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Processes to Make Vacuum Flasks

Processes to Make Vacuum Flasks

The core component of the vacuum flask is the liner. There are 4 steps to make a glass liner:


1. Preparation of vacuum flask


The glass material applied in vacuum flask production is commonly used soda lime silicate glass. We take uniform high-temperature glass liquid without impurities, and separately blow it into inner and outer vessels with a wall thickness of 1~2mm in the metal mold.


2. The glass liner of the vacuum flask


Production process: put the inner bottle into  the outer outer; seal the mouth of the bottles together; arrange a conduit at the bottom of the outer bottle for silver plating and air extraction. Glasses in this structure is called a glass liner. There are mainly three methods to make glass liners: bottom-pulling sealing, shoulder-shrinking sealing and waist-jointing sealing.


(1) The bottom-pulling sealing method: cut the top of the inner vessel and both the top and the bottom of the outer vessel; put the inner one into the outer from the bottom; fix them with an asbestos pad; seal the bottom of the outer bottle; connect it with small tail pipe; and melt and seal the mouth of the two bottles at last.


(2) The shoulder-shrinking sealing method: cut the top of the two vessels; put inner bottle into the outer one from its top; fix it with an asbestos pad; shrink the upper end of the outer vessel into the shoulder; seal the two vessels; and connect the small tail pipe at last.


(3) The waist-jointing method: cut the top of the the two vessels;split the outer bottle into two sections from its waist; put the inner bottle into the outer one; re-weld the waist; and connect the small tail pipe at last.


3. Silver plating for vacuum flasks


A certain amount of silver ammine complex solution and aldehyde solution as a reducer are poured into the interlayer of the glass liner through the small tail pipe; Thereafter, the silver mirror reaction is carried out. The reduced silver ions will deposit on the surface of the glass, forming a mirror-like thin layer of silver.


4. Vacuum the flask


Connect the tail pipe of the silver-plated double-layer glass liner with the vacuum system, and heat it to 300~400℃, so that the glass can release gases and residual moisture clinging to it. Meanwhile, pump out air with a vacuum pump till the vacuum degree of the interlayer space reaches 10-3~10-4mmHg. Last, melt and seal the tail pipe.


The modern vacuum flask was invented in 1892 by British physicist Sir James Dewar when he was conducting a research on liquefying gas. To liquefy gas at low temperature, he first needed to design a container that could isolate the gas from the outside temperature. Therefore, he asked glass technician Berg to blow a double-layered glass for him. After painting the inner walls of the two vessels with mercury, and then pumping out the air between them, a glass vacuum bottle was built. We later call this vacuum flask Dewar bottle which can retain the temperature of the liquid contained in it for a certain period regardless of whether it is cold or hot.

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