How to save raw materials and power during the coating process of Carbonless copy paper (also named as NCR paper) is directly related to manufacturing costs and enterprise benefits.

Here we elaborate on how to implement it from four aspects in this article.

1. Coating part

1.1 Air knife coating station

At present, most of the coating equipment used by Chinese NCR paper mills is air knife coating machines. However, the disadvantages of air knife coating are: firstly, it wastes coating chemicals; secondly, it causes dust pollution; thirdly, it causes serious noise; fourthly, it wastes electricity.

The strong airflow not only blows away the excess paint, but also turns it into smoke and dust. Without the need for a professional separation device for recycling, only the induced draft can be used for extraction, and even if it is recycled, it cannot be reused. According to survey statistics, the resulting coatings waste can reach up to 20%. The smoke collection system often blocks and fails, causing dust pollution in the workshop and surrounding environment.

A certain factory uses an air knife coating machine produced in Taiwan in the 1990s, with a speed of about 200m/min. In order to remove excess coatings, blow flat coatings, and maintain environmental cleanliness, each coating machine requires 52kW of power. With 300 working days per year and 24-hour operation, the annual power consumption reaches 400,000 kWh, worth over 400,000 RMB. It is clearly high energy consumption and not low-carbon.

1.2 Solutions

1.2.1 Reduce the workload of air knives

To avoid missed coatings, it is necessary to apply an excessive amount first, and then use an air knife to blow off the excess part. So, after coating, use a scraper to scrape off the excess paint, so that the air knife only needs to blow off a part, and the burden of the air knife will be greatly reduced. In this way, not only can the power of the Roots blower be reduced, but also the dust can be reduced. Therefore, the power of the dust removal blower can also be reduced. The use of the dust removal blower has been changed from the original large motor to about 3-4kW, which can reduce coatings waste, circulation, foaming, etc.

1.2.2 Use a suitable measuring rod

If a regular measuring rod is used, only a stainless steel rod with a diameter of 12-50mm or less is needed, which can scrape off some of the coatings and reduce the burden of the air knife, thereby reducing the loss of powder dust and coatings.

1.2 Replace with a with concave roller coater

Another disadvantage of air knife coating is that the solid content of the coating is low, only about 15% -26%, which requires a huge amount of energy during drying. Therefore, it is difficult to achieve significant savings and energy conservation in air knife coating. If the coating machine adopts a concave roller type coating head, the solid content can be increased by 40%~60%, which not only greatly saves steam, but also eliminates the above-mentioned pollution and a large amount of coatings loss.

The principle of concave roller coating head is shown in Figure 1. The surface of the concave roller is engraved with concave patterns, and the size of the concave patterns determines how much coatings can be accommodated per unit surface area. The scraper removes the coating outside the pattern and transfers a fixed amount to the surface of the coating roller; The coating roller is then transferred to the surface of the paper. Adjusting the linear speed difference between the coating roller and the paper feeding roller can change the coating amount within a section, and changing the size of the concave lines on the concave roller can move the section up or down

Figure 1 Concave roller type coating head

2. Drying part

2.1 Drying oven

The spray temperature, spray wind speed, and distance from the nozzle to the paper surface of the drying oven determine the drying efficiency. And the heat exchange efficiency of converting steam into hot air also affects the unit product steam consumption. The hot air drying systems of early NCR paper coating equipment were introduced in the late 1980s and early 1990s, and radiators were installed outside the oven, resulting in high heat loss; Basically, they are heated on one side; The distance from the air nozzle to the paper surface is mostly greater than 30mm; Spray air at a speed of over 10 m/min; Therefore, the measured heating efficiency of the system is only below 20%. In the 1990s, China introduced air floating ovens, which increased the thermal efficiency to around 25% and has now reached 50%.

The air floating oven has the following characteristics:

(1) The hot air channel and heat exchanger are both built-in, which not only makes the volume small and occupies less space, but also reduces heat loss,

(2) High spray speed, greater than 50m/min (ordinary oven less than 15m/min), high heat transfer coefficient.

(3) The closest distance between the paper web and the paper web is less than 10mm, which can greatly improve the heat transfer coefficient.

(4) The air volume of the upper and lower spray on the paper is different, that is, the air volume on the coated surface is greater than that on the non coated surface.

Figure 2 Air floating drying oven display

2.2 Combination of concave roller and air floating oven

A certain paper mill factory uses a mesh drying and air knife coating head to coat the CF paper, which is dried with 0.5MPa saturated steam. The steam consumption per ton of paper is 1.6t, and the calculated heating efficiency is less than 20%. Replacing the concave roller coating head with a floating air drying oven results in a steam consumption of only about 0.5t per ton of product, with a calculated heating efficiency of over 45%. The factory annually production is 17,000 tons of paper and saves 21,700 tons of steam throughout the year. This alone can save 4.86 million RMB annually. If we add the saved coatings, Roots blower, dust exhaust fan, etc., the energy saving is quite considerable.

Figure 3 Steam consumption data of paper under different coating and drying methods

The figure 3 shows the steam consumption data under saturated steam at 0.5MPa and with a coating amount of 5.5-6g/m². The solid content of the paper coated with air floating oven and concave roller is 40%; The solid content of the paper coated with the trawl type drying and air knife coating head is 20%. Through comparison, the combination of the concave roller coating head and the air floating oven has a significant energy-saving effect.

Due to the precise carving of stripes on the roller surface, the concave roller can apply very sticky paint to the coating surface with very little lateral flow, so there is no air mist generated by air knife coating, which does not waste paint or pollute the environment. The coating effect is very good. Due to the high solid content of the coating, the amount of steam required for drying can be significantly reduced.

1. Automatic paper roll unwinding and changing

The basic process in the paper industry is to unload the coated paper and replace it with a new roll for reprocessing after each roll of paper is produced. However, this seemingly simple processing process has some problems:

(1) After the changed roll of paper enters the oven, due to quality issues, the head and end are mostly scrap, defective, etc. Especially when the paper is just rolled into a paper tube or steel pipe, there are generally problems of wrinkling and uneven coating amount.

(2) After changing the coil, the speed needs to be increased again, and the hot air and exhaust systems need to be restarted. The temperature needs to be readjusted, which not only affects production efficiency but also causes energy waste.

(3) After each roll is processed, the workers need to re-roll and strip the paper. The labor intensity is high because the initial tension of the paper is difficult to control and it is easy to break the paper. Especially after changing the paper type, the tension and starting temperature are difficult to control. Sometimes, the paper cannot be taken for a long time, resulting in a large amount of waste paper and causing considerable trouble in production.

Our company, QINGDAO JIERUIXIN Machinery and Technology Co., Ltd., adopts Swiss BMB automatic roll changing technology to achieve non-stop roll changing, saving 100m of paper per roll. The success rate of automatic lap paper is high, and continuous production of paper not only improves the instantaneous energy cost at each machine start, but also reduces the labor intensity of workers and improves product quality

2. Unwinding brake power generation

The unwinding section generally uses magnetic powder brakes or friction plate brakes for braking. Regardless of the type of braking, it relies on friction to consume the kinetic energy of the paper roll, converting it into heating energy and dissipating it. The source of this kinetic energy is the motor that pulls the paper.

Friction braking is difficult to achieve satisfactory constant tension control because the diameter of the unwinding paper changes from large to small, so the torque also changes with the diameter. In order to maintain constant tension, it is necessary to reduce the tension as the coil diameter decreases. This tension adjustment can cause the volume and weight of the magnetic powder brake to be too large and even water-cooled, while the friction plate type has a gradient jump increase, resulting in unstable tension fluctuations. Reverse braking with a DC motor can effectively solve the above problems.

DC motors can automatically adjust the armature voltage, and during the adjustment process, the torque characteristic is a continuous and smooth curve, which can achieve automatic constant tension. At the same time, when the DC motor is driven and reversed, it is in a generating state, except for the unchanged torque characteristics, and the electrical energy can be fed back to the grid. That is to say, the electrical energy consumed by friction braking will no longer be wasted in the form of heat, but will be converted into electrical energy recovery.

Using a DC motor as the brake, taking the speed of 300m/min and the full range of unwinding tension of 400N as an example, the average braking power can be calculated to be 2kW. One machine can save 48 kWh of electricity per day, and the annual production of 300 days can save 14,400 kWh of electricity. In addition, it can save a lot of magnetic powder or friction pads, and no longer require regular maintenance, achieving the best braking performance. It is obvious that this move also significantly saves energy.

3. Conclusion

(1) The concave roller coating head can accurately apply high solid content coatings, saving steam during drying due to the low moisture content of the coatings, and providing a good production environment

(2) The high-efficiency air floating oven has high heating efficiency, and with the use of concave roller coating heads, it has a significant energy-saving effect.

(3) Automatic paper splicer unwinding device can save raw paper and improve production efficiency.

(4) The unwinding adopts a DC motor, which can feed back the electrical energy generated by the brake to the power grid.