HOW TO SOLVE BLOWN FILM PROBLEMS - LyondellBasell
HOW TO SOLVEBLOWN FILM PROBLEMS
How to Solve Blown Film ProblemsThis 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 ContentsPageBlown 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? . 301
at first glance, extremely simple. Theelements of the process (Figure 1) include 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 explanation 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 magnificent array of products — from a lightgauge, clear converter film to heavy gaugeconstruction film, which when slit andopened, may measure 40 feet or more inwidth.Elements of Blown FilmBlown Film BasicsThe 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 extruding molten resin into a continuous tube is,Basic Blown Film LineFigure 2Figure 12
Main Arena of ActionMore of the problems in blown filmextrusion 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. Elements 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 meaningless. 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.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:Blow Up Ratio (BUR) Bubble DiameterDie DiameterBUR 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:BUR 0.637 x Layflat WidthDie DiameterMelt Orientation TheoryThe final thickness reduction in themelt after blowing is indicated by adrawdown ratio.Drawdown Ratio (DDR) Figure 3To 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 allWidth of Die GapFilm Thickness x BURA 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).3
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.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 4½inches.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 (½ of thediameter).Collapsing the BubbleAlthough these ratios provide generalparameters, 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).Since the two triangles have verticalsides of equal length, D, but different baselengths, 4½ 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 20½ inches long, while thecenter C is 21½ 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 BubbleFigure 44
Fortunately, film made from low densitypolyethylene can stretch. The edge muststretch to permit the center to remain tautas it goes through the nips. If the edge doesnot speed up (stretch), the center will bebaggy and broad “smile” wrinkles willappear across the web.Less extensible film — stiff overwrapfrom resin with a density of 0.935 g/cm3, ora high density, paper-like film — does nothave the ability to stretch. The broad“smile” wrinkles appear if no attempt ismade to increase the edge velocity.However, if the edge velocity is too great,edge wrinkles occur.Normal procedure at this point is toclose down the collapsing frame. Thisprocedure decreases the angle A (seeFigure 4) and reduces the differencebetween the lengths of the center and edge.Decreasing the angle from 22 to 11 narrows the difference in length betweenthe center and edge to 1¼ %. At a 5½ angle, the difference is a mere 5/8%,essentially solving the problem, althoughnot completely.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 hair’s breadth. However,like many theoretical solutions, this one isjust not practical.Rotation of DieRotating the die and/or air ring asshown 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, level wound asfish line on a reel, and the result is acylindrical roll of film of perfect symmetry.RotationFigure 5Without rotation, these faults build upin one place on the roll of film, as fish lineon a reel without a level wind. The result isa roll of film with a surface looking like awood turning for a short thick balustrade.Unfortunately, rotation can introduceproblems of its own in that the gaugebands now gradually move across the faceof the collapsing frames. Such actioncauses the web to move back and forth5
between the frames and the layflat towander back and forth in the line downstream from the main nips. A web guide isrequired to finally track the layflat in astraight line so the film winds up as agood roll.Generally, broad gauge bands causedby a draft of air or a heat rise off the frontend of the extruder against the meltcannot be rotated because the melt itselfis not rotating through this fault. As aconsequence, the roll of film may betapered or have convex or concave facesas the different thicknesses of film buildup upon themselves.Again, as the bubble or die diameter isincreased, so is the transverse speed ofgauge bands across the faces of thecollapsing frames increased for a givenrotational speed. This can cause bubbleinstability, intermittent wrinkling in the nipsand web wander downstream. Thesethree problems can be corrected byreducing rotational speed.However, one rotation should nevertake less than the time it takes to build aroll of film. Otherwise, the gauge bandswill not have had time enough to beuniformly distributed across the entireface of the roll of film.Prevention ChecklistTo prevent problems in extruding blownfilm, purchase the proper equipment to dothe job. Then, make sure the equipment isinstalled properly, checked and maintainedregularly and scheduled efficiently so thathigh quality film can be produced at highoutput rates with minimum scrap.Checklists are excellent memoryjoggers for both production andmaintenance personnel. An operator shouldcheck each film line against a checklist atleast one time per shift. Lines should alsobe checked when there is an order change,and at start-up or at shutdown, since someequipment can be inspected only at thosetimes.All or part of the following checklistusually is incorporated in plant processdescriptions, plant operating standards orthe plant maintenance department’s schedule. The checklist cannot include everythingbecause each blown film extrusion shop isdifferent. However, regular use of thechecklist should minimize many potentialproblems.Resin, Additives and Regrind1.ConclusionMany problems occur in blown filmextrusion in the hot melt between the dieand the frost line and where the tube iscollapsed at the main nip. Other sectionsin this booklet deal more specifically withproblems such as uneven rolls, gaugebands, wrinkles, maintainng output, physicaland optical problems and solutions.2.3.4.6Are the polyolefin resin and additivesthe right grades for the film beingextruded?Are the quantities sufficient to complete the run?Are the resin loading system filtersclean or have they recently beenreplaced?Were the additive feeders emptiedand cleaned when out of service toprevent dribble from contaminating thenext resin to be run?
5.6.7.8.9.Were the recycle/regrind systems forsalvaging edge trim and roll scraprecently checked to make sure thatthe proportion of scrap to virgin resinis accurate?Are the scrap rolls of film compatiblewith the virgin pelletized resin beingused?Are all rolls of scrap clean and labeledby resin type?Was the equipment cleaned betweenresin changes to preventcontamination?Are all hoppers and boxes of resincovered, not only to preventcontamination of the resin, but also toprevent possible damage to theextruder by tramp metal or othermaterials?3.4.5.6.Extruder Drives1.2.3.Most extruders are equipped withsome type of variable speed drive forconsistent output control. Are allfluctuations in revolutions per minuteor power consumption, as indicated bythe screw tachometer or driveammeter, checked along with othercauses for extruder surging?Are all dirty air filters replaced?Otherwise, sensitive solid-state drivecomponents can overheat and burnout.Are the extruder transmissions andthrust bearings properly lubricated?7.8.Extruder Cooling1.Extruder Heaters and Controls1.2.Are temperature controllers for eachextruder zone checked for excessivetemperature override or insufficientheating?Are the melt temperature andpressure indicators and the screwtachometer and drive ammeter —which all indicate extruder operationstability — frequently monitored andmaintained?Are all heater bands and thermocouples around the extruder head,adapter and die checked for loosewires? Be sure the power is off when acheck is made.Is the wiring for the heater bandscorrectly connected, e.g., not “inseries” if “in parallel” is required orvice versa.Are thermocouple leads correctlyconnected to their correspondingextruder zones? For example, thethermocouple lead for zone 3 shouldnot be connected to the temperaturecontroller for zone 2.Are all the heater bands for a specificextruder zone of the same size andwatt density? Mixing different bands inthe same zones can cause cool areasor hot spots depending upon thermocouple location.Are the die heater band terminals notaligned in a row? If they are aligned ina row, the resulting cool spot can create a gauge band in the film.Is the extruder regularly checked forloose thermocouples, burned-outheater sections and loose or brokenwires? Most of the time, these problems are not readily visible and findingthem requires specific maintenance.2.7Is there a specific instrument thatchecks for overheating in the extruderdrives?If the extruder is water-cooled, are thefeed throats cool to the touch?Inefficient plumbing that does notforce all of the air out of the watercooled chambers of the feed throatcan cause poor circulation and hotspots on the surface of the feed throat.
3.4.5.6.7.Resin can melt and stick to these hotsurfaces, bridge and interrupt the resinfeed to the extruder. The result is areduction in output, surging, or in time,the complete loss of feed andshutdown of the extruder.Has the automatic barrel water coolingsystem been checked recently? Thiscan be done by lowering the set pointof the barrel zone temperaturecontroller for a moment, listening forthe pumps to start and observing atemperature drop on the instrument.If air is used in automatic barrelcooling, has the blower exhaust beenchecked for hot air output?If water cooling is used on the screw,are both the incoming and outgoingwater flows at the correct temperatures? Cooling water is normally fedthrough a siphon tube to the end ofthe screw. The outgoing water modifies the temperature of incoming waterbefore it reaches the end of the screw.Otherwise, if the incoming water weretoo cold, it could freeze the melt.Is the water flow in both feed throatand screw cooling areas controlled onthe output side only? Control on theinput side can lead to cyclic heatingand cooling, caused by the periodicdisplacement of water with steam(water hammering effect), leading tosurging.3.4.5.6.7.8.2.Is the air hose used to inflate thebubble removed before die rotationbegins?Tower and Line1.2.Are all the extruder water systemsfiltered and are all the filters checkedregularly?Rotator, Die and Air Ring1.Are the die and air ring level andparallel to each other, with the air ringconcentric with the die orifice?If the air ring is raised above the dieface, is there any dirt between the dieface and the bottom of the ring? Dirt inthis area can interrupt the air flow of meltand cause gauge bands in the film.Is there any dirt or contamination onthe lips and venturi tube of the airring? Die lips should be regularlycleaned with brass shim stock orknitted copper cleaner pads.If die and/or air ring rotator drives arein use, are the commutator rings andbrushes required for full rotation regularly cleaned?Are the power and thermocouplewiring checked for freedom of movement and any scuffing, if rotatingsystems are in use?Is a record kept of screens used anddate installed?Is the pressure gauge at the end ofthe barrel checked often? A highpressure indicates a filled screen. Alow pressure indicates a blow-out.8Is the bubble symmetrical around aplumb line dropped from the centerof the main nips to the center of thedie? This symmetry is essential forwrinkle-free film.Are the collapsing framessymmetrical with each other and thetops aligned with the main nip rolls?If not, the collapsing film can bedistorted as it enters the nip rolls,resulting in continuous orintermittent wrinkles on one or bothedges, depending upon themisalignment. Also, too large a gapbetween the top of the collapsingframe and the bottom of the niprolls allows the collapsed bubble topartially reinflate and cause edgeand/or full “smile” wrinkles acrossthe width of the film.
3.4.5.6.7.8.Is gussetting equipment easilyadjustable? Does it have a lockingIave a matte finish to provide arelatively friction-free surface forthe film to slide upon? Film tends tocling to a brightly finished metalsurface.Do the metal surfaces of equipmenthave a matte finish to provide arelatively friction-free surface for thefilm to slide upon? Film tends to clingto a brightly finished metal surface.Are the collapsing frames cleanand smooth with no sticky coating,dirt or dust? If the nip rolls aresteel and rubber, it is preferable topart the film from the smooth d
of defects when conditions are changed to accommodate new products or processing requirements. Reviewing the blown film extrusion process can prepare you to handle these problems. Blown Film Process Basics The process of producing film by extrud-ing molten resin into a continuous tube is, at first glance, extremely simple. The
3. Stretch Film 24 Key Findings 24 Stretch Film Demand 25 Stretch Film Production Methods (Cast, Blown) 26 Stretch Film Resins 28 Stretch Film Products 30 Demand by Product 30 Stretch Wrap 31 Stretch Hoods 32 Stretch Sleeve Labels 34 Stretch Film Applications 35 Demand by Application 35 Pallet Unitization (Mach
1920 - Nitrate negative film commonly replaces glass plate negatives. 1923 - Kodak introduces cellulose acetate amateur motion picture film. 1925 - 35mm nitrate still negative film begins to be available and cellulose acetate film becomes much . more common. 1930 - Acetate sheet film, X-ray film, and 35mm roll film become available.
Drying 20 minutes Hang film in film dryer at the notched corner and catch drips with Kim Wipe. Clean-Up As film is drying, wash and dry all graduates and drum for next person to use. Sleeve Film Once the film is done drying, turn dryer off, remove film, and sleeve in negative sleeve. Turn the dryer back on if there are still sheets of film drying.
2. The Rhetoric of Film: Bakhtinian Approaches and Film Ethos Film as Its Own Rhetorical Medium 32 Bakhtinian Perspectives on the Rhetoric of Film 34 Film Ethos 42 3. The Rhetoric of Film: Pathos and Logos in the Movies Pathos in the Movies 55 Film Logos 63 Blade Runner: A Rhetorical Analysis 72 4.
Film guide 5 Film is both a powerful communication medium and an art form. The Diploma Programme film course aims to develop students' skills so that they become adept in both interpreting and making film texts. Through the study and analysis of film texts and exercises in film-making, the Diploma Programme film
Factor each expression or equation, if possible. Solve for x if you are working with an equation. Solve for x, find the roots, find the solution, find zeros, and find x intercept mean the same. 1. Factor 4 20xx32 2. Solve xx2 1 3. Solve xx2 60 4. Solve x2 49 0 5. Solve xx2 2 1 0 6. Solve 2 5 3xx2 7. Factor 0y22 8. Factor 25 16xy22
Bandera co-extrusion lines can produce agrifilm as well as thicker film (e.g. waterproofing geomembrane): a valuable chance to have an all-year-round profitable co-extrusion line, regardless of agrifilm seasonality. AGRIFILM APPLICATIONS Greenhouse cover film Silo-bags film Silo-stretch film Solar sterilis
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