Fri, Jun 1st 2012 09:00 am
There is a fine line between
what constitutes thin and thicker gauge sheet.
As a general rule heavy cast film ends where double polishing thin gauge
sheet begins. This often occurs at a
threshold of 200 microns (8 mil MOL).
This threshold can be material dependent based on the thermo-physical
and mechanical properties of the resins being processed such that good
polishing can be achieved at the desired thickness.
Thin gauge sheet production presents
some unique challenges to the extrusion operator, including the following set
of commonly encountered considerations:
1. Raw Material Influences.
A properly configured extrusion system incorporates design features
permitting the achievement of stable operational conditions. This assures the overall delivery of a
homogeneous melt to the sheet die.
Variations in raw materials in the form of virgin and regrind blends can
prove to be challenging when attempting to achieve stable process conditions and
avoid undesirable levels of pressure oscillations (surging). Fluctuating head pressures will adversely
affect the uniformity of the melt bank that is essential to form between the 2
primary nipping chrome rolls. Typical
pressure oscillations of +/- 50 psig (approximately +/- 3 Bar) are acceptable
where as higher values presents challenging process conditions to occur. End user processes generates various amounts
of scrap or regrind that need to be recovered.
For instance, thermoforming can produce high levels of regrind. This is in part dependent upon the tooling
part density per unit of surface area within the tooling platen. Some tools can generate scrap levels as high
as 70+ %. Therefore it is essential to
be able to successfully process high levels of regrind on one occasion and
lower levels on another depending upon the specific amounts of regrind being
generated and recovered back through the sheet process. In terms of bulk density the measured variation
is likely to be a span of 2+/1 due to this thin gauge high regrind content when
compared to higher concentrations of virgin and heavier gauge regrind blends.
In addition to a good overall
feed screw design the extrusion system needs to regulate pressure stably,
maintain consistent temperature set points and thermally homogenize the melt
flow prior to delivery to the sheet die.
A back pressure valve located upstream of the gear pump inlet can permit
the build-up of head pressure to assure the feed screw metering flights are
effectively being filled when processing higher levels of regrind. This also permits adjusting it open when the
low bulk density regrind content diminishes during other processing
timeframes. All in all the key objective
is achieving a uniform melt pressure and homogeneity to the sheet die to
accurately control and minimize the melt bank between the primary nipping
2. Process conditions & set points. The process operating window is reduced when
attempting thinner gauge production when compared to heavier gauge production. This can be easily explained by examining how
the melt bank resides in the nipping area as it enters the roll stand. The melt bank has a tendency to freeze off
quicker given the reduced volume within the primary nip point. This rapid freezing reduces the nip point
flow that is needed in order to achieve a 'spreading' of the polymer uniformly
within the nipping point such that good sheet polish is achieved. It also creates high roll loads to occur
between the gap represented at the primary nipping point. Absent of the melt bank uniformity will cause
the formation of localized regions of shiny and dull spots to occur visually on
the surface of the sheet as the sheet is not being suitable polished across the
face width of the roll. In order properly
navigate these process requirements a tighter overall range of process control
becomes essential. This also leads to
repeatability throughout the production work week/month.
An overall tighter level of control
for the extrusion system is paramount (i.e.; extruder barrel thru the die) for optimally
controlling the process while keeping the chrome roll temperature settings high
enough to prevent rapid freezing to occur on the middle roll. The following summarizes typical melt
temperatures required for various polymers:
Recommended Melt Temperatures
PP (nucleated - clarified) 470-480
3. Die lip exit to roll nip distance. Ideally it is always desirable to operate
with minimal die lip exit distance to the entrance where the primary nipping
rolls meet. By minimizing this distance
it subsequently minimizes the adverse effects of pre-skinning (rapid freezing)
of the molten polymer and excessive neck-down of the melt 'curtain' that will
eventually form into the rolling melt bank.
And at the same time permits greater overall control and formation of
melt bank uniformity.
There are constraints that
must be managed in order to minimize this physical distance. The blunt cross sectional profile of the
sheet die is one limiting factor.
Externally protruding sheet die deckles and/or appendages are other
factors along with the physical diametrical size of the chrome rolls.
The sheet die can be profiled by using a
contoured body so its fits closely into the matting chrome rolls. Internal and/or 'pinch' shim deckles (i.e.;
pinch shim deckles reside between and inside of the lips and is usually made
from aluminum shim stock to suitably fill the die lip gaps) may be used to control the curtain width
entering the nipping area while avoiding protrusions from the die surface as
not to force the distance to widen. And
reducing the leading roll diameter such that the gap is minimized may also be
sheet width and sheet die deckling.
There are several accepted practices used for limiting the overall width
of the molten polymer curtain that exits the sheet die forms the rolling melt
bank between the nipping rolls. Internal
and pinch shim deckles were mentioned above as effective deckle methods. External deckles are also commonly used, but
protrude from the face of the sheet die
which increases the operating die exit to roll nip gap distance. Additionally increasing the die lip exit to
roll nip gap distance while increasing the overall line speed for the roll
stand will reduce the width of the molten curtain due to a phenomenon referred
to as 'draw down'.
By drawing the sheet width
down the thickness of the melt curtain will increase along with the edge bead
that subsequently forms. This is not a
desirable outcome for thin gauge sheet processing as it creates more problems
that is solves, including excessive sheet orientation which can lead to
dimensional stability issues and heavy sheet edge gauge bands making it
difficult to maintain a uniform melt bank and properly polish the sheet across
the entire width.
5. Roll construction & quality. Suitable roll design and construction is
essential when operating at higher roll loads.
Thin gauge sheet production requires high quality rolls and a system
capable of operating at high roll loads with precision. The roll journal and bearings used are
critical. C1 spherical tapered roller
bearings provide the best operating precision yielding a TIR (total indicated
run out) of 0.0005 inches or less. Roll
shafts and bearing journals must be designed accordingly. The overall roll shell construction and style
must accommodate precise rotation while under extreme loads. Required roll loads as high as 1000 PLI
(pounds per linear inch) MOL are not uncommon when processing thin gauge
sheet. High quality roll finishes are
also essential in order to reduce the variability that can be seen in coarser
roll finish treatments. Typically a
highly polished chrome super finish of 0.5 ra is recommended.
6. Roll closing forces and resultant nipping loads. The following chart summarizes the
recommended operating roll loads required to produce and maintain consistent
thin gauge sheet:
Material Thickness (mils) Operating Roll Load (PLI)
PP 10 - 14 800
- 12 600 - 800
PS 6 - 12 500
PLA 12 - 15 600
PETG 12 - 15 600
In order to generate suitable operating roll
loads such that the rolls remain closed a boosted pneumatic or hydraulic roll
actuation method is recommended.
Inadequate load generation will prevent the rolls from holding the
desired gap and a poor outcome will result.
7. Counteracting roll deflection (see Figure 1). The overall design and quality of steel
chrome rolls will determine the degree of deflection realized when placed under
load. Higher loads will result in
greater deflections. The level of
deflection may be significant when compared to the target gauge desired and can
result in an inability to produce high quality sheet. A deflecting roll will cause a relatively
high gauge band to form in the center of the sheet coinciding with the
deflection pattern that is being produced.
For example, if the sheet gauge target is 10 mils and the resultant roll
deflection is 3 mils the net variance is 30 % above the target gauge - which is
unacceptable. A reasonable gauge
performance would be +/- 3 percent of target throughout the range of thinner
There are three basic methods for counteracting
the adverse effects of roll deflection.
A crowned roll features a pre-ground camber to the roll. While this technique works it is limited to a
narrow range of products since the geometry of the camber is fixed and limits
the operational deflection conformity to a reduced processing window. Counter bending rolls feature design
attributes which attempts to bend the roll in an opposite direction to the load
such that it conforms to the mating roll thus minimizing the effects of the
deflection pattern. This technique also
possesses limitations as to the overall effectiveness across a wide operating
range. Roll loads less than 800 PLI are
required due to the spring constant and mechanical limitations of the operating
window for this roll design method. And
finally roll skewing employs a cross axis positioning of the roll such that the
roll deflection is wrapped about the mating roll thus creating a uniform gap
over a wide operating range. Given the
universal operating range for processing thin gauge materials the roll skewing
technique is highly versatile and therefore recommended. The adjustability feature operates over a
wide range of deflection patterns and subsequent sheet gauge targets.
8. Minimizing sheet sag and droop. The attitude of the sheet die position
relative to the roll stack will effect the direction that the molten curtain
exits the die due to gravitational effects.
There are several varieties of roll stand configurations and each has
desirable attributes that can be realized.
The vertical roll stand is widely used and has the greatest level of
versatility across a wide operating window of sheet processes. It is not the best configuration when
attempting the thinnest gauges of sheet as the gravitational effects are
extreme and cause sag and droop to occur leading to rapid freeze of and
pre-skinning of the melt bank on the middle roll. A j-stack arranges the roll stand such that
the die attitude approach is at a 45 degree angle from horizontal. This method permits a reduction of
gravitational effects and provides a greater center roll wrap to occur from the
unique web path that is realized.
However the tendency is for the melt curtain to droop onto the lower
roll since this arrangement is an up-stack whereby the web path traverses upward
throughout the roll stand. The final
configuration presented is a horizontal roll stand (see Figure 2). While this configuration is recommended to
yield superior thin gauge results it also possesses operational
limitations and constraints due to its geometry when processing heavier gauges.
The horizontally configured
roll stand eliminates the adverse effects of sag and droop and permits
introducing the melt bank precisely and directly into the primary roll nipping
location as presented in the referenced figure.
This allows the operator a greater range of processing when attempting
the challenges of thin gauge sheet production - and particularly when
processing tenacious materials that otherwise can present challenges on
different roll stand configurations discussed herein.
9. Controlling the melt bank position. No matter what roll stand configuration is
being used, controlling the melt bank position is essential in avoiding the
adverse effects of rapid freeze off and pre-skinning of the melt bank. Often an experienced operator will adjust the
height of the roll stand relative to the die lip exit such that the melt bank
can be manipulated to permit running with the melt bank against the middle roll
in order to reduce the adverse effects of rapid freeze off and pre-skinning
while also negating the adverse effects of sheet sag and droop.
An external air bar provides
an excellent tool for precisely manipulating and subsequently positioning the
melt bank by flipping it from one roll to the other as the melt bank will seek
the flowing air thus placing it against the middle roll. In the case of a vertical roll stand the air
bar positioning would be beneath the sheet die blowing upward against the
middle roll towards the pool of the melt bank at the nipping location.
10. Accurate speed coordination and control. Precise speed regulation and coordination is
essential in producing high quality thin gauge materials of all resin types. With a good digital drive control platform
roll to roll speed coordination should be on the order of +/- 0.01 % (+/-1/100
%) of set point. This becomes an
important factor since the processing window is narrower on these materials and
therefore requires a higher overall level of coordination and control
precision. Matching the roll speeds from
roll to roll is critical to assure minimal orientation is being introduced
between the rolls as well as for balancing the roll stand take off speed which
will minimize the adverse effects of draw down exiting from the die. Proper tension control between the main
chrome rolls and pull roll unit will also minimize unwanted cross machine
direction lines to form in the sheet in the form of faint chatter marks and
avoid introducing orientation from stretching the sheet. This tension should be maintained in a range of
2 - 4 PLI across the width of the sheet.
About Processing Technologies International LLC (PTi)
Processing Technologies International LLC, based in Aurora, Ill., is a leading global manufacturer of high-performance sheet extrusion machinery, serving more than 22countries. Established in 1988, PTi produces and services single-screw extrusion equipment for many end-use markets including packaging, construction, automotive, lawn and garden, office products, signage and displays, and appliances. PTi's extrusion systems are engineered to exacting standards and offer an exceptional range of design features which result in superior equipment performance. More information is available by calling (630) 585-5800 or visit www.ptiextruders.com.