Dynamic modeling of plasticating and purging processes in reciprocating screw injection molding machines

The screw changes its length to diameter ratio (LID ratio) during plastication in a reciprocating screw injection molding machine. Due to the time varying effective working length of the screw, the amount of re-circulating backflow changes with time. This causes a time varying oscillation in concentration of the screw output if a tracer is used. The time dependent variation of the output concentration is a measure of the residence time distribution of the resin in the screw, barrel and nozzle. From the residence time distribution one can also calculate the temperature and shear history of the resin.;The residence time distribution, temperature and shear history that the melt experienced during plastication can be modeled as a higher order system since the screw has the three functional sections as well as the dependent variables cannot be measured until the melt comes out of the nozzle during the melt injection or purging. To model the plasticating and purging processes, the plasticating and purging process was illustrated as a block diagram in which a series of blocks represented the appropriate transfer functions based on the results from the purging experiment. The dynamic model developed for this study consists of three different transfer functions: a dead time transfer function, an interacting second order transfer function and a non-interacting first order transfer function.;To determine the system's behavior and response of the dependent variable as a function of time; the model was excited with a rectangular pulse input where the magnitude of the rectangular pulse indicates the plasticating capacity and the duration of the pulse input is the plastication time when the melt flows as the screw rotates. During purging/melt injection, the melt flows through the nozzle, and is modeled as a first order system. Screw plastication occurs at a constant screw speed where the screw goes from zero rpm to its preset speed which is maintained during plastication and then in the same fraction of a second in time goes back to zero. This makes it a natural choice to use a concentration square pulse to excite the system. Alternatively one can use a step change when a tracer input goes from zero concentration to its maximum value at the feed throat at time = 0, as the screw rotates for a series of shots.;The results of this study showed the interacting second order system representing the transition and metering sections of the screw can model the oscillation due to the time varying effective working length of the screw, but the dynamic model does not account for the oscillation of the tracer concentration due to repetitive backflow by the remaining tracer with each plasticating shot. It takes 1 to 30 shots to clean out a tracer or color.