The requirements of many products, particularly in packaging applications, are such that they cannot be met by a single plastics. In order to meet the requirements often two or more materials have to be combined. There are a number of techniques to combine different materials; some of the more important ones are: co extrusion, coating, and lamination. We will discuss these in more detail.
Co Extrusion
Co Extrusion is a commonly used technique to combine two or more plastics passing through a single extrusion die. There are two major co extrusion techniques: the feed block system and the multi-manifold system. In the feed block system the different plastics are combined in the feed block module and then enter into a regular extrusion die with a single inlet, manifold, and outlet.
The advantage of the feed block system is that it is simple, inexpensive, and allows many layers to be combined. The main drawback of the feed block system is that the flow properties of the different plastics have to be quite close to avoid interface distortion. This limits the choice of materials that can be combined through feed block co extrusion.
In the multi-manifold system each plastics has its own entrance and manifold in the extrusion die. The different melt streams are combined just before the exit of the die, so that minimum interface distortion can occur.
The advantage of the multi-manifold system is that plastics with widely different flow properties can be combined. As a result, there is a wide choice of materials that can be combined through this extrusion technique. The disadvantage is that the design of the die is more complicated and, therefore, the die is more expensive.
Many multi-manifold dies are possible: flat film and sheet dies, tubing and pipe dies, blown film dies, and profile dies.
Extrusion Coating
In extrusion coating a molten layer of plastics film is combined with a moving solid web or substrate. The substrate can be paper, paperboard, foil, plastics film, or fabrics; the substrate can also be a multi-layer product.
Extrusion Lamination
Extrusion lamination involves two or more substrates, for instance paper and aluminium foil, combined by using a plastics film as the adhesive between the two substrates.
The extruded sheet or film can be laminated with a film on one side or both sides. The laminate can be paper, foil, mesh, or a number of other materials. With lamination many different structures of sheet or film products can be made. The laminate is unrolled from a payoff and combined with the film and immediately led into a set of nip rolls. After lamination the film is handled as a regular film
Blown Film Lines
A blown film line is quite different from a flat film line. In a blown film line a tubular film is extruded vertically upwards as shown here:
Air is introduced to the inside of the tube, as a result, the tube expands to a bubble with a diameter larger than the diameter of the die. The ratio of the bubble diameter and the die diameter is called the blow up ratio. Typical blow-up ratios used in LDPE film extrusion for packaging are in the range of 2.0 to 2.5:1. When the bubble has cooled sufficiently, the bubble is flattened in a collapsing frame and pulled through a set of nip rolls at the top of the collapsing frame. From there the layflat is guided over several idler rollers to the winder where the film is rolled up over a core.
One advantage of the blown film process is that it can produce not only tubular products (bags) but also flat film, simply by slitting open the tube. In some blown film processes the plastics is extruded downwards to produce films with special properties.
