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The technique of bonding different parts of ePTFE with a binder is only suitable for fluoropolymers that do not need to withstand large loads. On the contrary, other connection methods such as welding must be used.
Because it can not be melt processed, some complicated shapes or large-area splicing or joining of PTFE plates, cylinders, large-diameter pipes, etc., need to be processed by welding methods. Common welding methods include hot-press welding and hot-air welding. The latter is the most convenient way to process conventional thermoplastics, especially PVC.
1 hot press welding
Thermocompression bonding is a common method of joining ePTFE plates to plates. The welding parts of the two workpieces to be welded are placed between the metal welding knives, pressed up and down, and heated while being pressed tightly to fuse the welds. Long strip-shaped aluminum alloy welding knives can be used for welding between PTFE sheets. For aluminum PTFE sheets and ePTFE tubes, cylindrical aluminum alloy welding knives are used.
The welding method of the board and the board is lap welding and butt welding, and the welding method of the board and the tube is lap welding.
For lap welding, a small hole is made in a PTFE plate of 1 to 2 mm thick, and the plate is thermocompression-welded together with a flanged PTFE tube. This method can be used to make a container cover, which can also be used to attach a branch to the container as an inlet or outlet for the container. It is also possible to further manufacture a relatively complicated workpiece such as a reducer tee, a cross, or a communication tube on the side of the container. Of course, the diameter of the branch on the weld must be smaller than the diameter of the main pipe.
The quality assessment of the welding process is mainly to test the peel strength of the weld. When the welding temperature reaches (385 ± 5) ° C and the pressure is 1 2 MPa, the weld strength at the weld is about 90% of the strength of the substrate. The peel strength can reach about 1.5 MPa. If a layer of PFA film is applied to the surface of the weld as a hot melt adhesive, the temperature and pressure during welding can be appropriately reduced, and the peel strength of the weld at room temperature can be greater than 1.5 MPa.
The disadvantage of hot-press welding is that construction parts are sometimes difficult to adapt to construction equipment, especially large-size equipment cannot be moved to welding equipment, while hot-air welding is flexible and can be applied to construction parts of any size anywhere.
2 hot air welding
Using the hot melt of PFA, ePTFE can be welded to the hot air using a welding rod like polyvinyl chloride. The PFA electrode referred to herein is a translucent round bar having a diameter of about 3 mm extruded from a PFA resin, or a flat strip having a width of 14 mm and a thickness of 2.5 mm. The hot air welding machine has a hot air temperature of up to 600 ° C and a wind volume of 0.4 to 0.75 m 3 /min.
The welded part should be cut into a 70° to 90° bevel, and the welded surface should be cleaned with acetone to adjust the hot air temperature of the torch to 420 to 430 °C. At the same time, the PFA welding rod is erected at the welding place, and it is preheated first. After it is transparent, it is poured into the oblique mouth. The electric hot air is slowly moved from one end of the welded joint to the other end. During the moving process, the welding rod is melted and tightly combined with the welded part. together. When welding is carried out at high temperatures, a small amount of toxic gas is generated due to decomposition, and the operator is close to each other. Good ventilation and effective labor protection are very important.
The advantage of hot air welding is that it has strong adaptability. The disadvantage is that the weld strength is not as good as that of hot press welding. The hot air welding has a welding strength of 60% of the ePTFE substrate. Weld strength is closely related to the operation during welding. Due to the manual operation, it is especially necessary to prevent some points or partial small-section false welding caused by improper operation. There are two methods for testing the quality of welds. One is the electrostatic test, that is, the electric spark test. If there is an electric spark during the test, it means that there is a virtual or false solder joint; the other test method is the steam-water cycle test. Let the weldment pass steam and water of 2MPa pressure for 10min each time, and alternately test 500 times without leakage.