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.
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.
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