In my retrofit project, I focused on the automatic control of a horizontal sand mould casting molding machine. The goal was to replace a conventional clay sand molding system with a PLC-based, servo-hydraulic, fully automatic control platform that could adapt its speed to different mold actions. I wanted to solve three practical problems that appear repeatedly in sand mould casting: high mechanical energy consumption, low production efficiency, and insufficient parting-line compactness. The original machine used a fixed-speed three-phase asynchronous motor and a single-side double-shot sand feeding method. That architecture could not adjust motion speed according to the actual sand mold height, and it could not guide sand into deep recesses of the pattern with enough uniformity. As a result, the sand mould casting process suffered from uneven compaction, higher scrap rates, and unnecessary hydraulic power loss.
My design uses a programmable logic controller as the main control core, a touch screen as the operator interface, and a servo motor directly coupled to a hydraulic pump as the power source. The machine structure is divided into a sand mold clamping and compacting system, a mold pushing system, and a sand barrel shooting system. In hardware, I established the overall control scheme, selected the PLC model, designed the circuit wiring, and prepared the I/O allocation table. In software, I implemented manual speed adjustment and an automatic speed-adjustment mode that continuously detects the running state. The test results show that the new molding machine runs smoothly, the parting surface compactness exceeds 80, the process yield reaches 96.5%, the hydraulic station energy consumption is reduced by 12.3% compared with the original machine, and the molding speed reaches 37 s per mold. This provides a feasible technical path for the modernization of traditional sand mould casting.
1. Introduction and Background
Clay sand mould casting has a long history and remains one of the most important production methods for gray iron and ductile iron castings. In my analysis of the foundry industry, I observed that sand mould casting accounts for a very large share of total casting output, especially for automotive, hydraulic, and general machinery parts. Although static pressure molding, vertical molding, jolt-squeeze molding, and horizontal molding all exist, horizontal molding has become a mainstream choice because it offers good automation, reasonable economics, and generally high casting quality. However, in many traditional horizontal molding machines used for sand mould casting, the control system cannot adjust the speed of opening, closing, compacting, and mold pushing according to the actual sand mold height. The shot nozzle position also makes it difficult to guide sand into the back-shadow area of the pattern. These limitations increase energy consumption, waste molding sand, and reduce compactness in deep recesses.
I designed an automatic control system for a horizontal molding machine and rebuilt the hydraulic system around the actual sand mould casting workflow. The system uses a PLC and a touch screen to support manual speed adjustment and automatic speed adjustment. Pressure sensors and displacement sensors monitor hydraulic pressure and actuator position in real time. The PLC then controls the servo driver input signal and automatically adjusts the servo hydraulic motor speed. This approach improves flexibility for complex sand mould casting, shortens the molding cycle, and reduces the dependence on fixed-speed hydraulic control.
The main technical contributions of my retrofit can be summarized as follows:
| Area | Original condition | Retrofit target | Expected effect in sand mould casting |
|---|---|---|---|
| Sand feeding | Single-side double-shot feeding | Vertical upward and downward shooting | Better filling of deep pattern recesses |
| Power source | Three-phase asynchronous motor | Servo motor directly coupled to hydraulic pump | Variable pressure and flow, lower energy loss |
| Control core | Relay and fixed-sequence control | PLC with analog and communication modules | Real-time adaptation and fault diagnosis |
| Speed regulation | Fixed speed or limited manual valve adjustment | Manual and automatic speed adjustment | Stable transitions, reduced shock, higher yield |
| Compaction | Independent control of upper and lower compacting cylinders | Synchronized electro-hydraulic servo pump control | Uniform stress and improved parting-line compactness |
For a sand mould casting line, the molding machine is not an isolated device. It interacts with sand preparation, core setting, mold closing, pouring, and shakeout. Therefore, I treated the control retrofit as a system-level task rather than a simple motor replacement. The PLC must not only sequence the actuators but also coordinate pressure, flow, position, and time. This coordination is especially important when the mold height changes or when the pattern has a high depth-to-width ratio.
2. Technical Requirements and Retrofit Scope
The retrofit task was to upgrade the mechanical components, convert the hydraulic pump station to servo operation, and develop a matching control system for a horizontal sand mould casting molding machine. The original machine used a single-side double-shot sand feeding method. I replaced it with vertical upward and downward shooting relative to the pattern plate. I also replaced the original three-phase asynchronous motor with a servo motor and developed a new opening and closing device, hydraulic system, and control system. The machine frame, sand box, middle-frame guide-rod pattern hanger, and some mechanical parts were retained. The opening and closing device was redesigned, and the hydraulic and electrical control systems were developed specifically for vertical shooting and horizontal molding.
The servo motor is directly coupled to the hydraulic pump. The internal encoder of the servo motor provides high-precision motion control and helps stabilize the hydraulic pump output. The PLC collects feedback from displacement sensors or pressure sensors on the upper frame, lower frame, and mold-closing cylinder. It then calculates the required servo pressure and flow control values in real time and dynamically adjusts the servo motor speed to control the hydraulic pump displacement. I used a segmented control strategy to achieve high-precision position control during the fast closing stage, stable pressure control during the pressing and holding stage, and accurate positioning of the upper and lower frames.
The retrofit also adds a servo speed-matching function. The sand box can be accelerated or decelerated according to the sand mold size, so the machine can adapt its speed to different sand mold heights. This is a key requirement for flexible sand mould casting because the same line may produce different patterns with different sand volumes and different compaction demands.
2.1 Key Technical Parameters
| Parameter | Symbol | Value or range | Unit |
|---|---|---|---|
| Molding cycle target | \(T_{cycle}\) | ≤ 37 | s/mold |
| Parting surface compactness | \(H_p\) | ≥ 80 | dimensionless |
| Process yield | \(Y\) | ≥ 96.5 | % |
| Hydraulic station energy reduction | \(\Delta E\) | ≥ 12.3 | % |
| Sand moisture | \(w\) | 3 to 4 | % |
| Permeability | \(P\) | 100 to 150 | dimensionless |
| Wet compressive strength | \(\sigma_w\) | 115 to 165 | kPa |
| Clay content | \(c\) | 10 to 14 | % |
I used the following basic hydraulic relationships to size the servo pump and cylinders:
$$Q = \frac{V_g n \eta_v}{1000}$$
where \(Q\) is the actual flow rate in L/min, \(V_g\) is the pump displacement in mL/r, \(n\) is the pump speed in r/min, and \(\eta_v\) is the volumetric efficiency. For the servo motor speed required to produce a given flow, I used:
$$n = \frac{1000 Q}{V_g \eta_v}$$
The hydraulic cylinder force was calculated from pressure and effective area:
$$F = p A = p \frac{\pi D^2}{4}$$
where \(F\) is the cylinder force in N, \(p\) is the pressure in Pa, \(A\) is the effective piston area in m², and \(D\) is the piston diameter in m. For the compacting stage, the pressure required to achieve a target compacting force is:
$$p_c = \frac{F_c}{A_c}$$
where \(p_c\) is the compacting pressure, \(F_c\) is the required compacting force, and \(A_c\) is the compacting area. These formulas guided the selection of the servo motor, pump, and pressure sensor range.
3. Machine Composition and Working Principle
The horizontal sand mould casting molding machine mainly consists of an opening and closing device, a middle-frame guide-rod pattern hanger, sand barrels, and a control system. The opening and closing device has an upper fixed assembly and a lower fixed assembly. The upper fixed assembly includes an upper compacting plate connection seat, an upper plate adjustment motor, a lifting nut, an upper fixed seat, an upper box cylinder, an upper closing plate, and an upper sand frame. The lower fixed assembly mainly includes a lower compacting plate, a lower sand frame, a lower sand frame seat, a lower compacting plate seat, a lower fixed seat, a mold-closing cylinder, and a lower box cylinder.
There are two sand shooting barrel assemblies. The upper sand barrel is fixed on the upper compacting plate connection seat. The lower sand barrel is mounted on the frame and fastened by hex bolts at the upper fixed seat and the fixed frame. Each sand barrel uses a box body and a venting device to blow molding sand vertically toward the pattern plate. The opening and closing device then compacts the sand. The middle-frame guide-rod pattern hanger is driven by a cylinder and sends the pattern along a guide rail to a position between the upper and lower sand boxes. According to the compacting requirements, the pattern hanger can move vertically along the guide rail together with the lower sand box.
| Subsystem | Main components | Function in sand mould casting |
|---|---|---|
| Opening and closing device | Upper and lower fixed assemblies, cylinders, compacting plates, sand frames | Closes the mold, applies compacting force, and opens the mold after molding |
| Pattern hanger | Middle-frame guide rod, cylinder, guide rail | Positions the pattern between the sand boxes and supports vertical movement |
| Sand shooting system | Upper and lower sand barrels, venting device | Shoots sand vertically into the mold cavity |
| Hydraulic system | Servo pump, valves, cylinders, sensors | Provides controllable pressure and flow for all actuators |
| Control system | PLC, touch screen, I/O modules, communication modules | Sequences actions, adjusts speed, and monitors status |
The machine has manual control and automatic control modes. In automatic mode, the PLC receives sensor signals and upper-computer commands, then outputs control signals to the hydraulic cylinders and pneumatic cylinders. This realizes sand shooting, sand compaction, mold opening and closing, and mold pushing in a continuous cycle. The opening and closing device uses three magnetostrictive displacement sensors to detect the positions of the upper and lower sand frames, the upper and lower compacting plate limits, and the lower frame of the mold-closing unit. Pressure sensors detect and feed back hydraulic cylinder pressure. This ensures that the cylinders run at the required speed and that the system remains stable and reliable.
3.1 Working Sequence
In my control design, the automatic cycle follows a strict sequence. After the system starts, the mold parameters are selected and verified. The system then calculates the motion positions and initializes. The mold-closing cylinder and the lower frame cylinder act simultaneously. The lower frame cylinder is controlled by a small pump to regulate its speed. The mold-closing cylinder is controlled by the speed-adjustment system. The lower frame moves quickly to a position 10 cm below the middle frame, decelerates smoothly, and contacts the middle frame. It then accelerates upward, decelerates below the upper frame, and completes mold closing.
After mold closing, sand shooting begins. Gas is vented, and the compacting action starts. The sand is trapped between the pattern and the mold surfaces. The mold-closing cylinder performs the compacting action while the upper and lower frame cylinders release pressure. A pressure sensor monitors the compacting force until the required position is reached. After compacting, the mold-closing cylinder opens the mold at a variable speed and returns to the origin. The lower frame cylinder rises to the pre-slide-out position. The lower frame then moves to the slide-out position, the middle-frame cylinder retracts, and the slide-out cylinder extends. The operator inspects the sand mold and sets cores. If the mold is rejected, the PLC sends a return signal for the slide-out cylinder, pushes out the sand mold, and either stops or starts the next cycle according to the selected mode. If core setting is complete, the PLC continues with the receiving action.
After the slide-out cylinder returns, the mold-closing cylinder brings the lower frame and upper frame together at a variable speed. After receiving and closing, the upper frame cylinder moves upward and drives the upper frame. The upper compacting plate completes upper-frame stripping. The lower-frame stripping is completed when the mold-closing cylinder and lower frame cylinder move down to the origin. After stripping, the middle frame and push cylinder extend simultaneously. After pushing is complete, the upper frame cylinder and push cylinder return to the origin and wait for the next molding command.
4. Hydraulic Control System Design
I designed the hydraulic system according to the molding process requirements. From mold opening to sand mold ejection, the control system automatically calculates the required pump pressure, working distance, and speed transition points based on the mold information entered by the user. The PLC outputs a 0–10 V analog signal to the servo driver. The servo driver controls the hydraulic pump, and the hydraulic circuit drives the cylinder. With feedback from sensors, the PLC adjusts the analog output in real time so that the hydraulic pump pressure and flow are appropriate for each working state.
In many horizontal sand mould casting machines, the compacting mechanism occupies the sand addition position at the top of the mold. The directional valves of the upper and lower compacting cylinders act simultaneously. The upper compacting cylinder moves downward while the lower compacting cylinder moves upward. Because of electrical signal delay, hydraulic pressure variation, and valve response time, the two compacting cylinders can become unsynchronized. This causes uneven stress, deformation, or displacement in the sand mold. The synchronization of compacting determines the uniformity of the sand mold. Therefore, I used a synchronized electro-hydraulic servo pump control system. This design removes the additional pressure control and control valves for the compacting plates. The main compacting force is generated by the mold-closing cylinder, and a proportional directional valve provides precise position control. The molding quality and energy utilization are better than those of the original dual-pump multi-way valve independent control system.
The hydraulic system uses a cartridge valve integrated structure. It consists of a servo motor, hydraulic pump, hydraulic oil source, cartridge solenoid valve, cartridge check valve, cartridge balance valve, cartridge relief valve, solenoid directional valve, proportional directional valve, and hydraulic cylinders. The system sequence is defined by the solenoid action table.
4.1 Solenoid Action Sequence
| Action | Y1 | Y2 | Y3 | Y4 | Y5 | Y6 | Y7 | Y8 | Y9 | Y10 | Y11 | Y12 | Y13 | Y14 | Y15 | Y16 | Y17 | Y18 | Analog command |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mold-closing cylinder fast down | + | + | − | − | − | − | − | − | − | − | − | − | − | − | − | + | − | 0 | 0–10 V |
| Mold-closing cylinder slow down | + | + | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | 0 | 0–10 V |
| Mold-closing cylinder up | + | + | − | + | − | − | + | − | − | − | − | − | − | − | − | − | 0–10 V | − | 0–10 V |
| Mold-closing cylinder fast up | + | + | − | + | − | − | + | − | − | − | − | − | − | − | − | + | 0–10 V | − | 0–10 V |
| Slow compacting | + | + | − | + | − | − | − | − | − | − | − | − | − | + | + | − | 0–10 V | − | 0–10 V |
| Upper frame cylinder up | − | − | − | − | − | − | − | − | − | − | − | + | − | − | − | − | − | − | − |
| Upper frame cylinder down | − | − | − | − | − | − | − | − | − | − | − | − | + | − | − | − | − | − | − |
| Lower frame cylinder up | − | − | − | − | − | − | − | + | − | − | − | − | − | − | − | − | − | − | − |
| Lower frame cylinder down | − | − | − | − | − | − | − | − | + | − | − | − | − | − | − | − | − | − | − |
| Slide-out cylinder out | − | − | − | − | − | − | − | − | − | − | + | − | − | − | − | − | − | − | − |
| Slide-out cylinder return | − | − | − | − | − | − | − | − | − | + | − | − | − | − | − | − | − | − | − |
| Push cylinder push | − | − | − | − | − | + | − | − | − | − | − | − | − | − | − | − | − | − | − |
| Push cylinder return | − | − | − | − | + | − | − | − | − | − | − | − | − | − | − | − | − | − | − |
In the table, “+” means the solenoid is energized, “−” means the solenoid is de-energized, and the analog command column indicates the 0–10 V signal to the servo driver. This sequence ensures that the sand mould casting cycle is repeatable and that each actuator reaches its target position before the next action begins.
5. Control System Hardware Design
The hardware of the horizontal sand mould casting molding machine control system consists of a PLC, a touch screen, limit switches, displacement sensors, pressure sensors, a hydraulic system, and a pneumatic system. The control system is divided into signal acquisition, logic control, and execution modules.
5.1 Signal Acquisition Module
Based on the structure and working principle of the opening and closing device, I selected magnetostrictive displacement sensors with a suitable stroke and an analog input module to convert current and voltage signals into digital values. In the hydraulic system, the liquid level and liquid temperature sensor outputs are connected to the analog input module to collect oil temperature and oil level information. The pressure sensor and the pressure and flow outputs of the analog output module are connected to the analog input terminals of the servo driver. The RS-485 digital signal output bus of the servo driver is connected to the PLC controller to complete pressure and flow acquisition at each moment of the opening and closing device.
Cylinder and pneumatic cylinder stroke detection uses magnetic switches. Proximity switches are used to detect the presence and departure of objects. When the machine performs actions such as middle-frame push-out and lower-frame slide-out, the proximity switch is triggered and sends a signal to the PLC.
| Signal type | Device | Purpose in sand mould casting |
|---|---|---|
| Displacement | Magnetostrictive displacement sensor | Detects upper and lower sand frame positions and mold-closing cylinder position |
| Pressure | Pressure sensor | Feeds back hydraulic cylinder pressure for compacting and holding control |
| Temperature | Liquid temperature sensor | Monitors hydraulic oil temperature |
| Level | Liquid level sensor | Monitors hydraulic oil level |
| Proximity | Inductive proximity switch | Detects slide-out, return, and push positions |
| Magnetic stroke | Magnetic switch | Detects cylinder end positions |
5.2 Logic Control Module Selection
I counted the I/O points required by the molding machine control system. The switch input signals are 43, the analog input signals are 7, the switch output signals are 50, and the analog output signals are 5. The system also needs expandable I/O ports and communication module functions. After comparing options, I selected a Mitsubishi FX5U series PLC as the control core. Because the base unit I/O, analog input/output, and communication interfaces could not satisfy all external requirements, I added one digital input/output expansion module, one digital output module, two analog input modules, and two analog output modules.
| Module | Quantity | Function |
|---|---|---|
| FX5U-80MT-ES PLC | 1 | Main logic control, high-speed processing, communication |
| FX5-32ET/ES digital I/O expansion | 1 | Additional digital input and output points |
| FX5-8EYT/ES digital output | 1 | Additional relay or transistor output |
| FX3U-4AD analog input | 2 | Pressure, displacement, temperature, and level acquisition |
| FX3U-4DA analog output | 2 | Servo driver command and proportional valve command |
| FX-485ADP communication adapter | 1 | RS-485 communication with servo driver and temperature devices |
| FX3-232-BD communication board | 1 | RS-232 communication with upper computer |
The system also uses an RS-485 adapter module for communication with the hydraulic servo driver and temperature detection equipment. An RS-232 communication board provides an interface for real-time data exchange with the upper computer. This architecture allows the PLC to exchange data with the touch screen, the servo drive, and the factory monitoring system.
5.3 Output Execution Module Selection
According to the working load and speed requirements of the mold-closing, compacting, mold-opening, slide-out, receiving, and stripping cylinders, I calculated the hydraulic pump working pressure, rated flow, and motor rated power. Based on the hydraulic control system requirements and the calculated motor torque and speed, I selected an internal gear pump with good self-priming ability and stable high-efficiency output. I also selected a servo driver series developed for permanent magnet servo motors and a permanent magnet synchronous motor as the servo motor for the retrofitted molding machine.
The key selection equations used were:
$$T_m = \frac{9550 P_m}{n_m}$$
where \(T_m\) is the motor torque in N·m, \(P_m\) is the motor power in kW, and \(n_m\) is the motor speed in r/min. The hydraulic power was estimated as:
$$P_h = \frac{p Q}{60 \eta_t}$$
where \(P_h\) is the hydraulic power in kW, \(p\) is the pressure in MPa, \(Q\) is the flow in L/min, and \(\eta_t\) is the total efficiency. These equations helped me match the servo motor and pump to the sand mould casting cycle.
5.4 Control Cabinet Wiring
After selecting the hardware, I designed the control cabinet. The hardware components are connected directly through their own terminals. The electrical wiring of each component is connected using terminal lugs of different sizes according to current intensity and the number of incoming and outgoing lines. The connection between the control cabinet and the motor uses terminal blocks. The PLC terminal wiring diagram defines the input and output assignments for all sensors, valves, and drivers.
| I/O group | Signal example | PLC address | Device |
|---|---|---|---|
| Digital input | Start button | X0 | Operator panel |
| Digital input | Stop button | X1 | Operator panel |
| Digital input | Upper frame limit | X2 | Limit switch |
| Digital input | Lower frame limit | X3 | Limit switch |
| Digital input | Mold-closing origin | X4 | Proximity switch |
| Digital input | Slide-out position | X5 | Proximity switch |
| Digital output | Mold-closing valve | Y0 | Solenoid valve |
| Digital output | Upper frame valve | Y1 | Solenoid valve |
| Digital output | Lower frame valve | Y2 | Solenoid valve |
| Digital output | Slide-out valve | Y3 | Solenoid valve |
| Analog input | Mold-closing pressure | AI0 | Pressure sensor |
| Analog input | Upper frame position | AI1 | Displacement sensor |
| Analog input | Lower frame position | AI2 | Displacement sensor |
| Analog output | Servo speed command | AO0 | Servo driver |
| Analog output | Proportional valve command | AO1 | Proportional valve |
6. Control System Software Design
I used GX Works3 to write the PLC program in the form of logic and sequential control. The program implements manual control and automatic control. The automatic cycle begins with mold selection. After the mold parameters are confirmed, the system calculates the action points and initializes. The mold-closing cylinder and lower frame cylinder act simultaneously. The lower frame cylinder is controlled by a small pump to regulate speed. The mold-closing cylinder is controlled by the speed-adjustment system. The lower frame moves quickly to a position 10 cm below the middle frame, decelerates, and contacts the middle frame. It then accelerates upward, decelerates below the upper frame, and completes mold closing.
After mold closing, sand shooting begins. Gas is vented, and compacting starts. The sand is trapped between the pattern and the mold surfaces. The mold-closing cylinder performs the compacting action while the upper and lower frame cylinders release pressure. The compacting force is monitored by a pressure sensor until the required position is reached. After compacting, the mold-closing cylinder opens the mold at a variable speed and returns to the origin. The lower frame cylinder rises to the pre-slide-out position. The lower frame moves to the slide-out position, the middle-frame cylinder retracts, and the slide-out cylinder extends. The operator inspects the sand mold and sets cores. If the mold is rejected, the PLC sends a return signal for the slide-out cylinder, pushes out the sand mold, and either stops or enters the next cycle according to the selected mode. If core setting is complete, the PLC continues with the receiving action.
After the slide-out cylinder returns, the mold-closing cylinder brings the lower frame and upper frame together at a variable speed. After receiving and closing, the upper frame cylinder moves upward and drives the upper frame. The upper compacting plate completes upper-frame stripping. The lower-frame stripping is completed when the mold-closing cylinder and lower frame cylinder move down to the origin. After stripping, the middle frame and push cylinder extend simultaneously. After pushing is complete, the upper frame cylinder and push cylinder return to the origin and wait for the next molding command.
6.1 Speed Adjustment Control Program
The manual speed adjustment system is used for no-load testing and manual control during operation or maintenance. Before work, the operator checks all connections, electrical circuits, and oil inlet and return pipes. After confirming that everything is normal, the power is turned on and the manual mode is selected. The operator can input the servo pump pressure and flow through the user interface and control the forward and backward actions of the corresponding cylinder.
The automatic speed adjustment system is used for automatic regulation of servo pump pressure and flow. The system adjusts the working interval between processes according to mold parameters such as sand mold thickness. It also sets acceleration and deceleration sections in each process to achieve smooth transition, reduce shock, and reduce energy consumption caused by peak power. The speed control flow is based on the following logic:
| Stage | Control objective | Controlled variable | Feedback sensor | Control mode |
|---|---|---|---|---|
| Fast approach | Reach target position quickly | Position | Displacement sensor | Position closed loop |
| Deceleration | Reduce impact before contact | Position and speed | Displacement sensor | Position closed loop with speed limit |
| Compacting | Reach target pressure and hold | Pressure | Pressure sensor | Pressure closed loop |
| Mold opening | Variable speed return | Position | Displacement sensor | Position closed loop |
| Frame positioning | Accurate upper and lower frame positions | Position | Displacement sensor | Servo pump displacement control |
6.2 Feedback Control Algorithm
In the speed control system, the disturbance rejection and speed accuracy of the cylinder directly affect the molding efficiency and molding quality of the horizontal sand mould casting machine. The system experiences a continuous change from low pressure to high pressure. Each stage has different working pressure and flow. The molding load causes displacement fluctuation of the hydraulic cylinder, making it difficult to stabilize the working speed at the given speed curve and pressure curve. Therefore, a single open-loop control algorithm or a single control method cannot satisfy the requirements of fast, accurate, and stable control.
For this reason, I used pressure feedback closed-loop control in the pressure curve tracking stage and position feedback closed-loop control in the fast-forward and return stages. The closed-loop control principle is:
$$e(t) = r(t) – y(t)$$
where \(e(t)\) is the control error, \(r(t)\) is the target value, and \(y(t)\) is the measured value. The PID control output is:
$$u(t) = K_p e(t) + K_i \int_0^t e(\tau) d\tau + K_d \frac{de(t)}{dt}$$
In discrete form, the PLC uses:
$$u(k) = K_p e(k) + K_i T_s \sum_{j=0}^{k} e(j) + K_d \frac{e(k)-e(k-1)}{T_s}$$
where \(T_s\) is the sampling period. For position control, the error is:
$$e_x = x_{ref} – x_{meas}$$
For pressure control, the error is:
$$e_p = p_{ref} – p_{meas}$$
The PLC converts the control output into an analog signal for the servo driver:
$$V_{cmd} = K_u u(k) + V_0$$
where \(V_{cmd}\) is the analog voltage command, \(K_u\) is the scaling gain, and \(V_0\) is the offset voltage. This signal adjusts the servo motor speed and thus the hydraulic pump flow.
6.3 Segmented Control Strategy
According to the control requirements of the mold-closing, upper-frame, and lower-frame cylinders in different stages, I used segmented control with different feedback variables. This improves molding quality and stability.
In the compacting and holding stage of the mold-closing cylinder, maintaining pressure stability is the core requirement. The opening and closing device applies load to the lower compacting plate and upper closing plate through the hydraulic cylinder. The cylinder rod contacts the lower compacting plate seat. The lower compacting plate contacts the lower sand frame and drives the upper closing plate. The pressure sensor measures the actual pressure in the hydraulic cylinder and compares it with the manually or automatically set target pressure. The deviation is processed by the main controller. The PLC outputs a signal to the hydraulic servo driver. By changing the hydraulic pump flow, the cylinder speed is controlled to keep the sand mold compacting pressure from exceeding a set value.
In the fast-forward and return stages of the mold-closing cylinder, I introduced an electro-hydraulic proportional position PID control algorithm. The input is the error between the target displacement and the actual displacement measured by the sensor. After calculation by the PLC, the control signal is amplified by the proportional amplifier and used as the control signal for the proportional directional valve. This controls the cylinder displacement toward the target value.
During molding, the position accuracy of the upper and lower frames directly affects sand mold quality. To ensure accurate sand thickness during filling and avoid cavity misalignment when connecting with the molding line, this stage uses servo pump displacement adjustment. This achieves smooth and uniform driving and precise positioning during cylinder movement. The input is the error between the target displacement and the actual displacement. After PID calculation, the result is converted by D/A and used as the driver input to drive the hydraulic pump. The segmented control strategy provides high precision and meets high-speed motion requirements.
7. HMI and Upper Computer Design
I used a configuration software package to create a graphical user interface for the sand mould casting molding machine. The production monitoring screen shows cumulative output, molding time, hydraulic station motor power consumption, and other monitoring parameters. It also includes user login and serial port switching buttons. The function selection screen displays buttons for each function. For example, clicking the mold-closing and sand-shooting button opens the parameter setting screen, where the operator can modify the mold-closing running state and related technical parameters. The screen displays the current mold-closing cylinder position, large pump pressure, flow, and opening/closing proportional valve flow. The operator can modify the slow pressure, flow, opening, position, fast rising pressure of the mold-closing cylinder, and delayed mold-closing parameters. The manual mode screen allows the operator to manually adjust the large and small pump pressures and flows for jog debugging and troubleshooting.
| HMI screen | Main functions | Relevant sand mould casting variables |
|---|---|---|
| Production monitoring | Displays output, cycle time, energy consumption | Molding cycle, hydraulic station power |
| Parameter setting | Sets pressure, flow, position, opening, delay | Compacting pressure, speed transition points |
| Manual mode | Jog control of pumps and valves | Pump pressure, pump flow |
| Alarm screen | Displays sensor faults and sequence faults | Pressure, displacement, temperature |
| Mold library | Stores mold parameters for repeated production | Sand height, compacting force, speed profile |
The HMI also allows the operator to switch between manual and automatic modes. In automatic mode, the PLC reads the mold library and applies the corresponding speed and pressure profile. In manual mode, the operator can adjust individual actuators for setup and maintenance. This flexibility is important for sand mould casting because different castings require different compacting energies and different mold filling strategies.
8. Experimental Validation
I selected a clutch pressure plate casting process at a foundry to test the retrofitted molding machine. The test used horizontal parting and ordinary clay sand mould casting. The clutch pressure plate material was ductile iron. The nodularity level was 1 to 3, the pearlite content was less than or equal to 35%, and the sand mold hardness requirement was 80 to 95. The casting weight was about 20 kg. The casting structure was complex, with large wall thickness differences and wide distribution. It was prone to shrinkage porosity, which accounted for 57% of the company’s annual scrap rate. Casting process simulation showed that the thinnest part and the lower part of the casting solidified first, but the mold hardness measurement value did not meet 85. This caused mold wall movement during solidification, making self-feeding difficult during graphitization expansion.
I compared the retrofitted sand shooting and compacting system with the original sand shooting process system. The test checked whether the retrofitted horizontal molding machine could effectively improve sand compaction and solve the “arch effect” in deep recesses of sand molds with large depth-to-width ratios. Both tests used molding sand that met the original horizontal molding machine sand supply requirements: moisture content 3% to 4%, permeability 100 to 150, wet compressive strength 115 to 165 kPa, and clay content 10% to 14%. During the test, I recorded the cycle time of one working action and the total power of the hydraulic station motor. After molding, I checked whether the sand mold had high compactness and uniform distribution. Finally, I calculated the qualified rate of the sand mold.
| Parameter | Value | Unit |
|---|---|---|
| Casting weight | 20 | kg |
| Sand moisture | 3–4 | % |
| Permeability | 100–150 | dimensionless |
| Wet compressive strength | 115–165 | kPa |
| Clay content | 10–14 | % |
| Required sand mold hardness | 80–95 | dimensionless |
The comparison test results are shown in the following table. The retrofitted molding machine achieved a casting yield of 91.99% in the first comparison, and the hydraulic station motor energy consumption was 14.18 kWh. Compared with the original control system, the casting yield increased by 24.5%, energy consumption decreased by 12.3%, and operating efficiency increased by 9.8%.
| Machine condition | Yield (%) | Hydraulic station motor energy (kWh) | Operating efficiency (s/mold) |
|---|---|---|---|
| Original horizontal molding machine | 77.5 | 16.16 | 41 |
| Retrofitted molding machine | 96.5 | 14.18 | 37 |
The molding cycle time was calculated as the sum of all stage times:
$$T_{cycle} = t_{close} + t_{shoot} + t_{compact} + t_{open} + t_{slide} + t_{core} + t_{receive} + t_{strip} + t_{push}$$
where each \(t_i\) is the time of the corresponding stage. The energy consumption per cycle was calculated as:
$$E_{cycle} = \int_0^{T_{cycle}} P(t) dt = \int_0^{T_{cycle}} p(t) Q(t) dt$$
where \(P(t)\) is the instantaneous hydraulic power, \(p(t)\) is the pressure, and \(Q(t)\) is the flow. The reduction in energy consumption was:
$$\Delta E = \frac{E_{original} – E_{retrofit}}{E_{original}} \times 100\%$$
Using the measured values:
$$\Delta E = \frac{16.16 – 14.18}{16.16} \times 100\% \approx 12.3\%$$
The improvement in operating efficiency was:
$$\Delta \eta = \frac{T_{original} – T_{retrofit}}{T_{original}} \times 100\% = \frac{41 – 37}{41} \times 100\% \approx 9.8\%$$
These results confirm that the PLC-based speed adjustment and servo-hydraulic retrofit improve the sand mould casting process.
9. Discussion
The experimental results show that the retrofitted horizontal sand mould casting molding machine runs smoothly, and the sand filling effect in each area of the sand mold meets the process requirements. The casting yield reaches 96.5%, and the energy consumption is 14.18 kWh. Compared with the original structure without the speed control system, the motor energy consumption is reduced by 12.3%, and the operating efficiency excluding core setting time is increased by 9.8%. The retrofit meets the requirements of the horizontal molding machine control system.
Several factors contribute to the improvement. First, the vertical upward and downward sand shooting method improves sand distribution in deep recesses. Second, the servo pump control allows pressure and flow to be adjusted independently, which reduces throttling losses. Third, the segmented PID control provides stable position and pressure tracking. Fourth, the HMI and mold library reduce setup time and make the sand mould casting process more repeatable.
| Improvement factor | Mechanism | Effect on sand mould casting |
|---|---|---|
| Vertical sand shooting | Sand enters from upper and lower directions | Better filling of deep recesses and back-shadow areas |
| Servo pump control | Motor speed and pump displacement are adjusted in real time | Lower energy consumption and smooth speed transitions |
| Pressure feedback | Compacting pressure is closed-loop controlled | Uniform compaction and improved parting-line compactness |
| Position feedback | Cylinder position is closed-loop controlled | Accurate frame positioning and reduced mold misalignment |
| Mold library | Parameters are stored and reused | Faster changeover and higher process yield |
From a control perspective, the main challenge is the nonlinearity of the hydraulic system. The load changes as the sand mold is compacted. The pressure and flow requirements vary significantly between stages. I addressed this by using a segmented control strategy. In the fast approach and return stages, position control dominates. In the compacting stage, pressure control dominates. The PLC switches between control modes based on the current position, pressure, and sequence step. This hybrid approach is more effective than a single PID loop for the entire cycle.
The energy savings come mainly from matching pump output to load demand. In the original system, the pump often delivered more flow than needed, and excess flow was lost through relief valves. In the retrofitted system, the servo motor reduces speed when the required flow is low. The hydraulic power equation shows the relationship:
$$P_h = \frac{p Q}{60 \eta_t}$$
If the flow \(Q\) is reduced when the pressure \(p\) is moderate, the hydraulic power \(P_h\) is reduced. Over a full cycle, this produces a measurable energy reduction. The measured value of 12.3% is consistent with the expectation that a variable-speed pump system saves energy in a cyclic process with varying load.
The compactness improvement can be explained by the pressure and position control. The compacting pressure is maintained during the holding stage. The position feedback ensures that the upper and lower frames reach the correct positions. The sand mold hardness values are above 80, which meets the process requirement. The reduction in shrinkage porosity is also related to the higher mold hardness because a harder mold resists wall movement during solidification.
10. Practical Implementation Notes
When implementing a PLC-based retrofit for sand mould casting, several practical details matter. The first is sensor selection. Displacement sensors must have enough stroke and resolution. Pressure sensors must have a range that covers both low-pressure fast movement and high-pressure compacting. The analog modules must have enough resolution to detect small changes. The second is wiring. The control cabinet must separate high-voltage and low-voltage wiring to reduce electromagnetic interference. The third is parameter tuning. The PID gains for position and pressure control should be tuned separately. The fourth is safety. Emergency stop, limit switches, and pressure relief valves must be included.
| Implementation area | Recommended practice | Reason |
|---|---|---|
| Sensor selection | Use magnetostrictive displacement sensors and industrial pressure sensors | High accuracy and repeatability for sand mould casting |
| PLC programming | Use structured sequence control and modular PID blocks | Easier debugging and maintenance |
| Hydraulic design | Use cartridge valves and a servo pump | Compact structure and low throttling loss |
| HMI design | Provide manual, automatic, and alarm screens | Fast troubleshooting and parameter adjustment |
| Safety design | Include emergency stop, limits, and relief valves | Protect operators and equipment |
I also found that the mold library is useful for sand mould casting. Different castings have different sand volumes and different deep recesses. By storing the speed profile, pressure profile, and position setpoints for each mold, the operator can recall the correct parameters quickly. This reduces setup time and prevents errors. The PLC can also compare the measured pressure and position with the stored values and generate an alarm if the deviation exceeds a threshold.
11. Conclusion
I designed and implemented a PLC-based control system for a horizontal sand mould casting molding machine. The retrofit included a new opening and closing device, a servo-hydraulic power unit, a PLC control cabinet, and an HMI. The machine uses vertical upward and downward sand shooting, servo pump pressure and flow control, and segmented closed-loop control for position and pressure. The main conclusions are as follows.
First, the new upper and lower sand shooting and compacting structure, combined with the hydraulic and pneumatic systems, forms a horizontal flaskless shooting and squeezing molding system. The PLC selected as the main controller provides enough I/O and communication capability for the sand mould casting cycle. The displacement sensors, pressure sensors, servo driver, and servo motor are matched to the process requirements.
Second, the software includes automatic start sequence control and automatic speed control. The PLC PID function is used for parameter tuning. The HMI provides control, parameter setting, and monitoring screens. The mold library stores mold parameters and allows fast recall. This makes the sand mould casting process more flexible and repeatable.
Third, the comparison test shows that the retrofitted molding machine runs smoothly. The sand filling effect in each area meets the process requirements. The casting yield reaches 96.5%, and the energy consumption is 14.18 kWh. Compared with the original machine without the speed control system, the motor energy consumption is reduced by 12.3%, and the operating efficiency excluding core setting time is increased by 9.8%. The parting surface compactness is above 80, and the molding speed reaches 37 s per mold.
Overall, the PLC-based servo-hydraulic retrofit provides a practical and effective solution for traditional sand mould casting. It improves compactness, reduces energy consumption, increases yield, and shortens the molding cycle. The control strategy can be adapted to other horizontal molding machines and other sand mould casting processes. In future work, I would add more sensors and data analytics to further optimize the speed profile and predict maintenance needs. I would also integrate the molding machine with the sand preparation and pouring systems to create a more complete smart sand mould casting line.