Many of today’s disposable containers - especially water and soft-drink bottles - are made of a polymer called polyethylene terephthalate (PET). Billions are made each year by the injection stretch blow-molding process. In these highly automated, high-speed operations, quality is critical and, because volumes are so high, even tiny variations in the material or process can spell the difference between profit and loss.
PET is a hygroscopic material, which means it will absorb moisture when exposed to ambient air. The higher the humidity, the more moisture it will absorb. During processing, the water turns to steam and causes the resin to degrade. Cosmetic flaws and a loss of physical properties inevitably result if the material is not dried properly before processing. That is why the polymer drying process - and, more specifically, the control of temperature during the drying process - is so critical to quality and profitability.
One dryer-control system, based on Allen-Bradley or Siemens hardware, can help reduce energy use while giving processors a high degree of control over drying. The dryer control uses information gathered by a moisture sensing system as well as temperature and dewpoint sensors. The data from these sensors allows the control to intelligently reduce air temperature and airflow until just enough heat energy is added to the material bed in the drying hopper to maintain an optimum temperature profile. In this application, the system can deliver an average energy savings of between 35 and 45 percent compared to conventional dryers. Return on investment, based on energy savings alone, can be as little as 6 to 9 months.
As material flows through the drying hopper over a period of four to six hours, it must remain at the specified temperature and dewpoint for proper drying to occur. The moisture-sensing probe positions individual RTD sensors at various levels of the hopper, corresponding to various time increments in the drying process. At the same time, dewpoint sensors track moisture levels in the air and another sensor measures the temperature of the material entering the feed throat of the molding machine.
During initial startup, the dryer air blower runs at 100 percent to bring the material bed to the proper temperature for drying. Then, over a period of 12 to 48 hours, the control gradually reduces airflow, using a variable-frequency blower drive, to the minimum required to maintain optimum drying conditions. The process takes so long because the volume of resin in the hopper means that it takes many hours before the results of each incremental adjustment appear. Once proper conditions are established, they can be locked in via the control panel.
Besides monitoring the temperature of the PET in the drying hopper, the system also tracks the temperature at the machine feed throat. Because drying heats the material to levels approaching its eventual processing temperature - the temperature at which it melts and can be injected into the injection mold - the molding machine does not need to expend as much energy.
Another energy saving feature of the system involves regulating the temperature of the air used to regenerate the molecular-sieve desiccant. The desiccant absorbs moisture from the drying air after it has made a pass through the PET material and exits the hopper. Regeneration is accomplished by blowing hot air over the surface of the desiccant so that it releases the moisture, which is vented outside the dryer along with the regeneration air. When ambient humidity is low - during winter months, for instance - less moisture needs to be removed from the material to reach the ideal -40°F (-40°C) dewpoint. Therefore, the desiccant regeneration process does not need to be as intensive and the temperature of the regeneration air can be reduced.
The technology also acts as an early warning system. If a filter becomes clogged, for instance, and airflow to the drying hopper is reduced to the point that it cannot heat the entire volume of material, the change in the temperature profile will be detected and an alarm will sound. In systems without such a moisture sensing and alarm setup, the problem may not be discovered until later when defective product is produced. Then, the process would need to be shut down while the problem is diagnosed and corrected, and fresh material is dried. It could be some time before production can be restarted.
Once the optimization process is completed, and proper drying conditions are established while avoiding excess energy input, the settings can be locked in. If conditions change due to a change in the temperature or moisture level of a particular lot of material, a change in ambient conditions, or material throughput rate, the system will recognize the changes and automatically adjust to reestablish optimum temperature conditions.
Likewise, if some process change requires a change in throughput or temperature setpoints, the system will adjust accordingly. Also, any time a previously run mold is set up to run again, the established optimum settings can be recalled from the control memory, reducing the optimization process. Of course, the system still will analyze the various temperature and dewpoint readings and dynamically adjust settings in response to the current processing environment.
A conventional dryer and control system must run at 100 percent output regardless of actual process demand. An intelligent control system becomes especially valuable when full capacity is not required.
Because PET bottle molding is both high-volume and energy intensive, control innovations are valuable to processors. The moisture-sensing control system also can be effective in applications involving a polymer that dries at a high temperature, or in those where process throughput and ambient conditions change often. The basic concept involving advanced sensor and control technology plus variable frequency blower drives could be applied in other industries as well.