Description
About this Film Blowing Extrusion Machine
In the process of film blowing, a continuous tubular film is fabricated of which diameter varies from few centimetres to many metres. The principal polymers used in film blowing are the polyethylene (PE) which has the variations such as linear low and high density; LDPE, LLDPE, and HDPE etc.
Commercially, PE film has a significant demand and therefore over 70% of the current production is LDPE and LLDPE.
The majority of polymer films are manufactured by film blowing (blown film extrusion). A single screw extruder is used to melt the polymer and pump it into a tubular die. Air is blown into the center of the extruded tube and causes it to expand in the radial direction. Extension of the melt in both the radial and down-stream direction stop at the freeze line (frost line) due to the crystallization of the melt. The nip rolls collect the film, as well as sealing the top of the bubble to maintain the air pressure inside. This process is used extensively with polyethylene and polypropylene.
Polymer granules are plasticated and metered via an extruder, normally a single screw extruder. The melt is metered to an annular die and a thin-walled tube is extruded; the flowing melt is cooled by blowing air along the film and while the film is still molten,
Tools of Poly Bag Making
In the poly bag making plant various types of machines and accessories are used. They include Extruders, winders, cutters, colour printing machines, packaging machines, and others. the single machine unit which includes extruder, winder, collapsing plate, and bubble cage. Since the film blowing machine is the main machine of poly bag making some of its components are explained further.
Dies used for film blowing can be divided in to few types.
1. Cross head die
2. Bottom fed die (Spider die)
3. Spiral mandrel die
4. Rotating die
5. Banjo manifold die
6. Multi layer die
Air Ring
Air ring is designed to allow uniform air flow at high rates to provide exceptional cooling and stability without film chatter. It cools the bubble so as to reduce bubble temperature to 40oC at the time it arrives to nip rolls.
Bubble Cage
The bubble stabilizing cage is a vital component for optimum blown film bubble stability. The segmented rollers of the bubble cages, positioned between the die and the collapsers, provide a low friction contact surface, minimizing bubble drag and providing stability over a broad range of diameters.
Gusseting
For a number of applications, gusseted layflat film is
required for the production of polyethylene bags. Gusseting
is achieved by means of narrow wedges or plates which are
located within the angle formed by the collapsing frames;
these plates are forced into opposite sides of the bubble
just before the film passes into the nip rolls, so that it folds
in on itself. The gusseting attachments, which can be made
from wood or metal, should have smooth surfaces and
rounded edges to avoid scratching of the film.
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