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Solving 10 Common Challenges Encountered Processing Thin-Gauge Sheet

by Lukasz Jagiello
Fri, Jun 1st 2012 09:00 am
http://www.ptonline.com/articles/solving-10-common-challenges-in-extruding-thin-gauge-sheet  [ View Original Article ]
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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 rolls.

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                   

     Polymer                                   Melt Temperature F

                           PP                                               450-460

                           PP (nucleated - clarified)        470-480

                           PLA                                             410-420          

                           PET                                            540-550

                           PETG                                         450-460          

                           PS                                               440-450

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 employed.

4.         Targeted 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 - 1000

                PET              6  - 12                                     600 - 800

                PS                6  -  12                                    500 - 700

                PLA             12 - 15                                    600 - 800

                PETG          12 - 15                                    600 - 800

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 sheet gauges.

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.