In this section we will discuss:
• Typical components of an extrusion line
• Tubing and pipe extrusion lines
• Sheet and film lines
• Extrusion compounding lines
• Profile extrusion lines
It is obvious that the extruder alone is not sufficient to make extruded product. In addition to the extruder, we need upstream and downstream equipment to produce a useful product.
Typical elements of an extrusion line are:
• Resin handling system
• Drying system
• Extruder
• Post-shaping or calibrating device
• Cooling device
• Take-up device
• Cutter or saw
Besides the four main types listed earlier, there are quite a few more lines, such as fiber spinning lines, extrusion blowmolding machines, integrated sheet and thermoforming lines, and others.
Tubing and Pipe Extrusion Line
Dies for tubing and pipe were discussed earlier. Small diameter tubing, less than about 10 mm, is usually made without a calibrator - this is called “free extrusion.” Large diameter tubing and pipe is made with a calibrator or sizing device just downstream of the die.
The purpose of the sizing unit is to solidify the plastics in the calibrating section to a thickness sufficient to transfer the stresses acting on the product, while maintaining the desired shape and dimension. The main components of a typical tubing extrusion line are shown here.
A gear pump may or may not be used, depending on the precision that is required in the extrusion process. The internal air pressure of the tubing is controlled to achieve the correct values for the outside diameter and wall thickness. The diameter is often measured with a laser gage to allow close monitoring and control of the diameter. The diameter and the wall thickness are determined mostly by the extruder output, the puller speed, and the internal air pressure. Closed loop control systems are available that automatically set the appropriate values of gear pump speed, puller speed, and internal air pressure. After the puller, the tubing may be cut or it may be reeled up on a spool.
Film and Sheet Lines Using The Roll Stack
There are no major differences between the extrusion of flat film and sheet. The main components of a sheet line are the extruder, the roll stack, the cooling section, the nip roll section, and the winder. The roll stack contains three rolls that are often referred to as polishing rolls. They are used to exert pressure on the sheet and to impart the surface conditions of the rolls to the plastics sheet. If a smooth surface is required, smooth rolls will be used. If a textured surface is needed, a textured surface is used on the roll.
The rolls are normally cored so that the temperature of the rolls can be controlled. This is usually done with circulating oil. The temperature of each roll can be adjusted separately. The rolls can be in a vertical position as shown or they can be at an angle.
The cooling section consists of a number of rolls positioned in a frame; the sheet is guided over and under the rolls to keep the sheet flat.
At the end of the cooling section are the pull rolls or nip rolls; these are rubber rolls that pull the sheet from the roll stack to maintain a certain tension in the sheet.
After the nip rolls, the sheet is led to the winder that rolls the sheet on a core. Many different winders are available; some winders automatically transfer the sheet to a new core when one package is full.
Film Lines Using Chill Roll Casting
With thin film, the film is often cast on a chill roll rather than extruded into a roll stack. The main components of a cast film line are the, the film die, the chill roll unit, the thickness gauging system, the surface treatment unit, and the winder. The film is extruded downward onto the chill roll. The initial contact between the film and the chill roll is established by the use of an air knife. The air knife produces a thin stream of high velocity air across the width of the chill roll, the air stream pushes the film against the roll surface.
From the chill roll unit, the film is lead to a thickness gauging unit where the thickness of the film is measured across the width of the film. Next, the film passes through a surface treatment unit and then to the winder. A surface treatment unit may be incorporated to improve adhesion, for instance for a subsequent printing or laminating operation.
The most important adhesion promoters are:
• Flame treatment
• Corona discharge treatment
• Ozone treatment
• primers
