Polyester polymer chips such as PET are hygroscopic, which means they absorb moisture from the surrounding atmosphere. Too much moisture in chips can cause problems during injection moulding and extrusion. When plastic is heated, the water it contains hydrolyzes PET, reducing its strength and quality. This means that as much moisture as possible must be removed from the resin before processing the PET in the moulding machine. Under atmospheric conditions, resins may contain up to 0.6% by weight water.
PET pellets are dried before processing
The wood chips are loaded into the hopper, then hot dry air with a dew point temperature of about 50°C is pumped into the bottom of the hopper and it flows upwards over the pellets, removing any moisture en route. The most hot air leaves the top of the hopper and passes through the aftercooler first, as cold air removes moisture more easily than hot air. The resulting cold, moist air is then passed through the desiccant bed. Finally, the cold dry air leaving the desiccant bed is reheated in the process heater and sent back through the same process in a closed-loop. The moisture content of chips must be less than 30 ppm before processing. When PET is heated, any water present will rapidly hydrolyze the polymer, reducing its molecular weight and destroying its physical properties.
Online measurement and spot check
There are two techniques for measuring moisture during drying: online measurement and spot check.
1. Online measurement
Individual dryers are continuously monitored to ensure that the air supply to the PET is better than the specified dew point temperature limit of 50°C dew point to ensure that the chip material is effectively dried. Where precise measurements with automatic internal calibration are required, the HT-608 dew point sensor can be installed near the inlet of the drying hopper, and its small size and lightweight make it easy to install in ducts or tight areas to check for leaks in the dryer’s air path. High accuracy ±0.2 ° C (5-60 ° C Td), comparable to the quality of imported products, affordable, is a cost-effective alternative.
2. Spot check and calibrate
Regular spot checks with Hengko HK-J8A102 portable calibrated temperature and humidity meter can provide cost-effective assurance of product quality. It’s easy to use, simultaneous measuring the temperature, humidity, dew point, wet bulb and other data.Respond quickly to industrial standard dew points below 50℃.
The dew point measurement range of the temperature and humidity meter is: -50℃-60℃, and the large LCD screen is convenient for reading and reading. The measurement data is calculated once every 10 milliseconds, and the response speed is sensitive and the measurement is accurate.