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Current Online Topic: DRYING TIMES

Drying Times
IMM 09/01

We're somewhat new to the molding industry and lately we've been wrestling with drying times, especially with the arrival of spring and increases in temperature and humidity. Does anyone have any suggestions or guidelines on how to adjust drying parameters to compensate for changes in the environment?
Drying times require controls and procedures that ensure process control. I like to classify plastic compounds into three categories: spec dried (an unopened bag or drum ); wet material (usually an open bag or drum); and dried material (resides in the drying hopper). Next, consider how the material is added to the drying process and in what condition. If wet material is added to a dryer in the summer, and it is humid and the required drying residence time is not possible because you are using a marginal system, you will see splay very shortly after the wet material is added. Why is this? If you are using a conventional desiccant dryer you will notice the drying air enters the dryer at the bottom of the or at the center of the dryer, which result in a dewpoint increase at the feed throat. Eventually, the desiccant beds will shift and good dry material will continue. Basically, I think you need to look at the material being dried. Residence time is based on pounds-per-hour being processed through your equipment and good material storage control. We also have to assume the drying equipment is sealed and maintained. - J. Holmes, Boston Scientific Corp., Quincy, MA (508) 652-5126, holmesj1@bsci.com.

Store materials in a clean, dry (low relative humidity) environment. With moisture-sensitive materials (i.e., nylon 6/6, PET) make sure material is in sealed containers before processing. Monitor the to-process air dewpoint to maintain -25F or less during humid summer months. A material moisture analyzer is an excellent tool to determine the maximum level of moisture in your material that will not adversely effect your molding parameters. Nonhygroscopic materials (PP, PE and so forth) have surface moisture only and a hot air dryer should be used. Hygroscopic materials (ABS, nylon, PET, LCP, PEI) have internal pellett moisture and a desiccant or compress air dryer should be used. - D. Day, Day Plastics Machinery Inc., Hopkinton, MA (508) 435-5609, dpminc205@aol.com.

Technically, the drying parameters should not change with the seasons of the year. Each material has a recommended drying time at a particular drying air temperature, with a drying air dewpoint of -20F or lower. Sometimes, the material manufacturers forget to add that the airflow should be 1 1/2 cu ft/min for each pound of the material being dried per hour. Regrind may take more hours and a higher airflow. Under some very dry atmospheric conditions the efforts to dry the hygroscopic resins may by lessened, but for the vast majority of geographical locations drying should be the same all four seasons of the year. - W. Foster, Tessy Plastics Corp., Elbridge, NY (315) 689-2077, bfoster@tessy.com.

Always consult your material supplier for drying recommendations. Wet spring and summer air makes drying more difficult if you do not have a dehumidifying dryer or a source of dehumidified air. - F. French, Amitima, Manchester, MO (314) 520-1252.

The only way you can really know for certain if resign is dry is to check it with a moisture analyzer. Arizona instrument and other companies make some good models. - J. Schramski, Acra Inc., Traverse City, MI (231) 947-8733, jims@acrainc.com.