An extruder is a unit that compresses a product and then
pushes it through a die. Extruders are used in the hydrocarbon
especially in polymer production. One operational problem
that can occur is called blowback. A blowback
occurs when the processing conditions are such that the
machine can no longer push the material through the die. It
(the product) blows back from the discharge point
through the screw and out the feed end. This is similar to
what occurs when gases in centrifugal compressors surge back
to the product inlet. However, the blowback in extruders is
usually from steam or other volatiles present in the material
being extruded. Blowback can occur one time or it can be
repeated several times with a loud booming sound.
Causes for blowback.
Fig. 1 shows a typical extruder screw. Numerous
extruder-shaft failures, broken gear teeth, drive spline
fretting and other failures had occurred on this
motor-gear-extruder combination over the years. Different
causes were thought to explain these failures, and the unit was
rebuilt and put back into service many times. Usually, improper
startup techniques with cold product left in the extruder and
bumping the motor to free the product were given as
the primary failure causes.
1. Extruder blowback model.
At the request of the operating site, an analysis was done
to determine what effect blowback may have had on the system
torque and, thus, the loads acting on the extruder screw, gears
and bearings. An analysis of the springback of the screw
during blowback was specifically requested.
The model was developed on the idealized system, as shown in
Fig. 1.1 As the motor turns the screw, the shaft
torque, Ts (in-lb), winds up the long shaft of
length, Ls (in.), and diameter, Ds (in.),
and twists the screw u radians. The shaft stays twisted until
the blowback occurs.
Since not much was understood on what occurs during
blowback, it was assumed that, under the worst-case scenario,
the torque twisting the shaft goes to zero instantaneously,
meaning that no processing is being done even though the shaft
is still revolving. The product just slips without friction in
the barrel housing enclosing the screw. This is shown in Fig.
1. The shaft snaps back due to the unleashing of the potential
energy of the wound-up shaft when there is no torque to keep it
twisted. The shaft then unwinds until the potential energy is
used up. It then cycles at the fundamental torsional natural
frequency of the screw until the product starts to process
again, meaning torque is reinstated. The buildup takes place
over a much longer period, and it is assumed that the blowback
occurs in only one cycle.
This effect was calculated to be 25% of the mean torque
value of 150,000 in.-lb. Since it is not a long-term cyclic
event, it is probably not the root cause of the
failures.1 Because the extruder is not producing
product during the event, it wont be allowed to continue
the cycling without a correction. Many repeated blowbacks could
eventually cause a problem if allowed to continue. It was,
therefore, recommended to install a continuous torque
monitoring device to capture the blowback effect and the torque
Although these calculations did not point out the precise
cause of the failures, the logical thought processes eliminated
a number of possible causes. It helped provide a plan forward
if failures continued. One solution was to rewrite the
operating procedures to eliminate the possibility of blowback.
1 Sofronas, A., Case Histories in Vibration
Analysis and Metal Fatigue for the Practicing
Engineer, John Wiley & Sons, to be published in late
Dr. Tony Sofronas, P.E., was worldwide
lead mechanical engineer for ExxonMobil before his
retirement. He is now the owner of Engineered Products,
which provides consulting and engineering seminars. He
can be reached through the website
http://mechanicalengineeringhelp.com by clicking on the