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. To achieve this objective the Multi-Mull operates on a continuous basis and has automated process controls. When properly applied and adjusted, the Simpson Multi-Mull is a competitive advantage for the user foundry because it is capable of consistently producing higher qualities of quality molding sand at lower costs of operation than any competitive muller.
In practice, users reported the opportunity to improve the performance of the Multi-Mull due to the following issues –
- 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.
- 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 visa versa.
- 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.
- 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.
- 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.