Most plastics processors accept the premise that the moisture content of materials is important, especially in dealing with hygroscopic materials such as nylons (polyamides) and PC/ABS blends. Nylons are a particular challenge in that they need to be dried just enough, neither too much nor too little.
Moisture requirements are normally specified in terms in terms of the drying process, not the actual moisture levels. The instructions typically specify drying in terms of airflow, temperature, time (frequently 3-6 hours), and dewpoint (usually -40˚F). The actual (vs. theoretical) moisture levels attained as a result of this regime is measurable, but traditionally only offline, and with error-prone, labor-intensive lab procedures.
A fundamental development has occurred in moisture-measurement technology. It is now technically and economically feasible to measure the actual moisture level of material online, while it is being processed, and to make changes, either manually or automatically, to generate the actual moisture level desired.
A new mentality is required for processors and their customers to commit to measure actual moisture level in plastics, rather than using the traditional criteria and trusting that the outcome will be OK. Customers now have the opportunity to identify actual moisture levels that are appropriate rather than just what procedures should be conducted.
The technology comes from BryScan, and is exclusively licensed to Novatec for use in North America. Novatec is marketing this product as MoistureMaster™. We addressed this briefly in our March editorial, (Of Pellets & Magnets) in connection with the use of magnetic forces to perform these actual measurements.
Moisture control is especially critical to satisfy customers in demanding applications, notably medical, automotive and electrical components. Appearance problems that can result from under- or over- drying include splay, silver streaking, internal bubbles, sink areas, stress cracks, and fogging. Physical property losses can include elongation and decreases in impact strength and tensile strength.
Influences on moisture content are numerous, and some of them unavoidable. Processors can’t change the natural progression of the seasons or some of the daily changes in process. Problems can be caused by changes as simple as changing the bag or gaylord source during the day or changing the regrind percentage to use up regrind from the warehouse.
Also, the inherent levels of moisture in polymer materials as received by the processor can vary considerably. In polycarbonate (PC), for example, moisture levels can less than 500ppm in the winter, but as high as 3000 ppm in the summer. Resin temperature can change by 120°F throughout the year and as much as 50-60°F over the course of a day.
Hopper design, resin type, drying time, drying temperature and other considerations can change the outlet moisture. However, the processor can compensate for changes in the situation by using equipment that measures moisture online and indicates what drying changes are needed or makes them automatically. This delivers consistently dried material to the process.
Briefly, here is the basis for the automatic moisture measurement technology. Dielectric constants of water range from 36-80, and polymers range from 2-5. The BryScan 100 uses a magnetic field to measure moisture content within the resin. It measures the known dielectric constant of a particular resin vs. the relatively high dielectric constant of water. Moisture can be expressed as a percentage of resin weight or as parts per million. Sensors have been designed to operate in a range of temperature from 0 to 190°C.
With real-time in-line moisture control, the moisture in the pellet is measured as the pellets pass through the sensor. Trending charts alert the processor to change dryer settings or stop production if the moisture isn’t within acceptable limits. When integrated with dryers equipped for the purpose, the dryer automatically makes changes to settings in order to maintain proper moisture levels.
The system can effect changes before making a bad part. After all, the best way of dealing with a bad part is to not make it in the first place.
Merle R. Snyder
Editor, Plastics Drying Knowledge Center, April 2012
P.S. This commentary is mine, and the PDKC is sponsored by Novatec, whose personnel check this section for technical accuracy. This article is not a statement of Novatec policy. Please direct comments on this column to me at email@example.com.