Why we do it?
As we design inspection systems for the plastics industries, I am always searching for great knowledge bases on the cause of defects and problems in the extrusion and printing processes. The goal for OneBoxVision is to provide information to our clients that will help them improve. I came across a great paper from LyondellBasell. The following blog entry and supporting document is a mix of info from that white paper and some other sources. Hope you find it useful.
We will look at specific defects that can appear in tubular blown film and to suggest probable causes and solutions. An operator can become so familiar with a given film line that problems are solved intuitively, but training new personnel or bringing a new line on stream may raise difficulties.
Blown Film Process Basics
The process of producing film by extruding molten resin into a continuous tube is, at first glance, extremely simple. The elements of the process (Figure 1) include the resin pellets which are fed through a hopper into an extruder. Here, heat and friction convert the pellets to a melt which is forced through an annular or ring-shaped die to form a tube. The tube is inflated to increase its diameter and decrease the film gauge. At the same time, the tube is drawn away from the die, also to decrease its gauge. The tube, also called a “bubble,” is then flattened by collapsing frames and drawn through nip rolls and over idler rolls to a winder which produces the finished rolls of film.
However, anyone familiar with blown film extrusion knows this simplified explanation is less than half the story. The blown film extrusion system is, in fact, one of the most complex and sensitive of all plastics processing technologies. The tubular blown film process is efficient and economical, and can produce a magnificent array of products — from a light gauge, clear converter film to heavy gauge construction film, which when slit and opened, may measure 40 feet or more in width.
Main Arena of Action
More of the problems in blown film extrusion take place in the section of the tube illustrated in Figure 2 — from within the die to the far side of the nip rolls — than in any other portion of the line. Even though practice does not always follow theory, theory can help explain many of the problems encountered in extruding polyethylene into blown film. For example, blow-up ratio (BUR) used alone as a film-making parameter is meaningless.
BUR must be related to draw-down ratio and die gap. In Figure 3, all three of these parameters are used to illustrate a theory of melt orientation, an important factor in extruding the high quality film required by customers.
Blow Up Ratio (BUR) = Bubble Diameter/Die Diameter
To illustrate melt orientation, it is necessary to separate the blow-up and draw down functions. In reality, however, these take place simultaneously in the melt below the frost line. In this area almost all of the important characteristics of the film are fixed-orientation, shrink properties, clarity, gloss, strength, etc. The formula to obtain the BUR and draw down ratios and their meanings are as follows:
BUR = 0.637 x Layflat Width/Die Diameter
BUR indicates the increase in the bubble diameter over the die diameter. The die gap divided by the BUR indicates the theoretical thickness of the melt after reduction by blowing. Since it is difficult to use callipers on the bubble to measure its thickness unless you knock it down, a more practical formula is:
Drawdown Ratio (DDR) = Width of Die Gap/Film Thickness x BUR
The final thickness reduction in the melt after blowing is indicated by a draw down ratio. A third ratio, called the blow ratio (BR), is the increase of lay flat width over die diameter. BR is used less frequently, but can easily be confused in conversation with the more common BUR. A blow-up ratio greater than 1 indicates the bubble has been blown to a diameter greater than that of the die orifice. The film has been thinned and possesses an orientation in the transverse direction (TD). A draw down ratio greater than 1 indicates that the melt has been pulled away from the die faster than it issued from the die. The film has been thinned and possesses an orientation in the machine direction (MD).
In practice these numbers are only approximate because the melt swells as it leaves the die gap. The above calculations are made using the die gap dimension because the degree of swell varies with the resin used and processing conditions.
Collapsing the Bubble
Although these ratios provide general parameters, some incompatibility exists between the configuration of the bubble and that of the film after it has been collapsed over the various rolls. After film is wound, its size is called the lay flat width. Brief study of Figure 4 shows the reason for this incompatibility. The sketches show front and side views of a bubble 16 inches in diameter collapsed to a lay flat width of 25 inches (some numbers here are rounded off for ease of comparison).
In Figure 4, on the front, a right triangle is formed (shaded area) with the length of the vertical side equal to D, the distance between the nip rolls and the bottom of collapsing frame; the length of the base side is equal to half the lay flat width minus the radius of the bubble, or 4½ inches. On the side view, a right triangle is formed (shaded area) with the vertical side equal to D as before, but the base side is equal to the radius of the bubble, or 8 inches (½ of the diameter).
Since the two triangles have vertical sides of equal length, D, but different base lengths, 4½ inches vs 8 inches, the third sides of the two triangles (E vs C) must also have different lengths. In other words, the length of film that forms the edge of the lay flat (E) is not equal to the length that forms the center of the lay flat (C). Yet these unequal lengths must travel from the plane of their point of contact with the collapsing frame to the nip rolls in the same amount of time.
Tabulated data at the bottom of Figure 4 show the magnitude of this discrepancy in length. If the angle A, formed by the center line of the bubble and the edge of the collapsing frame is 22°, then distance D must be 20 inches for a collapsing frame long enough to accommodate the full bubble width. By calculation, the edge E is found to be 20½ inches long, while the center C is 21½ inches. The center of this section of film is one inch, or about 5%, longer than the edge.
To bring a lay flat out of the nips that actually lays flat, the edge of the film should theoretically travel faster than the center. In other words, the velocity of the film should gradually increase from the center until, at the edge, it is 5% greater than that of the center. With a line speed of 120 feet per minute (fpm) at the center, the edge must travel at about 126 fpm.
Fortunately, film made from low density polyethylene can stretch. The edge must stretch to permit the center to remain taut as it goes through the nips. If the edge does not speed up (stretch), the center will be baggy and broad “smile” wrinkles will appear across the web. Less extensible film — stiff over wrap from resin with a density of 0.935 g/cm3, or a high density, paper-like film — does not have the ability to stretch. The broad “smile” wrinkles appear if no attempt is made to increase the edge velocity. However, if the edge velocity is too great, edge wrinkles occur. Normal procedure at this point is to close down the collapsing frame. This procedure decreases the angle A (see Figure 4) and reduces the difference between the lengths of the center and edge. Decreasing the angle from 22° to 11° narrows the difference in length between the center and edge to 1¼ %. At a 5°angle, the difference is a mere 5/8%, essentially solving the problem, although not completely.
Closing down the collapsing frame however, doubles and quadruples the surface area of the frame in contact with the film. Unfortunately, films with a high surface coefficient of friction drag between the collapsing frames. As the center area of this warm film in contact with the collapsing frames increases, the additional drag distorts the flatness of the film, making it baggy at the center and difficult to print and convert.
The perfect theoretical solution to the bubble-to-lay flat problem is a collapsing frame 200 feet long with a zero coefficient of friction. In this frame, the length of the edge and center of the film would not differ by so much as a hair’s breadth. However, like many theoretical solutions, this one is just not practical.
Rotation of Die
Rotating the die and/or air ring as shown in Figure 5 can help mask errors built into the melt by process faults which cause variations in the film thickness, called gauge bands. By rotating the die and air ring, the gauge bands can be moved around the surface of the film as the bubble is extruded. The bubble itself does not rotate. The gauge bands are thus distributed across the face of the roll, level wound as fish line on a reel, and the result is a cylindrical roll of film of perfect symmetry.
Without rotation, these faults build up in one place on the roll of film, as fish line on a reel without a level wind. The result is a roll of film with a surface looking like a wood turning for a short thick balustrade. Unfortunately, rotation can introduce problems of its own in that the gauge bands now gradually move across the face of the collapsing frames. Such action causes the web to move back and forth between the frames and the lay flat to wander back and forth in the line downstream from the main nips. A web guide is required to finally track the lay flat in a straight line so the film winds up as a good roll. Generally, broad gauge bands caused by a draft of air or a heat rise off the front end of the extruder against the melt cannot be rotated because the melt itself is not rotating through this fault. As a consequence, the roll of film may be tapered or have convex or concave faces as the different thicknesses of film build up upon themselves.
Again, as the bubble or die diameter is increased, so is the transverse speed of gauge bands across the faces of the collapsing frames increased for a given rotational speed. This can cause bubble instability, intermittent wrinkling in the nips and web wander downstream. These three problems can be corrected by reducing rotational speed. However, one rotation should never take less than the time it takes to build a roll of film. Otherwise, the gauge bands will not have had time enough to be uniformly distributed across the entire face of the roll of film.
Many problems occur in blown film extrusion in the hot melt between the die and the frost line and where the tube is collapsed at the main nip. We will deal more specifically with problems such as uneven rolls, gauge bands, wrinkles, maintaining output, physical and optical problems and solutions in other blogs. If all equipment operates smoothly, film is now going through the main nips at rates fast enough to require a line speed increase. Many lines never operate at speeds more than one-half to two-thirds of their maximums. With the commonly used DC and eddy current variable speed drives, the slower line speeds can result in a loss of control and additional scrap.