Optimizing Hopper Airflow and Temperature Controls in Plastic Processing
Hopper airflow rates of 0.8 to 1.2 m3/hr per kg/hr combined with a dew point below -40 C prevent hydrolytic degradation and guarantee uniform drying.
Thermal Oxidation Index
Meaning
Numerical value representing the resistance of a polymer to heat and oxygen determines the stability of the material during processing. Temperatures during production often exceed the point where chemical bonds begin to break in the presence of air. A high thermal oxidation index indicates that a material can withstand long residence times in the barrel without significant yellowing.
Degradation Marker
Chemical changes result in the formation of carbonyl groups and chain scission. When the thermal oxidation index is low, the resin quickly loses its mechanical toughness and becomes brittle. Antioxidants are added to the virgin resin to improve this stability and allow for the use of regrind.
Process Variable
Residence time in the heated barrel is a factor for material health. If the machine is oversized for the part, the thermal oxidation index of the polymer is tested as the material sits too long at high temperatures. Black specks and carbonized deposits on the screw are physical signs that the material has exceeded its limits.
This degradation occurs most rapidly in the stagnant zones of the manifold.
Quality Testing
Laboratories use differential scanning calorimetry to measure the time until an exothermic oxidation reaction occurs. This test provides a baseline for comparing different grades of the same resin. Knowledge of the thermal oxidation index defines the maximum safe processing temperature and cycle time.