Why Can't PTFE Be Injection Molded?

August 04, 2024

Fluorine plastic-PTFE Polytetrafluoroethylene why can not injection molding

Basic introduction

English: Poly tetra fluoro ethylene, polytetrafluoroethylene, also known as teflon, Teflon. It cannot be molded by melt extrusion or injection molding. It has a wide range of operating temperatures, chemical resistance, electrical insulation, low friction, non-stick, weather resistance, flame retardant and other excellent properties. It is one of the best materials for corrosion resistance in the world today, accounting for 60~70% of the demand for fluoropolymer. The raw materials are mostly powdered resins or concentrated dispersions, which are thermoplastic polymers with high crystallinity (93-97%).

Molecular Structure

The molecular structure consists of carbon atoms (C) and fluorine atoms (F), and tetrafluoroethylene (TFE) is bound together in the form of linear chains. The arrangement of the atoms in the molecule is tightly symmetrical, and the bonds between the carbon atoms are protected by covering them with fluorine atoms that have no gaps. Due to its unique and stable molecular structure, the charge polarization is very small and it has properties such as non-stick, low friction, heat resistance, chemical resistance and electrical properties.

PTFE is white transparent or translucent, the higher the degree of crystallinity, the worse the transparency, the fluorine content is 76%.

Manufacturer

Developed by the U.S. DuPont in 1938, and in 1945 registered Teflon (Teflon) trademark and commercial production. At present, the world's major U.S. Como Teflon, Japan's Daikin POLYFLON, Japan Asahi Glass Fluon, and China's PTFE production and research started earlier, but due to a variety of factors, the scale of production and process technology overall level is relatively low, but in recent years there are many excellent domestic PTFE manufacturers, such as Shandong Huaxia Shenzhou, Zhejiang Juhua, Shanghai San Aifu, Sichuan Chenguang Kexin, Zhejiang Gerui and so on.

Product series

Molding Powder Series: Granular molding powder is used for molding sintering and extrusion to produce plates, rods, blanks and other general molded products.

Dispersed Fine Powder Series: Used for paste extrusion after adding additives to produce raw tapes, bi-directional stretch film, tubes, small diameter rods, cable insulation, etc.

Dispersed Liquid Series: Milky white liquid formed after PTFE microparticles are dispersed into water, used for impregnation, coating, production of fiberglass cloth coatings, metal coatings and other additives for resins.

Filler-containing Resin Series: Improvement of mechanical properties by adding glass fiber, graphite, bronze, carbon fiber, etc., used for molding and extrusion to produce special products.

Main Properties

Physical properties: the smallest surface tension in solid materials, non-adhesion of any substance; physiologically inert, non-toxic; non-sticky, excellent demolding, very difficult to adhere to any adhesive substances, even if attached can be easily removed; will not elute impurities; food, medical and high purity level of availability;

Mechanical properties: good fatigue resistance; room temperature tensile, bending, impact strength of low poor, easy to creep phenomenon, with cold flow; and filler composite can improve mechanical properties;

Abrasion resistance: minimum coefficient of friction in the solid; excellent slipperiness, water and oil repellency, non-sticky performance;

Thermal properties: melting point 327℃, can be used for a long time in the temperature range of -180℃ ~ 260℃; allows sudden cold and heat, or alternating hot and cold; low temperature is not brittle; high temperature of 400 degrees above the cleavage of highly toxic substances;

Combustion performance: very good flame retardancy, ultimate oxygen index 95% or more, meet UL-94 standard V-O grade, self-extinguishing;

Chemical stability: excellent chemical resistance, resistant to almost all chemicals, all solvents and all pharmaceuticals;

Electrical properties: dielectric constant and dielectric loss at all frequencies stable at low values, showing good insulation; breakdown voltage, volume resistivity and arc resistance is high; good electrical insulation, can resist 1500 volts of high voltage electricity; (HONY plastic add carbon black, graphite, carbon fiber can be processed to get conductive anti-static PTFE sheet rods.)

Weathering performance: excellent aging life due to chemical inertness, can be used semi-permanently outdoors; poor radiation resistance; excellent resistance to steam permeability;

Processability: Although it is a thermoplastic resin, it has a high melt viscosity, and even if it exceeds the melting point, it will only turn into a rubbery elastomer without flowing easily, and it is very sensitive to shear in the amorphous state, and is prone to melt rupture, so it is not possible to mold it in the melt-extrusion and injection molding methods, and it can only be used in sintering molding in a similar way as powder metallurgy, and can be used in extruded molding to make profiles. In addition, suspension dispersions and fine powders can be processed by paste extrusion, hydrostatic pressure, extrusion, impregnation, coating, and so on.

Application Classification

Profiles: rods, tubes, plates, blanks, building tent film, stretch porous film, etc.

Modification: added to other plastics to increase the lubricity of plastics and reduce friction.

Anti-corrosion applications: chemical vessels, pipe linings, corrugated expansion pipes, fittings, nozzles, agitators, valves and main parts of pumps, filtration materials, raw material tapes, generator stator and rotor lead-in tubes, impregnated fiberglass fabrics, metal coatings;

Sealing applications: sandwich gaskets, seating tapes, elastic sealing tapes, internal seals for shafts, piston rods, valves, turbine pumps;

Insulation applications: battery binders, thermocouple sheaths, high frequency and ultra-high frequency communication equipment, microwave insulating materials for radar, printed circuit substrates and insulating materials for motors and transformers (including gas transformers), air-conditioners, electronic stoves, all kinds of heaters, and insulation for cables and wires;

Anti-stick applications: kitchen pots and pans, baking molds for bread baking, frozen food storage trays, iron bottoms, photocopier pinch rollers;

Temperature-resistant applications: such as microwave oven couplings, rollers, refrigeration, air conditioning, oxygen generator, compressor temperature-resistant parts;

Medical use: human body substitute arterial and venous blood vessels, heart membrane, endoscope, clamp catheter, trachea, other tubes, bottles, filter cloth and other medical equipment;

Wear-resistant applications: oil-free bearings, sliding pads, piston rings, conveyor belts for assembly line equipment components;

Why can't PTFE be injection molded?

Molecular structure of polytetrafluoroethylene

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.

Author:

Ms. Tina

E-mail:

sales@honyplastic.com

Phone/WhatsApp:

+86 18680371609