Basic Theory of Blown Film Extrusion


Blown films are created by feeding plastics pellets into an extruder where
they are melted and homogenised before they are pumped through a circular
blown film die. The melted plastics form a continuous tube which is drawn
from the die. It is inflated and simultaneously cooled by rapidly moving air.
The tube, also called a “bubble,” is then flattened as it passes the collapsing
frames and drawn through nip rolls and over idler rolls to a winder which
pulls and winds the finished rolls of film.
A typical film blown film machine consists of the following five major units:
• Extruder unit—converts the solid pellets into hot melt.
• Die unit—forms the hot melt into tube.
• Cooling unit—cools down and solidifies the hot melt.
• Take-off unit—pulls and flattens the tube at constant speed.
• Winding unit—winds-up the flattened tube into finish rolls.

Polyethylene Films

The ethylene polymer is available in three main grades, low, medium and
high density film.
• Polyethylene, PE, film is slightly opaque, the opacity increasing With density.
• PE film is waxy to the touch, and is a good moisture barrier.
• Low and medium density PE films are flexible even when cold.
• PE is readily heat sealed.
• High density PE film is suitable for boil-in-the-bag packs, whereas low and medium density films are not.
• PE film that has a lower MFR means a higher molecular weight and better mechanical strength.

MACHINE SAFETY GUARDS & DEVICES

Safety devices in facilities, machinery and equipment are an essential means of ensuring worker safety. Foolproof devices, failsafe devices and others are used to ensure intrinsic (essential) safety. These devices are usually built into facilities, machinery and equipment so that workers cannot easily remove them.
The foolproof function is a safety mechanism designed with a focus on human factors. On the other hand, the failsafe function is a safety mechanism designed with a focus on the protection of facilities, machinery and equipment.
Types of safety devices typically used in blown film machine:
• Mechanical Interlock
• Electrical Interlock
All Dangerous spots of the blown film line must be secured by suitable
protective devices. If proper guards cannot be mounted due to the conditions at hand, these areas are secured by Emergency devices.
Guarding and Barriers
The purpose of machine guarding and barrier is to protect the machine operator and other employees in the work area from hazards created by moving parts, rotating parts, flying chips & sparks. Some examples of this are barrier guards, safety gates, etc.

Blown film air rings are used primarily to stabilize the bubble and secondarily to cool the melt. In plastics forming, a circular manifold distributes an even flow of cool air into a hollow tubular form passing
through the manifold.
An air ring is installed just above the die in an upward air cooling blown film machine. The air outlet called a lip (or slit) has an annular shape that surrounds the molten resin extruded from the die. Air is introduced into the ring by the blower, and the air is turned into a uniform flow inside the air ring. Then the air is blown through the lip against the molten resin for cooling. If the flow of air blown out of the lip is not uniform, it leads to nonuniform cooling. As a result, film thickness will be uneven. To prevent this, the air ring is designed to ensure uniform air flow over the entire lip.
As the molten resin is cooled, it becomes “frosty,” or less clear (transparent), and a solidification border appears. This borderline is called a frost line.

The width (lay-flat width) and thickness of the film to be made are always specified in blown film extrusion. The operators need to set the width and thickness to the specified values correctly and maintain them within the target level during the processing. Once the process is started, first adjust the lay-flat width to the specified value. The screw rotation speed shall be set to the predetermined speed and the amount of air to be introduced into the bubble shall be adjusted. Then the film thickness shall be next step.
In blown film extrusion, there are various kinds of operation requirements
that directly influence on the shape of the bubble. Examples of these requirements are extruder take-off speed and the amount of air that enters the bubble. In particular, they affect the bubble size and vary the film thickness and lay-flat width.

If a corona discharge is generated in air, ozone (O3) is created from the oxygen (O2) in the air. Ozone is a powerful oxidizing gas. This ozone oxidizes the surface of plastics film to make it easier for the ink to stick to the film. This method of treating a plastics film surface is called corona discharge treatment.
• The corona discharge treatment equipment is made up of a high frequency generator, an electrode, and a treater roll.
• The wetting tension of PE film and PP film that have not been surface
treated is around 30 to 35 mN/m. The tension appropriate for printing On these kinds of film is said to be normally around 40 to 45 mN/m. So,
what is wetting tension?
• Treated film surface can be checked by measuring the wetting tension of the film surface. The wetting tension is an indication of the wettability of a solid surface.
• Normally, wetting tension of a film that is not surface treated is around 30 to 35 mN/m compared to a treated film which has 40 to 45mN/m.

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