The Extrusion Coating Process

In the extrusion coating process, polyethylene is melted under heat and pressure in an extruder and the molten polymer is extruded through a slit die as a thin web. This web, at high temperature, is drawn down and coated onto  a flexible substrate in a nip-roll assembly formed by a water-cooled chill roll and a rubber-covered pressure roll. The substrate to be coated is fed continuously from an unwind reel over the rubber pressure roll into the nip where the laminate is formed by pressing the two layers together. The laminate is rapidly cooled by the chill roll and is taken up by a wind-up mechanism.

 

 

The Extruder

The purpose of the extruder is to deliver uniformly  heated homogeneous melt to the die, at an acceptable temperature and rate. The general principles for the extrusion of Alkathene LDPE resins are described in the publication: Extrusion. For extrusion coating, a single screw extruder is generally preferred with a screw length to diameter ratio of greater than 25:1. To facilitate die and extruder adjustment (especially for start-up and shut-down procedures), it is best to mount the extruder so that it can be moved away from the lamination and reeling equipment. Good mixing of the melted polyethylene is essential if  a good coating film with no defects or blemishes is to be obtained.

The Die

The slit die used in extrusion coating is normally center  fed, particularly when the die is wide; however, an end-fed design may also be used. The two main types of dies are  the coat-hanger die and the T-slot manifold die. The prime objective in an extrusion coating slit die is to have the melt leaving the die at constant velocity across the full width. When a center-fed die is used the uniformity of the delivery rate across the die can be controlled by restricting the flow in the shorter flow-paths, or by maintaining the temperature at the die extremities 5 to 10°C higher than that at die centre. However, the latter does not always produce the best quality film. Uniformity of flow is preferably achieved by contouring  the internal die dimensions or by adjustable choke bars. Adjustable die lips give maximum adaptability of the die when in use. Adjustable deckle rods inserted at each end of the die permit adjustment of the effective width of the die.

The Air Gap

The air gap, or draw distance, between the die lips and the nip to chill roll contact point is one of the most important areas in terms of the polymer’s coating performance. In the air gap, some of the properties of the laminate are most affected by extrusion conditions. It is here that the molten web is drawn down and necks-in (see pg. 8) and oxidation takes place. A large gap may cause premature cooling of the melt, increasing its viscosity which in turn may lead to poor adhesion to the substrate. However, if the air gap is too small then this can also result in a deterioration of adhesion strength. V-shaped dies allow the die lips to approach close to the nip, thus minimizing the amount of cooling and improving the control over the degree of adhesion.

Nip Assembly and Chill Roll

The nip (see Figure 3) is formed by the steel chill roll and the rubber-covered pressure roll. The chill roll is usually chromium-plated and highly polished, although matte  and “mirror pocket” surface finishes are sometimes used. It is designed to remove the maximum amount of heat  from the polyethylene, and its construction is usually  based on a double-shell arrangement with built-in spiral baffles to ensure effective and regular cooling. The cooling medium is water, circulated from a separate storage  tank in which it can be maintained at a set temperature.  It is essential to provide sufficient water to ensure  effective temperature control at the surface of the roll.  The diameter of the roll, which is determined by the working speed range, must be large enough to significantly reduce the temperature of the laminate during its brief period of contact. The temperature of the chill roll should be kept at approximately 15 to 30°C; if it increases to 55 to 60°C or higher, the polyethylene may stick to the chill roll, causing partial delamination between the coated web and the substrate, or even web breakage.

The rubber pressure roll is usually made from silicone rubber but neoprene rubber can also be used. The requirements of the pressure roll include that it needs to have good resistance to heat-ageing and work-hardening. The roll is actuated by air cylinders to exert an even pressure against the steel chill roll. It is also desirable  to have provisions for continuous cooling of the surface  of the pressure roll by having a water-cooled backing roll (see Figure 2). The position of the nip is usually adjustable relative to the die. To obtain maximum adhesion the nip should be set to ensure that the molten polyethylene meets the substrate just before the chill roll exerts its cooling effect.

Edge Trimming

It is not possible to coat the molten web to the same width as the substrate, due to two different effects which modify the edges of the web. One of these effects is known as edge weave. The other is the thickening at the edges of  the web to form an edge-bead. This is a consequence of the necking-in of the web between the die and the nip. To overcome these faults it is necessary to remove edge trim (consisting of polyethylene and/or substrate) continuously by means of slitting knives. When a neoprene pressure roll is used the polyethylene cannot be applied beyond the edge of the substrate because it sticks to the neoprene. Polytetrafluoroethylene (PTFE) adhesive tape is often applied around the ends  of the pressure roll to prevent sticking of the molten polyethylene, to improve the service life of the rubber covering and to allow extrusion of a wider web. With a silicone-rubber pressure roll, the molten polyethylene has a lower tendency to stick, particularly if the roll is cooled by contact with a water-cooled chill roll. This allows the polyethylene to be applied beyond the full width of the substrate, so that only the excess needs to be trimmed.

Reeling and Wind-up Equipment

 Equipment for the high-speed handling of reels of paper and other substrates is usually highly specialized and not designed to handle a range of materials. Static eliminators are always incorporated to dissipate any build-up of electrical charge. Information on reeling equipment is  best obtained from the machinery suppliers. Since the coating line must run continuously, flying-splice unwinds and rewinds are usually installed to permit automatic reel changes at full production speed.