Venting is one of several operations required for compounding polymers with
additives and fillers in order to achieve desired physical properties. The resulting compound,
in pellet form, must be free of voids and residual volatiles that would otherwise cause
defects in extruded or injection molded parts. How to ensure your venting is effective?
Compounders must realize that the “customer,” a single-screw extruder, usually does not
have any venting capability and that any volatiles within the pellet can only be released in
the final part—leading to surface defects, holes, etc. For hygroscopic polymers that
are prone to hydrolysis or degradation of molecular weight due to the presence of moisture,
efficient venting during compounding is critical to achieve acceptable physical properties.
Vent openings in the extruder barrel can be operated at atmospheric pressure or
under vacuum, according to the requirements of the process. Most typical compounding
processes include both types of venting. Machines can be equipped with multiple vacuum
vents for removal of large volumes of solvents or monomers while producing low residual
volatile levels in the finished product.
Most twin-screw compounding extruders utilize one or more atmospheric vents in
addition to a vacuum vent for degassing volatiles.
Various vent-port designs have been developed for co-rotating twin-screw
extruders to accommodate a wide range of applications and materials; these
include vent stuffers, elongated vent barrels, and side vents. Each of these designs
has evolved to overcome some problems associated with standard vent-port designs,
which are located on the top of the extruder barrel. Requirements for stable
operation of vents, whether atmospheric or vacuum, are a low degree of fill within the
screws in the vent area and a “melt seal” (filled screw) upstream of the vent opening.
Venting problems can be divided into two categories. Operational problems, whereby
polymer (and/or other raw material) is coming out of the extruder vent opening(s). This process
upset usually requires operator intervention to clear out vent ports or requires shutdown of
the line. Residual volatiles/gases remain within the compounded pellet as a result of
insufficient venting within the extruder barrel. In this case, the venting efficiency needs to
be improved to produce acceptable product quality.
Atmospheric vents are designed to release air, moisture, and other volatile gases following
the initial melting of polymers. The velocity of air and/or vapors flowing out of these vent-port
openings is a function of volume flow rate and the vent open area. When the vapor velocity
is too high (as a result of too much volume or too little open area), the exiting gases will tend
to entrain solids, resulting in melt coming out of the vent opening. The solution here is to
provide increased vent area and may require additional vent ports to accomplish this.
Atmospheric vents are required for side feeding of particulate fillers (talc, mineral fillers,
CaCO3 , etc.) downstream into the molten polymer. These vents are designed to release the
air that comes into the machine through the side feeder with the powders. The lower the
bulk density of the filler, the more air is introduced into the extruder barrel that must be vented.
If the vent open area is too small (relative to the volume of air that must be removed), the
resulting high exit velocity will tend to carry fines and powder out of the vent. Solving this
problem requires enlarging the vent area, as previously mentioned, and may also require
additional vent ports.
The presence of unmelted polymer at the first side feeder also results in filler coming out of the
atmospheric vent associated with that side feeder. To diagnose whether this is the cause of
the problem, a physical inspection of the melt quality at the side-feeder vent must be performed
to verify whether any unmelted resin is present. If this is the case, the solution requires
modification of the screw design in the upstream part of the screw where polymer is melted.
Vacuum vents are usually positioned near the pelletizing die and are designed to release any
residual vapors under vacuum conditions. It seems to be a very common problem that the melt
will come out of the vacuum vent and block the vent opening. When this occurs, the gases are
not removed from the melt, and the resulting pellets become porous with voids, etc. The vent
blockage requires an operator to manually clear the vent and in some cases may require
shutting down the line. There are several possible causes and solutions for this problem:
The melt may be reaching the vacuum vent as a direct result of increased backup length.
When the screen or die pressure increases (for example, when the screen becomes blocked
with contamination), the backup length increases accordingly until reaching the vent opening
in the extruder barrel. The melt will flow continuously from the vent opening even without
vacuum. The solutions to this problem are decreasing the pressure (e.g. increasing screen
area or number/size of die holes), increasing the pumping length of the extruder by moving
the vacuum vent one barrel upstream, or installing a melt pump to pressurize the downstream
equipment. Note that most of these solutions are capital intensive. This same condition (backup
length reaching the vacuum vent opening) will occur as the pumping screw elements at the end
of the machine become worn. In the case of screw wear, the melt would be coming out of the
vent more often over time, eventually becoming a chronic condition.
Melt will also flow out of the vacuum vent if the supportable pressure of the melt-sealing
elements is less than the vacuum pressure. This condition results in melt being “pulled” out
of the machine, since the vacuum pump is sucking air through the extruder barrel. Since there
are no pressure sensors installed on the extruder barrel, the only clue that this is the cause of
the problem is to watch the vacuum gauge.
If the gauge is steady over time, this means the vacuum system is “tightly sealed.” If you can
see the gauge dropping off, this is a sign that air is being pulled through the system (and the
vent port will now be full of molten plastic). If the melt is flowing out of the vacuum port only
when vacuum is applied, this is a symptom that the vacuum sealing elements are not creating
sufficient pressure. Changing the screw design to use more restrictive elements for the vacuum
seal is the only solution.
There are some specialty materials that present unique characteristics when exposed to vacuum
at high temperature. Such materials tend to expand and foam upon reaching the barrel opening
and will not flow readily back into the screws. These materials will come out of the machine under
all conditions when using conventional open vent ports. These compounds can only be processed
under vacuum using vent stuffers: mechanical twin-screw systems prevent the melt from
expanding outside of the extruder screw channel yet allow for gases to move axially through
the screws of the vent stuffer, which is installed within the vent opening.