Before starting to explain general extrusion processes applied to polymers to produce sheets and films, we need to know what is the film and what a is sheet.
To say ‘film’ to a polymer product, the thickness of the products must be below 0.5 millimeters. Above 0.5 millimeters to 12.5 millimeters, it is called a sheet. In film production, generally low-density polyethylene is used.
The three sheet and film extrusion processes are; slit die extrusion process, blown film extrusion process, and calendering.
Actually, the slit-die extrusion process is a plastic extrusion process that has a very specific die configuration. Because of the shape of the die opening, this process is called a ‘slit’ die. Slit refers to the very thin and wide shape of die opening to obtain the required sheet and film sections. The lengths of the slit die can be up to a few meters, and the narrowness of the slit opening can be below 0.4 millimeters.
Uniform cooling must be established in the silt die extrusion process. If there is non-uniform cooling of the extruded sheet from the slit die, the temperature gradient and pressure gradient around the slit die can cause non-uniformity in the shape of the extruded sheet.
Uniform cooling is established by the use of quenching of sheet directly after the slit die. This quenching solidifies the sheet abruptly. Another process is the cooling of the sheet on rollers. In this cooling system, rollers much cooler than usual to abruptly solidify the extruded sheet, right after slit die opening.
The calendering process to produce thin layers of plastics is the most efficient way by the most expensive way. The production rates of calendering of plastics such as general rubbers or rubbery thermoplastics reach up to 3 meters per second.
In the calendering process, there are a set of rollers in which the starting form of material is rubbery. When this rubbery form enters into sets of rollers, it takes the shape of film or sheet after each step by decreasing the thickness.
The most important products that are produced with the calendering method are inflatable boats, PVC curtains, vinyl table cloths, etc.
This process to produce plastic films and sheets is more complex than other processes. These are feeder that feeds the polymer melt onto guide rolls. Air is blown inside this polymer melt to obtain a bubble. The flow of polymer melt is in the upward direction and after a point, solidification of the bubble occurs. This point is called as ‘frost line’. This bubble was then quenched by collapsing rollers. Then it’s wound to windup reel.
With air blown sheet extrusion process, many more strong sheets and films can be obtained to use in the packaging industry. But the control over the dimensional tolerances is harder than other processes.
As you see that there are three main methods to produce sheets and films from plastics.
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