The crystalline melting point of PTFE is 327℃, but the resin must be above 380℃ to be in a molten state. In addition, PTFE has extremely strong solvent resistance. Therefore, it cannot use either melt processing or dissolution processing. Generally, the production of its products can only be done like metal and ceramic processing. The powder is first compacted, and then sintered and machined, or formed by extrusion, Isobaric forming, coating forming, and calendering forming.
The crystalline melting point of PTFE is 327℃, but the resin must be above 380℃ to be in a molten state. In addition, PTFE has extremely strong solvent resistance. Therefore, it cannot use either melt processing or dissolution processing. Generally, the production of its products can only be done like metal and ceramic processing. The powder is first compacted, and then sintered and machined, or formed by extrusion, Isobaric forming, coating forming, and calendering forming.
Compression Molding
Compression molding is the most PTFE molding processing method currently used for PTFE. The molding process is a method in which certain molding materials (powders, pellets, fibrous materials, etc.) are placed in a metal mold and molded at a certain temperature and pressure. The polymer processed by compression molding is not limited by its molecular weight, and almost all plastics can be processed by compression molding. The main characteristics of compression molding are: low cost, simple equipment, low investment, and not limited by the molecular weight of the processed plastic; the disadvantages are low production efficiency, high labor intensity, and unstable product quality. The relative molecular weight of PTFE is too high and the fluidity is extremely poor. When other processing methods are immature, countries around the world mainly use compression molding to process PTFE products.
The specific process of compression molding is divided into ① press-sinter-press method; ② sinter-press method; ③ rapid heating-press method; ④ press and sinter simultaneously.
Hydroforming
The hydraulic method is also called equalization method and equal pressure method, which is to add PTFE resin evenly between the rubber bag and the mold wall, and then inject the liquid (commonly used water) into the rubber bag, and the rubber bag that generates pressure expands to the mold wall and compacts Resin becomes a method of preforms. This method can be used to manufacture larger sleeves, bottomed storage tanks, hemispherical shells, tower columns, large slabs, etc., and can also be used to manufacture tees, elbows, profiled materials, etc. with an integrally lined PTFE composite structure. Products with complex shapes. The main advantage of hydroforming is the simple structure of equipment and molds. An ordinary water pump replaces a high-tonnage press. The product is uniformly compressed, has a consistent density, can manufacture large-scale components, has complex shapes, and simple lining structures.
Push forming
Pushing is also called paste extrusion molding. The 20-30 mesh sieved dispersing resin is mixed with organic additives (toluene, petroleum ether, solvent oil, etc., the proportion of resin weight is 1/5) to make a paste. Pre-compressed into a thick-walled round simple blank, then put it into the press material, and use a plunger to form it under heating. After drying, it is sintered at a temperature of 360~380C, and after cooling, a tough push tube, rod, and other products are obtained. Push products are limited to rods with a diameter of 16mm or less and pipes with a wall thickness of 3mm or less.
Spiral extrusion molding
The screw extruder of PTFE powder is different from the extruder used for other thermoplastics. The extrusion molding of Zengtong thermoplastics is to advance the material by means of the rotation of the screw, while also playing the role of compression, shearing, and mixing. , The material is also melted by the heat generated by the sheer force and the external heating of the material. The screw of the PTFE screw extruder only plays the role of conveying and pushing, so that the material passes through the head of a single-screw extruder with double-threaded, equal pitch and equal depth, and then enters the die for sintering, Cooling and molding with the pressure provided by the backpressure device to achieve the continuous purpose. When processing PTFE with a single screw extruder, great difficulties are often encountered. Because the friction coefficient of PTFE powder is very low, it causes slippage during the feeding process, which greatly reduces the conveying capacity of the screw. In addition, due to friction and heat generation, the powder may stick to the screw or barrel, making feeding more difficult and unstable.
In recent years, twin screws have also been used in the processing of materials of this special nature. Its feeding principle is different from that of single-screw extruders. It has a positive conveying effect and can overcome the slippage of UHMWPE powder in the screw. Greatly improve the screw feeding capacity. Among them, the counter-rotating twin-screw extruder has better mixing and homogenization effects than the same-rotating twin-screw extruder, but due to its larger separation force, the shearing effect at the screw gap is greater, causing the material to overheat and extrude. The molecular weight of the output material can be reduced by about 40%. If the courtyard is made larger and the screws are not engaged, the material will stick to the hotter metal. However, the use of co-rotating twin-screw extruders does not have the above problems. The shearing effect of the material in the extruder is relatively small, and the heat required for plasticization is all from an external heating source, so it can be accurately controlled so that the overheating degradation of the material during the extrusion process can be minimized, and at the same time to maintain The flow of materials in the machine head is normal and stable, and the design of the section size of the machine head should be adapted to the volume of the material conveyed by the screw. The rotation speed of the screw cannot be fast either, generally around 10 revolutions per minute. In order to avoid the material suddenly sticking to the metal surface, the extrusion temperature must be strictly controlled.
Plunger extrusion molding
Plunger extrusion molding processing plastics is an old method in plastic processing. Since the emergence of plastic materials, people have begun to use this method to process plastics. To process PTFE with a plunger extruder, a quantitative resin is pressed into the mold, the plunger is reciprocated, and it is pressed into a preform. In this way, multiple preforms are formed in the die. Due to the mutual friction between the PTFE resin and the friction between the PTFE resin and the die wall, resistance is generated. At the same time, the volume expansion of the preform when sintered in the die also generates resistance, so that it is a preform of one segment. Sintering and cooling under pressure form a continuous whole. The advantages of this method are: there is no shear during the molding process, the relative molecular weight is reduced, the quality of the product is good, and it is not limited by the relative molecular weight, and it can process large diameters with a maximum diameter of 500mm and a wall thickness of only 3mm. Pipe. However, due to the small contact area between the raw material and the heating part during the sending process, the heating efficiency is low, which limits the speed of its extrusion.
Post time: Jun-14-2020