Calculating Direct Machine Downtime Penalties from Moisture Induced Part Defects
Calculate downtime penalties by summing unabsorbed press hourly burden, loaded labor, scrapped resin mass, purge cycles, and hot runner clearing expenses.
Dew Point Monitoring
Meaning
Continuous measurement of the temperature at which water vapor in the drying air stream begins to condense is a primary process control in polymer preparation. This measurement, known as dew point monitoring, provides an immediate indication of the moisture-absorbing capacity of the desiccant beds. A reliable dew point sensor, typically installed in the return air line of the resin dryer, must maintain a reading below minus forty degrees Celsius.
This low moisture level is necessary to ensure that the air circulating through the polymer hopper is dry enough to extract residual water from the pellets.
Sensor Technology
Capacitive ceramic or metal oxide sensors are commonly employed to measure the trace moisture content in the process air. These instruments change their electrical capacitance in response to the partial pressure of water vapor in the gas stream. The sensor must withstand high temperatures and potential contamination from polymer additives that volatize during the drying process.
Regular calibration of these sensors is required to prevent measurement drift and ensure accurate process control.
Process Variable
Fluctuations in the measured dew point indicate potential system failures such as desiccant contamination or air leaks. When the dew point rises above the target threshold, the drying air cannot effectively remove moisture from the polymer pellets. This failure leads to incomplete drying of hygroscopic materials, causing cosmetic and structural defects in the molded parts.
Operators can use the dew point trend data to schedule desiccant replacement before product quality is compromised.
System Integration
Integrating the moisture sensor with the dryer control system enables automatic adjustment of the regeneration cycle based on demand. Instead of switching desiccant beds on a fixed timer, the system only initiates regeneration when the dew point of the outlet air rises. This demand-driven control reduces energy consumption by avoiding unnecessary heating cycles.
It also extends the service life of the desiccant material by reducing thermal stress.
Desiccant Dryer Operation and Basic Moisture Management in Hygroscopic Polymers
Maintaining desiccant dryers at -40°C dew point prevents hydrolytic degradation, preserving melt viscosity and mechanical performance in hygroscopic polymers.