How is mineral insulated cable made ?

Mineral insulated thermocouple cable is used to insulate thermocouple wires from one another and from the metal sheath that surrounds them. The MIMS cable has two or four thermocouple wires running down the middle of the tube, the tube is then filled with magnesium oxide powder and compacted to ensure the wires are properly insulated and separated.

It has some advantages: first, the response time is fast, the minimum number of milliseconds can be reached, which is suitable for the detection of microcomputer control; second, the chemical stability is good and the deviation of the indexing drift is small after using for a long time, thus due to long life; third, the mechanical properties is good which can withstand strong vibration and impact; fourth, the high pressure resistance is greater than 3000MPa; fifth, the insulation properties is good under high temperature conditions; sixth, it is suitable for measuring temperature and automatically controlling temperature under the condition of high pressure, high temperature at the same time; seventh, it can be bent freely with small outer diameter, suitable for complex, narrow bending, miniaturized occasions to measure and control temperature; therefore, mineral insulated cable has been widely used.

The manufacturing process of mineral insulated cable is combined the bare thermocouple wire, insulating material (generally magnesium oxide powder) and sheath tube to form a solid body. After the assembly, the drawing machine is used (chain puller and cylinder puller) toreducethe diameter gradually from the maximum diameter of 23mm to 8mm and the minimum diameter is reduced to 0.25mm. During each diameter reduction process, the metal is compressed and deformed, a deformation stress is generated, the cable will be broken if the deformation stress is not eliminated. In order to eliminate the deformation stress, it is necessary to anneal in a heat treatment furnace to restore the metallographic structure, and the reduction process is continued until the desired outer diameter is reached.

Since the requirement of material accuracy is very strict,currently we imply box furnace, the belt type annealing furnace or the short circuit annealing. But the results does not reach the stability process requirement, so the high grade cable can not be come out.

We provide a method for manufacturing mineral insulated cable with gas shield to achieve the goal of high quality and low production cost. The thermocouple wire is inserted into the prefabricated hole of the insulated material and the metal sleeve is sleeved on the outside of the insulating material, its diameter is reduced and heat treated to reach the outer diameter meeting the requirement.

The thermocouple material is heated and cooled equally, uniformly, so that the entire wires can be reached to data requirements by continuous heat treatment. By gas protection and continuous heat treatment, the qualified product rate is above 70%, so the production efficiency is increased and the cost is reduced.

When feeding the qualified thermocouple wire is first straightened and cleaned, to be penetrated into the hole of pre-made Mgo powder porcelain column, then is inserted into the inner hole of the metal sleeve, the diameter reduction process will bestartedafter the assembly is competed. The diameter of 23mm to 16mm is rolled into the rolling point machine, and then penetrated into the mold to reduce the diameter on the chain drawing machine until the diameter is reduced to 8mm, the roller drawing machine is used to reduce the diameter to 3mm instead when the diameter is less than 8mm,the vertical drawing machine is used to reduce less than3mm until 0.25mm. After each reduction, hardening occurs due to metal deformation stress, which need to be softened by heat treatment to restore the metallographic structure. After the heat treatment, the reduction need to be continued to the required diameter. The protective gas in the heat treatment is usually a mixed gas of nitrogen or hydrogen, so the bright surface can be reached.

The temperature of heat treatment is also critical, its range is 750-1200C and the temperature intelligent meter is used to automatically control the display and control temperature.

Whether the heat treatment step using gas protection is related to the surface quality of mineral insulated cable. Because the MIMS cable is a precision material used in automated instruments, its surface quality is very important, not only its appearance does not allow metal spalling, pitting or pulling defects, but also the outer surface brightness to reach the mirror surface.

If the gas protection is not used, the metal will be generated an oxide layer above 750C, the surface of mi thermocouple cable will cause metal peeling and pitting on the surface. At the same time, due to repeated heat treatment, the outer thickness is thinned, which affects the mechanical strength that reduces the service life. Therefore, the whole process of heat treatment must be continuous and protected by gas.

In the above mentioned reduction treatment, the diameter is gradually reduced from a large diameter of 23mm to a minimum of 0.25mm, and molds having different apertures are to be disposed. In order to enable the mi thermocouple material to penetrate into the hole of the mold, the tipping machine must be designed according to the different specifications. According to the reduction ratio, the tipping machine has multiple holes which require a reasonable compression ratio and rolling. The surface is smooth and no burrs to avoid pulling the mold, The chain puller uses pneumatic automatic clamping device and trolley automatic shuttle device to reduce labor intensity and increase the production efficiency.

During the drawing process, the drawing die and the thermocouple material need to be well lubricated so that the drawn workpiece is sufficiently lubricated in the die hole. If the lubrication is poor or the inner hole of the mold is not smooth, the surface of the coupler is roughened, so a lubrication device is also required.

From the diameter of 23mm to the minimum of 0.25mm, the type and quantity of drawing machines and molds can be designed and configured according to the specific conditions of the production site and equipment. get more information from www.super-instrument.com.

For more special applications, it is recommended to consult our professional engineers, and we can provide more specific suggestions and solutions for you.