Davis Standard J Stack
The Types of Sheet Extrusion Roll Stands: Advantages and Disadvantages
Part III of III
Types of Sheet Roll Stands
There are almost as many terms for the roll stands as there are roll stand configurations. Typical 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 calender”. The term calender does however seems to be used more in Europe and may be considered a borderline misnomer, as it actually denotes a gross sizing of the melt bank and subsequent sheet.
If having choices in life is a good thing, the guys who manufacture roll stands have been very good to sheet processors. Over the years 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. There are also many more terms coined to describe the various sheet roll stand configurations. (See Figure1. and 2.)
One Large roll
By definition, sheet is a web with a gauge of 10mils (0.010 inches) or greater. Below 0.010 inches the web is considered to be film. Film is produced on a one roll stack (if you don’t count the stripping roll.) This process is commonly referred to as (film) casting. Sheet has also been produced using the cast film process. One hundred mil is about as thick as the cast process will take you, that might be pushing it , depending on the polymer. There are also other limitation when attempting to produce sheet on a cast line. The inability to polish both sides of the sheet is a limitation and there is also loss of precise gauge control across the web when thicker sheet is produced. Using one large roll will also produce longer roll residence time for one side of the sheet only, this may be a problem maintaining flatness with certain polymers over various line speeds and sheet gauge. The benefit of producing sheet on a cast line includes less stress being introduced to the sheet through the absence of a nip (kiss roll) and a simpler roll stand design.
Two rolls are better than one.
Adding a second roll would promote increased roll contact, thus improving the quality of the primary surface, as well as improving control of the thickness across the sheet.Using two rolls system however, will bring its’ own limitations. For example, the die will lose some of the close approach to the nip. The kiss roll will also introduce higher stress/strain on the sheet, unlike the lower levels from a casting process. The cooling is still only on one side and flatness may be difficult to control with certain polymers at desired line speed. Some of these issues may be controlled by adding downstream fans, blowers, water bath or water spray.
Now add a third roll and you have the common roll stand. But let’s not stop there, when we can now develop permutations of the three roll stack. So here they are, with as many pros and cons as I could come up with.
Figure1. Click on to enlarge