The Sheet Extrusion Process- Part 2
Many control points within the roll stack need to be optimally set, to run a high quality sheet. Let’s take a look at they key points.
The Plastic Sheet Extrusion Process
Product and Production Requirements
Before we compare the
How effective the stack will ultimately be,
Most importantly, beyond the basic sheet stack functions we also need to consider what sheet characteristics need to be produced, and what maximum production rate must be achieved.
Key consideration involves;
What is the intended resin(s) to be used, as the properties will have a very strong influence on the necessary stack configuration selected?
Low melt strength will cause sag and premature contact with
Typical resins of low melt include nylon, APET engineering resins and high melt flow rate (index) resins.
If the resin has a high melt point and low conductivity it will require longer cooling times.
How fast the heat will leave a particular
Fast or Slow setting Resin
If a selected resin is known to set up fast, then you will either need to run faster line speeds or use smaller rolls.
This allows the sheet to be processed free of undesirable stress and the
Resin with process temperatures close to their Tg such as PC for instance, will quickly set.
A resin that will crystallize. The crystallization can have various effects on sheet surface. Generally semi- crystalline will contain higher heat content, thus requiring longer roll contact for cooling.
This means either higher extruder rates and thereby faster line
This can get a bit tricky however. For example, the semi- molten web must quickly arrive and pass through the second nip before the sheet crystallizes fully, so that the top sheet surface will be flat, polished and stress free
These resins will not crystallize, however they cool and densify quickly due to the lower heat content typical of amorphous resins. Again, you will be faced with striking a balance with the roll wrap, to roll size.
The web as it sets will need to stay on the roll long enough to cool to a manageable solidified level, yet not so long that it either takes a set and curls in the machine direction, and/or curls in the transverse direction.
Some resins are inherently glossier than others.
A hot melt on a hot roll surface will produce a polished sheet surface, however if the web is moved from the roll before the sheet
Stiffer material and thicker sheet, from fast setting resin, may require larger rolls for the gentle arc.
Finished Sheet Requirements
The wider the sheet to be produced, the larger the roll diameter required to help offset roll deflection.
Thin sheet will also set much faster than a thick sheet as it passes through the first nip. Care must be taken to run the web through the first nip as close to the nip point as possible, with equal roll contact on both sides of the sheet, this is important in order to maintain lower stress levels in the finished sheet.
Polish or Double polish
When a double polish is required you may need to move the web to the exit roll quickly, in order to produce a glossy top side of the web. For example PP, a semi crystalline resin, must be polished before it begins to crystallize.
Emboss or Double emboss
Emboss is performed on the master (second)
If the kiss roll is selected to impart embossing on the top of the sheet (on a down stack) the sheet cannot be too thick. Thicker sheet will pick up the embossing but the residual sheet heat will then wash the embossing out,
If the second side emboss cannot be achieved in the top nip, the bottom (third) roll may be used. When the sheet is thin, the line speed must be fast enough, or the roll diameters small enough, to transport the web to the second nip before the sheet is too cold to accept an emboss.
The installation of infra-red heaters can also be positioned on the web, on the downside half of the master roll, to prevent the web from cooling prior to passing the second emboss nip.
Double sided Embossed thicker sheet should not be a problem, as a small bank can be run on the second nip.
Finished Sheet Stress levels
The production of flat sheet requires that the sheet possesses a minimum amount of internal stress during and after production to ensure flatness and prevent any post warpage.
Stock used for thermoforming and printed applications are good examples for when sheet low in residual sheet stress is demanded.
As a rule of thumb, higher rates require larger and/or more rolls. There are exceptions to this and a clear understanding of the resin’s properties will help you to more accurately anticipate when the exceptions may occur.
For example resin
Hand in hand with the roll size, thought must be given to the roll stack configuration, which affects the residence time (roll wrap) on the roll surface.
Stringing the line
Consider the time necessary, and the facility with which a line can be strung up.
Quick Roll change
The ease of changing rolls out for sheet surface requirements, as well
Ease of cleaning the rolls
Easy and safe access for cleaning the rolls is beneficial, not only when the line is down, but also while the line is producing.
Hot Nip Laminations
Provisions for a “let off station” on the
Post cooling rolls, water sprays or baths, blower, and fans.
These are all alternative methods to provide additional cooling to your line.
An air knife is used
Open second nip
You may run the second nip open when polish on only one side is needed or when trying to maintain an emboss pattern on the top side of the sheet (in a down stack).
Compound angle die nose
Achieving close nip point approach is often
A die such as this will commonly have a nose tapering down to allow the lip to nip distance to be kept short.
For thin sheet with the lowest of internal stresses a polishing belt is sometimes used in place of the roll stack.
This technology is offered by
Process Control Points within the Roll Stack
The critical control points of the sheet extrusion process
Specific to the roll stack that are highly influential to sheet quality are listed below.
Lip to nip distance
This is a variable factor and can be adjusted during the run.
So it does not crystallize for a Semi Crystalline resin,
Additionally the closer you can keep
This helps to more accurately line up the section of die lip to the corresponding bank, so bolt adjustments
Melt Curtain Approach angle to roll nip
Point of contact at nip roll contact
Adjustable to some extent, depending upon your stack and die face and stack travel in the x and y direction.
Why should the entrance angle give no droop,
Affects angle into the nip
Affects the point of contact to the nip
Avoid once side making too much of an early contact with the master roll.
Roll Wrap angle total cooling per sheet side
Cooling time per side of sheet
Why are why one size cool
Total roll Wrap angle Total Cooling time
This is a fixed variable based on the roll sizes and the collective wrap angle.
Why total cool side
Affect the bending arc of the web
Large roll with a gentler arc are often required for thick sheet.
Running without a kiss roll is some times
The number of rolls will be dictated by the heat content and heat conductivity of the plastic.
Size of rolls
The larger the roll diameter the greater
Exit angle off the last roll
Last roll is adjustable via the roll conveyor.
Differential roll speed
Enogu to keeproll closed