HOW TO SOLVE BLOWN FILM PROBLEMS

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HOW TO SOLVEBLOWN FILM PROBLEMS

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How to Solve Blown Film Problems

This technical brochure covers some of the most common blown film problems and theirprobable solutions. It is hoped that the information contained here will be of assistance to youin your film operations.

Table of Contents Page

Blown Film Basics ........................................................................................................................ 2Prevention Checklist .................................................................................................................... 6Dies and Air Rings ....................................................................................................................... 10Maintaining and Improving Film Output ....................................................................................... 13Roll and Film Defects .................................................................................................................. 17Visible Roll Defects...................................................................................................................... 19Gauge Variations ........................................................................................................................ 22Bubble Trouble and Blemishes ................................................................................................... 26Roll Defects Whose Fault? .................................................................................................... 30

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Blown Film Basics

The goal of this brochure is to describespecific defects that can appear in tubularblown film and to suggest probablecauses and solutions. However, a reviewof the process of blown film extrusion isworthwhile.

An operator can become so familiarwith a given film line that problems aresolved intuitively, but training newpersonnel or bringing a new line on streammay raise difficulties. Some inherent andhalf forgotten quirk of the process (andthere are many) may be of noconsequence under familiar conditions,but can become the unrecognized causeof defects when conditions are changedto accommodate new products orprocessing requirements. Reviewing theblown film extrusion process can prepareyou to handle these problems.

Blown Film Process BasicsThe process of producing film by extrud-

ing molten resin into a continuous tube is,

at first glance, extremely simple. Theelements of the process (Figure 1) in-clude the resin pellets which are fedthrough a hopper into an extruder. Here,heat and friction convert the pellets to amelt which is forced through an annularor ring-shaped die to form a tube.

The tube is inflated to increase itsdiameter and decrease the film gauge. Atthe same time, the tube is drawn awayfrom the die, also to decrease its gauge.The tube, also called a bubble, is thenflattened by collapsing frames and drawnthrough nip rolls and over idler rolls to awinder which produces the finished rollsof film.

However, anyone familiar with blownfilm extrusion knows this simplified expla-nation is less than half the story.Theblown film extrusion system is, in fact,one of the most complex and sensitive ofall plastics processing technologies. Thetubular blown film process is efficient andeconomical, and can produce a magnifi-cent array of products from a lightgauge, clear converter film to heavy gaugeconstruction film, which when slit andopened, may measure 40 feet or more inwidth.

Basic Blown Film Line

Figure 1

Elements of Blown Film

Figure 2

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Main Arena of ActionMore of the problems in blown film

extrusion take place in the section of thetube illustrated in Figure 2 from withinthe die to the far side of the nip rolls than in any other portion of the line. Ele-ments in this section are labeled and arereferred to again in this booklet.

Even though practice does not alwaysfollow theory, theory can help explainmany of the problems encountered inextruding polyethylene into blown film. Forexample, blow-up ratio (BUR) used aloneas a film-making parameter is meaning-less. BUR must be related to draw-downratio and die gap. In Figure 3, all three ofthese parameters are used to illustrate atheory of melt orientation, an importantfactor in extruding the high quality filmrequired by customers.

Blow Up Ratio (BUR) = Bubble Diameter Die Diameter

To illustrate melt orientation, it isnecessary to separate the blow-up anddrawdown functions. In reality, however,these take place simultaneously in the meltbelow the frost line. In this area almost all

BUR = 0.637 x Layflat Width Die Diameter

of the important characteristics of the filmare fixed-orientation, shrink properties,clarity, gloss, strength, etc.

The formula to obtain the BUR anddrawdown ratios and their meanings are asfollows:

Drawdown Ratio (DDR) = Width of Die Gap Film Thickness x BUR

BUR indicates the increase in the bubblediameter over the die diameter. The die gapdivided by the BUR indicates the theoreticalthickness of the melt after reduction byblowing. Since it is difficult to use caliperson the bubble to measure its thicknessunless you knock it down, a more practicalformula is:

The final thickness reduction in themelt after blowing is indicated by adrawdown ratio.

A third ratio, called the blow ratio (BR),is the increase of layflat width over diediameter. BR is used less frequently, butcan easily be confused in conversation withthe more common BUR.

A blow-up ratio greater than 1indicates the bubble has been blown toa diameter greater than that of the dieorifice. The film has been thinned andpossesses an orientation in thetransverse direction (TD).

A drawdown ratio greater than 1indicates that the melt has been pulledaway from the die faster than it issuedfrom the die. The film has been thinnedand possesses an orientation in themachine direction (MD).

Melt Orientation Theory

Figure 3

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In practice these numbers are onlyapproximate because the melt swells as itleaves the die gap. The above calculationsare made using the die gap dimensionbecause the degree of swell varies withthe resin used and processing conditions.

Collapsing the BubbleAlthough these ratios provide general

parameters, some incompatibility existsbetween the configuration of the bubbleand that of the film after it has beencollapsed over the various rolls. After filmis wound, its size is called the layflatwidth. Brief study of Figure 4 shows thereason for this incompatibility. Thesketches show front and side views of abubble 16 inches in diameter collapsed toa layflat width of 25 inches (somenumbers here are rounded off for ease ofcomparison).

In Figure 4, on the front, a righttriangle is formed (shaded area) with thelength of the vertical side equal to D, thedistance between the nip rolls and thebottom of collapsing frame; the length ofthe base side is equal to half the layflatwidth minus the radius of the bubble, or 4inches.

On the side view, a right triangle is formed(shaded area) with the vertical side equal toD as before, but the base side is equal to theradius of the bubble, or 8 inches ( of thediameter).

Since the two triangles have verticalsides of equal length, D, but different baselengths, 4 inches vs 8 inches, the thirdsides of the two triangles (E vs C) mustalso have different lengths. In other words,the length of film that forms the edge ofthe layflat (E) is not equal to the lengththat forms the center of the layflat (C). Yetthese unequal lengths must travel from theplane of their point of contact with thecollapsing frame to the nip rolls in thesame amount of time.

Tabulated data at the bottom of Figure4 show the magnitude of this discrepancyin length. If the angle A, formed by thecenter line of the bubble and the edge ofthe collapsing frame is 22, then distance Dmust be 20 inches for a collapsing framelong enough to accommodate the fullbubble width. By calculation, the edge E isfound to be 20 inches long, while thecenter C is 21 inches. The center of thissection of film is one inch, or about 5%,longer than the edge.

To bring a layflat out of the nips thatactually lays flat, the edge of the filmshould theoretically travel faster than thecenter. In other words, the velocity of thefilm should gradually increase from thecenter until, at the edge, it is 5% greaterthan that of the center. With a line speed of120 feet per minute (fpm) at the center, theedge must travel at about 126 fpm.

Theory Geometry of theCollapsing Bubble

Figure 4

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Closing down the collapsing framehowever, doubles and quadruples thesurface area of the frame in contact with thefilm. Unfortunately, films with a high surfacecoefficient of friction drag between thecollapsing frames. As the center area ofthis warm film in contact with the collapsingframes increases, the additional dragdistorts the flatness of the film, making itbaggy at the center and difficult to print andconvert.

The perfect theoretical solution to thebubble-to-layflat problem is a collapsingframe 200 feet long with a zero coefficientof friction. In this frame, the length of theedge and center of the film would not differby so much as a hairs breadth. However,like many theoretical solutions, this one isjust not practical.

Rotation of DieRotating the die and/or air ring as

shown in Figure 5 can help mask errorsbuilt into the melt by process faults whichcause variations in the film thickness,called gauge bands.

By rotating the die and air ring, thegauge bands can be moved around thesurface of the film as the bubble isextruded. The bubble itself does not rotate.The gauge bands are thus distributedacross the face of the roll, lev