PFA transparent heat shrink tubing (PFA heat shrink tube forming process)

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The bending fatigue life of PFA heat-shrinkable tube is about 100 times that of other fluoroplastics, with a high continuous use temperature (260°C). It has non-stick, heat-resistant, weather-resistant, corrosion-resistant, anti-fouling, moisture-resistant, flame-retardant and good electrical insulation properties, and is widely used in electronics, electrical appliances, machinery, chemicals, electroplating, pharmaceuticals, printing, paper-making and other fields.

PFA transparent heat shrink tubing (PFA heat shrink tube forming process)

PFA heat shrink tube forming process

  1. Selection of raw materials

Not all PFA resins can be used as raw material for heat shrinkable tubes. It requires PFA resins with a uniform molecular weight distribution, similar distance between molecular chains and good ductility.

The melt index of the raw material is a very important technical parameter for the selection of raw materials. It is not only an important criterion for measuring the molecular weight of fluorine resin, but also related to its mechanical properties and processing performance. The smaller the melt index of the resin, the larger the molecular weight and the better the mechanical properties, especially the ductility. In the extrusion process, the performance of the base tube has a low extrusion rate, as far as the blowing process is concerned, because the base tube has a high mechanical strength laterally, can improve the blowing rate and blowing efficiency. However, the melt index is too low and the extrusion process is prone to melt breakage, resulting in a defective base tube that is difficult to blow. In order to ensure a good blowing effect, generally choose the melt index of PFA resin in (1.5~6) g/10min.

1.2 Molding process

The general fluoroplastic heat shrink tube forming process consists of two parts: the base tube manufacturing and blowing forming.

1.2.1 Preparation of the base tube

The forming process for the base tube used for PFA heat shrink tubes is similar to the production process for PFA tubes, while the forming process conditions are not identical and generally low temperature, low speed extrusion is used. The screw speed should not be too fast in order to control the lower extrusion volume. The process is: PFA pellets an extrusion a horizontal tube head a cooling set - traction a cutting in order to get in line with the blowing with the base tube, the pipe stretching ratio should be controlled in 3 ~ 7 as good, melt cone length control in the range of 10 ~ 20mm more appropriate, more than 20mm, molten tube billet due to self-weight and sagging, affecting the uniformity of wall thickness, wall thickness is not uniform will not get the same blowing rate of heat shrinkage Tube. The molten billet must be cooled quickly to reduce the crystallinity of the base tube, which is conducive to blowing; the compression ratio of the head should be in the range of 1.4 to 3 to ensure complete plasticisation of the melt and dense.

In short, as the base tube for blowing, the surface must be clean, uniform wall thickness, does not allow the surface filiform and large black spots, impurities, etc.. The wall thickness of the base tube is generally controlled within the range of 0.20-0.60mm, and the best wall thickness should be (0.25-0.40)±0.02mm

1.2.2 Blowing and forming (expansion process)

The blowing process of fluoroplastic heat-shrinkable tubes is developed on the basis of the blowing process of general-purpose plastics, and generally adopts the method of blowing with compressed air inside. According to the different processes of blowing and forming can be divided into two kinds.

(1) Continuous blowing: on the blowing machine, a certain length of the base tube is continuously blown through the blowing mould, which is heated and pressurised, and cooled and shaped. The advantages of this method are: low investment, only need to replace the blowing mould to blow different specifications of the shrink tube, high production efficiency, and can get a certain continuous length of heat shrink tube.

(2) Intermittent blowing: the base tube cut to a certain length (generally 1 m), and then fixed in the blowing mould, both ends sealed, one end ventilated, heated, pressurised blowing and forming. The advantages are: the ability to blow special specifications of the shrink tube, and heat shrink tube appearance quality (and the appearance of the base tube quality close to), roundness; disadvantages are: low production efficiency, product length also has limitations.

1.2.2.1, continuous blowing and forming process

Continuous blowing process flow is as follows.

Base tube a heating plus a European expansion - cooling and shaping - inspection - packaging - finished blowing process process parameters are: temperature, pressure, traction speed

(1) temperature

According to the basic principle of the blowing process, the blowing temperature of PFA base tube should be between the glass transition temperature and the melting temperature, the best blowing temperature is 130℃~190℃. If the temperature is lower than 130°C, a higher pressure is required. If the pressure is too high, the pipe is prone to rupture; if the temperature is higher than 190°C, the tensile strength and elongation of the pipe decreases as the temperature rises, and thus it is prone to rupture when blowing.

(2) Pressure

There is also an optimum blowing pressure under suitable blowing temperature conditions. In the case of constant blowing temperature, the size of the blowing rate depends on the size of the gas pressure in the base tube, and the size of the air pressure is a function of the diameter of the base tube and the wall thickness. The diameter of the base tube is the same, the size of the required air pressure depends on the wall thickness of the base tube, the greater the wall thickness, the greater the pressure; the wall thickness of the base tube is the same, the size of the required air pressure depends on the outer diameter of the base tube, the smaller the outer diameter, the greater the pressure, because the smaller the diameter of the base tube, the greater the surface tension. The general blowing pressure is (0.25~1.0) MPa.

(3) traction speed

In the blowing temperature, pressure is certain, traction speed and blowing rate is inversely proportional. However, when the tube blowing rate has reached the requirements, traction speed within a certain range, the blowing rate does not have a significant impact. In practice there is such experience: blowing process temperature, pressure, traction speed should be carried out in the best range, and their value should be the temperature of the low value, the pressure of the high value, traction speed of the high value for good.

1.2.2.2, intermittent method of blowing forming process

Intermittent method of blowing process is as follows: base tube - flanging, loading head - pressurization, heating - insulation - cold shaping test a product

(1) the main factors of the intermittent blowing process are temperature, pressure and holding time. The influence of temperature and pressure on the process is similar to that of the continuous blowing process and will not be repeated here.

Turning the edge and fitting the head is the first step in the intermittent blowing process. If the turning edge is not good, there will be air leakage and pressure relief in the later blowing process. In the flanging process, the temperature of the flanger should be well controlled, if the temperature is too high, the base tube will melt and affect the effect of the flanging, therefore, the flanging is a key step in the success of the blowing. Fitting the head is a guarantee that the base tube and the blowing mould can remain sealed. If the head is not fitted properly, during the heating process, due to the softening of the base tube, the base tube retracts under pressure, making a gap between the bore and the blowing mould, producing air leakage and blowing will not be possible. The length of insulation time is an important aspect of ensuring product qualification rate and production efficiency, insulation time is too short, the base tube heating time is not enough, the temperature is not uniform to reach the process temperature requirements, blowing is difficult to carry out; insulation time is too long, affecting production efficiency, generally controlled in 1 ~ 1.5 hours.

(2) heat shrinkage tube is bamboo knot-shaped is a common phenomenon in the intermittent method of heat shrinkage tube forming. The phenomenon of bamboo joints is mainly in the heating process, the temperature of the billet along the longitudinal distribution is not uniform. Located at the ends of the temperature is high, the middle of the temperature is low, so that in the expansion process, the high temperature of the place first expansion, the low temperature of the place after the expansion, resulting in the phenomenon of bamboo knots.

In order to solve the phenomenon of bamboo knots in the blowing process, it is necessary to use a hot air circulation oven, which is characterised by the circulation of hot air in the oven to ensure a consistent temperature everywhere in the oven. Avoid the phenomenon of uneven heating during the heating process.