Multi-Mull Discharge Door Controls Upgrade

• Improvements in both control methodology and mechanical elements
• Accurately and continuously measures the true energy usage at the drive motor
• Accurately determines the energy required to retain a constant mass

The primary design objective of the Simpson Multi-Mull was to enable the preparation of large quantities of tightly controlled molding sand using less energy (horsepower), space and capital than any other muller or mixing system.

Users reported the opportunity to improve the performance of the Multi-Mull due to the following issues:

1. To maintain optimal performance of the muller during major changes in sand demand it is required that an operator intervene to manually reset mechanical adjustments of the inner discharge door mechanism. This requires a skilled, knowledgeable operator and some muller downtime. In many foundries the adjustment was just not done, and system performance suffered. Major changes in sand demand could result from a change in the number of active molding machines requiring sand due to a major maintenance event or reduced production schedules. If the Multi- Mull were set up to run at a high capacity but sand demand was low, frequent starting and stopping of the muller resulted in poor sand control and increased muller maintenance.
   
   
2. The design using a combination of a fixed inner door and the floating outer door lacked the sensitivity required to maintain close tolerance control of molding sand properties during periods of minor changes in demand. Minor changes in demand could result from temporary demand changes due to a machine stoppage for a pattern change or a change from a low to a high sand/metal ratio job or vice versa.
   
   
3. Prior to the year 2000 the supply of most Multi-Mulls took place with separately provided compactability controls resulting in redundant components (panels, PLC’s, etc.) and minimal interfacing between the muller and its primary process control. The redundant controls increased the cost of operator training & maintenance and reduced the overall performance of the system.
   
   
4. The original discharge mechanism of the Simpson Multi-Mull was designed to maintain a constant retained mass of sand inside the muller by controlling the rate of discharge according with a constant amperage draw on the drive motor. The amperage was monitored from one phase (also referred to as one leg) of the three-phase motor. However, fluctuations in amperage can occur across all three legs of a three-phase motor. Some foundries can experience changes in the power supply within their plants.
   
   
5. Unbalanced electrical loads and fluctuations in power demand often create variations in the available power. Sometimes, those variations can be as much as 15%. These variations reduce the true power consumed by the Simpson Multi-Mull drive motor, thereby reducing its efficiency. Monitoring amperage alone typically does not reveal the problem and controlled retention suffers. For the demanding conditions of modern foundries, the old amperage monitoring method, discharge door design and control logic were not precise enough.

Variations are often due to changes in the number of or speed of the molding lines and/or changing sand to metal ratios. In order to allow medium to high volume foundries to continue to enjoy the advantages of continuous mulling but also to optimize control over sand preparation during frequently changing conditions, Simpson has updated the discharge door and its control logic to meet these challenges. The object of the new discharge door system is to reduce or eliminate the requirement for manual adjustment to compensate for a majority of changing conditions. The new door allows any foundry to easily enhance performance of their Simpson Multi-Mull.

For better control and more efficient operation the original double door and amperage meter system have been redesigned. They have been replaced by significant improvements in both control methodology and mechanical elements. The control of retention and discharge is no longer accomplished by monitoring single leg amperage alone. The new discharge door control system accurately and continuously measures the true energy usage at the drive motor, processes this information by comparing it to a set-point value, and adjusts the new discharge door to retain a precise mass of sand in the Simpson Multi-Mull at all times. In order to accomplish this, the new system must first accurately determine the energy required to retain a constant mass. Utilizing a power transducer, the control system first monitors three phase power at the drive motor. Unlike the old system that measured amperage from one phase, the new system monitors amperage, voltage, and phase angle from all three phases of the drive motor. By monitoring all three phases it is possible to eliminate the fluctuations found by monitoring amperage alone. Additionally, the power transducer can be adjusted to overcome the fluctuations in plant electrical power, which hampered the old door design. This provides precise monitoring of the power utilized to retain a consistent retained load of sand inside the Simpson Multi-Mull. This utilization is expressed as a percent kilowatt (or percent horsepower) demand. As the Simpson Multi-Mull operates, the transducer continuously supplies the percent kilowatt demand to the control system. This information is sent through a three-mode control loop (also known as a PID Loop) and is continuously analyzed. The percent kilowatt demand is compared to a set point value. This value is preset by the user depending on the sand quantity and properties required. The three-mode controller is programmed to continuously move the new discharge door into proper position so that the percent kilowatt demanded from the drive motor equals the set point value. These values are continuously compared, and over time, the three-mode controller tightens the span of control. This improves retention accuracy, reduces excessive movement of the discharge door, reduces muller stopping and starting, and minimizes sand property variations. Door control set-points are input into a PLC rather than with manually set with conventional needle type meters.

In order to accomplish precise positioning of the discharge door, a new design has been created. The new door consists of a single, vertical discharge gate. This door is positioned by an electro-mechanical actuator in response to the three-mode control loop. This actuator, complete with a variable frequency drive for positioning, is designed to quickly respond and locate the door to retain a constant retention time at all times manual intervention. This significantly improves the performance of the Simpson Multi-Mull during varying foundry conditions. More consistent sand properties result, even when slight fluctuations in sand composition and demand occur. If more dramatic changes in retention time are required (for example, when one of the two or more molding machines on the sand plant are taken off-line), the new discharge door system can be more quickly and easily balanced.

For existing Simpson Multi-Mull installations, the system has been engineered to easily replace the old door. For new installations, the redesigned discharge door system comes standard on all new Simpson Multi-Mulls.