0 Introduction
Currently, most cardboard manufacturers in China still use traditional manual glue-making machines, where glue quality relies heavily on worker skills. However, with increasing market competition, the efficiency requirements for cardboard production lines are rising, and manual glue-making methods are no longer sufficient. This article introduces a glue preparation control system designed for use with cardboard production lines. It enables fully automated control of the glue-making, glue-feeding, and glue-application processes. Because the glue is prepared according to pre-set raw material ratios and procedures, it avoids glue quality instability caused by human error, ensuring the peel strength of the cardboard boxes. Furthermore, the automated glue-making system is significantly more efficient than manual glue-making, meeting the process requirements of high-speed production lines.
1. Overview of Process Equipment and Flow
1.1 Main process equipment
As shown in Figure 1, the main process equipment of the glue preparation production line consists of storage tanks (including three tanks for starch, alkali solution, and resin), transfer tanks (including three tanks for borax, alkali solution, and resin, and their shared weighing device), a main tank and its weighing device, and four finished product tanks and their metering devices.
This adhesive preparation production line is equipped with an alkali-refining system (using carbon steel tanks), a stirring device, an alkali storage tank, and two alkali pumps. The alkali transfer tank and pipelines are all made of stainless steel. After the caustic soda flakes are refined into liquid alkali, they are stored in the alkali storage tank. The liquid level in the alkali transfer tank can be automatically controlled by a PCC (Precision Control System). When it falls below the set low limit, the alkali pump will automatically start to replenish the alkali from the storage tank.
The starch storage tank is made of carbon steel and is equipped with a screw conveyor, a vibrating feeder, and high and low level sensors. The main tank is equipped with a steam heating device and an automatic temperature control system, which includes temperature detection elements, secondary temperature instruments, and a steam shut-off valve, and uses a PCC (Polycarbon Controlled Controller) to control the temperature.
Each glue storage tank is equipped with a liquid level detection and stirring device. The glue weight in each glue storage tank can be displayed and monitored in real time on a touch screen in the glue preparation room.
Figure 1. Main process equipment and layout of the adhesive preparation production line.
1.2 Overview of the Process Flow
As shown in Figure 1, materials are first added from the storage tanks of borax, resin, and alkali to the transfer tank. All three transfer tanks are fixed to a frame supported by three sensors and share a common weighing device. At room temperature, both resin and alkali are liquids, and their transfer tanks are equipped with high and low level floats. The first time, only resin is added. Adding stops when the liquid level reaches the high level float. The weight displayed on the transfer tank's weighing instrument at this point is the weight of the resin, which is also its maximum value. The second time, only alkali is added. Adding stops when the liquid level reaches the high level float. The weight displayed on the instrument at this point minus the weight of the resin added the first time is the weight of the alkali. This calculated value is saved as the current weight of the alkali. Borax is a solid and is manually added to the borax transfer tank, but it is also measured in the same way. The starch storage tank is equipped with high and low level sensors and a vibrator. When the high level is reached, no more starch can be added to the tank; the low level signal prompts for addition. The vibrator's function is to loosen and disperse the starch, preventing it from bridging. When the material level drops below the minimum allowable level, the vibrator will automatically activate to shake off the starch adhering to the tank walls.
When producing adhesive, besides adding a certain proportion of water and starch as base materials to the main tank in several stages, the main ingredients added are borax, resin, and alkali solution. These three ingredients are added separately. If alkali solution is being added, the weight in the transfer tank will decrease; this difference is the weight of the alkali solution injected into the main tank. The current weight of the alkali solution in the transfer tank is the original weight minus the weight of the alkali solution injected into the main tank. This value is stored as the current weight of the alkali solution.
The other two materials are added to the main tank in the same way.
Each cardboard production line actually only needs one type of adhesive formula, and the formulas for different cardboard production lines are also different. Currently, the adhesive equipment is equipped with 4 finished product tanks (with steam heating devices). The adhesive formula in each finished product tank is different. These 4 tanks are located next to different cardboard production lines and are connected to the main tank through pipes. The heated adhesive is drawn from the main tank into the corresponding finished product tank by an air pump.
The system features an automatic formula correction function. During material addition, deviations can be monitored on the screen, and the system automatically corrects subsequent addition ratios based on the formula proportions to ensure accuracy. It can communicate with the user's production management system, automatically determining the glue production quantity based on pending orders through parameter settings to reduce glue waste. After the initial formula input, user adjustments can be made without calculating material changes; simply inputting the glue variation result will automatically modify the formula. An automatic glue blowing function automatically blows glue and resets the system after each can of glue is delivered to prevent hose blockage.
2. Composition and Configuration of the Control System
2.1 System Composition
As shown in Figure 2, this control system is based on B&R's B2003 series PCC, uses a touch screen as the human-machine interface, and is integrated with Mettler Toledo's PANTHER weighing instrument, main tank temperature controller, and water and starch conveying control devices. In addition, the system is also equipped with pumps, shut-off valves, and level and material level sensors.
2.2 System Configuration
This PCC is configured with one BP704 (4-slot baseplate), whose CPU is mounted at the far left of the baseplate. It uses a dual-width CP476 module with a 24V DC power supply. This module has one RS232 and one CAN interface, along with status indicator lights. It also has four screw-in module slots; for expansion, the screw-in modules are inserted into the slots and secured with screws. The screw-in modules can be analog or digital modules, or communication expansion modules.
One slot houses an IF321 module with one RS485/RS422 interface for data communication with instrument stations #1, #2, and #3. The other slot houses an AI774 (4-channel analog input module) for acquiring the liquid level (corresponding to weight) data from the four finished product tanks. Additionally, the PCC is equipped with two DM465 modules (16-channel input and 16-channel transistor output digital mixed-signal modules) and one DM435 module (8-channel input and 8-channel transistor output digital mixed-signal module). The system's human-machine interface (HMI) is a Power Panel 4PP320.1505-31 color TFT 15” touchscreen (24V DC, configured with one Ethernet 10/100M, one RS232, and two USB interfaces).
Figure 2. Composition of the automatic control system of the glue-making machine
The weighing system utilizes two sets of Mettler Toledo multi-bucket, multi-material batching weighing instruments (model Panther 2000 T600, maximum weighing capacity range 5-10000 kg, graduation value 0.005-10), one set for the main tank and one set shared by the intermediate tank. Each weighing instrument consists of three load cells and a secondary instrument with logic control functions (including a power supply for the sensor bridge). These instruments feature high accuracy, high reliability, professional material handling capabilities, and a user-friendly human-machine interface, playing a crucial role in improving production efficiency. Mettler Toledo's control scheme employs advanced bus technology, enabling convenient high-speed data communication with relevant PLC (or PCC) systems using fieldbuses such as Profibus, RS232, RS485, or MODBUS-Plus. This results in a weighing and batching system with strong real-time performance, high reliability, and flexible configuration, easily meeting the weighing and batching needs of various users.
The glue temperature control unit of the main tank is equipped with a platinum resistance temperature measuring element, a steam control valve and a secondary temperature instrument; the water addition control is equipped with one fast and one slow water addition valve; the starch conveying control is equipped with a screw conveyor and a vibrating feeder, and is equipped with high and low material level sensors.
3. Working principle and main functions of the control system
3.1 System Operation Mode
The system features both manual and automatic operating modes, which can be preset before production and switched between as needed during production. After each can of glue is delivered, automatic glue blowing occurs. This highly automated design enables fully automatic control of the glue preparation and application process. From glue demand, formulation parameters, and preparation to delivery, the entire process is controlled by the PCC system, requiring no dedicated personnel.
3.2 Main Functions and Features of the System
The system has key functions such as multi-formula storage, automatic formula selection, parameter setting, multiple cost reports, automatic weighing and dispensing control, and automatic glue delivery control.
It features an automatic formula correction function. When adding ingredients, the deviation during the addition process can be monitored through the operation screen, and the system will automatically correct the subsequent addition ratio by changing the formula proportions to ensure the accuracy of the formula.
The system can also communicate with the customer's production management system. Through data exchange, the amount of adhesive to be produced is automatically determined based on the order of paper products to be manufactured, thereby reducing adhesive waste. After the initial formula is entered, if the customer needs to adjust the formula, there is no need to recalculate the changes in materials; they only need to directly input the changes in adhesive, and the program will automatically modify the formula.
An external USB storage device was used to record historical data for several important process parameters.
3.3 Working principle of automatic weighing and batching
The automatic weighing and dispensing unit is the core of this adhesive preparation control system, determining the quality of the adhesive formulation and production efficiency. The system employs two Mettler Toledo electronic weighing devices, whose secondary instruments are integrated with the PCC (Pressure Control Center) to control the weighing, dispensing, and batching of all materials. This allows for more precise control of the entire batching process, ensuring accurate execution of the adhesive formulation.
Considering that the data communication between the PCC's CPU and each instrument station uses the relatively low-speed RS485 serial communication method, if the PCC were to handle the digital filtering of the weighing values, it would have to continuously acquire data from each PANTHER weighing terminal multiple times in each program scan cycle, which would take a considerable amount of time. This could affect the accuracy of the digital filtering of the weighing values and directly impact the weighing results. Therefore, in the system design, the PANTHER weighing terminal is assigned the task of digital filtering of the weighing values, and its unique TraxDSP™ digital filtering function is fully capable of handling this task. In each scan cycle, the PCC only needs to acquire a single weight data point that has already undergone digital filtering from the PANTHER terminal via the RS485 bus network and perform subsequent processing (see flowcharts 4 and 5).
(1) Weighing and injecting materials and dispensing adhesive in the transfer tank
The transfer tanks for borax, resin, and alkali are all relatively small in size. Due to the requirements of the weighing process, they are fixed on a platform supported by three weighing sensors and share a single weighing instrument. All inlet and outlet pipes are connected using flexible metal hoses to avoid additional stress affecting weighing accuracy.
The three Mettler Toledo weighing sensors each have a range of 1000 kg and an accuracy of 0.05%, resulting in an overall weighing system accuracy of 0.5%. The weight output signals from these sensors are connected in parallel (0~40mA) and transmitted to the PANTHER industrial weighing terminal (secondary instrument). The terminal's high-precision regulated power supply serves as the bridge power for these sensors. The PCC (Power Control Center) can set and acquire weighing and batching parameters via RS485 bus network data communication. The PCC also controls the start/stop of each feeding pump and feeder, and the opening/closing of the feeding solenoid valves via digital input and output contacts (as shown in Figure 3). PANTHER employs Δ-Σ analog-to-digital conversion and digital processing technology to provide various applications for industrial weighing needs: weighing display, checkweighing, sorting, and setpoint control. The PANTHER weighing terminal's unique TraxDSP™ digital filtering patented technology provides real-time, stable weight values. Even with moving devices attached to the connected scale, the accurate weight data can be obtained by adjusting the weighing terminal's parameters. PANTHER weighing terminals can be used in harsh industrial environments.
Figure 3. Simplified schematic diagram of the weighing and dispensing/batching control unit at the transfer station.
As described in the process flow section 1.2, under the control of the PCC, resin, alkali solution, and borax are sequentially added from the storage tank to the transfer tank according to the pre-input adhesive formulation. Then, according to process requirements, resin, alkali solution, and borax are added from the transfer tank to the main tank. The actual batch weights of these three materials are obtained through multiple weighings and calculations. The flowcharts for weighing and dispensing the materials and weighing and dispensing the adhesive are shown in Figures 4 and 5, respectively. The logic in these two figures is continuous.
Figure 4 Flowchart of the weighing and filling procedure for the transfer tank
Figure 5. Flowchart of glue weighing and dispensing procedure
(2) Weighing of the main tank
Employing Mettler Toledo sensors and PANTHER industrial weighing terminals, the main tank is supported by three sensors, whose weight output signals are connected in parallel to the secondary instrument to measure the weight of the main tank. Each sensor has a range of 2000 kg and an accuracy of 0.05%, while the overall weighing system has an accuracy of 0.5%. Weighing parameters can be set on the HMI touchscreen.
(3) Weighing of finished product cans
Since the accuracy requirement for measuring the weight of the finished product tanks is not high, an indirect weighing method is adopted. This involves installing a pressure transmitter (output signal 4-20mA) on the measuring pipe leading out from the bottom of each tank, with a full-scale range corresponding to 2000kg and an accuracy better than 1%. The liquid level inside the finished product tank is measured indirectly using the pressure transmitter, and then the weight of the material in each tank is calculated based on the tank's geometry and the density of the relevant liquid.
(4) Compensation for ingredient batching error
The system uses PCC logic to control the operation of each pneumatic feeding valve. Due to the delay in the operation of the feeding valve and the delay in the material falling, if no corresponding measures are taken, it may lead to material control deviation. Therefore, the system uses a batching error compensation program for adjustment, which can better control the accuracy.
3.4 Water addition control
The adhesive formulation requires a relatively large amount of water. To reduce dosing errors, the dosing speed cannot be too fast, but a dosing speed that is too slow will affect the adhesive production efficiency. Therefore, the system is set to automatically switch between fast and slow water dosing modes. In fast water dosing mode, the fast water dosing valve is opened to ensure water dosing efficiency. Before the dynamic metering value approaches the set value, it automatically switches to slow water dosing control mode to ensure control accuracy.
3.5 Starch conveying control
The starch storage tank is equipped with a screw conveyor and a vibrating feeder, and features high and low level sensors. The vibrator loosens and disperses the starch to prevent clumping during feeding. When a high level signal is output, no more starch should be added to the tank, while the system automatically supplies starch when the level drops below the minimum.
3.6 Main Tank Adhesive Temperature Control
To maintain the fluidity of the adhesive in the main tank, in addition to a stirring device, steam heating and corresponding temperature control equipment are also installed. This temperature control unit is equipped with a temperature measuring point (using a platinum resistance thermometer), a steam control valve, and a secondary temperature instrument with an RS485 bus communication interface (see Figure 2). The temperature setting range is 10~50 °C (which can be set via a touchscreen), and the control deviation is within +/- 2 °C. The PCC communicates with this temperature measuring instrument via the RS485 bus, and they cooperate to achieve two-position intermittent automatic temperature control with a dead zone according to a pre-programmed procedure.
4. Conclusion
This system, employing a PCC with a time-sharing multi-tasking mechanism and a high-precision weighing device, ensures the cyclical execution of certain rapid tasks and the accuracy of weighing and dispensing. The application of advanced automation technology ensures the stability of adhesive quality and the peel strength of bonded cartons, while reducing production costs. The equipment has operated normally since its commissioning, with an extremely low failure rate.
