Dynamic viscosity purging of a hot runner manifold

Purging of hot runners in injection molding consumes an inordinate amount of time, materials, and money. The research to improve the purging efficiency was based on the premise that the melt rheology could be modified via changes in processing conditions such as injection speed, melt temperature and resin additives or purge compounds. Measurement of the outlet concentration of a pulse inlet of mica particles in the resin injected at the feed hopper showed that a melt flow instability caused the poor purging performance for either purging the barrel or the hot runner. Flow theory during plastication does not predict an oscillating pulse decay at the outlet. Instead it was found that the quality of the melt is established during plastication and the check ring or valve isolates the melt flow completely form the purging phase when a sufficient shot has been generated. The instability during plastication is similar to surging in a single screw extruder caused by forcing the extruder to operate beyond its output range. Surging in extruders is caused by slippage of the solid bed in the feed section of the screw due to high die pressure. No high melt/nozzle pressure was found in injection molding so a new cause had to be found. In injection molding plastication it is the time varying working length of the screw that causes a backflow and recontamination of the shot as the melt is plasticated. The working length of the screw is at a maximum when plastication starts and the screw recovers until the desired shot is plasticated. As this happens, the working length decreases and the working length of the solid bed, transition and melt/metering section change. This causes an additional backflow of the material that recontaminates the shot. Data have shown that the contamination is eliminated by doing a screw suck-back retraction without rotation and then starting plastication with a constant maximum working length of the screw. The purging time as measured by the number of shots can be decreased by a factor of three. The process procedure can be programmed into all reciprocation screw molding machines whether they are hydraulic or all electrics with servo-motors. The hardware in terms of microprocessor, hydraulics or servo motors is standard. This procedure will also work for reciprocating screw blow molding machines. This procedure not only improves purging but also improves the shot melt quality in production while molding. The constant screw length assures the melt experiences a more uniform time, temperature, and shear history.