Many processors have their own procedure for starting up and shutting down an extruder or injection molding machine and, depending on the process, there may be reasons for the specific method. But from the point of view on the effect of the screw itself, the following procedure will produce the best overall results.
When starting up an extruder or injection machine that has been in operation, it is best that all of the barrel and downstream components have reached the set zone temperatures for a minimum of a half hour for machines up to about 90mm (3.5-in.), and 1.5 hours for large machines up to 150-mm (6-in.). Some machine manufacturers actually preset the machine with a "soak delay" to make sure that the equipment is not "cold started".
Here is my recommended startup procedure:
• After all of the zones have gone through the proper soak time, the screw rotation can be initiated.
• The screw speed should not exceed 15 to 20 rpm, so as not to damage the contact surfaces of the screw and barrel.
• As soon as the screw reaches the set speed, the slide gate on the hopper can be opened. Note: At this time, the screw channels should be partially empty because of the shut-down procedure which will be discussed next.
WARNING: During this time when the screw is recharging the channels, no one should be standing in front of the die or the injection nozzle of the equipment.
• As the screw channels start to refill, the motor load will gradually increase. This is an indication that extrudate will be exiting the end of the die or nozzle shortly. On an extruder, it is important to observe the head pressure gauge during this time because it will also indicate that the screw channels are filling.
• During the recharging of the screw, any air that may have been trapped inside the screw channels will be eliminated during the refilling of the channels. Once a steady flow of polymer exits the die or nozzle, the screw speed can be increased to the desired operating speed.
The shut-down procedure is just as important as the start-up procedure for both an extruder and an injection molding machine. By properly shutting down the equipment, the start-up will be
much quicker and most effective.
Here is my recommended procedure:
• 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.
• During this time of "running out the screw", the screw speed should be reduced to 15-20 rpm so as not to damage the contact surfaces of the screw and barrel.
• 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.
• The feed throat 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 as the barrel zones should be turned off). If the shutdown is only for a short period (less than 8 hours) the screw cooling should also be left on.
• Now the equipment can be totally shut down.
As mentioned earlier, shutdown is very important because the main objective is to evacuate resin from the feed section and as much of the remaining portion of the screw as possible.
If the feed section of the screw is not evacuated, resin will begin to melt onto the root of the screw.
This will cause a melt block and require a significant amount of time (depending on the resin) to work itself off the root of the screw. In the extrusion process, screw cooling will help to eliminate melt block development in most cases. In the injection molding process, the screw should be left in the forward position so that there is not a large inventory of resin in front of the screw to be melted before an "air shot" can be done.
Finally, by evacuating as much of the screw as possible, the heat-up time will occur much more quickly and reduce the risk of screw breakage by trying to rotate the screw before all of the resin near the discharge end of the screw has melted.
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. He was inducted in the Plastics Hall of Fame in 2012. Contact: (724) 355-3311; firstname.lastname@example.org; twwomer.com.