SMART-DT Dewpoint Meter for Polymer Particle Drying Control

Polymer particles (e.g. PET) are the basic raw material for plastic injection molding machines and extruders. Before the production process begins, the particles must be dried to reduce free moisture and to improve the quality and appearance of the finished product. Strict control of the drying process is critical to the quality and appearance of the product.

PET is very hygroscopic in its solid form and is typically capable of absorbing up to 0.6% of its weight in atmospheric moisture. In order to obtain a quality molded product, the moisture content of the particles must be reduced to less than 0.003% by weight prior to production molding. This is because if the moisture content of the particles exceeds this level, the polymer will hydrolyze when heated, the molecular weight will decrease and its physical properties will be affected.

Therefore, prior to production in injection molding machines and extruders, the polymer particle feedstock is usually dried in a feed hopper and then heated to melting temperature. Drying is performed by hot air, which is first passed through a dryer containing a desiccant and then through the particles for a period of time. The dew point of the hot air is usually below -40°C. The particles are then transferred to the feed hopper of the molding machine. 

The drying process is not absolutely reliable. If it fails, it takes some time before the deterioration of the product quality due to higher than permissible moisture content of the raw material is noticed. If the finished product of poor quality is passed further down the production chain, the molded part will definitely be in it and the cost of such failure is very high.

The causes of failure can be: clogged air filters, return air cooling, heater failure, leakage of ambient air. 

Therefore monitor the plastic dryer closely to ensure it stays at a low dew point. The MZD SMART-DT dew point meter is installed on each dryer to continuously monitor, display and alarm online.

Another low-cost solution is to use the MZD SMART-DT portable dew point meter to periodically check each dryer on site and keep records.

The 5 micron stainless steel sintered cartridge, used as the MZD SMART-DT online dew point meter, can also be used in the MZD SMART-DT portable dew point meter to protect the probe from particulate contamination, such as dryer desiccant or polymer dust.