The general FEP heat shrink tube manufacturer molding process consists of roughly two parts: base fep tube manufacturing and blow molding.
FEP shrink tubing forming process
Preparation of the base tube – blowing and shaping (expansion process)
I. Fep extruder Preparation of base tube
The forming process of the base tube used for FEP heat shrink tube is similar to the production of FEP tube process, but the forming process conditions are not exactly the same, generally using low temperature, low speed extrusion. The screw speed should not be too fast in order to control the lower extrusion volume. The process is:
FEP pellets a extrusion a horizontal Fep extruder head a cooling and setting a haul-off a cutting.
In order to get in line with the blowing of the base tube, FEP tube stretching ratio should be controlled at 3 ~ 7 as good, molten cone length control in the range of 10 ~ 20mm more appropriate, more than 20mm, molten 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-shrinkable tube. The molten billet must be quickly cooled 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 plasticization of the melt and dense.
In short, as the base FEP 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 FEP hose is generally controlled within the range of 0.20-0.60mm, and the better wall thickness should be in (0.25-0.40)±0.02mm.
2 Fep extruder Blow molding (expansion process)
FEP heat-shrink tubing blowing process is developed on the basis of the general plastic blowing process, generally using the internal compressed air blowing method. According to the different blow molding process can be divided into two types:
2.1 Continuous blowing
In the blowing machine, a certain length of the base tube continuously passes through the blowing mold, blowing under heating and pressure, and after cooling and shaping.
The advantages of this method are: low investment, only need to replace the blowing mold to blow different specifications of the shrink tube, high production efficiency, and can get a certain continuous length of heat-shrink tube.
2.2 Intermittent blowing
FEP heat shrink tubing base tube cut to a certain length (generally 1 m), and then fixed in the blowing mold, both ends sealed, one end ventilated, heated, pressurized blowing molding. Advantages: able to blow special specifications of shrink tube, and heat shrink tube appearance quality (and the appearance of the base tube quality close to), roundness; disadvantages: low production efficiency, product length also has limitations.
3 The process parameters of the blowing process are mainly: temperature, pressure, traction speed
According to the basic principles of the blowing process, FEP heat shrinkable tube base tube blowing fep material properties temperature should be between the glass transition temperature and melting temperature, the better blowing temperature of 130 ℃ ~ 190 ℃. If the temperature is lower than 130 ℃, you need higher pressure, the pressure is too high, the tube is easy to rupture; if the temperature is higher than 190 ℃, the tensile strength and elongation of the tube with the increase in temperature and reduce, and thus blowing is also easy to rupture.
Under fep material properties suitable blowing temperature conditions, there is also a better blowing pressure. In the case of constant blowing temperature, the size of the blowing rate depends on the size of the gas pressure in the base FEP tube, and the size of the air pressure is a function of the diameter of the base tube, 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 FEP hose, the greater the surface tension. The general blowing pressure is (0.25 ~ 0.1) MPa.
3.3 Traction speed
When the blowing temperature and pressure are certain, the traction speed is inversely proportional to the blowing rate. But when the FEP heat-shrink tubing 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 a better range, and their values should be a low value of temperature, pressure, high value of traction speed is good.
4 the choice of blowing process
Need to select the appropriate process according to product requirements, generally speaking, the requirement for continuous product length, the appearance of the requirements are not high, do not require garden neatness, choose continuous blowing process. On the contrary, the intermittent method should be selected.
5 To fully consider the impact of the crystallinity of the base FEP tube blowing
FEP is a crystalline fluorine plastic, its crystallinity are above 70%, with the fep polymer increase in crystallinity, elastic modulus, hardness have different degrees of increase, while tensile strength, elongation, flexural resistance are correspondingly reduced. This is very unfavorable to the production of heat-shrink tube , because the blowing base tube requires high tensile strength and elongation. Therefore, in the production of the base tube, should try to reduce the crystallinity of the base tube, so that the tube is flexible, so that the blow molding.
6 The appropriate extrusion process conditions is a key factor in improving the blowing rate
Extrusion, the use of large stretching ratio of extruded FEP tube, longitudinal and transverse properties of large differences, is not conducive to blow molding. Therefore, in order to improve the blowing rate, it is necessary to control the lower extrusion volume and use a smaller stretching ratio to achieve the purpose of improving the transverse strength of the fep hose, which can increase the blowing rate to 50%.
7 FEP tube rupture is a common phenomenon in the process of producing FEP heat shrink tubes
7.1 Blowing temperature is too high
So that the base tube strength decline caused by rupture if the blowing mold temperature distribution is not uniform, but also easy to cause rupture.
7.2 Blowing pressure is too high
Continuous blowing makes the base tube over-expanded, and the cooling section of the blowing mold expands and dies, and the tube is stretched, which easily causes rupture. Intermittent blowing, the pressure exceeds the transverse strength limit of the base tube, causing rupture.
7.3 In the intermittent blowing method
Pay attention to keep the parallelism between the base tube and the blowing mold when loading the head to avoid local overheating of the base tube during the heating process and rupture under the pressure.
In summary, FEP heat shrink tube manufacturers by selecting the appropriate process and process parameters, the use of continuous blowing and intermittent blowing can develop a large blowing rate, longitudinal shrinkage rate qualified FEP heat shrink tube