Drying is an absolute necessity to prepare polyesters for molding.
Polyesters and other polycondensation polymers can absorb moisture from the air. If pellets are not dried, the moisture will react with the molten polymer at processing temperatures, resulting in a loss of molecular weight. This loss leads to lowered physical properties such as reduced tensile and impact strengths. Therefore desiccant dryers must be used to dry the pellets prior to processing in an injection molding machine.
Note that molded parts may not show any noticeable defects such as splay but may still exhibit lower physical properties without proper drying.
Pellets to be dried need to be in the hopper at the conditions shown on the data sheets for each specific polymer. If the dryer is turned on from a cold start, it must warm up to the proper temperature and the dew point of the air must be reduced to –30°C (–20°F) or below before drying time can be counted.
Dryness of air
Dry air comes from the desiccant beds in the closed air circulation loop of the dryer/hopper system. Desiccant beds must be heated and regenerated before they can dry incoming process air. After regeneration, it is beneficial to cool down the regenerated bed with closed loop (previously dried) air as opposed to ambient air.
Returning process air from the top of the pellet hopper is filtered before it is blown through the desiccant bed and on to the heater and hopper. Dryers used for polyesters should be equipped with aftercoolers to cool the returning process air. Return air temperature should be below 65°C (150°F) to increase the desiccant’s affinity for moisture, thus improving efficiency.
The usual airflow rate requirement for drying is 0.06 cubic meter of hot dry air per minute for each kilogram of material processed per hour (0.06 m3/min per kg/h) or 1 cubic foot of hot dry air per minute for each pound of material processed per hour (1 cfm per lb/h). For example, if 109 kg (240 lb) of material is used per hour, airflow should be at least 6.7 m3/min (240 cfm). Minimum airflow to ensure good air distribution is usually about 2.8 m3/min (100 cfm) for smaller dryers.
Dew point meters measure only the dryness of the air, not the dryness of the plastic pellets in the hopper. Use of a calibrated dew point meter along with measurements of temperature, airflow, and time can give an accurate indication of whether the plastic pellets are being dried properly.
Gravimetric (weight loss) type moisture meters are instruments that measure the moisture inside the pellets. These meters can give a general indication of the effectiveness of the drying system in reducing the moisture level in the plastic pellets. However, most are usually not entirely accurate enough to use as a quality control method to ensure adequate dryness of polyesters while preventing degradation during processing. A moisture level in the range of 0.020%– 0.030% (200 to 300 ppm) is desired, and this should be determined by this gravimetric moisture test in combination with a calibrated dew point meter or other analytical test.
Common dryer problems
- Poor airflow caused by clogged filters
- Air passing through the middle of the load rather than dispersing through the pellets caused by unfilled hopper
- Supply/return dry air lines allowing ambient “wet” air to contaminate dry air
- Wet air contamination through loader on top of hopper
- Lack of cooldown on air returning to the bed in absorption process. (Air should be cooled below 65ºC (150ºF) to increase the desiccant’s affinity for moisture, thus improving efficiency. An aftercooler is required when drying some polymers.)
- Reduced desiccant effectiveness caused by worn-out or contaminated desiccant
- Nonfunctioning regeneration heater and/or process heater
- Blower motor turning backwards
- The airflow is not being shifted when controls call for a dessicant bed change; i.e., one bed stays in process continuously and, thus, is not regenerated, resulting in poor or no drying.