The Sheet Extrusion Process- Part 3
In this final section of this three part post, the pros and cons of the the various Roll Stand configuration are discussed.
Types of Roll Stacks and their Advantages and Disadvantages.
Types of Roll Stacks
There are almost as many terms for the roll stacks as there are stack configurations. Common terms used throughout industry to refer to the common roll stack include: “the roll stand, the roll stack, the polish rolls, the chill rolls, the cooling stack, the three roll stack, the chrome rolls and the calendar”.
If having choices in life is a good thing, the guys who manufacture roll stands have been very good to sheet processors, since they have provided a generous array of roll stack configurations to choose from. Processors require flexibility and gradually through evolution of the roll stack, various process requirements can be now more readily achieved.
However sheet up to about 0.060 has been produced using the cast film process. Additional limitations would include a maximum sheet thickness and the Inability to polish both sides of the sheet. A loss of precise gauge control across the web when sheet thicker sheet is produced can also be expected. The benefits include less stress introduced to the sheet through the absence of a nip (kiss roll).
Two rolls are better than one.
Adding a second roll would promote increased roll contact, thus impacting the quality of the primary surface, as well as improving control of the thickness across the sheet.
Using two rolls systems however will bring its’ own limitations.
Now add a third roll
But let’s not stop there, when we can now develop permutations of the three roll stack.
Roll Stack Configuration Figures
Vertical Down Stack
Vertical Up Stack
Vertical Down Stack with small Kiss Roll
Vertical Up Stack with small Kiss Roll
Offset Up Stack
Incline Up Stack with Offset Roll
Three Roll Horizontal Stack
Four Roll Horizontal Stack
Vertical Down Stack with Post cooling
Vertical Down Stack with Air knife
Variable Position third roll (Pivoting Roll or Swiveling Roll Stand)
Variable Position first roll with a pivoting third roll (Orbital Roll Stand), Not shown.
Various manes for
(“the roll stand, the roll stack, the polish rolls, the chill rolls, the cooling stack, the three roll stack, the chrome rolls and the calendar”.).
1. Vertical Down- Stack
Traditional invented 1930s good for heavy sheet, as gravity acts on sheet to hold it to roll surface. Disadvantage is the primary sheet surfaces leave the bottom roll stack facing downward and making contact
High extruder center line required.
When large rolls for higher output are needed, the die will be elevated and a platform for the operator to perform adjustments will be required.
With very large rolls the die can be installed with the R bar and die lip bolts facing downward so the operator could stand underneath and make adjustments.
High polished surface is on the bottom of the sheet as it is cooling on the surface of the middle roll.
The primary sheet surface coming off the middle roll is forced to make contact with the conveyor rollers .This can be a disadvantage as the sheet surface can become marred. This may be more of a problem for slower cooling resins.
Used for HIPS and ABS.
Amorphous resins such as these will cool and set faster and may be less apt to distort on the conveyor rollers, compared to say a semi crystalline resin like PP.
If the extruder center line is high enough, it may be possible to hot nip laminate to the primary surface from a portable let off station, directly beneath the die, feeding into the first roll nip.
May be advantageous for heavy sheet where the melt curtain is pulled to the middle roll and adheres to the roll face with gravity assist.
Some believe it is good for thick sheet, as gravity assists sheet being pulled down conveyor experiencing a Low stretch to pull rolls. The level of stress that pull rolls would introduce in a down stack is not as severe. This stack is also good for textured (embossed) sheet, as the
Low extruder center line.
Die can be adjusted easier with the operator at floor level.
Stringing the line requires cat walk.
Rolls can be cleaned easier during operation behind the roll stack.
Space behind the roll stack can be used for a let off unwind for hot nip laminating substrates to the bottom side of the sheet.
Mezzanine is required for let off station to hot nip laminate to the top side of the sheet.
Sheet polished side is facing up so it does not scratch or mar on conveyor rollers. This is even more beneficial for resin that cool and set slowly such as PP and HDPE
Embossing with the middle roll allows the textured surface to be face up away from the conveyor roller reducing potential for loss in grain.
Generally recommend for thick sheet. This may be due to the larger diameter rolls required for cooling and the ability to keep the center line of the extruder lower.
The down ward conveyor allows gravity to assist in the uniformly transport the web so pull rolls can be slowed. This allows the sheet to shrink under less tension thereby creating a sheet with less internal stress.
Can elevate conveyor roller to reduce residence time on the exit roll or de elevate to increase wrap on the exit roll.
Can use a rubber roll in the kiss roll position and cool it via a water trough.
3 and 4 .Vertical Down-Stack and Vertical Up-Stack Roll Stand with small kiss roll
Allows a close nip contact point, which allows low melt strength resin to be placed closer to the roll nip point.
Cuts down the primary roll contact time of the melt curtain prior, to passing through the nip. It also shortens the lip to nip distance which cuts down melt curtain cooling prior to the nip passage.
With low and medium melt strength resin, premature contact with the middle roll can occur. To avoid this you must make the die center line higher than the roll nipping point (lower the roll stand with a Down stack configuration or raise the roll stand with an Up stack configuration).
When the kiss roll is of same diameter as middle roll you must then back the roll stack away to prevent the kiss roll from colliding with the top roll. Unfortunately increasing the distance from lip to nip creates the same problem as the wrong centerline of the die. As you move the sheet rollstand back away from the die the material then sags and makes premature contact with the middle roll, the further away the roll is from the die, the greater the droop in the melt curtain. This results in a lower contact point on the middle roll further from the roll nip.
By using a kiss roll with a smaller diameter than to that of the middle (center, primary, master) roll, you can lower the stack and so that the relationship between the roll nip and the die lips(centerline) is one
This proper positioning will produce better cross section sheet gauge and higher primary sheet surface polish.
5. Offset Up-Stack Roll Stand
Same benefits as an Up Stack but due to the additional wrap on the exit roll, more cooling is available. This would allow faster line speeds of thinner sheet, or the ability to run thicker sheet achieving a cool surface temperature
6. J Stack Roll Stand
Advantages of the up stack with the additional benefit of a close nip approach for thin gauge sheet or sheet run from low melt strength resins, such as nylon, polyester and high melt index resins.
Offers most cooling as total wrap angle is the greatest.
J Stack (down)
Good nip entrance.
7. Incline Up-Stack Roll Stand
Used to achieve a close nip approach for thin gauge sheet and/or sheet run from low melt strength resins, such as nylon, polyester and high melt index resins.
Offers shorter roll residence time when faster removal from the rolls to maintain high polish surface or
45 degree angle incline is the most common
60 and 30 degree incline also available, they offer more and less roll
Good Nip Entrance, good for medium melt strength material.
8. Incline Down-Stack Roll Stand
The incline downstack may be more desirable
When very wide sheet is required, larger diameter rolls are desirable to offset roll deflection. Deflection in the rolls can create undesirable heavy gauge in the
Unfortunately with larger rolls comes longer residence time on the roll surface, which can be a disadvantage for some resins. Or when the extruder’s capacity is not high enough to provide increased line
10. Three Roll Horizontal Roll Stand
Horizontal roll stacks are generally
With a horizontal roll stack the negative effect gravity has on the melt
This offers the best nip entrance as the vertically fed extrudate enters the nip uniformly with neither side touching the rolls prematurely.
11. Four Roll Horizontal Roll Stand
Small first roll allow
12. Vertical Down- Roll Stand with Post cooling rolls
For high rates with slow cooling resin HDPE also need uniform control of cool side to side
Or very high line speed with thin packaging grade resins, PP, that don’t set fast
Controls sheet curl (flatness better than
Down side the rolls will be OD smaller diameter which on very think sheet would be subjected to arc that may stress sheet.
Or use many small rolls to ensure the equal
13. Vertical Down- Roll Stand with air knife
The kiss roll is replaced by the air knife. The air is used to hold the sheet in contact with the middle roll
The cross section sheet gauge may not be controlled as
This is important when sheet must not distort in secondary and must have low shrinkage, for example in thermoform /print registration applications often seen in PP packaging.
14. Variable position third roll (Pivoting Roll or Swiveling Roll Stand)
This offers the flexibility
Tailor the residence time to the sheet thickness or the
Swivel stacks offer adjustable roll wrap (residence time).
Variable position first and third roll (Orbital Roll Stand)
The first and the third roll can be adjusted relative to the middle roll. The kiss roll is adjusted but then fixed, however the exiting roll has the ability
This offers the maximum in roll wrap adjustability. As a result the roll