Abstract: This paper introduces a new drying and energy-saving system by analyzing the process factors that affect the drying effect, such as drying temperature, air inlet volume, exhaust air volume, and wind speed. It can ultimately achieve an energy saving of 50% to 80% in printing and compounding processes.
In the flexible packaging process, the drying system is the largest energy-consuming part of the gravure printing press and the dry laminating machine. The energy consumption of heating and air intake accounts for 70% to 80% of the total energy consumption of the entire machine. It is also the place where organic solvents accumulate and are discharged.
Most gravure printing machines and dry laminating machines use electric or steam heating systems, such as hot air ovens, solar energy, heat pumps, etc. These systems generate hot air that quickly evaporates organic solvents, enabling rapid drying of ink and adhesive layers. Proper setting of drying parameters is not only crucial for ensuring print quality but also essential for meeting hygienic standards regarding residual solvent levels. Therefore, the drying parameters include the drying temperature, air intake, air flow, and wind speed, which are key elements in the compounding process.
First, the current printing compound drying principle and technology:
1. Drying Temperature
The volatilization of a solvent is a physical process in which a liquid changes into a gas. In an open environment, solvents naturally evaporate. In gravure printing and compounding, the equipment runs at high speeds, often exceeding 300m/min. To meet the fast-drying requirements on the film, enterprises generally increase the oven temperature to accelerate solvent evaporation and shorten the drying time. The criteria for setting the drying temperature are that the substrate should not deform under tension and the temperature should remain stable (within a deviation range of ±1°C). The temperature, air pressure, and air volume deviation of each air outlet in the same oven should be minimized.
Low temperatures result in low solvent evaporation rates, leading to inadequate drying. High temperatures may cause substrates to deform, especially when the ink or adhesive layer is thick, resulting in surface drying while internal drying remains insufficient, thus reducing product quality. For single-layer substrates like OPP, NY, and PE, and PVC substrates, it is recommended that the drying temperature does not exceed 65°C, and a reasonable machine tension parameter must be matched with the drying temperature.
If the drying temperature is unstable, large temperature differences can generate internal stress on the substrate, affecting the flatness and consistency of the finished product, thereby impacting product quality.
2. Intake, Exhaust, and Wind Speed
These three data can be combined into one, called the circulation amount. The wind speed is determined by the air intake and exhaust volume under conditions of a constant oven structure. The structure of the oven is known, and the number and size of the air nozzles are constant. As the air intake and exhaust volume increase, the wind speed increases. South Korea's BOOSUNG Engineering company found in their study of various printing equipment manufacturers' intaglio printing machine ovens that there are two forms of oven outlets: nozzle structure and hole structure. The nozzle structure oven allows wind speed to be monitored, while the hole structure oven can only be used as a reference for the total exhaust air volume.
Adjusting the air intake aims to ensure adequate drying and save energy. It also creates a relative negative pressure inside the oven.
When the difference between the air inlet and exhaust volume is set, the oven can be in either positive or negative pressure states.
(1) Positive Pressure: Inlet air volume > Exhaust air volume
In a positive pressure state, the drying process may lead to ink skinning, excessive heat loss, and increased odor in the workshop. Hot air from the oven can blow into the gravure ink tray. At the same time, since the hot air entering the oven takes away volatile solvents, the exhaust volume cannot timely discharge them, causing an increase in solvent concentration in the oven and a decrease in drying efficiency.
As shown in the figure above, the drying temperature in the oven is 70°C, and the ambient temperature is 20°C. When the positive pressure is too large, a large amount of 70°C hot air overflows, wasting energy.
(2) Negative Pressure: Inlet air volume < Exhaust air volume
Generally, the inside of the oven is required to reach a negative pressure state so that the exhaust air volume is slightly larger than the intake air volume, allowing hot air containing volatile solvents to be taken out of the oven, maintaining a certain solvent concentration in the oven to create a rapid volatile drying condition.
However, if the negative pressure is too high, 20°C air from the environment is sucked into the oven in large quantities to neutralize the 70°C process temperature required in the oven. This leads to a decrease in drying efficiency, and excessive exhaust again enters the oven, causing a waste of energy about 30%. Similarly, dust in the air can enter the oven, leading to contamination of printed objects.
Second, Drying Energy-Saving New Technology
Shanghai Tepeng Energy Saving Equipment Co., Ltd. has developed a new heat pump energy-saving system using clean energy + temperature, air volume data + oven inlet and outlet air volume balance. This achieves scientific management, guarantees a 50% to 80% energy-saving rate, and improves equipment usage efficiency.
Heat pump energy-saving system features include: (heat + wind + balance) data (quantification). Regarding this, the following reform measures can be taken: equipping the air outlet with an accurate measuring instrument (FMS), which measures the air volume accurately (accuracy ±0.3%) on-line and uses the frequency converter to control the air volume in real-time. Balance - Pass: The relative balance of the intake air volume and the exhaust air volume has not been taken seriously before. In traditional equipment, simply adjusting the damper in the exhaust pipe to adjust the balance problem. However, the balance of intake air volume and exhaust volume close to 1:1 has a considerable impact on the performance of dry residual solvent indicators, energy consumption, and operating environment.
Retrofit measures: Each set of drying unit is equipped with a digital balance meter and an automatic exhaust air valve. According to the data displayed on the online balance meter, the automatic air damper in the exhaust air is adjusted to adjust the balance of the intake and exhaust air.
Third, Summary
The drying process of flexible packaging accounts for 70%~80% of the energy consumption of the whole machine. It is the most promising place for energy saving and greatly reduces production costs. By using clean energy + oven inlet and outlet to control air volume balance, it can make the environment clean and reduce odor. At the same time, through system control such as frequency converter, PLC, thyristor, FMS, automatic exhaust valve, etc., to realize the balance of temperature and airflow, and use digital settings to manage scientifically and ultimately reduce undesirable products. Improve product quality and achieve 50% to 80% energy savings.
Shanghai Tepeng special techniques from BOOSUNG Engineering company, BOOSUNG business since 1983, 30 years in development, a variety of hot air drying system equipment supply flexible packaging industry, intaglio printing machine, laminating machine and coater, and after years of efforts, the company launched the energy-saving hot air dryer product (E-SAVER) using the international patent "Heat exchange element with heat recovery device" and became a global hot air drying company. Invention Patent No. 1 (PCT/KR2009/006395). Special Friends of Energy was established in Shanghai in 2013. So far, it has produced more than 200 sets of energy-saving heat pump systems and has served all domestic large and medium-sized packaging companies. The effective energy-saving rate is 50% to 80%.
(Shanghai Tepeng Energy Saving Equipment Co., Ltd.)
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Suzhou Herstar Medical Technology Co., Ltd. , https://www.hosunherstar.com