The Blown Film Process
The resin is available in 1000-2000# boxes called Gaylords brought in by truck or more typically, purchased in railroad cars. The higher volume reduces cost. Resin may be stored in the rail car and drawn directly from it into the building or more commonly vacuumed into a silo standing beside the train tracks and right outside the building. The resin is exactly the same as the film, except it arrives in the form of a hard pellet from the supplier. In the extrusion process, the pellets are melted down and air forced through the air ring “blows a bubble” with it transforming the hard pellets into thin flexible film that can be wound into rolls. They may add things to it in the process like antiblock, slip, a colorant, an antistat or flame retardant to meet the customer’s needs and application.
In the film extrusion process, these pellets are vacuumed into the extruder from their storage site and are melted down using a carefully controlled melt profile. It’s gradual, with “extruder barrel zones” set to increasingly higher temperatures. A Process Engineer and QC Department will record these setting for future run uniformity. “The screw” pushes the resin horizontally through the various barrel die zones as it melts and forces the molten material toward the extrusion die. The “screen pack“, which is just as it sounds: a metal screen or layers of them, act as a filter holding back any unmelted pellets or chunks of contamination. If the temperature profile is not set correctly and / or the screen pack fails, “gels” may show in the film. These are usually un-melted pieces of the resin pellets. With the resin fully melted and filtered by the screen pack, the force of the screw turning pushes the molten material around a tube or sleeve called “the mandrel” and between the round “die ring“. This is what forms sort of the donut shape of hot poly to get things going. A thick ring of molten plastic squeezing out between the solid mandrel and lip of the die ring now forms the tubing. Operators will gather this and tie it off like the lip of a balloon as it exits the die. With the tube sealed off, by blowing air into the bubble from the bottom, a hot bubble of plastic is formed and is pulled upwards. The material gathered is hoisted up the extrusion tower so the whole mass can be pulled back down over idler rollers and toward the winders. They control the size of the bubble, and hence “layflat” or width of the film via size of the die and amount of air flow from below. The term “blow up ratio” is your bubble diameter divided by your die diameter. It is typically 2:1 or 3:1. As we continue upward, an “air ring” blows air into the bubble from the underneath. The bubble of molten poly begins to cool fairly quickly; typically about 30″ above the top of the die as it exits the extruder. You can see the poly change from hot and crystal clear to kind of a hazy or frosty appearance; and hence this line is called the “frost line“. If the bubble breaks, which it can and often does, the crew has to start all over again. The gauge is controlled by the gap in the die ring, machine speed and air flow. The die may slowly rotate to prevent “lanes of high or low gauge” developing in the roll called “gauge bands“. Many extruders rotate the film at the top of the bubble instead. This is called an “oscillating haul off”. A thing called a “beta gauge” or other device electronically measures the gauge of the film around the bubble and sends the information back to the die and operator. It tells him whether the film is on target and whether changes need to be made to bring the thickness of the film into spec. Today’s sophisticated machines send the information to the die and extruder ‘brains’ which can automatically make necessary changes.
As the bubble continues straight up in the air it chills and sets up. A “collapsing frame” or “air bearing ladder” begins to gradually flatten the tube. When the tube reaches the top of the machine, it gets pinched completely flat by passing through rollers. It then turns downward and heads back toward the ground. Now what you have, is a flat tube heading for the winder. If you want “tubing“, the operators simply wind the flattened bubble or tube onto the core. If you want “sheeting“, the operators set blades at each fold in the tubing and slit that off. They re-circulate “the trim” right back into the extruder, re-melt it and blow it again rather than wasting it. By slitting off both folds of the tube, you end up with two sheets, front and back of the tubing. Each is drawn to a separate winder and you get Master Roll #1 and Roll #2 of Single Wound Sheeting or SWS. Things like “J-Sheeting” are made in a similar way where only one side of the tube gets slit. Operators will set the lip size desired on the slit side and wind it up on the core. The “corona treater” is usually installed at the point where the film makes its downward turn toward the wind up. The electrical discharge from the ‘high voltage corona’ does two or three things: 1) It polarizes the surface of the poly 2) roughens the surface and 3) burns off any surface contamination. The treatment makes the surface ideal for ink and or adhesive adhesion. Treatment level is measured in terms of dynes with water as a reference point. 38 to 42 dynes is a typical film surface tension reading for printing or laminating.
In co-extrusion the film supplier is blowing two or more materials at the same time through a special co-ex die. Here, materials are not “mixed”, which is possible as in a co-polymer and sometimes desirable as in EVA modified Low Density Poly, but co-extrusions result in layers. This may include something like a Nylon / poly, where you want the tough Nylon on the outside but LD inside for easy heat sealing or Poly/ Nylon/ poly.
There is quite an art and science to controlling all of the variables mentioned here! This is the expertise of film extrusion. If the resin is not good; the melt profile is incorrect; the air flow is contaminated or chill temperature is off, or any one of these variables is off, all sorts of things can happen. The visuals may be bad, the film may “block” (stick together), it may contain gels, or be tacky because it was over treated. The gauge will go bad if the machine rate or air flow changes and a myriad of problems may occur unless everything is just right and the process uniformly can be repeated from run to run. So, just like we depend on our customers to supply us the specifications, requirements and application for packaging products, from a film point of view, we need to know all of the details in order to produce quality films that will meet the end requirements.