Discussion on the factors affecting the thickness

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Discussion on the factors affecting the thickness of BOPET film (Part 2)

2.3 the influence of the casting process on the thickness

the so-called casting refers to the process of rapidly cooling the melt into amorphous sheets through external force in order to achieve good tensile effect, as shown in Figure 4

2.3.1 selection of L-D (die lip cold drum) gap

the melt flows out of the die lip and is cast into a thick sheet on CD. Due to the traction of CD and the high voltage adsorption of pinning, the melt undergoes pre stretching during sheet casting. With the increase of L-D gap, the MDO outlet film will be narrowed, which further improves the tensile ratio of TDO without changing the original track width. According to formula (1), it can be found that the increase of drawing ratio will make the drawing more sufficient, the uniformity of drawing will be improved, and the thickness of film will tend to be improved. In actual production, considering the thickness deviation and productivity, the L-D gap has been adjusted from the initial 4mm to the current 15mm

2.3.2 position adjustment of pinning (SI)

in order to make the melt uniformly quench to an amorphous state, it is generally necessary to use the high electric pressure field of pinning system to make the melt uniformly adsorb on CD in an instant. The quality of uniform adsorption determines the number of spherulites in the thick sheet. The more spherulites, the worse the production stability, on the other hand, the local thickness change and uniformity of the film. Table 1 shows the relationship between Si position and thickness deviation at full load of 12b01

it can be seen from table 1 that when the si-die distance is 20 mm and the si-cd distance is mm, the thickness deviation of the film is the smallest and the production stability is good

2.3.3 Dieline control

when the melt at high shear rate flows out through the die lip, the elastic recovery phenomenon will occur due to the off mold expansion, and the odd polymer will condense above and below the die lip under the off film expansion. When its number accumulates to a certain point, with the traction of the thick sheet, it will randomly cast with the melt along the die lip, and irregular Thick and thin stripes with different lengths are the formation process of die line. Because the die head line greatly affects the thickness uniformity of the product, according to the mechanism that the off die expansion is affected by the shear rate and the cold drum traction, we mainly take the measures of reducing the export expansion by increasing the buffer failure, increasing the die lip opening and L-D spacing, so as to reduce the die head pressure and improve the pre drawing ratio. Good results have been achieved in production. With 12/μ M film as an example, the die line tends to disappear when the die lip opening is controlled at about 2.7mm

2.3.4 confirmation of bolt state

the adjustment of film thickness is generally divided into manual rough adjustment and automatic fine adjustment. Automatic fine adjustment is to adjust the heating power or temperature of the heating element on the thermal expansion bolt above the die head to expand when heated and shorten when cooled, so as to achieve accurate thickness control by adjusting the opening of the die lip. In order to ensure that the thermal bolt is in a better controlled state, on the one hand, the bolt power value is centered at 45% and controlled within the range of 20% - 80% for automatic fine adjustment; On the other hand, when 90% of the power value is 10%, it is necessary to turn off the automatic control, restore the heating power to the center and manually adjust the bolt. If the thickness is still not ideal after manual adjustment, it should be considered to confirm the mechanical state of the bolt itself to decide whether to replace the bolt. Only combining these two aspects can we ensure the most basic bolt conditions with controlled thickness

2.4 effect of APC (automatic profile control) on thickness

2.4.1 positioning of mapping diagram

the so-called mapping diagram refers to the corresponding position relationship between each adjusting bolt on the die head and the TD direction of the stretched film. Its accuracy is very critical to the control of film thickness. Due to the rapid cooling and cooling of the melt during the casting of the sheet, the nonlinear pre stretching as shown in Figure 5 will occur. When the prepared sheet is stretched longitudinally and transversely, due to different factors such as temperature and tensile stress, the film will again be stretched unevenly in the transverse direction. In order for the thickness gauge to correctly control the thickness of each bolt, it is necessary to accurately locate each bolt and the stretched film in the transverse direction

the commonly used positioning method in production is to select some areas on the OS, C and DS sides of film a to mark longitudinally, and then measure the stretched marking data on the C film to make the corresponding mapping diagram. With 12 μ M as an example, if the track width is 4.42m, then mark 3. The mapping diagram after the indication error shows "front positive and rear negative" or "front negative and rear positive" on the dial and individual points are out of tolerance is shown in Table 2

it can be seen from table 2 that corresponding to each die bolt, the mapping diagram gives the corresponding control range. According to this corresponding relationship, the purpose of accurately controlling the thickness can be achieved

2.4.2 influence of control mode on thickness

bopet thickness control is realized through the thickness curve measured by the continuous ray thickness gauge at the traction station, and through the data processing of the computer for the control of the heating power of the die bolt. Due to the large distance between the traction station and the die head, the lag time has a great relationship with the production speed, so the time control constant of the controller is very important. If the time constant is short, the heating power of the die bolt will change too quickly, resulting in severe fluctuations in thickness, which will affect the fluctuation of the die back pressure, and then affect the thickness fluctuation of the whole width; On the contrary, if the time constant is too long, the response time is too long, which will cause the thickness of a certain point to be on the high or low side for a long time, resulting in the phenomenon of untimely control. The time control constants of some common varieties are shown in Table 3. The advantages of new materials comprehensive industrial clusters in Bohai rim, Chang Sanjiao, zhusanjiao and other regions are prominent

in addition, because the ray thickness gauge is very sensitive, the mechanical walking device of the probe of the thickness gauge will have a great difference between the Jilin provincial government's ban on production and sales and the provision of disposable non degradable shopping bags throughout the province during the round-trip process. Therefore, when processing the measurement results, in order to eliminate the interference caused by accidental factors, it is often used to round-trip several times and superimpose the scanning results, To control the heating power of die bolt. Especially in the production of thin films, due to the high production speed and sensitive control, the results of four scans are often superimposed to reduce the frequent adjustment of the heating power of the die bolt, so as to realize the adjustment and stable control of the film thickness

3 conclusion

a) in order to make the stretching stable and the thickness uniform, it is necessary to choose the appropriate stretching ratio and stretching temperature

b) L-D distance, pinning position, bolt state and die line control have great influence on thickness deviation in the process of casting

c) in order to make the APC system work effectively, it is necessary to map different varieties and take corresponding control methods

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