I. Project Background and Equipment Specifications

This case study involves the WDL-RG series elliptical resistance crucible furnace, available in 400 kg, 600 kg, and 800 kg capacity configurations. The equipment is designed for factory workshop environments with an ambient temperature of 0 to 60°C and relative humidity of 40% to 70%. The rated aluminum liquid temperature is 650 to 700°C.

All three configurations achieve a full-load melting time of no more than 4 hours. Heating power increases with capacity: the 400 kg model is rated at 66 kW, the 600 kg model at 90 kW, and the 800 kg model at 105 kW; corresponding heating currents are 99 A, 135 A, and 158 A respectively. The input power supply is uniformly 380V AC, 50Hz, three-phase five-wire.

The control cabinet measures 650×600×400 mm, with an S7-200smart PLC and touch screen as the main control system. Temperature measurement uses 2 channels with a range of 0 to 1300°C. Aluminum liquid temperature control accuracy during the holding stage is ±5°C. Protection mechanisms include leakage protection and over-temperature protection.

II. Cascade PID Temperature Control Architecture

The core feature of this equipment's temperature control is the adoption of a cascade PID control structure rather than single-loop PID. There are two temperature measurement channels in the system: one for furnace atmosphere temperature and one for aluminum liquid temperature.

The cascade control action path is as follows: the outer loop uses aluminum liquid temperature as the main control parameter, and the inner loop uses atmosphere temperature as the auxiliary parameter. The setpoint of the inner loop is dynamically generated by the PID output of the outer loop. The engineering significance of this architecture lies in the fact that aluminum liquid temperature changes with relatively large lag, while atmosphere temperature responds faster. If the heating power were adjusted directly based on aluminum liquid temperature deviation, overshoot or sluggish response would be likely. With atmosphere temperature introduced as the inner loop, the system can respond quickly through the inner loop before the aluminum liquid temperature deviates significantly, ensuring both response speed and overshoot suppression.

The PID control interface on the touch screen provides three independent input boxes for P, I, and D parameters, with a real-time temperature curve displayed at the top of the interface, supporting sliding of the time axis to view historical data within 0.5 hours. When parameters are adjusted, the temperature curve provides real-time feedback to help operators judge the adjustment effect.

III. Three Process Modes Related to the Crucible

The equipment has three preset working modes, switched via a drop-down menu on the touch screen. The design of all three modes revolves around thermal stress management of the graphite crucible.

The sintering and solidification mode is a dedicated program for the first heating of a new crucible. This program is divided into 6 heating steps, with the starting setpoint rising progressively from 50°C to 800°C, and the corresponding heating rate decreasing progressively from 36°C/s to 6°C/s. The design logic of this decreasing heating rate is that faster heating can be tolerated in the low-temperature stage, while the rate must be reduced near the sintering temperature to avoid thermal stress cracking of the crucible caused by excessive temperature differential between the interior and exterior. The upper right corner of the interface verifies operating prerequisites in real time, with items that do not meet the standards warned in black font and items that pass marked in green font.

The preheating mode is a secondary preheating program after the equipment completes the first sintering and solidification. Its running time is shorter than that of the sintering and solidification mode, but the temperature curve settings, condition verification rules, and temperature monitoring parameters are completely consistent with the sintering and solidification mode. This design enables the equipment to restore working temperature along a controlled heating path when restarted after shutdown, avoiding direct full-power startup from a cold furnace.

The working mode is the standard state after the equipment enters normal production. When the furnace temperature reaches the preset operating threshold and the thermal field uniformity meets the requirements, the system confirms the state switch through audible and visual prompts.

IV. Parameterized Requirements for Crucible Installation and Preheating

The selection of the crucible base directly affects crucible life. The base shape should match the crucible bottom, with a size approximately 2 cm larger. Crucible bases of appropriate height should be selected for different types of crucible furnaces. Using discarded crucible bases will greatly shorten the crucible's service life.

For installing a resistance furnace crucible, the crucible should be placed in the center of the furnace, with the bottom of the crucible higher than the bottom row of heating elements. Insulation cotton should be used to seal the area between the top of the crucible and the furnace rim.

The parameter path for the first preheating of the crucible is as follows: without adding any material, preheat to approximately 200°C for 30 minutes, then dry for 4 hours. The purpose of this step is to evaporate moisture adsorbed on the crucible surface, preventing cracking during initial use. After preheating, the crucible should be heated to 800 to 900°C (about 2 hours) as soon as possible and held at that temperature for 1 hour, then raised or lowered to the working temperature. This rapid passage through the temperature range is designed to move the graphite crucible quickly through the oxidation-prone temperature band.

After the first preheating, the crucible usually does not need to be preheated again unless the crucible becomes damp again.

V. Thermal Stress Constraints in Charging and Operating Procedures

The metal material must be dried before it is added. When adding materials, place them gently; do not throw or pour metal ingots into the crucible.

Place the metal ingot vertically into the crucible, ensuring sufficient space between the ingot and the crucible wall for expansion. Do not place the ingot horizontally in the crucible, as the expansion of the ingot during heating may damage the crucible. You can first add some small pieces of metal into the crucible as a base for the larger ingot.

During continuous smelting, some molten metal can be retained in the crucible to accelerate the melting process. However, large pieces of metal should be avoided incorporating into small amounts of solution, as this could cause the solution to cool rapidly or solidify, damaging the crucible. When adding liquid metal, avoid overfilling; generally, the crucible should not be filled to more than 80% capacity.

Additives should only be added after the metal has completely dissolved. Additives should never be added to an empty crucible or to solid metal, nor should they be allowed to directly contact the crucible wall or outer rim. The prescribed dosage must be strictly followed, as excessive use of additives will significantly shorten the lifespan of the crucible.

The size of the crucible tongs must match the size and shape of the crucible, and they must not squeeze the upper edge of the crucible. The crucible tongs should be clamped in the lower middle part of the crucible – if too high, the crucible may fall off or be damaged; if too low, it will be difficult to operate during pouring. When moving a hot crucible, it should be placed on a high-temperature resistant plate or transport tool. Do not place a hot crucible on a cool surface to avoid damaging the bottom of the crucible.

When it is necessary to temporarily stop using the crucible for reasons such as removing slag or repairing the crucible furnace, the molten metal inside the crucible should be emptied. If the molten metal is left to solidify inside the crucible, it will burst due to expansion during the next heating.

VI. Slag Removal Frequency and Crucible Corrosion Control

During the smelting process, additives and impurities in the molten metal will form slag, which adheres to the crucible wall. Because the slag contains additive components, it will corrode the crucible wall, so the slag should be removed once a day.

Specialized tools should be used to remove molten slag before it solidifies. Solidified slag will be difficult to remove. When removing slag, avoid scratching the crucible wall. When multiple layers of slag solidify and remain on the crucible wall, they will corrode the crucible, affect its thermal conductivity, and cause the crucible to crack during heating and expansion, thus shortening the crucible's service life.

VII. Control System Interface and Data Recording

The control cabinet panel is equipped with a three-color indicator light: red for Level 1 error alarm, yellow for Level 2 warning alarm, and green for automatic operation indication. The LED digital display shows the current ambient temperature during preheating and sintering modes, and the current aluminum liquid temperature during operating mode.

The touch screen system features hierarchical user permissions, with different permission levels corresponding to different operating scopes. The main interface displays equipment status, heating system status (including power regulator contactor open/closed state and regulator readiness status), real-time three-phase current monitoring values, and the current and set values of atmosphere temperature and aluminum liquid temperature. The atmosphere temperature setpoint upper limit is 950°C, and the aluminum liquid temperature setpoint upper limit is 900°C.

The energy consumption statistics interface records the cumulative electricity consumption since the equipment was put into use, as well as the daily electricity consumption of the last two weeks. The I/O monitoring interface displays the real-time action status of PLC input points. The alarm event interface sounds the buzzer immediately when a fault occurs; if the fault cannot be eliminated temporarily, clicking the mute button silences the buzzer, and a new fault will automatically unmute and sound again. After the fault is cleared, clicking the reset button clears the current alarm.