In modern commercial production and distribution, packaging is indispensable for products, and cardboard box packaging is the most widely used in this field. Common cardboard box packing machines mainly include gluing and stapling types. The gluing type works by using a melt adhesive system to melt hot melt adhesive into a liquid. Then, a solenoid valve controls the nozzle valve core, and under the action of compressed air, the molten hot melt adhesive is sprayed in a strip onto the surface of the cardboard box. After the hot melt adhesive cools, the bonding is complete. Because the gluing process does not use metal nails, it is more conducive to ensuring the safety of the packaged items, which is especially important for soft or liquid contents. It also significantly improves the strength of the cardboard box, which is beneficial for long-distance transportation. Furthermore, since there is no need to remove nails, the recycling and reuse of cardboard boxes is environmentally friendly. The quality of hot melt adhesive sealing control in this method directly affects product quality, production efficiency, and production costs. Based on the application requirements of industrial sites, a PLC-based adhesive spraying control system was designed. This system can flexibly adjust the spraying length, spraying method, and spraying time according to different cardboard box specifications, achieving various adhesive dispensing forms such as spray points, spray strips, or sprays.
1. Analysis of hot melt adhesive properties
Ethylene-vinyl acetate copolymer (EVA) is a commonly used hot melt adhesive in spray adhesive systems. It is a solvent-free, water-free solid fusible polymer. EVA is solid at room temperature, but transforms into a flowing liquid with a certain viscosity when heated to a certain temperature. This liquid is sprayed onto the surface of an object and can be bonded and cured in a short time by pressing. Its main components are EVA resin, tackifier, and viscosity modifier. The EVA resin, as the main component, directly determines the performance of the hot melt adhesive, such as bonding strength, melting temperature, tensile strength, and temperature deformation resistance. Tackifiers are generally polymerized rosin or terpenes, which are added mainly to prevent the EVA resin from reducing its bonding strength and penetration into the surface of the bonded object when the temperature drops. To achieve the optimal balance between melting temperature, colloid flowability, wettability, and solidification speed, microcrystalline wax or paraffin wax is often used as a viscosity modifier. The temperature and physical state characteristics of hot melt adhesives are shown in Figure 1.
In Figure 1: 80–135℃ is the softening zone; when heated to 80℃, the colloid begins to soften and melt. 135–200℃ is the melting zone; at this point, the colloid melts into a flowable liquid. By applying a pressure of 5×10⁴–1×10⁵ Pa to this liquid, the length of the colloid sprayed onto the surface of the object being bonded can be controlled. Throughout the entire colloid spraying process, the heating temperature must not exceed 200℃, otherwise the colloid may burn. The cooling and curing of EVA hot melt adhesive is usually completed at room temperature, with an ideal temperature of 15–26℃ and humidity maintained at around 50%.
2PLC-controlled glue spraying system design
The main process of a carton-gluing cardboard packing machine is spraying hot melt adhesive onto the cardboard. Accurate control of parameters such as the spraying position, spraying length, and spraying time directly affects the quality, appearance, and cost of the formed carton. The control principle of the hot melt adhesive system is shown in Figure 2. The controller uses temperature and pressure detectors to determine the temperature and pressure of the hot melt adhesive. Once the working requirements are met, it drives the solenoid valve to switch direction. Then, compressed air is sent to the nozzle through the adhesive hose, adhesive distributor, and switch. Molten EVA is sprayed onto the surface of the carton, and the motor drives the spray head to move, thus spraying a strip of adhesive of a certain length onto the carton surface. After spraying is completed, the solenoid valve resets, the compressed air is cut off, and the adhesive spraying stops. Relevant process control parameters can be set and displayed through the human-machine interface.
2.1 System Hardware Configuration
From the perspective of the processing characteristics and cost-effectiveness of spray adhesive, this study comprehensively compared the control features of PLCs, microcontrollers, and industrial PCs. The Mitsubishi FX1N-40MR PLC was selected as the controller to realize the data acquisition, conversion, and control functions of the spray adhesive system. The Mitsubishi FX1N-40MR PLC features a compact structure, strong versatility, and flexible configuration. It can function as a standalone PLC controller, achieving high-speed discrete control and complex process control on a single platform; or it can function as an I/O substation, connecting with multiple PLCs via fieldbus to form a distributed large-scale control system, enabling batch operation control of the production line.
The dot-jet adhesive control system described in this paper is applied to the bonding of cartons. It requires accurate continuous and intermittent glue spraying on three glue-receiving surfaces of a carton (two sides and one top). The I/O allocation of the PLC control system is shown in Table 1. The system requires input points including those for activating glue spraying, indicating the machine is ready to run, confirming sufficient heating and compressed air, and detecting when a door is opened or an emergency stop is pressed. The system requires output points including those for controlling the top and side glue spray valves, controlling the engagement or disengagement of the top glue spray motor clutch, stopping the machine during heating/cooling or when insufficient compressed air pressure, and stopping the machine when a door is accidentally opened or an emergency stop is pressed.
2.2 System Software Design
The PLC operates in a cycle scanning mode. The main program is executed in each scanning cycle, while the top spray adhesive control module, side spray adhesive control module, air pressure and heating detection module, emergency stop and safety door opening and closing detection module are designed as subroutines for calling.
A PLC scan cycle consists of three stages: input scan, program execution, and output refresh. First, pressing the equipment start button activates the air compressor and glue-dispensing system. The air pressure relay and temperature controller monitor the operating temperature and pressure. When these parameters reach their set values, the normally open contacts of the air pressure relay and temperature controller close, completing the initial conditions for glue dispensing. Otherwise, glue may not be dispensed or may overflow instead of spraying. Next, the PLC reads process parameters input through the control panel, such as the glue dispensing method, dispensing time, and dispensing volume, while simultaneously checking if the safety doors of the dispensing head are fully closed. Because the dispensing chamber temperature reaches 150℃ and operates in a spraying mode, a protective safety door made of plexiglass is installed around the dispensing head. Under normal operating conditions, the safety door is closed, ensuring safety and facilitating real-time observation of the equipment's operation. If the safety door is suddenly opened for any reason, the equipment stops immediately. After the safety door closes normally, pressing the reset button resumes operation. Then, the solenoid valve is energized and reverses direction, opening the dispensing head and allowing the molten glue to be sprayed out under its own pressure.
The control of the glue spraying length is actually achieved by controlling the motor to drive the glue spraying head along a set trajectory. The continuous rotation of the motor and the synchronous glue spraying of the spraying head can obtain a glue strip of a specific length. By setting the glue spraying time in the PLC, the glue amount can be adjusted arbitrarily within the required range. By controlling the glue spraying head to work intermittently, discontinuous glue segments can be obtained on the side of the carton. The opening and closing of the glue spraying head is controlled by the switching of a solenoid valve. When the melt glue pressure reaches the set value, the solenoid valve is energized and switches, and the glue spraying head opens to start spraying glue; when the set time is reached, the solenoid valve is de-energized and switches again, and the glue spraying head closes.
2.3 Compensation Algorithm for Precision Control
Solenoid valves are actuators with significant time delays; both opening and closing require a certain amount of time, although this time is only on the order of milliseconds. When the nozzle moving speed is low, the error caused by the delayed opening or closing of the solenoid valve is small. Considering production efficiency, increasing the nozzle moving speed becomes inevitable, but this also significantly increases the error. To minimize the impact of this error, a mathematical model for error correction and compensation is established based on the analysis of the relationship between nozzle moving speed, spraying pressure, and solenoid valve closing characteristics. This model provides a lead and lag correction amount related to relevant influencing factors to compensate for the position and length errors caused by the solenoid valve delay. Since the functional relationship between the parameters cannot be directly abstracted, a high-order polynomial approximation method is used to construct the corresponding compensation curve as shown in equation (1), and the least squares method is used to fit and solve the polynomial.
Let x represent the speed of the spray nozzle movement and y represent the corresponding compensation amount. Then select m experimental data (xi, yi), where i = 1, 2, ..., n. Construct the relationship matrix as shown in equation (2) using the least squares method, and solve the linear equation system about a0, a1, ..., am to obtain the approximate functional relationship between x and y.
Let ωi = 1, that is, take an algebraic polynomial with {1, x, ..., xm) as the basis function for fitting. Considering the balance between accuracy and speed, after experimental verification and analysis, m = 2 and n = 4 are selected. The coefficients a) and a1 are solved using the Cramer algorithm, and the equation of the compensation curve can be obtained:
y=φ(x)=a0+a1x(3)
By calculating the error compensation curve equations for multiple glue spraying control heads, the simultaneous multi-station glue gluing process control of multiple glue spraying heads can be realized.
3. System Debugging
The debugging of the PLC control system is divided into two parts: hardware and software. Hardware debugging mainly checks whether the electrical control components are working normally and reliably, whether the wiring connections are correct, and whether the anti-interference measures are reasonable. Software debugging is carried out by first debugging the modules and then the overall system, gradually analyzing whether the program operation meets the control requirements and eliminating the occurrence of abnormal situations. Actual operation on a carton gluing production line shows that the PLC-controlled glue spraying system meets the requirements of actual production, with high system reliability, easy expansion, convenient maintenance, and strong anti-interference ability.
4 Conclusion
The hot melt adhesive spraying process for sealing cartons has strict sequential control requirements. Using a PLC to control the spraying process can largely eliminate the shortcomings of traditional relay contactor control systems. Through research on factors affecting spraying speed and accuracy, and utilizing the analysis results of an error compensation model for real-time control adjustments, the spraying control system can perform multi-directional, multi-form, and high-accuracy spraying of various carton sizes, demonstrating excellent flexibility and reliability. Based on a reasonable analysis of the controlled object and considering cost requirements, the system also possesses good scalability; with appropriate adjustments to the system's hardware and software, it can be adapted to new processing methods and controlled objects.
