Proper Shut-down and Start-up Methods
30. November 2011 10:37
1) After the zones have all gone through the proper soak time, the screw rotation can initiate.
2) The screw speed should not exceed 15 to 20 rpm, so as not to damage the contact surfaces of the screw and barrel.
3) As soon as the screw reaches the set speed, then the slide gate on the hopper can be opened. At this time the screw channels should be partially empty because of the shut-down procedure, which will be discussed next.
4) As the screw channels start to refill, the motor load will gradually increase, this is sign that extrudate will be exiting the end of the die or nozzle shortly. On an extruder it is important to observe the headpressure gauge during this time because it will also indicate that the screw channels are filling.
5) During the recharging of the screw, any air that many have been trapped inside of the screw channels be eliminated during the re-filling of the channels. Once a steady flow of polymer exits the die or nozzle, then the screw speed can be increased to the desire operating speed.
1) Close the hopper slide gate and stop the flow of material into the feed throat of the equipment. On an extruder you will notice that the drive load will start to decline. For the injection molding process, several “air shots” can be performed.
2) During this time of “running out the screw,” the screw speed should be reduced to 15–20 rpm so not to damage the contact surfaces of the screw and barrel.
3) When it is visible that the amount of extrudate coming out of the die or nozzle has totally diminished, the screw rotation can be stopped.
4) The feedthroat cooling should remain on, unless the equipment is going to shut down for an extended amount of time then it can be turned off just was the barrel zones should be turned off. If the shut-down is only for a short period (less than 8 hours) then the screw cooling should also be left turned on.
5) Now the equipment can be totally shut down.
As mentioned earlier, the shutdown is very important because the main objective is to evacuate resin from the feed section of the screw (and also as much of the remaining portion of the screw as possible).
Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; firstname.lastname@example.org; twwomer.com