The plastic product manufacturing process uses some variation of the extruder system. Of the different product types, plastic bag manufacturing is one of the most sensitive operations to manage. Plastic pellets are fed into the extruder where it is heated to melt. The extruder screw then feeds the molten plastic to a ring with hot air blowing through it to blow a plastic bubble. The bubble is fed up into the cooling rack where proper tension must be applied to keep the size and thickness consistent. Anywhere along this process, many issues can occur that will ruin the product and quality.
This sensitivity was on display at a plastic bag manufacturing plant, where the extruder screw was identified as the primary culprit. The plant had seven production lines; all were sensitive and frequently shut down. When a voltage sag occurred — typically caused by distribution level electric utility faults — the contactor controlling the extruder screw motor de-energized. This would stop the flow of molten plastic, which would eventually harden in the screw. The shutdown occurred without maintenance personnel being aware until it was too late. The only way that maintenance personnel could remove the hardened plastic was to chip it out. Labor was the highest cost of the shutdown; the plastic was cheap and in some cases could be reused.
With very low profit margin, high-cost voltage sag mitigation equipment would not be practical in this situation. Instead, a much lower cost option must be used.
In some extruders, motor contactors and control relays are the only sensitive components. In this case, the two types of coil hold-in devices are low cost, providing ride-through down to 25% of nominal voltage. Although the low-cost approach to this problem may not eliminate all shutdowns, it reduces the intensity of the impact of voltage sags, which is a win-win for the customer.
Some customers rely on permanently installed power quality monitoring equipment to notify plant operations of power system events. This allows operations to be restarted quickly.
Glenn is a power quality consultant for EC&M. He can be reached at [email protected].
