Molding method for PTFE by mould pressing

The crystallization melting point of PTFE is 327℃, but the resin cannot be in the melting state until it is above 380℃, and the melt viscosity is as high as 1 010 Pa*S. In addition, PTFE has strong solvent resistance. Therefore, it can neither melt processing method, can not be dissolved processing method, usually the production of its products can only be like metal and ceramic processing - sample, first powder compaction, then sintering and mechanical processing, or through extrusion molding, isotactic pressing molding, coating molding and calendering molding and other ways of processing.

1.  Molding

Molding is currently the most widely used molding method for PTFE. Molding technology is a certain molding materials (powder, granule, fibrous material, etc.) into the metal mold, in - - fixed temperature, pressure - - a method of forming. Molded polymers are not limited by their molecular weight, and almost all plastics can be molded. The main features of molding are; Low cost, simple equipment, low investment, not limited by the molecular weight of the processed plastics; The disadvantages are low production efficiency, high labor intensity and unstable product quality. PTFE is of high molecular weight and extremely poor fluidity. When other processing methods are not mature, PTFE products are mainly processed by molding all over the world.

In the molding can be subdivided into five methods according to the difference of specific process: (1) pressing - sintering a pressing method (also known as free sintering method);(2) sintering - pressing method;(3) rapid heating a pressing method;(5) simultaneous pressing and sintering method.

2.  Hydroform method

Hydraulic pressure method, also known as equalization method, isobaric pressure method or rubber molding method, is to add PTFE resin evenly between the bag and the mold wall, and then to the bag into the liquid (commonly used water), the pressure of the rubber bag to the mold wall expansion, compacting the resin and become a preformed product - a method. This method can be used to manufacture large-volume sleeve, bottom storage tank, hemispheric shell, tower column, large slab, etc., as well as complex products with PTFE composite structure, such as tee, elbow and profile. The main advantages of hydraulic molding are the simple structure of the equipment and mould - a common water pump replaces the high-tonnage press, and the products are compressed evenly and densely - resulting in the manufacture of large components, complex shapes and simple lining structure.

3.       Push molding

Push pressure is also known as paste extrusion molding, 20-30 mesh sieve of disperse resin and organic additives (toluene, petroleum ether, solvent oil, ratio of resin weight of 1/5) mixed into a paste, prepress into thick wall cylindrical blank, then put into the push press machine barrel, under heating with the plunker push molding. After drying and sintering at temperature of 360~380°C, strong and tough push and pressure tube and bar products are obtained after cooling. The pushing and pressing products are limited to the rod with diameter of 16mm or less and the pipe with wall thickness of 3mm or less.

4.       Spiral extrusion molding

The screw extruder of PTFE powder is different from the extruder used by other thermoplastic plastics. The extrusion molding of ordinary thermoplastic plastics is to push the material forward with the help of screw rotation, and meanwhile to compress, shear and mix the material. The material is also melted by the heat generated by shear force and the external heating of the material cylinder. However, the screw of PTFE extruder only plays the role of conveying and pushing, so that the material passes through the head of a single-screw extruder with double-head thread and the same pitch and depth, and then enters into the mouth mold for sintering and cooling, and forms with the pressure provided by the counter-pressure device to achieve the continuous purpose. It is often difficult to process PTFE by single screw extruder. The low friction coefficient of PTFE powder causes skidding during the feeding process, which greatly reduces the conveying capacity of the screw. And because of friction heat, may also make powder adhesion to the screw or barrel, making the feeding more difficult and unstable.

In recent years, twin-screw has also been applied in processing materials of this special nature. Its feeding principle is different from that of single-screw extruder, and it has a positive transport function, which can overcome the sliding problem of UHMWPE powder in the screw and greatly improve the feeding capacity of the screw. The counter-rotating twin-screw extruder has better mixing and homogenization effect than the same-direction twin-screw extruder, but due to its larger separation force, the shearing action at the gap of the screw is larger, which causes the material to overheat, and the molecular weight of the extruder can drop by about 40%.If the gap is large and the screw is not engaged, the material will stick to the hot metal. However, the use of the same rotation of the twin screw extruder, there is no such problem. Material in the extruder by the shearing action of smaller, plasticizing the required quantity of heat, all from plus heat source, and thus can be precise control, which can make the material in the extrusion process to minimize heat degradation, at the same time in order to maintain the material flow of normal and stable in the nose, the nose section size of design should be compatible with the volume of a screw conveying material. Screw speed is not fast, generally about 10 revolutions per minute. In order to avoid material suddenly sticking to the metal surface, the extrusion temperature must be strictly controlled.

5.        Plunger extrusion molding

Plunger extrusion processing plastic, plastic processing is a relatively ancient method, since the emergence of this material, people began to use this method to process plastic. PTFE is processed with a plunger extruder by pressing the quantitative resin into the inlet mold, making the plunger reciprocate and pressing it into a preformed product. So back and forth, in the mouth mold to form a multi - stage pre - molding products. Due to the friction between PTFE resin and the friction between PTFE resin and the mold wall, and the volume expansion of the preformed product during sintering in the mold, the preformed product is sintered and cooled into a continuous whole under pressure. The advantages of this method are as follows: no shearing occurs in the molding process, the relative molecular weight decreases less, the product quality is good, and is not limited by the relative molecular weight. However, due to the small contact area between raw materials and heating parts in the extrusion process, the heating efficiency is low, which limits the extrusion speed.

6.        Other processing methods;

PTFE can also be processed by injection molding, calendering molding, coating molding or secondary molding.