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Suko UHMWPE Rod ram extruder

THREE COMMON PROBLEMS WITH PTFE TUBES

 PTFE tube or Teflon tube, a semi-transparent, chemically inert and non-toxic material, has been mostly used in applications requiring resistance to high temperatures, including medical and health care, chemical, aviation, national defense, cutting-edge science and technology, and mechanical engineering, etc. One of its unique features is temperature resistance and its working temperature is from -190℃ to +260℃, so it can be used in elevated temperature applications which require high temperatures of over 200℃.  PTFE tubes have very good chemical resistance to common solvents, acids, alkali and bases, and excellent dielectric insulation properties.

Do you meet any problems when using a PTFE tube? Here SuKo Polymer Machine Tech Co., Ltd., a well-known China PTFE tube manufacturer and exporter, explains three typical problems to help you make better use of PTFE tubes.

1. How to connect PTFE tubes?

The continuous advancement of anti-corrosion technologies and innovation of anti-corrosion materials promote PTFE to be an attractive anti-corrosion material due to its excellent chemical stability, and high temperature and chemical corrosion resistance. But PTFE material has high lubricity and non-adhesive property, making it difficult to connect PTFE tubes. To solve this problem, hot air welding method and manual flanging method can be adopted to connect PTFE tubes.

2. Precautions of PTFE tube installation

The lined plastic is soft when drilling, but it is more troublesome to force a rigid drill bit, which will easily destroy the internal steel lined plastic pipe and cause edges to be hard to be flush, so it is recommended to use a cutter to open the hole. Lined plastic residues will fall into the original PTFE tubes during the hole opening process, so you should pay more attention during the operation, otherwise it will lead to blocked end water points. No electric welding tool should be used for punching holes or installing branch pipes, otherwise lined plastic of steel PTFE tubes will be destroyed.

3. How to treat the surface of PTFE tubes?

The surface of a PTFE tube should not come into contact with aggressive materials such as oil, solvents, and corrosive materials, etc. The surface should be cleaned with water and detergent and rinsed well after staining. The use of solvents and corrosive cleaning materials is prohibited.
You may encounter other problems when use a PTFE tube except these three problems, and don’t worry. You can find SuKo and will get necessary guidance and help.

Chemical recycling of waste polymer materials

Chemical cycle is one of the important methods of polymer material cycle, which refers to the degradation reaction of polymers under the action of heat and chemical reagents to form low-molecular weight products, which can be further utilized, such as monomer repolymerization, oil products can be further processed. At present, the main methods of chemical cycle are chemical degradation. Chemical degradation can be divided into depolymerization, pyrolysis, hydrogenation and gasification.

The present situation of chemical cycle development of polymer materials

1.       Step - by - step polymer material

The progressive polymer materials mainly include polyester and polyurethane, which are represented by polyethylene terephthalate. Mainly used for film, fiber and fabric, beverage bottle, etc. Waste materials can react with polyols in the presence of catalysts and the products can be condensed with unsaturated polyacids to make unsaturated polyresin. Different esters can be obtained by alcoholysis with different alcohols, either as monomers or as plasticizers. PET can be hydrolyzed under acidic or alkaline conditions. It can be hydrolyzed under normal pressure in strong acid (such as sulfuric acid and nitric acid) medium. The hydrolysis rate is fast. If it is hydrolyzed in an aqueous solution of alkaline (such as NaOH), 3-sh should be reacted at 21 2500C and 1.4-2.0mpa. After the reaction, TPAO weak alkali (such as hydrogen and oxygen) can be precipitated, and can also be used to hydrolyze PET waste to obtain monomer. Saponification reaction under atmospheric pressure has been applied in silver and TPAO polyurethane is from recycled PET film condensation polymer materials, can be hydrolyzed into the pluralistic alcohol and amine, with a special extruder hydrolysis, dibasic acid can be obtained by the purification of the product and diamine, diamine and reaction with phosgene, preparation of diisocyanate, used for foam production. But the cost of this process is large and the recovery benefit is not high. PU alcoholysis is a widely used method at present. PU waste materials can be alcoholyzed to obtain polyol mixture, which cannot be separated effectively at present, but this product can be used as a component in the manufacture of foam plastics and elastomers.

2.       Additive polymer material

Polystyrene (ps) in addition to used as paint, binder, but also used to crack styrene ps under the action of heat can be cracked into styrene, the yield of 65%, above. Japanese scientists have cracked PS in solvent method, decomposed it at 400 to 500 ℃ for 1 20min, and the condensate obtained can be distilled into styrene with a purity of 96%.High purity styrene can be obtained by fractionation using metal oxide as catalyst in melting state (>350 ℃).In addition, lead alloy is used as heating medium to crack ps. Under the action of appropriate catalyst or irradiation, polyolefin can undergo chemical reactions to form materials with good performance, such as polyethylene (PE), which can be crosslinked with crosslinking agents (such as peroxides) to produce PE materials with good performance. Waste polyolefin can be chlorinated, and the chlorinated polyolefin can be used in binders and coatings. In addition, polymer polymerization to make oil is a common cycle method.

3.       Hybrid polymers and composites

A common method of making use of a variety of polymer mixtures is cracking them to make oil. The mixture is cracked at high temperatures to produce gas and oil, which can be used as fuel or refined directly at the refinery. Chemical plants have high requirements on the organochlorine content of oil, generally no more than 10 * 10-6. However, the organochlorine content of oil obtained from cracking of plastic mixture can reach (50 to 200) * 10-6, so it is very important to dehalogen before cracking or during cracking. In addition, waste plastics often contain heavy metal element compounds, cracking oil refining to consider the poisoning of the catalyst. Most of the resins of composite materials are thermosetting resins such as unsaturated polyvinegar resin, epoxy resin and phenolic resin, etc. The waste materials of composite materials are not only used as powder filler, combustion to take heat and chemical auxiliary fuel, but also used for cracking and recycling oil products and raw materials. For example, after the decomposition of glass fiber reinforced plastics at 380 ℃ at atmospheric pressure of 50 ℃, the further decomposition at 450 ℃ and 550 ℃ can obtain oil products. Due to the large amount of residue, a special decomposition furnace needs to be designed to complete the pyrolysis process, and the research work is still under way. Another example is the pyrolysis of phenolic resin in the experimental vulcanization bed (722 ℃), the products include aliphatic hydrocarbon (mass fraction is 5 · 24%), phenol (8, 25%), carbon black (42).2%), gas (24, 3%), etc.

Chemical cycle process and equipment

1.       Reaction Still

Reaction kettle is a common chemical equipment for chemical circulation, and its matching equipment is condenser, storage tank, distillation tower, etc. The products of raw materials (such as polyolefin) degraded in the reaction kettle can be monomer, chemical raw materials, etc., such as PET pyrolysis and then polymerization into PET, or unsaturated polyvinegar resin can be produced. According to the demand, the reactor can be designed into a tank reactor to facilitate heating and slag removal. The reactor can also be designed as a tubular reactor, which can increase the cracking temperature, shorten the time, and continuously crack, suitable for the cracking of ps, PMMA and other polymers.

2.       Fluidized bed reactor

Fluidized bed reactor is a kind of bed reactor. W. kanaminsky et al., Germany, used propane combustion to heat carrier gas or water vapor, and used heater to heat sand and carrier gas to 500 ° c at 00 ° c. The carrier gas should be enough to promote sand fluidization in the reactor. The polymer is squeezed into the fluidized bed by the extruder, and the polymer material is cracked in the fluidized bed. The resulting gas and carrier gas are separated by condensation and separation, and the pyrolysis products are obtained. Fluidized bed cracking apparatus has the advantages of fast heating, high efficiency, even cracking temperature and closed system. Cracking reaction is best carried out under inert carrier gas. If air is used as carrier gas, the product is easy to be oxidized and the thermal energy of the obtained oil is 10700 lower.

3.       Extrusion cracking equipment

Extrusion cracking equipment is composed of two extruders in series. The first extruder has a vent hole. The waste polymer materials were cracked at low temperature on the first extruder. The main purpose was to remove HCI from the waste materials. The intermediate products of cracking are then entered into the second extruder for high-temperature cracking, which turns the polymer into a low-molecular compound or oil or gas. After separation, HCG cracking reaction can be carried out continuously, which can be decomposed mechanically and thermally. In addition, new devices are being researched and developed.

Extrusion process of PTFE insulated wire

Ptfe is one of the types of fluorine plastic, used for processing and insulation layer, as a result of ptfe in melting point 327 ℃, and higher than the melting point 360 ℃ using disposable push extrusion sintering molding methods, process is more complex, process quality control involves many factors, once the control is not good, can make the extrusion process to or appear all sorts of quality problem; Once the extrusion and sintering are not successful, it will cause the material part or all scrap, ptfe price is expensive, so it has caused a big waste. Therefore, it is necessary to control the extrusion process of ptfe reasonably, comprehensively and effectively, which is a necessary condition to ensure product quality and reduce waste.

Based on the application and processing practice of ptfe material in electric wires and cables and the practical experience, the paper gives a brief overview of ptfe insulation extrusion process, hoping to help production technicians to make correct selection and control in ptfe extrusion process.

Technological process：                                                                                   

1. Sieve F4 powder    

                                                         

Ptfe powder is the powder particles after polymerization, soft, strength is very small, when in packaging container stored easily by weight or turbulence vibration in transit in the cluster, blending uneven phenomenon, so before mixing must adopt certain specifications of the mesh screen to sieve of ptfe powder, filter out larger knot cooking starch material, the specifications of the mesh screen in 8 and 12 mesh is advisable. If the agglomeration is serious, the powder should be placed in a lower temperature, after a period of time, if the agglomeration shows signs of loosening, then screening. The powder should be stored in a low temperature environment, and should be handled gently in the process of handling, so as to avoid severe agglomeration of ptfe powder and it cannot be used. In addition, environmental hygiene should be ensured and impurities or dust should not be mixed into ptfe powder. Due to the strong electrostatic adsorption of ptfe powder, it is easy to adsorb air or small impurities and dust around, so to keep the environment clean without dust, the packaging container surface should be clean before opening the cover

2. Toner or pulp treatment

When insulation needs coloring for identification, a certain proportion of toner or slurry should be added to ptfe powder. Toner is after fine grinding of high temperature resistant organic or inorganic pigment powder, easy to absorb moisture, agglomeration, so in advance should be heated treatment, remove moisture, and screen, filter out the larger particles. Generally, 180 and 200 mesh screen specifications are appropriate. If the screen screen is smaller than 180 mesh, then the pigment particles after filtering are larger, which is not conducive to the insulation performance of the insulation layer. The screen mesh is more than 200 mesh. Although the insulation performance of insulation layer is guaranteed, it is difficult to filter. Color paste is a mixture of toner and oily liquid, toner in oily liquid suspension state, but stationary for a period of time will precipitate, so before use should be shaken and mixed, so that the precipitate toner particles evenly suspended in oily liquid for use.

3. The ingredients

Batching is an important preparation before mixing, according to the ratio of polytetrafluoroethylene powder and extrusion aid, or color powder or color paste ready, after the mixture.

There are more kinds of extruder, such as yan 20 aviation gasoline, toluene, petroleum ether, or other volatile solvent oil, organic solvent, etc., different extruder volatilization speed is different, according to the actual situation of the equipment, extrusion speed to choose the appropriate extruder. The extrusion assistant volatilizes too fast, although it can shorten the drying time and improve the extrusion speed, it requires strict operation time for the preparation before extrusion, and the preparation operation time should not be too long, otherwise the extrusion assistant of the blank will be ineffective due to excessive volatilization .Extruder is too slow to evaporate, and although there may be sufficient preparation time, the drying time must be extended or higher requirements for the equipment capacity are required. The proportion of extruder should be 18% and 23%, and should be adjusted according to the specifications of the conductor, insulation layer thickness, extrusion speed and extrusion pressure. Generally, the wire specification is larger, the insulation thickness is thicker, the extrusion speed is faster, the compression is smaller, the proportion of the extruder can be appropriately small; On the contrary, the wire specification is smaller, insulation thickness is thinner, extrusion speed is slower, compression is larger, the proportion of the extruder can be appropriately larger.

Toners cost less and are still commonly used today. Using pigments to dye, teflon insulation dispersion in toner as paste, toner and ptfe powder is dry mixing, and toner additives and relatively little, easy to cause mixing cluster, dispersion is poor, is not easy to mix, will lead to serious existence chromatism different part of insulation, so its size as small as possible when choosing toner requirements, general imported toner quality is better. In the toner mixing, should choose the pigment powder temperature tolerance above 400 ℃, otherwise after high temperature sintering (sintering temperature generally basic in 350, 400 ℃) may appear the product fading phenomenon. Generally, inorganic pigments have good high-temperature resistance, while organic pigments have poor high-temperature resistance. Therefore, the use of inorganic pigments as toner is more reliable, but its environmental protection is poor. The matching ratio of toner is generally not more than] %, in the actual production can be according to the type of toner, toner quality, color requirements, insulation layer thickness, etc.

Although the cost of color paste is high, it is conducive to the uniformity of the mixture and is an ideal choice, which can effectively ensure product quality, simple operation, high efficiency and less waste. When mixing the color paste, the color paste can be added to the extrusion aid and premixed evenly. Due to the relatively large amount of extruder, it is wet mixed with ptfe powder, so there is no problem of toner agglomeration, and the dispersion is very good, it is easy to mix evenly. The proportion of color paste is generally between 0.3% and 0.5 ‰, with 3 ‰ being the majority. Only a few special cases require the proportion of 0.5 ‰.When the insulation layer edge layer thickness is relatively thin, the proportion of color paste should be appropriately increased by 0.1%, 0.2.

4. The mixture

Mixing is to mix polytetrafluoroethylene powder with extrusion aid, or color powder or color paste evenly, to ensure that the extrusion insulation lubrication and color uniformity. In order to facilitate uniform mixing, the volume of general mixing bottle (wide mouth bottle) is appropriate to 4, 5 liters, the loading quality is about 800, 1 000 g, the loading volume accounts for about 2/3 of the mixing bottle volume is good, so there is enough space for mixing.

The natural color mixture (mainly used to extrude the insulation of radio frequency cable) does not need to add toner. When mixing, put ptfe powder into mixing bottle first. Used in to make people try not to contact with mixing bottle of bottle wall, in person to squeeze aid available clean sheet tool will gently mixing in a bottle the center of the surface of ptfe powder to bottle wall, make its surface center to form a depth of about 2, 3 c of conical hole, then people directly used in a certain proportion in the taper hole; After that, the mixing bottle shall be capped and strictly sealed to prevent the extruder from volatilizing. Firstly, the mixing bottle shall be shaken and mixed manually for no less than 5 minutes, and then the mixing bottle shall be placed on the mixing machine to swing and roll for more than 30 minutes until the mixing is even.

5. Compact

The effect of pressing (material) billet is to press loose porous polytetrafluoroethylene powder into a relatively compact material billet, eliminating the pores and air contained in it for extruding use. Generally should be in the environment of 20 ℃ above the blank pressing, will be homogenized powder slowly pour into the pressure cylinder cylinder, at the same time should ensure that the heart rod and pressure cylinder cylinder concentric, and gently shake the pressure cylinder in the material surface, and then add pressure cover on it. The pressure should be appropriate, too much or too little will affect the extrusion quality. If the pressure of the blank is too large, the extruder is easy to be overpressed, and the lubrication performance during extrusion is reduced, the extrusion pressure is increased, and the shear stress of the material is larger, leading to excessive fibrosis, which is not conducive to the stability of the outside diameter of the extrusion insulation, and the shrinkage rate of insulation sintering is larger, which is also easy to cause insulation cracking.

6. Squeeze out

The preparation work for ptfe insulation extrusion mainly includes selection of compression ratio, preheating and heating of extrusion cylinder and die, adjustment of needle gap between die sleeve and die core, charging blank, conductor inspection, conductor piercing, adjustment of thread tension, combination sealing of machine head and extrusion cylinder, etc., which will be introduced in the following.

The compression ratio of different brands of ptfe powder during extrusion is also different. the proper compression ratio should be determined according to the properties of ptfe powder to ensure the process performance and quality during extrusion. Extrusion compression ratio refers to the ratio between the annular area of the extrusion die and the annular area of the blank. Generally, extruder cylinders are divided into three sizes: small, medium and large. The diameter of small cylinder is about 38 mm, medium cylinder is about 50 mm, and large cylinder is about 65 mm.In the actual production, the specifications of the extruder cylinder should be selected according to the sectional area of the extruder insulation layer.

In order to maintain a stable temperature of the blank and facilitate the volatilization of the extruder after extrusion, the extrusion cylinder and mold should be preheated properly. The preheating temperature of the extrusion cylinder should be 30 or 40 ℃, and the preheating temperature of the mold should be 50 or 60 ℃.

The extrusion should be according to the specifications of the corresponding matching mold and according to the location of the assembly, and the mold sleeve bearing line neck population and needle tube between the distance (gap) to the appropriate position. Too large or too small clearance will affect the quality of the extruded insulation layer. If the gap is too small, the velocity of extrusion of ptfe material to the outside of the mold will be accelerated, and the material will be excessively cut, leading to excessive fibrosis, resulting in the decrease of the transverse strength of insulation layer, the product is prone to longitudinal cracking or internal cracks of insulation, and the insulation and conductor coating is not tight (there is "loose cover").The gap is too large, will make ptfe material to the outside of the mold extrusion velocity slow down, material pressure on the conductor increases, often occur extrusion instability or cause conductor compression rolling and produce "pine" problem, and the conductor can not pass through the needle plug phenomenon.

Extruder Screw Design

The single most important mechanical element of a screw extruder is the screw.The proper design of the geometry of the extruder screw is of crucial importance to the proper functioning of the extruder. If material transport instabilities occur as a result of improper screw geometry, even the most sophisticated computerized control system cannot solve the problem. Screw design is often still considered to be more of an art than a science. As a result, misconceptions about certain aspects of screw design still abound today. Since the theory of single screw extrusion is now well-developed, the design of screws for single screw extruders can be based on solid engineering principles. Thus, screw design for single screw extruders should no longer be an art, but a science based firmly on the principles of polymer processing engineering.      

Unfortunately, people involved in screw design are not always up-to-date on extru-sion theory. As a result, many extruder screws in use today perform considerably below maximum possible performance, solely because of improper screw design.An example is the still-common use of the square pitch extruder screw. This is ascrew with constant pitch with the pitch being equal to the diameter of the screw;this pitch corresponds to a helix angle of 17.66. It can be demonstrated quite easily that the square pitch is far from optimum with respect to melting and melt convey-ing for a number of polymers. This fact has been known since the early 1950s, yet most extruder screws in use today still use the constant square pitch design.

      A factor that may have contributed to the state of affairs in screw design is that there has not been a comprehensive text dealing with screw design. The objective of this chapter is to demonstrate how extrusion theory can be used to properly design extruder screws. Hopefully, this will provide a solid foundation, based on engineer-ing principles, from which better and more effective screw designs can be developed in the future. The principles of screw design are not only important in designing new extruder screws, but also in the analysis of processing problems of an existing extrusion line. It is important to be able to recognize whether a problem is related to poor screw design or to another part of the process. Thus, knowledge of the basic principles of screw design is important to essentially every person involved with extruders.

Development of key technology of coaxial twin screw extruder

1.       Spiral groove depth

The depth of screw groove is an important parameter of the twin - screw extruder. Usually it is expressed indirectly by the ratio of screw diameter and screw root diameter. The value is an important parameter to indicate the technical level of the twin screw extruder. At present, the ratio of the external diameter of the screw to the root diameter of the screw in the latest generation (sixth generation) coaxial twin-screw extruder represented by WP of Germany is D/Di = 1.55.There are many factors limiting D/Di, in addition to the center distance of the gearbox output shaft of the driving screw, the screw itself also has a greater impact on D/Di: such as the shear resistance of the screw mandle-shaft, the torque transfer mode between the screw element and the mandle-shaft, the structure of the screw element itself, etc.

(1)     The breakthrough in material selection, processing technology and heat treatment performance of mandrel makes the torque capacity of mandrel bearing greatly improved, and the length-diameter ratio of screw increases from the initial 36 to 50, or even higher.

(2)     The technological breakthrough of the special gear drive system of the co-directional twin-screw extruder provides the power transmission guarantee for the torque input required by the deep groove screw under the condition of the constant center distance, making the screw D/Di goes to 1.55.

(3)     The torque transfer mode between screw and mandrel is the key to realize the deep screw groove. The torque transfer between the screw element and the mandrel was initially connected by flat keys, which not only limited the deepening of the screw groove and increased the free volume of the screw, but also the strength of the flat keys could not meet the requirements of some production operations. Using multi - key torque transmission in the form of involute can greatly improve the depth of the screw groove under the same outer diameter of the screw. In recent years, while learning from foreign advanced technologies, China has made great breakthroughs in processing and manufacturing. At present, it has been able to manufacture involute spline structure with the same connection form with foreign screw, thus making great breakthroughs in the depth of screw groove.

(4)     The breakthrough of element structure. The structure of the kneading block solves the key technology of manufacturing, its structure changes from the original single piece type, the assembling type to the present integral type, thus guarantees the deepening of the screw groove from the strength of the element, also makes the adjustment of the screw combination and the assembly of the screw more reliable and convenient.

To sum up, breakthroughs in the aspects of gear transmission system, shear strength of mandrel, torque transmission form of screw element and mandrel, and design and manufacture of screw element ensure the successful development of a new generation of high-speed and deep screw slot extruder.

2.       New screw element

Synthetic twin screw extruder is the core part of the screw, after the advent of synthetic double screw extruder, and development of screw and screw element has always been a developer research and the main direction, screw structure from the integral development early to "building block", three thread from single head to head, later when the double structure, type of screw elements, structure and function differences. Develop self-cleaning type mixer type screw element, edge type mixer type screw element, toothed screw element, pressure regulating screw element, etc., of the twin screw extruder screw element made greater innovation, supplement and perfect, strengthen and improve the direction of the twin screw extruder mixing and plasticizing effect, dispersion, shear, pressure, and adjustable.

3.       Gear box torque rating parameters

The torque grade parameter M/A3 of the gear box is an important performance indicator parameter of the coaxial twin-screw extruder. Its value represents the technical level of the coaxial twin-screw extruder and represents the torque output capacity of the gear box under a certain screw center distance. Due to the particularity of the transmission form of the coaxial twin-screw extruder, the torque of the gear is input into the gear box by a high-speed shaft. Through deceleration and torque distribution, the torque is transmitted to the twin-screw by two output shafts with limited center distance under the condition of great axial force. Therefore, the ratio parameter M/A3 between the center distance of the output shaft and the output torque has become an important symbol to measure its technical level. So the design of gear box of twin screw extruder is very difficult compared with the design and development of general gear transmission system. At present, China has made great breakthroughs in gear transmission mode, torque uniform distribution and axial force solution. The torque grade parameters of the experimental extruder have been changed from 4.Seven goes up to eight.8. It lays a good foundation for the development of high speed and high torque gear box of production extruder.

In terms of the service life of the gear box, by improving and driven by new type of multi-channel way, not only greatly improve the transmission capacity, and through the reasonable arrangement of the axis, solved the gear and thrust bearing load distribution, greatly reduced the gear drive and work process of the additional force, making long-term restricting the service life of the gear box of the anti-fatigue performance of the gear, thrust bearing system performance are greatly improved, after adopts multiplex drive, output the stress of the bearing diameter of axle direction decrease greatly, therefore, in many ways to improve the gear, the thrust bearing system, the torque distribution part of the bearing service life, Designed life up to 10 a.

4.       Screw speed

Screw speed is another important parameter to measure the performance grade of the extruder with the same direction and it is an important way to improve the performance index of the extruder. It directly affects the torque grade parameters and production capacity of the equipment. The screw rotation speed increase mainly depends on the innovation and development of gear transmission system drive, the ability of the screw torque under and anti-fatigue performance, by developing a new type transmission way and optimize the innovation design, selection of key parts imported bearing, means, the strict screening of screw spindle materials and the improvement of heat treatment process, and the screw element and spindle, involute spline breakthrough made in the form of uniform stress distribution, making it possible to screw rotation speed increase, has been in a small high-speed experimental synthetic twin-screw extruder machine through industrial test.

5.       Production capacity

The technical level and performance of the equipment are aimed at improving production capacity, reducing energy consumption and improving reliability. Production capacity is the main factor to evaluate the performance of a unit:

(1) improve the screw speed, the same type of production can be doubled;

(2) the use of deep screw groove screw, increase the screw free volume, improve the unit speed and unit time transport capacity;

(3) the use of twin screw and gear pump combined technical route, improve the reliability of the transmission box, reduce the axial force borne by the gear, eliminate the limit of the axial force to reduce the output of the factor, improve the output of the extruder.

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.

Polymer PTFE production process Ram-punch extrusion

With the development of technology and economy, ptfe has been applied in more and more fields, and has become an indispensable material to solve many key technologies in scientific research, military and civil fields and improve the production technology level. In order to continuously improve the production efficiency of polymer ptfe, we began to adopt the advanced production process, which is suitable for high viscoelastic material —— Ram-punch extrusion.

According to the principle of "forging", plunger stamping extruder mainly adopts plunger with small cross section area and high frequency stamping to push material into the barrel of extruder. Compared with traditional hydraulic column extruder, it has the characteristics of high stamping frequency, good melt plasticizing effect, good product quality, simple equipment and process.

According to the forming characteristics of Ram-punch extruder, the extrusion process is analyzed and studied. The physical model of the whole extrusion process of plunger ram extruder is established, which is divided into five sections: solid feeding section, melting section, shunt section, forming section and cooling section. The influence of die length and pressure vibration frequency on extrusion process was analyzed. Analysis results indicate that: although there are large fluctuation of pressure at the entrance of the melt, but on the whole pressure from inlet to outlet were reduced gradually, thus ensure the enough pressure to push the material extrusion, and the closer it gets to the finish mold outlet pressure and speed fluctuation is smaller, so it can ensure the continuity of extrusion, finally get good quality products. The length and stamping frequency of the die have certain influences on the extrusion pressure and speed. Due to the high viscosity of PTFE, a longer die should be selected, and the stamping frequency between 5-6hz is appropriate.

Ram plunger extrusion machining polymer PTFE are very new machining method , but in the process of material forming process, also cannot leave the material flow and deformation, rheological behavior and the law, understanding the process of molding process of reasonable selection, operation and optimization control and improve molding equipment has very important practical significance. Plunger stamping is not a steady-state machining method, and there is vibration in the stamping process. It is of great significance to study the behavior response of polymer under the action of vibration field, and to discuss how much impact the vibration field will have on the flow stability, so as to study the dynamic forming of polymer materials and optimize production.

Measurement for shear viscosity spectrum of polymer melts by using screw extruder capillary

Most polymer materials are processed in the melt state, which involves melt flow and deformation, which not only affects the processing process itself, but also affects the final performance of the product. Therefore, the study of rheological properties of polymer materials has been a hot topic. Accurate measurement of rheological parameters is the basis of in-depth study of rheological properties.

Shear viscosity is an important parameter to characterize the rheological behavior. The so-called shear viscosity of polymer melt is the ratio of shear stress and shear rate that melt is subjected to in the process of flow. Polymer melt causes pseudoplastic fluid, and its flow behavior has the characteristics of shear thinning. It is usually necessary to use the relationship curve between shear viscosity and shear rate, namely shear viscosity spectrum, to fully reflect the processing characteristics of polymer melt.

The basic method of measuring melt viscosity is to try to make the melt flow through a long and thin capillary tube, such as a round capillary tube. The shear stress can be calculated by measuring the pressure drop at both ends of the melt as it flows through the capillary tube. The shear rate can be calculated by measuring the flux of melt per unit time. Thus the melt viscosity can be obtained.

The conventional way to get the melt out of the capillary tube is to use piston propulsion. The advantage of this method is that it USES fewer test materials and can obtain higher shear stress. The high pressure capillary rheometer is based on this principle [4].However, the disadvantage of this test method is that the material cannot be tested under the actual processing conditions, and it is difficult to obtain the rheological properties of the polymer melt when it is processed. Especially in the study of blending modification of several polymer materials, the polymer melt needs the strong shearing action of screw to achieve the purpose of blending. High pressure capillary rheometer is not suitable for testing such materials.

The screw extrusion capillary rheological test device can solve the above problems. The device USES the propulsive force of the screw to make the polymer melt flow through the capillary tube. Therefore, the shear viscosity of polymer melt can be measured under conditions closer to real processing. This method is particularly suitable for the measurement of the rheological properties of thermoplastic materials and their mixtures. Because the measurement simulates the real experimental environment, the obtained test parameters can more accurately describe the behavior of materials in actual processing.

Shear viscosity spectra of polymer melts can be measured using specialized test instruments, such as high pressure capillary rheometers, or combined revolutions. However, these devices are expensive and limited in practical use, especially in the application of large-scale industrial production. In fact, it is not necessary to rely on the special test instrument, as long as the shear viscosity test principle, you can use a simple small single-screw extruder and capillary mold, constitute a low-cost shear viscosity spectrum test device. Combined with computer data processing, the shear viscosity spectrum of polymer melt can be easily and quickly obtained. This method is especially suitable for small and medium-sized enterprises to carry out product development and raw material inspection.