Polytetrafluoroethylene is generally formed by compression molding or by ram extrusion. The resin obtained by the dispersion polymerization method may be subjected to paste extrusion and calender molding. The specific process conditions are as follows: the preforming pressure varies with the shape and size of the product, but usually 9.8-34.3 MPa, the resin with the filler is 29.4-93 MPa, and the thickness for the pressing direction is about A 600mm large blank also requires a holding time of 20 to 30 minutes.
In the PTFE compression molding, the sintered blank is placed in a mold preheated to a sintering temperature, a hot pressing method of cooling while being pressurized, an automatic compression molding method of bidirectional extrusion, and pressurization using a fluid, etc. Press molding method.
Extrusion is a method of using rods, tubes and profiles of pre-sintered PTFE. Usually divided into vertical or horizontal, but because the horizontal production tube material is unevenly filled, it is often used in the vertical extrusion process. The process conditions are as follows:
1 round bar diameter is 13 to 40mm, mold size is 15.1 to 46.0mm, heating length is 500-1100mm, heating temperature is 400-350°C, cooling length is 150-200mm, and extrusion speed is 4-1m/h .
2 pipe diameter is Φ15-Φ40/Φ8-Φ25, the inner diameter of the cylinder and the outer diameter of the mandrel are Φ16.5-44.5/Φ8.9-27.8, the heating length is 350-800, and the heating temperature is 400. At 350 ° C, the cooling length is 150-250 mm, and the extrusion speed is 3.8-1.4 m/h. The development direction of PTFE extrusion technology is to further increase the extrusion speed and to form multiple products at one time.
Dispersed PTFE is usually prepared by paste extrusion. First, the dispersed PTFE resin and the extrusion aid such as crude solvent gasoline (b.p. is 90-120 ° C) or white oil are mixed and placed in a glass or stainless steel bottle. Roll for 30 minutes, let it evenly disperse and absorb, place for 8-12h, press into a blank placed in the extruder cavity, pressure 0.69-3.43MPa dwell time 30-60 seconds. The extrudate made by the process is subjected to a continuous drying oven (100 to 300 ° C) to remove the extrusion aid. After sintering in a sintering furnace (360-380 ° C), it is rapidly cooled to obtain a good product.
After the paste is extruded into a small rod (Φ10-Φ30), it can also be sent to a calender for calendering before sintering, which is a so-called raw tape product. The specific molding conditions are 100 parts of resin, 27 parts of auxiliary agent, compression ratio (R, R) 30, head temperature 50 °C, extrusion shape 50×14 mm, calender roll diameter 500 mm, roll temperature 70 ° C, calendering speed 28 m /min, calendered into a 500 x 0.1 mm raw tape.
The fluorine-containing resin other than the polytetrafluoroethylene may be molded by a thermoplastic resin such as molding, transfer molding, injection, extrusion, blow molding, or the like.
1 molding method: This is an ancient plastic molding method. It is suitable for the production of round bars, plates, casings and other products with simple shapes and small batch sizes.
2 Transfer Molding: Transfer molding is a method of closing a mold and then molding the article by pressing the molded material grease into the mold with a high pressure. Fluoroplastic transfer molding is suitable for the production of small batches of large valves, pumps and other products. Transfer molding can form high-precision products. The disadvantage is that the raw material consumption is large. The molding pressure used is usually below 9.8 MPa, and the processing temperature is similar to the injection molding temperature.
3 injection molding; injection molding is the same as usual plastic. It is suitable for processing carriers, instrument joints, sockets, pipe fittings, bobbins, special-shaped bearings, valves, pump casings and impellers.
4 extrusion molding: extrusion molding is suitable for forming films, pipes, rods, monofilaments, wire sheathing and other products.
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