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Film extrusion Info.
Film extrusion
Film extrusion by the chill-roll process:
The principal components of chill-roll plants are the extruder, the slot die and the chill roll, with the melt being stretched and cooled on the surface of the chill roll. These are followed by downstream equipment that includes the take-up unit and the winder.
The polymer that has been plasticized in the extruder is distributed in the die and then applied to the chill roll. This generates thin films (generally in a thickness range of 10 µm to 250 µm). The film is then cooled further on a number of cooling rolls. Following this, the film is wound onto a film winder.
Composite films are made by various different processes which serve to combine different materials. The coextrusion process is the most widely-used of these processes. Alongside this, however, a number of coating methods are also employed.
Blown film extrusion:
Extra-wide and tubular films are produced on blown film units.
On these units, a tube of melt emerges perpendicularly in the upwards direction from a side-fed blown film die. Air is introduced into the inside of the tubular film in order to blow it up to the required dimensions, and the tube is then cooled with air from the outside on a cooling ring. At the top end of the system, the tube, which has now cooled down sufficiently, is folded and guided over deflector rolls onto the winder. A calibrating basket is incorporated between the cooling ring and the layflat unit for purposes of defining and stabilizing the circumference of the bubble.
Fig. 1 Flexible film
Advantages of the process
* Very high productivity
* Permits the combination of a number of different materials and properties
Materials used for packaging perishable foods such as meats, cold cuts, ham, and cheese products need to fulfill a wide range of requirements, including the provision of a gas and aroma barrier, a moisture barrier to prevent the contents from drying out, resistance to oils and fats, neutral taste, optical properties such as transparency and gloss, suitability for printing and labeling, mechanical properties such as toughness and resistance to buckling fracture, thermoformability and sealability, smooth passage through the latest packaging machines with short cycle times, sales-promoting product presentation, durability and freshness up to the final consumer, and also problem-free, environmentally-compatible disposal after use. It is scarcely possible to have all these many and varied requirements fulfilled by a single material, yet by combining a number of products offering complementary properties, it is possible to produce tailor-made packaging materials with an optimum price-to-performance ratio. Multi-layer films made up of PA plus polyolefins plus related products fulfill the requirements imposed on food packaging in an ideal manner.
Drying the material
Durethan is supplied in moisture-proof containers and does not normally need to be pre-dried. Material from already-open containers, however, must be pre-dried in a dry-air dryer. Vacuum drying is essentially possible too, but this requires relatively long drying times. Degassing is virtually never used, since the moisture content of the material up to the point of degassing can lead to an undesirably high level of hydrolytic degradation.
Chill roll extrusion units are used to manufacture flat film. This is generally composite film produced in three to seven layers by coextrusion and/or lamination.
Extruder
The screw diameters employed are between 45 mm and 150 mm. Standard commercial three-section screws are suitable for processing Durethan, although preference should be given to high-performance screws that are fitted with shearing and mixing elements. The screw length should be at least 25 D, and better still, between 30 and 33 D in order to guarantee optimized plasticizing and conveying properties. Three-section screws should have a flight depth ratio of between 3:1 and 4:1 (feed section to metering section). Degassing is hardly ever employed, since the moisture content of the material up to the point of degassing can lead to excessively high hydrolytic degradation.
Melt filter
The use of screen packs between the extruder and the die has proved advantageous. The best screen packs are those with mesh sizes of 0.8 - 0.1 - 0.1 - 0.1 - 0.8 mm.
Throughput control
Contrary to the trend in sheet extrusion, where use is generally made of melt pumps, it is throughput control that has become established in film extrusion. The throughput is monitored by continual weighing of the material taken up by the screw (gravimetric metering) and the adjustment of the extruder screw speed. At times, use is also made of melt pumps in order to keep the flow of melt constant and to offset any throughput fluctuations in the extruder.
Coextrusion adapters, coextrusion dies
When it comes to flat film extrusion, adapter systems have become established in preference to coextrusion dies. A distinction has to be made between fixed adapters, vane adapters and sliding adapters. In view of the frequent product changes normally required in extruders and the different viscosities of the melt that result from this, use is made primarily of flexible vane adapters or sliding adapters. These permit the flow of the individual melt streams to be adjusted during running production.
Slot dies
Single-channel slot dies are generally employed with coextrusion adapter systems. The design of the slot die must make allowance for the specific requirements of multi-layer flows. The manifold geometry is generally designed to be independent of the operating point, and the thickness is normally adjusted by means of a flexi-lip. The adjustment can be performed either manually or automatically. High-grade slot dies permit film thickness tolerances of ±3 to ±5 %.
Chill roll unit
The chill roll unit is generally equipped with one matte casting roll and two downstream cooling rolls which can be run at varying speeds relative to the casting roll so as to achieve good flatness during step-by-step cooling, whatever the film thickness. The temperature of the rolls is between 10 and 90 °C depending on the raw material employed. The temperature differential over the roller width should not exceed ± 1 K. An air knife ensures that the film comes into contact with the casting roll over its full width, making for uniform cooling on the casting roll.
Measurement and adjustment of film thickness
The overall film thickness over the width of the film is generally measured on a contact-free basis by means of a radiometric or capacitive measuring facility. If individual layer thicknesses are to be measured in composite films, then infrared measuring technology can be the right solution. This requires the individual polymer layers to have a dissimilar chemical make-up and to display unambiguous absorption peaks at different wavelengths. The film thickness is controlled by linking the thickness measurement to an automatic die (automatic adjustment of the lip gap) by means of an appropriate control facility.
Take-off, winders
Chill roll units are available in different working widths of between 900 and 3500 mm with take-off speeds of up to some 300 m/min. State-of-the-art winders have pre-selectable winding characteristics and will perform several different types of winding (contact, split and central winding). They ensure high-quality winding even in multi-reel mode, along with high speeds. Both the removal of the finished reels and the application of new reel cores to the winding shaft can now be performed fully automatically.
Fig. 2 Flat film unit [made by Reifenhäuser, Troisdorf/Germany]
When Durethan is used in blown film extrusion, it is generally coextruded into multi-layer film in combination with a number of other polymers. Three, five and seven-layer film are the standard types of film produced today. In terms of their size and the technical facilities they offer, the extrusion plants employed are conditioned to a large extent by the specified properties and quality of the film.
Extruders
In order to be able to achieve differently-structured films on a blown film unit, it is necessary to be able to process different polymers on the extruders. This is why most manufacturers have now adopted standard extruders. These can be equipped with grooved-bush or smooth-bore barrels. If Durethan is used in grooved-bush extruders, it is necessary to dispense with cooling in order to avoid an inadmissibly high power consumption by the screw, since the polyamide pellets are considerably harder than polyolefin ones. Where mixing tasks are to be performed, such as the admixture of amorphous polyamide, the use of barrier screws and additional mixing sections has proved advantageous.
Throughput control
Throughput-controlled blown film units are now state-of-the-art. The principal and ancillary components are metered primarily by gravimetric means. Melt pumps are rarely encountered in blown film units.
Dies
The majority of multi-layer dies are designed in the form of spiral manifold dies. On dies of this type, the melt streams supplied by the extruders are divided up into a number of separate streams. The manifolds lead into spiral-shaped channels with a steadily-decreasing depth, while the gap between the mandrel and the outer part of the die constantly increases in the direction of extrusion. This is followed by an outer die ring which shapes the tube to the required diameter and determines the gap width. Over the past few years, increasing use has been made of diskpack and stacked dies. These have spiral manifolds that are stacked one on top of the other instead of spiral manifolds wrapped around each other in a radial configuration. This design has the advantage of allowing the temperature of the individual elements to be controlled separately and to be correctly adjusted to the raw material in question. Apart from this, the configuration can be subsequently adjusted and extended to cope with changed production requirements. The residence times of the melt are also shorter and more uniform than those with conventional spiral manifolds.
Cooling and calibration
The output rate and film quality are largely governed by the cooling of the melt. Use is made here of external cooling rings and internal-air exchangers with the appropriate cooling units. Apart from the actual cooling operation, a number of manufacturers also use the external cooling facility to control the thickness profile in the circumferential direction. Either the quantity of air or the air temperature is controlled on a segment-by-segment basis. A further means of thickness control involves partially modifying the die temperature at the die orifice. In most cases, the thickness measurement required for thickness control is performed with capacitive systems and in some case with radiometric systems.
Layflat and take-off
Rotating reversing-rod systems are now used for film take-off and for laying the film flat in a uniform manner.
