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Analysis of composite quality control of cast polypropylene film

unstressed cast polypropylene film (CPP) is an unstressed flat die extruded film produced after melt casting quenching. It has the characteristics of good transparency, high gloss, good flatness, balanced MD/TD properties, high productivity, easy heat sealing, etc. after surface treatment (usually EDM), it can be used in aluminum plating, color printing, composite and other aspects, and is widely used in food Packaging of textiles and daily necessities. At present, the main products of domestic CPP manufacturing enterprises include vacuum aluminum plating type, general composite type, cooking type, special purpose CPP film, etc

for the quality control of CPP products, the performance of production line equipment, production process, raw materials and production operation factors are particularly important

at present, the international famous manufacturers of CPP production line mainly include the following companies: reifenhauser in Germany, Battenfeld in the United States, Mitsubishi Heavy Industries in Japan, Collins in Italy (c011 INS), lenzin, etc. The different main design ideas of different production lines lead to great differences in equipment performance, which is crucial to product quality

from the point of view of production process, the influence on film quality mainly includes: melt temperature, air box, vacuum chamber, cold roll temperature, EDM treatment, winding parameters, etc. The next interview will be analyzed from these aspects

effect of melt temperature on film properties: with the increase of melt temperature, the haze of the film decreases, and the longitudinal (MD) tensile strength increases, but the transverse (TD) tensile strength of the film decreases, which may cause the film to become brittle in winter and increase the off machine friction coefficient of the film. Therefore, the melting temperature must be controlled within a reasonable range, so as to ensure the balance of film quality. Generally speaking, the melting temperature of PP material should be controlled at 230-260 degrees

effect of air box on film performance: the role of air box is to form a uniform air layer between the film and the surface of the casting roll, so that the film can be evenly adhered to the roll and cooled, so as to ensure stable production. The space relative position and inclination angle of the air box and the casting roll must be appropriate, and the air volume of the air box must also be appropriate. Excessive assembly leads to increased jitter of the melt curtain, making the thickness of the film unstable; Too small causes the film not to stick to the roll, so that the film is not cooled sufficiently, which is easy to cause wrinkle, water ripple and other appearance quality problems

effect of vacuum chamber on film properties: the function of vacuum chamber is to extract the oil smoke formed by small molecules volatilized by the melt at high temperature and make the film stick to the roll evenly. Too large vacuum can easily lead to appearance quality problems such as scratches on the film surface, and too small vacuum can easily lead to poor roll sticking, affecting the appearance and physical and chemical properties

the influence of casting roller temperature on film properties: with the increase of temperature, the film stiffness increases, and the friction coefficient decreases, but the transparency decreases, and the transverse elongation decreases; Too low temperature will easily lead to the increase of friction coefficient, film astringency, and affect the winding quality; Too high temperature is easy to reduce the transverse elongation of the film, making the product brittle in winter, and it is easy to break the film in the process of aluminum plating and lamination. Therefore, the temperature should be set according to different types of products, thickness and production speed

the surface of the casting roll and other cooling rolls is easy to accumulate small molecular precipitates in the raw materials, which pollute the surface of the cooling roll and affect the appearance quality of the film: for this problem, it is necessary to stop the machine for cleaning, and check and confirm the cleanness of the roll surface before starting the machine, so as to prolong the continuous production time and reduce the loss caused by the increase of start-up and shutdown

EDM treatment: PP and PE materials are not subject to surface treatment, and the surface wetting tension is lower than 32 dyne. CPP products should meet the requirements of printing, compounding and aluminizing, and the surface wetting tension is generally required to be above 38 dyne. After EDM treatment, the surface wetting tension of the film gradually decreases with the passage of time due to the precipitation of small molecules and additives in the raw materials, as well as the influence of environmental temperature, humidity, dust and other factors. Set appropriate spark power according to different products, thickness and production speed. The power is too low, and the surface wetting tension declines too fast; If the surface wetting tension is too high, it will not increase infinitely with the increase of power. On the contrary, it will cause the loss of physical and chemical properties of the film and the defects of appearance quality. In addition, appropriately increasing the temperature of corona treatment roll can also appropriately improve the surface treatment effect and slow down the decline rate of surface wetting tension

winding parameters: the winding parameters must be set according to the film thickness, production speed and other factors. In the process of CPP processing, the winding parameters directly affect the product quality. In general, the winding tension is too large, and the experimental data made in the future if there is a gap in the screw is not conducive to the relaxation of the film. Under the condition of film shaping, according to the statistics of more than 150 years, it is easy to have cold stretching. If the winding tension is too small, it will be detrimental to the slitting process - the slitting tension is difficult to control. Therefore, the winding parameters should be set according to different products and different thicknesses to achieve the best winding effect

in terms of the selection of raw materials, CPP films need to use proprietary resins. Generally, resins with a melt flow rate (MFR) of 6 ~ 12g/10min are required to have a certain melt fluidity. It is generally divided into homogeneous

PP, binary copolymer PP and ternary copolymer PP

for three-layer co extruded CPP film, it can be divided into corona layer, core layer and heat sealing layer. Generally, the corona layer adopts binary copolymerized PP, which requires a certain degree of anti adhesion, and strictly controls the cross-linked high-molecular materials of low molecules. Through the cross-linking agent, the linear high-molecular chain becomes the content of network structure, so as to ensure the maintenance of surface treatment effect. Homopolymer PP is generally selected as the core layer to ensure that the film has excellent rigidity, and the good optical properties (high transparency and high gloss) of CPP film are mainly determined by the core layer. In addition, from the perspective of cost control, the price of homopolymer PP is relatively low, and the core material accounts for about 60% - 70% of the total film weight. Due to the increasing number of high-speed packaging machines, ternary copolymer PP is generally used as the heat sealing layer, which must have good heat sealing performance, that is, the so-called low-temperature heat sealing property, and at the same time, it should also have the characteristics of smoothness, anti adhesion, less precipitates and so on

from the perspective of production operation, it is necessary to ensure the cleanness of raw materials. In addition to selecting appropriate filtration, filtration mesh number and appropriate raw materials to control the crystal point of film appearance quality defects, ensuring that the raw materials are clean and free from pollution is also an important measure to control the crystal point caused by impurities

during the inspection of film appearance in the production process, it is also important to take corresponding adjustment measures immediately after finding abnormalities to prevent batch nonconforming products

(author/Jiang Wei)

-- flexible packaging weekly

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