how to drving PET resin.
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| PET, ABS, nylon and PC are examples of
hygroscopic resin. They absorb water moisture in the air. If the moisture
is not got rid of before injection molding, the resin will hydrolize so
the strength becomes substandard and cosmetic appearance suffers, causing
Under a microscope, one could find a hygroscopic resin pellet is filled with capillaries. Once moisture gets into the capillaries, only heating could vapourize the water and get it out. During heating, temperature, duration and the source of incoming air are critical in determining the residual moisture content. Measured by weight, the residual moisture in PET resin should be below 50 ppm. When this happens, the absolute humidity of the drying air has a dew point of -40oC.
The simplest drying method is to use the hopper dryer. Indoor (moist) air is sucked in, heated to 160oC and blown upward through the hopper bottom. After drying for 5 hours, injection molding could begin. As the source of air is the atmosphere, the drying effect is dependent on the air humidity. The drying is unstable and in any case could not reach the 50 ppm requirement.
An oven works on the same principle as the hopper dryer. The difference is resin pellets are spread out in flat pans which increase their contact area with the air and improves the drying efficiency. After drying at 160oC for 5 hours, the pans are emptied into the hopper dryers for use.
Dehumidifying dryer recycles the air for reuse. Before reuse, the moisture in the recycled air is removed by desiccant. The desiccant is in turn regenerated by an independent hot air stream which enables it to continue removing moisture from the recycled air.
Dehumidifying dryer could attain the drying requirement of PET resin.