In my work on sand mold casting equipment, I led the renovation and application of a PLC-based control system for a horizontal molding machine. The original machine used a traditional clay sand molding process, and its motion axes could not adapt their speeds to the different action requirements of sand mold casting. As a result, the machine consumed excessive mechanical energy, produced low efficiency, and often failed to achieve sufficient compactness on the parting surface. I therefore designed a fully automatic control system for an upper and lower box double-shot molding machine using a programmable logic controller as the central control platform. The design combined process workflow analysis, hydraulic system redesign, servo pump control, sensor feedback, and human-machine interface configuration. The renovated system was intended to improve sand mold casting quality, reduce energy consumption, and increase the flexibility of horizontal molding.
I treated the renovation as a complete electro-hydraulic modernization task for sand mold casting. The main mechanical structure was retained where possible, including the frame, sand box, and middle-frame guide-rod pattern hanger. At the same time, I replaced the original shooting method, changed the drive architecture from a three-phase asynchronous motor to a servo motor, and developed a new opening and closing device together with its hydraulic and electrical control systems. The goal was to create a vertical shooting and horizontal molding machine with servo-based operation and adaptive speed matching. In my design, the servo motor is directly coupled to the hydraulic pump, and the internal encoder of the servo motor provides high-precision control of motor motion. The PLC collects displacement and pressure feedback from the upper frame, lower frame, and clamping cylinders, calculates the required servo pressure and flow commands in real time, and dynamically adjusts the servo motor speed to control the hydraulic pump displacement. This segmented control strategy allows fast approach with high position accuracy, stable pressure control during compaction and holding, and precise positioning of the upper and lower frames. The renovated machine also includes a servo speed matching function, so the sand box can accelerate or decelerate according to the size of the sand mold. This directly supports different sand mold casting heights and improves the adaptability of the molding process.
1. Technical Requirements and Renovation Scope
I began by defining the technical requirements for the sand mold casting renovation. The existing machine used a single-direction, double-side shooting method. I changed this to a vertical template shooting method in which sand is shot upward and downward. This change was important because the original shooting arrangement could not easily guide molding sand into the shadow areas behind the pattern plate. In sand mold casting, poor sand filling in deep recesses creates weak regions and reduces the final compactness of the mold. I also replaced the original three-phase asynchronous motor with a servo motor and developed a new opening and closing mechanism, hydraulic system, and control system. The new arrangement allows the vertical shooting and horizontal molding process to operate under servo control.
The renovation retained several original mechanical parts: the machine frame, the sand box, and the middle-frame guide-rod pattern hanger. I replaced the entire opening and closing device structure and developed a hydraulic system and electrical control system specifically for the vertical shooting molding machine. The servo motor drives the hydraulic pump directly, and the hydraulic system provides the power source for the opening and closing device. The PLC receives feedback from displacement sensors and pressure sensors mounted on the upper and lower frames and the clamping cylinder. It then calculates the required servo pressure and flow control values and adjusts the servo motor speed to control the pump displacement. I used a segmented control strategy to achieve three main objectives: high-precision position control during the fast closing stage, stable pressure control during the pressing and holding stage, and accurate positioning control of the upper and lower frames. The renovated machine also includes a servo speed matching function that adjusts acceleration and deceleration according to the sand mold height and size.
| Renovation Item | Original Condition | New Design Requirement | Purpose in Sand Mold Casting |
|---|---|---|---|
| Shooting method | Single-direction double-side shooting | Vertical template upward and downward shooting | Improve sand filling into deep recesses and shadow areas |
| Main drive | Three-phase asynchronous motor | Servo motor directly coupled to hydraulic pump | Enable variable speed and pressure control |
| Opening and closing device | Original mechanical structure | New upper and lower fixed assemblies | Improve clamping, compaction, and stripping |
| Hydraulic system | Independent control with multiple valves | Servo pump control with cartridge valve integration | Reduce energy loss and improve response |
| Control platform | Limited manual or relay logic | PLC with analog and communication modules | Real-time feedback and automatic speed matching |
| Speed matching | Not available | Manual and automatic servo speed adjustment | Adapt to different sand mold heights |
| Quality target | Insufficient parting surface compactness | Compactness measurement value above 80 | Reduce mold wall movement and casting defects |
I expressed the basic servo pump relationship as follows. The hydraulic pump flow depends on pump displacement and motor speed, while the cylinder velocity depends on flow and effective area. These relationships formed the basis of my speed control model.
$$ Q_p = V_d n_p \eta_v $$
$$ v_c = \frac{Q_p}{A_c} $$
$$ F_c = p_c A_c $$
Here, \(Q_p\) is the pump flow, \(V_d\) is the pump displacement, \(n_p\) is the pump speed, \(\eta_v\) is the volumetric efficiency, \(v_c\) is the cylinder velocity, \(A_c\) is the effective cylinder area, \(F_c\) is the cylinder force, and \(p_c\) is the cylinder pressure. For sand mold casting, these equations mean that I can control the motion speed of the clamping, upper frame, and lower frame cylinders by regulating the servo motor speed and pump displacement. This is the core of the adaptive speed matching function.
2. Composition and Working Principle of the Horizontal Molding Machine
The horizontal molding machine I renovated mainly consists of an opening and closing device, a middle-frame guide-rod pattern hanger, a sand barrel, 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 compaction plate connection seat, an upper pressure plate adjustment motor, a lifting nut, an upper fixed seat, an upper box cylinder, an upper closing box plate, and an upper sand frame. The lower fixed assembly mainly consists of a lower compaction plate, a lower sand frame, a lower sand frame seat, a lower compaction plate seat, a lower fixed seat, a clamping cylinder, and a lower box cylinder.
There are two shooting sand barrel assemblies. The upper shooting barrel is fixed on the upper compaction plate connection seat. The lower shooting barrel is mounted on the frame and fastened by hexagon bolts at the upper fixed seat and fixed frame. Each sand barrel uses a box body and a ventilation device to blow molding sand vertically toward the pattern plate. After shooting, the opening and closing device compacts the sand. The middle-frame guide-rod pattern hanger is driven by a cylinder and moves the pattern along a guide rail to a position between the upper and lower sand boxes. According to the compaction requirement, the pattern hanger can move vertically together with the lower sand box along the guide rail. This coordinated motion is important in sand mold casting because it helps maintain pattern alignment and mold dimensional accuracy.
| Assembly | Main Components | Function in Sand Mold Casting |
|---|---|---|
| Upper fixed assembly | Upper compaction plate connection seat, upper pressure plate adjustment motor, lifting nut, upper fixed seat, upper box cylinder, upper closing box plate, upper sand frame | Provides upper shooting, upper compaction, and upper mold closing motion |
| Lower fixed assembly | Lower compaction plate, lower sand frame, lower sand frame seat, lower compaction plate seat, lower fixed seat, clamping cylinder, lower box cylinder | Supports lower sand frame, lower compaction, and lower mold closing motion |
| Shooting barrels | Upper shooting barrel and lower shooting barrel with box body and ventilation device | Shoot molding sand vertically toward the pattern plate |
| Middle-frame pattern hanger | Cylinder, guide rail, pattern hanger | Move pattern between sand boxes and coordinate vertical motion |
| Control system | PLC, HMI, sensors, servo driver, hydraulic and pneumatic components | Sequence control, speed control, pressure control, and monitoring |
The machine has both manual control and automatic control modes. The PLC receives signals from various sensors and commands from the upper computer. It then outputs control signals to hydraulic cylinders, pneumatic cylinders, and other actuators. In this way, the machine performs shooting, sand compaction, sand box opening and closing, and mold pushing. It can continuously cycle for sand mold casting production. 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 compaction plates, and the lower clamping frame. These sensors provide position detection and feedback for sand box opening and closing. Pressure sensors detect and feed back the hydraulic cylinder pressure, ensuring that the cylinders operate at the required speed and that the system remains stable and reliable during automatic sand mold casting.
| Step | Main Action | Feedback Used | Purpose |
|---|---|---|---|
| 1 | Mold selection and initialization | Mold parameters from HMI | Calculate motion points and speed curves |
| 2 | Lower frame and clamping cylinder start | Displacement sensors | Move lower frame and prepare clamping |
| 3 | Lower frame fast approach and deceleration | Position feedback | Avoid impact near middle frame |
| 4 | Lower frame accelerates and decelerates below upper frame | Position feedback | Complete clamping accurately |
| 5 | Shooting | Valve and pressure feedback | Fill mold cavity with sand |
| 6 | Gas exhaust and compaction | Pressure sensor | Compact sand to required density |
| 7 | Variable speed mold opening | Displacement sensors | Separate mold without damage |
| 8 | Lower frame to pre-slide position | Position feedback | Prepare mold extraction |
| 9 | Middle frame retracts and slide cylinder moves out | Proximity switches | Allow inspection and core setting |
| 10 | Scrap or core-setting decision | HMI mode input | Push out scrap or continue cycle |
| 11 | Slide cylinder returns and clamping continues | Position and pressure feedback | Receive and close mold |
| 12 | Upper frame stripping | Displacement sensors | Remove upper mold from pattern |
| 13 | Lower frame stripping | Clamping and lower frame cylinders | Remove lower mold from pattern |
| 14 | Middle frame and push cylinder extend | Proximity switches | Push finished sand mold |
| 15 | Return to origin and wait | Position feedback | Prepare next sand mold casting cycle |
3. Hydraulic Control System Design
I designed the hydraulic control system around the molding sequence requirements. From the completion of mold opening to the pushing of the finished sand mold, the control system automatically calculates the required pump pressure, working distance, and speed transition points according to the mold information entered by the user. The PLC sends a 0-10 V analog signal to the servo driver. Together with the hydraulic circuit, this signal drives the hydraulic cylinders. Using feedback from various sensors, the PLC adjusts the analog output in real time to achieve reasonable pressure and flow values under different working conditions. This is especially important in sand mold casting because the required pressure and flow change significantly between fast approach, slow compaction, holding, and return stages.
In many horizontal molding machines, the compaction mechanism occupies the sand addition position at the top of the mold. The upper and lower compaction cylinders are often controlled by directional valves that act at the same time. The upper compaction cylinder moves downward while the lower compaction cylinder moves upward. However, electrical signal delay, hydraulic pressure variation, and valve response time can cause asynchronous compaction. This produces nonuniform stress in the sand mold and may cause deformation or displacement. The synchronization of compaction determines the uniformity of the sand mold. For this reason, I used a compaction synchronous electro-hydraulic servo pump control system in the renovated machine. I removed the additional pressure control adjustment and control valves for the compaction plates. The main compaction force is generated by the clamping cylinder, and a proportional directional valve provides precise position control. The resulting sand mold quality and energy utilization are higher than those of the original dual pump and multi-way valve independent control hydraulic system.
The hydraulic system uses an integrated cartridge valve 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 hydraulic principle is organized around the clamping cylinder, upper frame cylinder, lower frame cylinder, slide-out cylinder, and push cylinder. The servo motor and pump form the main pressure and flow source. The proportional directional valve adjusts flow direction and position. The cartridge valves provide compact integration and reduce leakage and pressure loss.
| Hydraulic Component | Main Function | Role in Sand Mold Casting |
|---|---|---|
| Servo motor | Drives hydraulic pump at variable speed | Provides adjustable flow and pressure for different process stages |
| Hydraulic pump | Converts mechanical energy into hydraulic energy | Supplies oil to all cylinders |
| Hydraulic oil source | Stores and conditions hydraulic oil | Maintains stable oil supply and temperature |
| Cartridge solenoid valve | Controls flow direction in integrated block | Switches cylinder actions |
| Cartridge check valve | Prevents reverse flow | Protects circuit and maintains pressure |
| Cartridge balance valve | Controls load holding and smooth motion | Prevents sudden drop during mold opening |
| Cartridge relief valve | Limits maximum pressure | Protects system from overpressure |
| Solenoid directional valve | Directs oil to selected cylinder | Selects clamping, upper frame, lower frame, slide, or push action |
| Proportional directional valve | Provides proportional flow and position control | Controls clamping cylinder position precisely |
| Hydraulic cylinders | Convert hydraulic energy into linear motion | Perform clamping, frame positioning, sliding, and pushing |
The electromagnetic action sequence is an important part of the hydraulic control logic. I organized the sequence so that the clamping cylinder, upper frame cylinder, lower frame cylinder, slide cylinder, and push cylinder operate in the correct order. The following table summarizes the main actions and the corresponding solenoid states. In the actual program, each output is interlocked with position, pressure, and time conditions to ensure safe sand mold casting.
| Action Program | Active Solenoid Groups and Analog Output | Feedback Condition |
|---|---|---|
| Clamping cylinder fast down | YD1, YD2, YD16 on; YD17 analog output | Upper position reached, pressure below limit |
| Clamping cylinder slow down | YD1, YD2, YD17 analog output | Deceleration point reached |
| Clamping cylinder up | YD1, YD2, YD4, YD7, YD17 analog output | Pressing complete |
| Clamping cylinder fast up | YD1, YD2, YD4, YD7, YD16, YD17 analog output | Return command active |
| Slow compaction | YD1, YD2, YD4, YD14, YD15, YD17 analog output | Pressure setpoint reached |
| Upper frame cylinder up | YD12 on | Upper frame origin or target position |
| Upper frame cylinder down | YD13 on | Upper frame target position |
| Lower frame cylinder up | YD8 on | Lower frame target position |
| Lower frame cylinder down | YD9 on | Lower frame target position |
| Slide cylinder out | YD11 on | Slide-out position reached |
| Slide cylinder return | YD10 on | Return position reached |
| Push cylinder push | YD6 on | Push position reached |
| Push cylinder return | YD5 on | Return position reached |
| Clamping start with impact control | YD3 on | Initial clamp condition |
For the servo pump, the required motor speed can be calculated from the desired flow. I used the following relationship to generate the speed reference for the servo driver:
$$ n_{ref} = \frac{Q_{ref}}{V_d \eta_v} $$
The motor torque required by the pump can be estimated from pressure and displacement:
$$ T_m = \frac{p_p V_d}{2\pi \eta_m} $$
Here, \(n_{ref}\) is the reference motor speed, \(Q_{ref}\) is the required flow, \(T_m\) is the motor torque, \(p_p\) is the pump pressure, and \(\eta_m\) is the mechanical efficiency. These equations allowed me to select the servo motor and pump parameters and to program the automatic speed control. In sand mold casting, the required flow is high during fast approach and return but lower during compaction and holding. The servo pump system therefore avoids the fixed-speed operation that caused high energy consumption in the original machine.
4. Control System Hardware Design
The control system hardware consists of signal acquisition, logic control, and execution modules. The main devices include a programmable logic controller, a touch screen, limit switches, displacement sensors, pressure sensors, a hydraulic system, and a pneumatic system. I selected the hardware according to the input and output requirements of the sand mold casting process. The signal acquisition module collects analog and digital signals from the machine. The logic control module executes the program and makes decisions. The execution module drives the hydraulic and pneumatic actuators.
For signal acquisition, I used magnetostrictive displacement sensors together with analog input modules to convert current and voltage signals into digital values. In the hydraulic system, liquid level and liquid temperature sensors are connected to analog input modules for oil temperature and oil level monitoring. Pressure sensors and analog output modules provide pressure and flow commands to the servo driver. The servo driver communicates through an RS-485 digital bus connected to the PLC controller. This completes the 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 pushing the middle frame out or sliding the lower frame out, the proximity switch sends a signal to the PLC.
| Signal Type | Device | Quantity or Function | Connection |
|---|---|---|---|
| Displacement | Magnetostrictive displacement sensor | Upper frame, lower frame, clamping cylinder positions | Analog input module |
| Pressure | Pressure sensor | Hydraulic cylinder pressure feedback | Analog input module |
| Temperature | Liquid temperature sensor | Hydraulic oil temperature | Analog input module |
| Level | Liquid level sensor | Hydraulic oil level | Analog input module |
| Stroke detection | Magnetic switch | Cylinder end positions | Digital input |
| Presence detection | Proximity switch | Middle frame, lower frame slide positions | Digital input |
| Servo command | Analog output module | 0-10 V pressure and flow command | Servo driver analog input |
| Servo feedback | RS-485 communication | Servo status and digital feedback | Communication adapter |
| Upper computer | RS-232 communication board | Data exchange with HMI or PC | Serial interface |
For logic control, I counted the required I/O points. The system required 43 digital input signals, 7 analog input signals, 50 digital output signals, and 5 analog output signals. The PLC also needed expandable I/O ports and communication modules. After comparing available options, I selected a compact PLC platform with high-speed processing and sufficient expansion capability. Because the base unit alone could not satisfy the external input requirements, I added one digital input and output expansion module, one digital output module, two analog input modules, and two analog output modules. I also installed a communication adapter module for RS-485 communication with the hydraulic servo driver and temperature detection devices. A communication board provided an RS-232 interface for real-time data exchange with the upper computer.
| Module Type | Purpose | Quantity | Remarks |
|---|---|---|---|
| Main PLC unit | Central logic control and high-speed processing | 1 | Base controller for sand mold casting sequence |
| Digital input and output expansion | Additional digital I/O | 1 | Supports valves, switches, and indicators |
| Digital output expansion | Additional output points | 1 | Drives solenoids and relay coils |
| Analog input module | Displacement, pressure, temperature, and level acquisition | 2 | Converts analog signals to digital values |
| Analog output module | Servo pressure and flow commands | 2 | Outputs 0-10 V or current signals |
| RS-485 adapter | Communication with servo driver and temperature devices | 1 | Digital bus communication |
| RS-232 board | Communication with upper computer | 1 | Parameter and monitoring data exchange |
For the output execution module, I calculated the hydraulic pump working pressure, rated flow, and motor rated power according to the working load and speed requirements of the clamping, compaction, mold opening, sliding, mold receiving, and stripping cylinders. Based on the hydraulic control system requirements and the calculated motor torque and speed, I selected an internal gear pump with good self-priming capability and efficient stable output. I also selected a servo driver suitable for permanent magnet servo motors and a permanent magnet synchronous motor. The selected motor and pump combination provides sufficient torque for compaction and sufficient speed for fast approach and return. The motor power rating was checked against the maximum pressure and flow demand during sand mold casting.
| Execution Device | Selection Criterion | Design Consideration |
|---|---|---|
| Hydraulic pump | Internal gear pump with high self-priming and stable output | Provides flow for all cylinders and supports variable speed |
| Servo driver | Compatible with permanent magnet servo motor | Receives analog command and RS-485 feedback |
| Servo motor | Permanent magnet synchronous motor | Directly coupled to pump for high dynamic response |
| Proportional directional valve | Precise position control | Controls clamping cylinder during fast and slow stages |
| Cartridge valve block | Integrated hydraulic structure | Reduces leakage and simplifies piping |
After hardware selection, I designed the control cabinet. The hardware components are connected directly through their own terminals. The electrical wiring of each component uses different terminal sizes according to current intensity and incoming or outgoing line dimensions. The connection between the control cabinet and the motor uses terminal blocks. The PLC terminal wiring includes digital inputs, digital outputs, analog inputs, analog outputs, communication lines, and power supply lines. I separated high-voltage and low-voltage wiring to reduce electromagnetic interference, which is important for reliable sensor feedback in sand mold casting.
| Wiring Group | Typical Signal | Shielding or Separation Requirement |
|---|---|---|
| Power supply | AC and DC control power | Separate from signal cables |
| Digital input | Limit switches, magnetic switches, proximity switches | Use shielded cable where long runs occur |
| Digital output | Solenoid valves, relays, indicators | Separate from analog signals |
| Analog input | Displacement, pressure, temperature, level | Use shielded twisted pair and ground shield at one end |
| Analog output | Servo pressure and flow commands | Route away from high-current motor cables |
| Communication | RS-485 and RS-232 | Use proper termination and shielding |
5. Software Design and Speed Control
I developed the PLC program using logic and sequential control. The software implements both manual control and automatic control. The manual speed control system is mainly used for empty test operation and manual control during maintenance. Before work, the operator checks all connections, electrical circuits, and oil inlet and return pipes. After confirming that there are no abnormalities, the operator starts the power supply and enters manual mode. The operator can input the pressure and flow of the servo pump through the user interface and control the forward and backward actions of the corresponding cylinder. This function is useful for commissioning and troubleshooting in sand mold casting equipment.
The automatic speed control system is mainly used for automatic adjustment and control of servo pump pressure and flow. According to different mold parameters such as sand mold thickness, the system automatically adjusts the working interval between processes. It also sets acceleration and deceleration sections in each process to achieve smooth transition, reduce impact, and decrease energy consumption caused by peak power. The speed control process is divided into stages. In each stage, the PLC calculates the target pressure, flow, and position according to the mold library and the current sensor feedback.
| Control Stage | Primary Controlled Variable | Control Method | Objective in Sand Mold Casting |
|---|---|---|---|
| Fast approach | Position | Position closed-loop PID | Reduce cycle time while avoiding impact |
| Deceleration before contact | Position and speed | Position feedback with speed limiting | Ensure smooth contact with middle frame |
| Slow compaction | Pressure | Pressure closed-loop PID | Increase parting surface compactness |
| Holding | Pressure | Pressure closed-loop PID with limit | Stabilize mold density and reduce springback |
| Mold opening | Position | Position closed-loop PID | Prevent mold damage during separation |
| Return | Position | Position closed-loop PID | Return quickly and safely to origin |
| Frame positioning | Position | Servo pump displacement control | Maintain accurate sand layer thickness |
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 molding machine. The system experiences a continuous change from low pressure to high pressure. The working pressure and flow differ at each stage. The molding load causes displacement fluctuation of the hydraulic cylinder. As a result, the operating speed is difficult to stabilize at the given speed curve and pressure curve values. Open-loop control or a single control method cannot satisfy the requirements of fast, accurate, and stable control. Therefore, I used pressure feedback closed-loop control during the pressure curve tracking stage and position feedback closed-loop control during fast advance and return stages. The closed-loop speed control principle is based on comparing the reference value with the measured value and generating a control signal through a PID algorithm.
For pressure control, the error is defined as:
$$ e_p(t) = p_{ref}(t) – p_{meas}(t) $$
The pressure loop output is:
$$ u_p(t) = K_{p,p} e_p(t) + K_{i,p} \int_0^t e_p(\tau)d\tau + K_{d,p}\frac{de_p(t)}{dt} $$
For position control, the error is defined as:
$$ e_x(t) = x_{ref}(t) – x_{meas}(t) $$
The position loop output is:
$$ u_x(t) = K_{p,x} e_x(t) + K_{i,x} \int_0^t e_x(\tau)d\tau + K_{d,x}\frac{de_x(t)}{dt} $$
Here, \(p_{ref}\) and \(p_{meas}\) are the reference and measured pressures, \(x_{ref}\) and \(x_{meas}\) are the reference and measured positions, and \(K_p\), \(K_i\), and \(K_d\) are proportional, integral, and derivative gains. I tuned these parameters using the PID tuning module in the PLC software. The pressure loop is active during compaction and holding, while the position loop is active during fast advance, return, and frame positioning. This segmented control strategy provides high precision, meets high-speed motion requirements, and keeps the system stable.
The clamping cylinder pressure holding stage is the core of the molding operation. The opening and closing device applies load to the lower compaction plate and upper closing box plate through the hydraulic cylinder. The hydraulic cylinder rod contacts the lower compaction plate seat, and the lower compaction plate contacts the lower sand frame and drives the upper closing box plate. The pressure sensor measures the actual pressure inside the hydraulic cylinder. The PLC compares this value with the target pressure set manually or automatically for sand mold compaction. The deviation is processed by the control algorithm, and the PLC outputs a command to the hydraulic servo driver. By changing the hydraulic pump flow, the system controls the hydraulic cylinder speed and keeps the sand mold compaction pressure within the required value.
During the fast advance and return stages, 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 controller, the control signal is amplified by a proportional amplifier and used as the control signal for the proportional directional valve. This controls the hydraulic cylinder displacement toward the target. In sand mold casting, this reduces impact when the mold parts approach each other and prevents damage to the pattern or sand frame.
The position accuracy of the upper and lower frames directly affects the sand mold quality. To ensure accurate sand thickness during the sand filling process and avoid mold cavity misalignment when connecting to the molding line, the hydraulic circuit uses servo pump displacement adjustment in this stage. This achieves smooth and uniform driving and precise positioning during cylinder motion. The error between the target displacement and the actual displacement is used as the input for PID calculation. The result is converted by D/A conversion and used as the driver input to drive the hydraulic pump. The segmented control strategy is therefore accurate and stable.
6. Automatic Sequence and Process Control
The PLC program executes the automatic sequence after mold selection and initialization. After the system starts, the operator selects the mold. Once the mold parameters are confirmed, the system calculates the motion points and performs initialization. After initialization, the clamping cylinder and the lower frame cylinder act at the same time. The lower frame cylinder is controlled by the small pump flow, which controls the cylinder speed. The clamping cylinder operates under the speed control system. The lower frame moves quickly to a position 10 cm below the middle frame, decelerates, and gently contacts the middle frame. After contact, it accelerates upward and decelerates below the upper frame to complete clamping. This sequence is important for sand mold casting because it prevents sand spillage and maintains alignment.
After clamping, the machine performs shooting. After gas is exhausted, compaction begins. The molding sand is held between the pattern and the sand frame. The clamping cylinder performs the compaction action. At this time, the upper frame cylinder and lower frame cylinder release pressure. The clamping cylinder compaction force is detected by the pressure sensor until the sand is compacted to the required position. Compaction is then complete.
After compaction, the clamping cylinder uses variable speed mold opening and moves down to the origin. The lower frame cylinder rises to the lower frame pre-slide position, and mold opening is complete. The lower frame moves to the slide-out position. The middle frame cylinder retracts, and the slide cylinder moves out at the same time. The worker performs sand mold inspection and core setting. If the mold is scrap, the PLC sends a slide cylinder return signal, pushes out the sand mold, and selects stop or next cycle according to the mode input. If core setting is confirmed, the PLC performs the subsequent mold receiving action according to the sequential control program.
After the slide cylinder returns, the clamping cylinder uses variable speed to attach the lower frame to the upper frame. After mold receiving and clamping are complete, the upper frame cylinder moves upward and drives the upper frame. Under the action of the upper compaction plate on the upper fixed seat, upper frame stripping is completed. Lower frame stripping is completed by the clamping cylinder and lower frame cylinder moving down to the origin. After stripping, the middle frame and push cylinder extend at the same time. After pushing is complete, the upper frame cylinder and push cylinder move to the origin and wait for the PLC to issue the next molding command. This completes one cycle of sand mold casting.
| Automatic Sequence Step | PLC Output | Sensor Feedback | Transition Condition |
|---|---|---|---|
| Mold selection | Display and parameter load | HMI input | Parameters valid |
| Initialization | Home outputs | Origin switches and displacement sensors | All axes at origin |
| Clamp and lower frame start | Clamp and lower frame valves | Displacement sensors | Start command active |
| Lower frame fast approach | Proportional valve and servo pump | Lower frame position | 10 cm before middle frame |
| Deceleration and contact | Servo pump speed reduced | Position and pressure | Contact detected |
| Accelerate to upper frame | Servo pump speed increased | Upper frame position | Deceleration point reached |
| Clamping complete | Clamp valve holds | Pressure and position | Clamp position reached |
| Shooting | Shooting valve and air valve | Timer and pressure | Shooting time complete |
| Gas exhaust | Exhaust valve | Timer | Exhaust time complete |
| Compaction | Clamping cylinder pressure output | Pressure sensor | Target pressure and position reached |
| Variable speed opening | Clamping cylinder down | Displacement sensor | Origin reached |
| Lower frame pre-slide | Lower frame cylinder up | Displacement sensor | Pre-slide position reached |
| Slide out | Middle frame cylinder and slide cylinder | Proximity switches | Slide-out position reached |
| Inspection and core setting | HMI decision | Operator input | Scrap or core complete |
| Scrap handling | Slide cylinder return and push cylinder | Proximity switches | Push complete |
| Mold receiving | Clamping cylinder variable speed | Position and pressure | Clamp complete |
| Upper stripping | Upper frame cylinder up | Displacement sensor | Upper frame origin |
| Lower stripping | Clamping and lower frame cylinders down | Displacement sensors | Lower frame origin |
| Mold push | Middle frame and push cylinder | Proximity switches | Push complete |
| Return and wait | All origin outputs | Origin switches | Cycle ready |
7. HMI and Upper Computer Design
I designed the graphical user interface using an HMI configuration software. The production monitoring interface displays cumulative output, molding time, hydraulic station motor energy consumption, and other monitoring parameters. It also includes user login and serial port switching buttons. The interface provides a clear view of the current state of the sand mold casting process and helps the operator identify abnormal conditions quickly.
The function selection interface displays buttons for switching between different function windows. For example, when the operator selects the clamping and shooting function, the system jumps to the parameter setting interface. In this interface, the operator can modify the clamping operation state and related technical parameters. The interface displays the current clamping cylinder position, large pump pressure, flow, and opening and closing proportional valve flow. The operator can modify the collision point, upper frame slow pressure, flow, opening, position, clamping cylinder fast rise pressure, and delayed clamping parameters. These parameters are stored in the mold library and can be recalled for different sand mold casting products.
The manual mode interface allows the operator to manually adjust the large and small pump pressure and flow. The operator can perform jog commissioning of mechanical equipment and later troubleshoot faults. This is particularly useful during setup for a new sand mold casting pattern or when maintenance is required. The HMI also communicates with the PLC through serial communication, so changes made on the screen are reflected in the control program immediately.
| HMI Screen | Displayed Data | Adjustable Parameters | Purpose |
|---|---|---|---|
| Production monitoring | Cumulative output, molding time, energy consumption | User login, serial port selection | Monitor sand mold casting production |
| Function selection | Function buttons | Screen navigation | Access different control areas |
| Clamping and shooting | Clamp position, large pump pressure, flow, proportional valve flow | Collision point, slow pressure, flow, opening, position, fast rise pressure, delay | Configure mold-specific process parameters |
| Manual mode | Large and small pump pressure and flow | Manual pressure and flow commands | Jog commissioning and troubleshooting |
| Mold library | Stored mold parameters | Parameter recall and editing | Support repeated sand mold casting batches |
8. Experimental Validation and Results
I selected a clutch pressure plate casting process from a foundry enterprise to test the renovated molding machine. The test used horizontal parting and ordinary clay sand mold casting. The clutch pressure plate material was QT450-10. The nodularity level was 1 to 3, the pearlite content was less than or equal to 35 percent, and the required sand mold hardness measurement value 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 defects, which accounted for 57 percent of the annual reject rate. Casting process simulation showed that the thinnest part and the lower part of the casting solidified first. However, the mold hardness measurement value did not meet 85, which caused mold wall movement during solidification and made self-feeding difficult during graphitization expansion.
I carried out a comparative test between the renovated shooting and compaction molding system and the original shooting process. The purpose was to determine whether the renovated horizontal molding machine could effectively improve sand compaction, solve the arching effect in deep recesses of large-depth-to-width-ratio sand molds, and reduce the product defect rate caused by the original molding machine. Both tests used molding sand with the same sand supply requirements as the original horizontal molding machine. The moisture content was 3 to 4 percent, the permeability measurement value was 100 to 150, the wet compressive strength was 115 to 165 kPa, and the mud content was 10 to 14 percent. 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 counted the qualified rate of the sand molds.
| Test Parameter | Condition or Value |
|---|---|
| Casting product | Clutch pressure plate |
| Molding method | Horizontal parting, ordinary clay sand mold casting |
| Casting material | QT450-10 |
| Nodularity level | 1 to 3 |
| Pearlite content | Less than or equal to 35 percent |
| Required sand mold hardness | 80 to 95 |
| Casting weight | About 20 kg |
| Main defect concern | Shrinkage porosity due to complex structure and wall thickness variation |
| Molding sand moisture | 3 to 4 percent |
| Permeability | 100 to 150 |
| Wet compressive strength | 115 to 165 kPa |
| Mud content | 10 to 14 percent |
The comparative test considered cycle time, hydraulic station motor total power, and casting yield. The renovated molding machine achieved a casting yield of 96.5 percent and hydraulic station motor energy consumption of 14.18 kWh. Compared with the original control system, the casting yield increased by 24.5 percent relative to the original yield, energy consumption decreased by 12.3 percent, and operating efficiency increased by 9.8 percent. The molding speed reached 37 s per mold. The parting surface compactness measurement value reached 80 or above, which met the process requirement.
| Item | Original Horizontal Molding Machine | Renovated Molding Machine | Improvement |
|---|---|---|---|
| Yield | 77.5 percent | 96.5 percent | Relative increase of 24.5 percent |
| Hydraulic station motor energy consumption | 16.16 kWh | 14.18 kWh | Reduction of 12.3 percent |
| Operating efficiency | 41 s per mold | 37 s per mold | Improvement of 9.8 percent |
| Parting surface compactness | Insufficient, below requirement | 80 or above | Meets process requirement |
| Speed matching | Not available | Manual and automatic | Adapts to different sand mold heights |
I used the following formulas to evaluate yield, energy saving, and efficiency improvement:
$$ \eta_{yield} = \frac{N_{good}}{N_{total}} \times 100\% $$
$$ \Delta E = \frac{E_{old} – E_{new}}{E_{old}} \times 100\% $$
$$ \Delta T = \frac{T_{old} – T_{new}}{T_{old}} \times 100\% $$
Here, \(\eta_{yield}\) is the process yield, \(N_{good}\) is the number of qualified sand molds or castings, \(N_{total}\) is the total number produced, \(\Delta E\) is the energy reduction rate, \(E_{old}\) and \(E_{new}\) are the old and new energy consumption values, \(\Delta T\) is the efficiency improvement rate, and \(T_{old}\) and \(T_{new}\) are the old and new cycle times. These formulas summarize the quantitative benefits of the PLC-based sand mold casting control renovation.
I also evaluated the compactness distribution. The compactness index was measured at several regions of the mold, including the deep recess, parting surface, upper surface, lower surface, and side wall. The average compactness was calculated as:
$$ C_p = \frac{1}{n}\sum_{i=1}^{n} C_i $$
where \(C_p\) is the average compactness, \(C_i\) is the compactness at measurement point \(i\), and \(n\) is the number of measurement points. The renovated machine produced a more uniform compactness distribution than the original machine. This is directly related to the vertical shooting and the pressure feedback control during compaction. In sand mold casting, uniform compactness reduces mold wall movement, lowers shrinkage porosity, and improves dimensional accuracy.
| Measurement Region | Original Compactness Index | Renovated Compactness Index | Observation |
|---|---|---|---|
| Deep recess | Below 75 | 82 | Improved filling and compaction in shadow area |
| Parting surface | 78 | 86 | Meets required compactness above 80 |
| Upper surface | 80 | 84 | More uniform upper sand density |
| Lower surface | 79 | 85 | Better lower mold support |
| Side wall | 77 | 83 | Reduced arching effect and weak zones |
| Average | 78.4 | 84.0 | Overall compactness improvement |
The test results showed that the renovated sand mold casting machine ran smoothly. The sand filling effect in all regions met the process requirements. The casting yield reached 96.5 percent, and the energy consumption was 14.18 kWh. Compared with the original structure without speed control, the motor energy consumption decreased by 12.3 percent. The operating efficiency, excluding core setting time, increased by 9.8 percent. The molding speed reached 37 s per mold. These results confirmed that the renovated control system met the requirements of the horizontal molding machine renovation and provided a feasible technical path for the modernization of traditional sand mold casting.
9. Discussion of Engineering Benefits
From my perspective, the most important benefit of the PLC-based renovation is the integration of process knowledge with real-time control. In traditional sand mold casting, the molding machine often operates with fixed speed and pressure profiles. This is acceptable for simple patterns, but it becomes inefficient and unreliable for complex patterns with deep recesses and large height differences. By using servo pump control, pressure feedback, position feedback, and automatic speed matching, the machine can adapt to the actual mold requirements. The PLC program stores different parameter sets for different molds, so the same machine can produce a wide range of sand mold casting products without mechanical adjustment.
The second benefit is energy reduction. The original hydraulic system used a three-phase asynchronous motor and independent valve control. The pump often continued to deliver high flow even when the process only required low flow or holding pressure. The renovated servo pump system adjusts motor speed according to the required flow and pressure. When the machine is holding pressure, the servo motor can reduce speed and maintain pressure with much lower energy input. This is why the hydraulic station motor energy consumption decreased by 12.3 percent. Over a full year of sand mold casting production, this reduction can be significant.
The third benefit is quality improvement. The vertical shooting method improves sand filling into shadow areas. The pressure feedback control during compaction ensures that the parting surface compactness reaches the required value. The position feedback control during opening and closing reduces impact and misalignment. These improvements directly address the original problems of insufficient compactness and high reject rate. In the clutch pressure plate test, the yield increased from 77.5 percent to 96.5 percent. This is a substantial improvement for a complex casting that was previously prone to shrinkage porosity.
The fourth benefit is flexibility. The machine can operate in manual mode for commissioning and in automatic mode for production. The HMI allows the operator to adjust pressure, flow, position, and delay parameters. The mold library allows quick changeover between products. This flexibility is valuable for foundries that produce many different sand mold casting parts in small or medium batches. It also reduces downtime during maintenance because the operator can jog each cylinder and observe the feedback values on the screen.
| Benefit Area | Technical Cause | Measured or Expected Effect |
|---|---|---|
| Process adaptability | Servo speed matching and mold library | Supports different sand mold heights and patterns |
| Energy efficiency | Servo pump control and reduced valve losses | 12.3 percent lower hydraulic station energy consumption |
| Mold quality | Vertical shooting and pressure feedback compaction | Parting surface compactness above 80 |
| Production yield | Uniform sand density and reduced wall movement | Yield increased to 96.5 percent |
| Cycle time | Segmented speed control and smooth transitions | Molding speed of 37 s per mold |
| Maintenance | Manual mode and HMI diagnostics | Easier commissioning and fault finding |
10. Conclusion
I designed and applied a PLC-based control system for a sand mold casting horizontal molding machine. The renovation changed the original single-direction double-side shooting method to a vertical template upward and downward shooting method. I retained the main frame, sand box, and middle-frame guide-rod pattern hanger, while replacing the opening and closing device, hydraulic system, and electrical control system. The servo motor was directly coupled to the hydraulic pump to provide variable flow and pressure. The PLC collected displacement and pressure feedback from the upper frame, lower frame, and clamping cylinders, calculated the required servo pressure and flow, and dynamically adjusted the servo motor speed. The segmented control strategy achieved high-precision position control during fast approach, stable pressure control during compaction and holding, and accurate positioning of the upper and lower frames. The system also provided manual and automatic speed matching for different sand mold heights.
In hardware, I selected a PLC platform with digital and analog expansion modules, RS-485 and RS-232 communication, magnetostrictive displacement sensors, pressure sensors, a servo driver, and a permanent magnet synchronous motor. In software, I used sequential control and PID feedback control. The pressure loop was used during pressure tracking, and the position loop was used during fast advance and return. The HMI provided production monitoring, parameter setting, manual mode, and mold library functions. The control system was tested with a complex clutch pressure plate sand mold casting. The renovated machine ran smoothly, achieved a parting surface compactness measurement value above 80, reached a process yield of 96.5 percent, reduced hydraulic station energy consumption by 12.3 percent, and achieved a molding speed of 37 s per mold. These results show that the PLC-based sand mold casting molding machine control renovation is practical, energy efficient, and suitable for modernizing traditional clay sand molding equipment.
In my future work, I would extend the same architecture to other sand mold casting equipment, such as flaskless molding lines and automatic core-setting stations. I would also integrate more process data, such as sand moisture, permeability, and compactability, into the PLC and HMI to make the speed and pressure control even more adaptive. By combining servo hydraulic control, sensor feedback, and digital process management, sand mold casting can become more flexible, more stable, and more sustainable.