Proper Shut-down and Start-up Methods

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 their particular approach. But from the point of view of the effect on 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 before, it is best that all of the barrel and downstream components have reached the set zone temperatures for a minimum of 30 minutes for machines up to about 3.5 in.; for large machines such as 6 in., the zones should be at temperature for about 90 minutes. Some machine builders actually preset the machine with a “soak delay” to make sure that the equipment is not “cold started”.

Follow this procedure:

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.

SAFETY WARNING: During the time that the screw is recharging the channels, no one should be standing in front of the die or the injection nozzle of the equipment.

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.

The shut-down procedure is just as important. Properly shutting down the equipment will make the start-up much quicker the next time.

Follow this procedure:

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).

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 to occur and require a significant amount of time, depending on the resin, to work itself off. In the extrusion process, screw cooling will help elimination formation of the melt block in most cases. In injection molding, the screw should be left in the forward position so that there is not a large inventory of resin in front of the screw, which will need to be melted before an “air shot” can be done.

Finally, by evacuating as much of the screw as possible, your heat up time will be significantly reduced. Also, you’ll be reducing the possibility of screw breakage that can occur when you try to rotate the screw before all of the material near the discharge of the screw has melted.

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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; tim@twwomer.com; twwomer.com