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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
rejects.
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. |
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