Glass Tube Sealing
Small diameter glass tubes are often used in the medical device industry to hold either an electronic/electrical circuit or a sensor that is to be placed inside the human body. Glass is an ideal material for such applications since it is transparent, inert to body fluids, insulating, can be hermetically sealed, and does not interfere with electrical signals. A small length of glass tube, cut to size by the tube manufacturer, is rinsed clean before sealing one end to form a small test-tube. The required components are then inserted into the tube through the open end; care should be taken to make sure that the free edge is not contaminated by any chemicals used to secure the internal components in place. The free end is then sealed shut to form a hermetic enclosure.
The sealing process requires the glass to be heated beyond the glass transition temperature so that it soften and is able to deform without excessive melting. Using a continuous beam of Nd:YAG laser light has proven to be an ideal tool capable of producing the right conditions to seal the glass tube without introducing any contaminants. In addition to being a non-contact process, the laser light can be absorbed uniformly throughout the transparent glass wall thickness thus producing uniform heating and avoiding thermal shock. The laser energy can also be controlled to produce gradual cooling to avoid any residual stresses. Residual stress state can be checked by dipping the sealed glass tubes in oil having similar refractive index and then viewing light passing through the tubes under a polarizer. Residual stresses present will appear as bright zones whereas a stress-free region will appear transparent.
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