**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 the 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 for heating and air intake accounts for 70% to 80% of the total energy consumption of the entire machine. It is also the area where organic solvents accumulate and are discharged.
Most gravure printing machines and dry laminating machines use electric heating or steam heating systems. New energy-saving heating technologies, such as hot air ovens, solar energy, and heat pumps, have been introduced. These systems use hot air to quickly evaporate organic solvents, enabling rapid drying of ink and adhesive layers. Properly setting the drying parameters is not only crucial for ensuring good print quality but also essential for meeting hygiene standards regarding residual solvent levels. Therefore, key drying parameters include drying temperature, air intake, airflow, and wind speed, which are critical in the compounding process.
**First, the current printing and compound drying principles and technology**
**1. Drying Temperature**
The evaporation of solvents is a physical process where a liquid changes into a gas. In an open environment, solvents naturally evaporate. In gravure printing and compounding, the equipment operates at high speeds, with printing speeds exceeding 300 m/min. To adapt to the fast operation of plastic film gravure printing machines and laminators, and to ensure the ink dries quickly on the film, enterprises generally increase the oven temperature to accelerate solvent evaporation and reduce the time required for drying.
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 deviations 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, causing the surface to dry faster while the interior remains wet, thus reducing product quality. For single-layer substrates like OPP, NY, and PE, which are prone to stretching, and PVC substrates, which tend to shrink, 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, which in turn impacts product quality.
**2. Air Intake, Exhaust, and Wind Speed**
These three data can be combined into one concept called "circulation amount."
The wind speed is determined by the air intake and exhaust volume under constant oven structure conditions. The oven structure is known, and the number and size of air nozzles are fixed. As the air intake and exhaust volume increase, the wind speed also increases. South Korea's BOOSUNG Engineering company found in their study of various printing equipment manufacturing companies' gravure printing machine ovens that there are two types of oven outlets: nozzle structures and hole structures. Nozzle structures allow for wind speed monitoring, while hole structures only provide reference data for total exhaust air circulation.
Adjusting the air intake serves two purposes: first, to adjust the air volume according to the printing area of each printing unit to achieve adequate drying and save energy; second, to create a relative negative pressure inside the oven.
When the difference between the air intake and exhaust air volume is set, the oven can be in either positive or negative pressure states.
**(1) Positive Pressure**
Inlet air volume > Exhaust air volume
When the oven is in a positive pressure state, excess hot air overflows, leading to energy waste. This can also increase the odor in the workshop and blow hot air into the gravure ink tray. At the same time, since the hot air entering the oven takes away volatile solvents, the exhaust volume cannot discharge them in a timely manner if it is too small, resulting in increased solvent concentration in the oven and reduced drying efficiency.
As shown in the figure above, when the oven temperature is 70°C and the ambient temperature is 20°C, a large amount of 70°C hot air overflows due to excessive positive pressure, wasting energy.
**(2) Negative Pressure**
Inlet air volume < Exhaust air volume
Generally, the oven is required to maintain a negative pressure state during the printing process, so that the exhaust air volume is slightly larger than the intake air volume. This helps remove hot air containing volatile solvents from the oven, maintaining a certain solvent concentration to create a rapid evaporation drying condition.
However, if the negative pressure is too high, large amounts of 20°C ambient air are sucked into the oven, neutralizing the 70°C process temperature required in the oven. This results in a drop in temperature in parts of the oven below 70°C, decreasing drying efficiency and causing energy waste of about 30%. Additionally, dust in the air can enter the oven, causing contamination of printed materials.
**Second, Drying Energy-Saving New Technology**
Shanghai Tepeng Energy Saving Equipment Co., Ltd. has developed a new heat pump energy-saving system based on currently used drying technology. By using clean energy + temperature, air volume data + oven inlet and outlet air volume balance, it achieves scientific management, guarantees a 50% to 80% energy-saving rate, and improves equipment usage efficiency.
The features of the heat pump energy-saving system include:
1. Core elements of hot air ventilation and drying: (heat + wind + balance) data (quantification)
To implement this, the following reform measures can be taken:
- Install accurate measuring instruments (FMS) at the air outlet to measure air volume accurately (accuracy ±0.3%) and use frequency converters to control air volume in real-time.
- Balance - Pass: The relative balance of the intake and exhaust air volumes has not been given enough attention before. Traditional equipment simply adjusts the damper in the exhaust pipe to address the balance issue. However, a near 1:1 balance of intake and exhaust air volumes significantly impacts the performance of dry residual solvent indicators, energy consumption, and operating environment.
Retrofit measures include equipping each drying unit with a digital balance meter and an automatic exhaust valve. Based on the data displayed on the online balance meter, the automatic air damper in the exhaust air is adjusted to maintain the balance of intake and exhaust air.
The specific parameters may refer to the operation interface of the recycled waste heat hot air drying system produced by Shanghai TePeng Energy Saving Equipment Co., Ltd., using the patented technology of Korea BOOSUNG Engineering Co., Ltd.
Heat - Temperature: Detected by the temperature probe. It is important to install the temperature probe near the oven inlet to avoid large temperature errors within the oven. Good insulation measures are also necessary.
Wind - Air Volume: Traditional equipment lacks air flow measurement devices. Operators manually control the air valve to adjust the air volume. To ensure drying quality, operators often use excessive hot air and need to re-adjust after changing the printing plate. Rarely adjusted, leading to energy waste.
**Third, Summary**
The drying process in flexible packaging accounts for 70% to 80% of the total machine energy consumption. It is the most promising area for energy savings, greatly reducing production costs. By using clean energy + controlling air volume balance at the oven inlet and outlet, it can make the environment cleaner and reduce odors. At the same time, through system controls such as frequency converters, PLCs, thyristors, FMS, and automatic exhaust valves, the balance of temperature and airflow can be achieved. Digital settings enable scientific management, ultimately reducing undesirable products, improving product quality, and achieving 50% to 80% energy savings.
Shanghai Tepeng Energy Saving Equipment Co., Ltd., using techniques from BOOSUNG Engineering, has developed energy-saving hot air dryer products (E-SAVER) based on international patents. Since 1983, BOOSUNG has been developing hot air drying systems for the flexible packaging industry, including gravure printing machines, laminators, and coaters. After years of efforts, they 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 leader in hot air drying. The invention patent number is PCT/KR2009/006395. Special Friends of Energy was established in Shanghai in 2013. So far, more than 200 sets of energy-saving heat pump systems have been produced, serving all major domestic packaging companies. The effective energy-saving rate is 50% to 80%.
(Shanghai Tepeng Energy Saving Equipment Co., Ltd.)
Lifting Columns
Suzhou Herstar Medical Technology Co., Ltd. , https://www.hosunherstar.com