Characterization of the Temperature Profile during Dosing of Different Rubber Compounds under Steady State Conditions by Means of a Specially Designed Screw Test Stand

The distribution of the relative curing degree is one of the dominating structural characteristics in injection molded rubber parts. It influences almost all part properties including tensile strength, hardness, compression set and dynamic properties. In addition, the material temperature distribution significantly influences (1) the reduction of the incubation time during processing and (2) the curing degree homogeneity of the produced part. Thus, it is substantial to know the temperature distribution of the dosed rubber compound prior to injection to be able to ensure the processability of the used material. The apparatus used in this work is a specially designed screw test stand consisting of a plasticizing cylinder and a downstream throttle valve. The design of the plasticizing cylinder and the temperature control is identical in comparison to a vertical Maplan rubber injection-molding machine. In addition, it is equipped with pressure and temperature sensors. Furthermore, the throttle valve offers the ability to measure the temperature distribution of the dosed material as well as to adjust the back pressure. To determine the temperature distribution of the dosed material it is necessary to (1) heat up the plasticizing cylinder to the testing temperature (e.g. T=80°C), adjust (2) the back pressure (e.g. p=50 bar) and (3) screw rotational speed (e.g. n=80 min-1) and (4) perform the measurement under steady state conditions. Therefore, a defined injection cycle with a dosing volume of 150 cm³ was repeated five times to check the steady state conditions prior to the temperature measurement. Firstly, it could be shown that steady state conditions could be reached after three injection cycles due to a very effective temperature control of the plasticizing cylinder. Secondly, a temperature rise could be observed during dosing.