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Tips of the trade: Prevent chatter marks on your extruded sheet

So many things can go wrong during sheet extrusion that it's almost a miracle processors are able to produce as much good product as they do. Chatter marks can be a common problem when processing sheet, but with this checklist you'll be able to locate and fix a weak spot quickly. 

So many things can go wrong during sheet extrusion that it's almost a miracle processors are able to produce as much good product as they do. Chatter marks can be a common problem when processing sheet, but with this checklist you'll be able to locate and fix a weak spot quickly. 

(Editor's note: The K show approaches, fiscal years are closing and budgets are being prepared; as a result of all of these, plus a broad economic recovery, capital investment is on the minds of many processors. We have asked experts from some of the industry's leading suppliers to offer their tips on what processors should consider as they buy new equipment or upgrade their current equipment. Thanks to Richard Risch, president at extrusion line manufacturer Mega Machinery Inc., for offering these tips. Have your own Tips of the Trade? Willing to share them? Contact us at [email protected].)

There are numerous possible causes of chatter marks in extruded plastic sheeting. This checklist helps to identify them by groups: mechanical, thermal, and "variables."

Mechanical
Chain drive systems—it is common for chain drive systems to mark off on the web. Typically one can see consistent marks and measure the frequency, then compare this number with sprocket teeth or chain links. A Ramsey double-faced silent chain will minimize the chatter, but likely not eliminate it totally. This will vary for different material and gauge ranges. 

Individual roll drive systems—generally much better in not imparting vibrations to the web. However, in polycarbonate glazing applications, special reducers, motors, and controls are required to achieve zero web defects. The web chatter marks will have a consistent frequency, which will be time dependant. A technician may identify the source of the vibration by listening at each power transmission device for a noise that matches the interval of the chatter marks. If there is no noise, measurements must be taken to match the roll circumference to the defect. Then work back to the causes.

Bearings—bad bearings will cause chatter marks which may or may not have a constant frequency.

Vibration—pumps and motors may translate vibrations to the web. Where possible, disable one by one and check for improvement. If the source is identified, it should be vibration dampened or isolated. Another source of vibration could be water hammer delivered to the roll. All air must be removed from the circuit to eliminate this possible cause. 

Tension—Excessive tension between the pull rolls and chrome rolls can cause the pull roll drives to translate vibrations to the chrome rolls. Reduce the tension to a minimum and check for improvement. Also check for the frequency of the timing belt lug interval. If it matches the chatter marks, then the belt drive system requires adjustment.

Extrudate delivery—the melt coming from the die may have a "heartbeat." This is generally due to a bad screw design. Melt pumps will dampen screw pulsations 10:1 and in most cases eliminate this influence. Check the extruder and melt pump drives for consistent performance.

Thermal
Extrudate delivery - improper die temperature can cause stick-and-release that may be evident in the sheet. Typically the cause will be an overheated die.

Roll temperatures—it is critical for the web to pass smoothly from one roll to the next. It should be seamless. Each proceeding roll should be hotter than the prior roll. The web should stick to the last roll and release with an even and consistent 'peel' from the chrome surface. If the sheet is too cold to stick, raise the roll temperatures. Check to see that the web does not follow one roll before releasing to the next. The web should not be visibly shrinking on the roll as it travels around, as this would indicate that it is not sticking to the roll.

Drafts—air currents will cause imperfections in the sheet. These are usually not as consistent as one would find in a mechanical event. Check for drafts, and isolate the web with guards in the die area if necessary.

Variables
Roll speeds—the beauty of individual roll drives is the ability to turn the rolls at different speeds. However, some speed ratios may be the cause of chatter marks. On an up-stack machine, the bottom roll has a major influence on the inventory of melt in the nip with the center roll. (This would be the top roll on a down-stack machine.) Incorrect speed could cause a rolling bank, or a squeegee effect on the web that may not be perceptible.

Try various ratios between these rolls while maintaining a 1:1 speed ratio on the last roll with the center roll. Another solution could be to ignore the rolls speeds, but balance the amp load between all four drives including the pull rolls. If the pull rolls' drive were larger than the chrome rolls, this value would have to be scaled. When the polish rolls are too far out of speed synchronization, the sheet has to slip on one of the rolls, which results in a mark which can be amazingly uniform. However, polymers shrink as they are cooled so generally the third roll is run a little slower than the two nip rolls. 

Roll pressures—the roll nip pressure may have an influence. Fixing this will require trial-and-error testing. 

Misalignment—the die to rolls alignment should be checked, as well as all roll-to-roll alignment which is in contact with the web in a molten state. Once the web is frozen solid any misalignment will have no effect on the surface finish.

Hopefully these tips help you to see even more success in your sheet extrusion line. [email protected]

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