Back pressure is a huge factor in getting the most out of mechanical purging compounds. It helps increase the cleaning power of the purge by using the power of the machine. high back pressure e allows us to run the screw with high rotation speeds without allowing the screw to be loaded backwards creating a pull. This way, the mechanical purge is able to scrub all the threads of the screw and the walls of the barrel with the assistance of the rotation of the screw. But as I saw on a recent customer visit, there are some situations where this isn't a good option on an injection molding machine. Some machines have limits on the amount of back pressure that can be applied to hold the screw in the forward position (zero position).
In these cases where the back pressure has limits, the screw will not stay in the zero position and begins to retract when dosing a shot. At this point we would recommend performing the shot manually to overtake the screw and put it back in the forward position. This adjustment should be made with an increase in injection speed (push speed) so that the same action of overtaking the screw behaves like a high-speed injection shot. This high-speed, horizontal movement will also assist mechanical purging and administer its scrubbing action, but focusing specifically on the check valve and nozzle area.
The two rotation actions and short shots are usually run as two separate steps, starting with the rotations and ending with the short shots. In these situations where the screw has to be moved anyway (even manually), it is advisable to do it with greater speed and in a single step; this way the screw and barrel are cleaned with the two recommended actions.
The set of high rotation speed and high injection speed is responsible for managing the cleaning offered by mechanical purges such as ASACLEAN.
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