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.
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, PTFE 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.
Post time: Jan-01-2018