Many companies 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.

1. Machine Start-Up & Shut-Down
How does your method affect the screw?
Many companies 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.
STARTUP
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 ½ hour for machines up to about 90mm (3.5"), and 1-½
hours for large machines such as 150mm (6").
Some machine manufacturers actually preset the machine with a
"soak delay" to make sure that the equipment is not "cold started".
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.
SHUTDOWN

2. 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.
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.
REASON FOR PROCEDURE
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.