical industries, processed food, etc. are some common examples. Glass has very brittle nature and due to internal strain, it may show some volatile behaviour. Defective glass bottles may rupture unexpectedly and may remain silent. You cannot predict. But we cannot take the risk as broken glass pieces are dangerous, especially if the packing is being used domestically. To avoid any product failure, it would be better, if the packings are tested prior filling and circulating.

What is a Polariscope and how does it work?

Polariscope strain vieweris an optical device having 2 Polaroid lenses or sheets. The sheets are designed in such a way that electrons allow only parallel lights to pass through it. All other rays will be absorbed by the electron which is aligned in a parallel line. It has 2 parallel sheets and an incident light source. The Polaroid sheets have parallel lines along their axis. This is why the sheets allow the light to pass through if they are in-line with the incident light. Thus, a test sample is free from defects, the sheets allow the light to pass through it. On the other hand, if there is any defect, the light would get refracted double and electrons will absorb that light. This absorption of light creates black isoclinic impression or birefringence on the other side.

Polariscope strain for Glass and Plastics – PSPL G20

PSPL G20 is the Presto’s latest model launched for analysing defects in plastics and glass samples. The best thing about this model is that it can test the final product. Glass items like drinking glass, decorative pieces, beakers, test tubes etc. can be easily tested. The operator just needs to put the sample between polaroid lens and light. If there is any defect, the light would refract from its path and birefringence effect can be seen. The diameter of the field view is 190 mm. The sample needs to be tested from different angles. It has a provision that body of the polariscope can be rotated to get the perfect angle.

Polariscope for plastic preforms – PPS 400

This device is specifically designed for testing defects in preforms. In PET preforms, the light would refract double through a strained area. In this model, there are two slots to keep the sample. One is for 45° and other is for 90°. Different type of defects creates different birefringence pattern. Thus, to understand the defect type in a right manner, it is important to observe the sample from different positions. The viewing area of the device is 260mmX240mm. It has two type of incident lights, monochromatic and sodium lamp.

This model is designed with computer compatibility and uses software to provide pictorial views and images. Data logging and recalling are possible which helps in creating a report for the entire batch of preforms. It’s a table top model and weighs not more than 20kgs.

What is the importance of Strain viewing in glass? 

Quality check process is incomplete without testing process. Ridges and furrows are some common defects that are easily visible with naked eyes. It’s a form of external defects. Some internal strains are due to manufacturing defects like air bubbles, uneven thickness, etc. When pressure is exerted on strain points, they tend to crack. As glass is used for critical applications. We cannot take chance with their quality testing. Product failure during transit or storage may spoil other products in the lot. 

Another reason for incorporating defect identification is contraction and expansion due to climatic change. When decorative items are transported overseas, from extremely cold region to hot areas or vice-a-versa, they might experience thermal shock. From defective areas, the strength of items is less. Any minimum physical impact may result in damage to the whole item. This increases rejection and cost of transportation. If out of 50, 15 items fail during transit, that would lead to increase transportation cost.

Other applications of Polariscope

It is widely used in gemology industry to identify the defects in expensive gemstones. Based on the internal defect, the quality of gemstones is identified.

Polariscope strain viewer is very simple to operate the device. But it requires a very skilled operator to study and understand the pattern that is being formed. Presto offers 2 models for analysing internal strain in the sample.