Most plastic resin looks dry. Looks can be deceiving. If plastic parts are injection molded with resin that has too much moisture in it, problems can occur - problems like ugly-looking parts, or parts that are too brittle. So if monitoring moisture is important, how is that done for plastic resin?
Usually, injection molders use loss-on-drying balances to check their products for moisture. One problem: when you dry a dry product, the weight doesn't change much. When we dried lexan resin for 4 hours at 100 degrees C, here's what happened:
As you can see, the sample weight changed by only 7mg or 0.1%. Water activity is a different way of looking at sample moisture. It measures the relative humidity of a sample instead of how much water is in it. When we measured the water activity of the same lexan sample you see above, we saw this:
The difference between the two Lexan samples is clear - the water activity changed by more than 60%.
You can apply this extra power to your plastic resin samples using a simple process. First, take a water activity reading on the resin before drying. Then dry per the manufacturer's specifications. Then, take a reading on the resin after drying. This gives you your "target". For your next batch, all you need to do is takewater activity readings on your resin every half hour or so. Once you've reached your "target" you can stop drying and start injection molding. You'll save time and also energy cost by not overdrying your resin.
Even if you don't understand the science behind water activity, it's easy to use a water activity meter. All you need is about 2-4 grams of sample. Put it in a sample cup and close the sample chamber to start a reading. In about 5 minutes, you'll have your reading. Operating the instrument doesn't require special training, reagents, sample preparation, or weighing.