Polyester-based resins are hygroscopic -- meaning the pellet can and will absorb moisture. Drying is necessary to prepare the plastic pellets for processing.
If pellets are not dried, the moisture will react with the molten polymer at processing temperatures, resulting in loss of molecular weight. This loss leads to lowered physical properties such as reduced tensile and impact strengths.
Dessicant type dryers are used to dry the pellets prior to processing.
Typical dryer requirements:
| Type of dryer: |
Automatic, desiccant |
| Air dryness: |
Dew point of –40°C (–40°F) or lower. |
| Air temperature: |
Maximum 65°C (150°F) at the hopper inlet. |
| Circulation: |
0.062 (m3/min)/kg/h (1.0 cfm per lb/h) of plastic to be dried and not less than 2.8 m3/min (100 ft3/min) per unit.
|
| Monitors: |
(a) hopper inlet thermometer
(b) dew point monitor. |
| Hopper size: |
6–10 times the hourly poundage to be dried. (Hold-up capacity depends on average incoming pellet moisture content.)
Example: If a 454-g (1-lb) part is being run on a 1-minute cycle, 27.3 kg (60 lb) of dry material will be needed each hour. If 6 hours is required for drying, at least 164 kg (360 lb) of material must be in the hopper continuously [27.2 kg/h 6 h (60 lb/h 6 h)]. |
| Hopper shape: |
For good air and pellet flow, a tall, slender hopper should be used versus a short, large diameter design. The hopper should have a cone in the bottom to help prevent channeling of the material through the center of the hopper. |
The Bottom Line
Drying saves money in the long run. It will help reduce scrap rates and the amount of rework needed in a normal production environment. It should also reduce customer complaints and field failures and help eliminate most inefficiencies in the processing operation.
