PTFE molecule CF2 unit arranged in a sawtooth shape, due to the fluorine atom radius is slightly larger than the hydrogen, so the adjacent CF2 unit can not be completely according to the trans cross orientation, but the formation of a helical twisted chain, fluorine atoms cover almost the entire surface of the polymer chain, the formation of shielding so that the smallest of the hydrogen is very difficult to enter the C - F bond. At the same time, the fluorine atom has the highest electronegativity ( 4. 0), the atomic radius is small ( 0. 135nm), the bond length of C - F is short ( 0. 138nm), and the dissociation energy of C - F is high ( 452KJ / mol), so it is very difficult to break the C - F. These characteristics determine the various properties of P TF E.
Why can't PTFE be injection molded?
The main reasons why PTFE polytetrafluoroethylene cannot be injection molded include its high melting point, large melt viscosity, and the shape stability it maintains in the molten state. These properties make PTFE unsuitable for conventional plastic molding processes such as injection molding.
High melting point: PTFE has a melting point of approximately 327°C and its melt viscosity is several orders of magnitude higher than that of ordinary thermoplastics. This means that at high temperatures, PTFE flows extremely poorly and is difficult to heat through an injection molding machine and then inject into the mold to form the desired shape.
Shape stability in the molten state: In the molten state, PTFE is able to maintain its original shape, similar to a jelly state that cannot flow. This characteristic makes PTFE unable to be molded by injection molding like other thermoplastics.
In addition, PTFE's processing dimensional stability is not ideal, its coefficient of linear expansion with temperature changes and very irregular changes, hot and cold shrinkage changes, which further limits its application in injection molding.
Molding process of polytetrafluoroethylene
PTFE crystallization melting point of 327 ℃, but the resin to be above 380 ℃ to be in a molten state. In addition, PTFE has a strong solvent resistance. Therefore, it can not use the melt processing method, also can not use the dissolution processing method, usually the production of its products can only be like the processing of metals and ceramics, the first powder compaction, and then sintering and mechanical processing, or through extrusion molding, isobaric molding, coating molding and calendering molding and other ways of processing.
1.Molding
Compression molding is PTFE is currently the most widely used molding process. Molding process is a certain amount of molding with raw materials (powder, granules, fibrous materials, etc.) into the metal mold, at a certain temperature, pressure molding method.
2.Hydraulic molding method
Hydraulic method, also known as equalization method, equal pressure method, is the PTFE resin uniformly added to the rubber bag and the mold wall, and then injected into the rubber bag of liquid (commonly used water), resulting in pressure to the rubber bag to the mold wall expansion, compaction of the resin and become a pre-molded product of a method.
3.Push molding
Push also known as paste extrusion molding, 20-30 mesh sieve dispersion of resin and organic additives (toluene, petroleum ether, solvent oils, etc., the proportion of resin weight 1/5) mixed into a paste, pre-pressure into a thick-walled round simple blank, and then put into the push press material in the brief, under the heat of the plunger to push the molding. After drying in 360 ~ 380C temperature sintering, cooling to get a strong push-press
tube, rod and other products. Push products are limited to the diameter of 16mm below the rod and wall thickness of 3mm below the tube and so on.
4.Screw extrusion molding
PTFE powder screw extruder is different from other thermoplastics with the extruder, had through the extrusion of thermoplastics with the help of the screw rotating role of the material forward at the same time also play a role in compression, shear, mixing, the material is also subjected to the shear action of the heat generated by the heat and the material briefly outside the heating so that it melts. The PTFE screw extruder screw only play a role in conveying and pushing the role of pressure, so that the material through a single-screw extruder with a double thread, equal pitch and depth of the head, and then into the mouth of the mold sintering, cooling, and with the help of counter-pressure device to provide pressure molding, to achieve the purpose of continuous.
5.Plunger extrusion molding
Plunger extrusion molding processing plastics, plastics processing is considered to be a relatively old method, since the emergence of such materials as plastics, people have begun to use the method to process plastics. Plunger extruder processing PTFE is the amount of resin pressed into the mouth mold, so that the plunger reciprocating movement, will be pressed into the pre-molded products. The plunger moves back and forth, forming multiple segments of preforms in the die.
