The Fatal Flaw in Dew Point Measurement for Injection Molding

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Courtesy of Arizona Instrument LLC

Dew-point measurement is a common and important metric in the plastic drying process. A good dew point meter can tell you whether your dryer is working normally or if it’s in need of repair. Dew point meters do this by measuring the humidity in the air that flows through the dryer. What dew point meters won’t tell you anything about, however, is how much moisture is actually present in the resin that you’re trying to dry. Some molders operate on the assumption that the resin must be dry if the air around it is dry; these molders are setting themselves up for some unpleasant surprises.

Why the industry relies on dew-point measurement

Dew point meters are common among plastics molders for some very good reasons: they’re relatively cheap, they’re widely available and they provide a quick, easy to understand measurement that can help confirm your dryer is operating correctly. Dew-point sensors are often pre-installed in resin dryers, and more advanced dew point meters can even be used to automatically control the dryer itself by automatically switching valves and activating subsystems based on the current dew point.

Why dew point measurement isn’t good enough

As soon as you begin to rely on dew point measurements to tell you whether or not your resin is dry, however, you are setting yourself up for a number of potentially expensive problems. After all, dew point meters measure humidity in the air, not the moisture in the plastic itself. If you work with tight schedules, demanding customers or expensive resins, there’s not a lot of room for error in the molding process. The only way to be certain that your resin is at the correct moisture content before molding is to check the moisture in the pellets themselves using a dedicated moisture analyzer.

Dew point monitors are incredibly useful for assessing the overall health of your drying process. What they cannot do, however, is tell you the actual moisture content of the pellets you will be molding. For that, you need a dedicated moisture analyzer. Arizona Instrument offers a number of moisture analyzers to help you properly monitor this crucial variable in the injection molding process.

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