Blown film

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General view of a blown film line at the K'01 trade fair in Düsseldorf

The term blown film refers to films of thermoplastic plastics which are made using a blown film.

Construction of a blown film line

Extruder

Die head of a 3-layer tubular film line with a highly transparent 3-layer composite film made of PP and PE

The most important component of a film line is the extruder . This essentially consists of a heatable metal cylinder in which the plasticizing screw rotates. The task of the screw is to convey the plastic granulate or the melt produced downstream, to melt and homogenize the material via shear (friction) and to build up the pressure that is necessary to push the melt through the narrow tool gap. In the intake area, the plastic granulate is fed onto the screw via a funnel, melted and mixed as it continues to be conveyed through the extruder. Depending on the design of the system and the material processed, pressures of around 600 bar arise directly in front of the screw tip. Common processing temperatures range from 150 ° C ( LDPE ) to approx. 250 ° C (HDPE). Throughputs from 5 kg / h to 1500 kg / h depending on the extruder size. An extruder with a screw diameter of 60 mm achieves a throughput of around 200 kg / h with LDPE and a screw speed of 200 min −1 (conveying rate 1 → 1 kg per revolution and hour).

Tool

Inner workings of a 3-layer tubular film tool

After the extruder, the melt is pressed through a tool with a ring nozzle . The resulting melt tube is inflated with air and cooled by cooling air from the outside and, if necessary, from the inside. The width and thickness of the film are also specified here. The cooled film tube is laid flat and then wound up. The state of the art are foils with up to 11 layers, which are placed one on top of the other in the die. The photo shows three stacked melt spiral distributors of a three-layer blow head from the Reifenhäuser company.

cooling

With the help of a cooling ring, the melt hose emerging from the tool is evenly cooled from the outside. In large systems, the hose is cooled from the inside at the same time, i. H. there is an exchange of air inside the bladder. In the lower area, cold cooling air flows into the bladder, the air, which has meanwhile been heated, is sucked off after approx. 4 m above through a pipe in the bladder. The cooling supply and exhaust air is regulated by three ultrasonic sensors that keep the diameter of the bubble constant. A calibration basket with Teflon or felt rollers that clamps the bladder in place is used to stabilize the bladder . The shape of the bladder (conventional driving style or long neck) is determined by the material, tools and cooling.

Flat lay / deduction

When processing HDPE, the flattening consists of wooden slats, with LDPE mainly of rolls. The film web is pulled off by motor-driven rubber rollers at the upper end of the lay-flat. The thickness of the film is determined by the take-off speed. The final film thickness is determined in the thermoplastic area of ​​the plastic (below the calibration basket). During production, it is unavoidable that minimal thickness tolerances arise over the circumference of the bubble, which would add up during winding. In order to prevent these so-called piston rings and to be able to produce a smooth foil wrap, the entire lay-flat unit including the trigger is rotated back and forth by 360 ° (reversed), thus distributing the thick and thin areas evenly over the circumference. Alternatively, systems are also known in which the blow head is rotatably mounted. The take-off is fixed and the reversal and thus the thickness distribution takes place via the blow head rotation.A disadvantage of this system is that environmental influences such as drafts, which adversely affect the thickness distribution, are still visible on the lap. Blown film lines can reach heights of over 15 m.

Turning bar system

Turning bar system

Since the film web can be rotated by up to 360 ° after leaving the haul-off due to the reversing, it must be brought back into a position with turning bars (horizontal or vertical) where it can be transported further in the fixed part of the system.

Winder

Winder

After the flattened film web is fed back to the ground floor via guide rollers, it still has to be wound up. As a rule, the flattened film tube is trimmed at the edges using razor blades and the two webs of film produced are wound separately on two winding points. Depending on the winder configuration, there are the following winding options: contact, gap and central. Subsequently, the wound film rolls are delivered directly to the end customer, or are further processed into an end product (bags, garbage bags, etc.) or as laminating films with other materials such as paper, aluminum foil or another plastic film, glued over the entire surface and sometimes in flexo beforehand - or gravure printing.

Full tube / half tube / single film

(Vented) blown film laid flat is wound onto cardboard cylinder cores with a diameter of 5 to 10 cm and referred to as a full hose - the cross-section corresponds to that of a squeezed "O". If a hose is cut into two parts lying next to each other, two half-hoses are created - each with a lying "U". If both folds of a full hose are cut off, a pair of flat films of the same width is created, which can be wound up separately. Sacks of exactly the same width (and variable length) can be produced from a full hose - by means of a cross-weld seam and cutting. Half-tube, on the other hand, is used to create carrier bags of a certain depth and a seamlessly stable base with welded seams on the sides that determine the width.

Flat film is used to wrap magazines for individual mailing, for example: film runs on a conveyor belt , the goods are placed on top of it piece by piece, the film flanks are folded up, overlapped (to form a tube), and then welded across. If the packaging of bulk goods , from a little colored powder up to 25 kg of road salt, is assumed to be full hose, the sack open at the top is formed first, and then filled and closed from above during horizontal transport. Flat film, on the other hand, runs from top to bottom, is continuously welded to the side of the tube while the product (with portion-forming) also trickles in from above.

For pressure-sensitive corn flakes , the sack is already in the outer carton while it is being filled. Filling goods can also be liquids, pastes and gases. Hoses made of composite material with paper or metal foil, for example for beverage cartons, can only be produced by means of a longitudinal weld seam.

Film properties

With blown film, the tear strength in the longitudinal and transverse directions is very similar. There are films that only consist of one layer (so-called mono-blown film) and films that are made of several layers (so-called coextrusion blown film). With a coex blown film, it is possible to combine the positive properties of different materials in one film. For example, a ham package consists of 5 different layers:

  • Outside: a printable layer, mostly polypropylene or polyethylene
  • between the outside and the middle: adhesion promoter made of ethylene vinyl alcohol or ethylene vinyl acetate
  • Middle: a barrier layer made of polyamide to "lock in" the aroma
  • between center and inside: adhesion promoter
  • Inside: a food-safe layer with good sealing properties so that the film can be welded to the lower part of the packaging, usually made of polyethylene

Applications

Blown films are mostly made from polyethylene . They are used for:

  • packagings
    • Medicine & Pharmacy
    • Food
    • Non-food & hygiene
    • Pet food
    • Waste packaging (on garbage dumps, wrapped like silage bales in a field)
    • Cling film
  • Agricultural film (e.g. for strawberry or asparagus fields)
  • Agricultural stretch film (wrapping of the silage / hay bales on fields)
  • Trash bags
  • Carrier bags
  • freeze bag
  • Shrink hoods
  • Pallet transport lock (stretch film)
  • Foil finishing, laminating foils

Films for food packaging, so-called barrier films, are made up of multiple layers. Layers of PE provide the water vapor barrier , layers of PA or EVOH provide the gas barrier. In order to ensure good cohesion of these different materials, an adhesion promoter (HV) is often used between the layers . A typical barrier composite z. B. for meat packaging looks like this: PE-HV-PA-EVOH-PA-HV-mPE

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