Detlef Merklinger, of coating equipment specialist Pagendarm, looks at constructing a line for labelstock
The construction of an in-line coating line (where it is possible to siliconize the substrate and then coat the result with a PSA hotmelt) is by no means new. However, the trend to in-line equipment designed to produce labelstock based on dispersion adhesives is now becoming established in the field of PSA hotmelt.
On the basis of a recently delivered in-line system for the Asian market, this article takes a fresh look at the machine elements needed to manufacture high quality labels using hotmelt.
The basic requirements for the design of the machine were a maximum coating width of 2,100mm and a maximum production speed of up to 750m/min, depending on the silicone system used. The substrates to be handled are glassine paper, PE coated papers and PET films.
To ensure rapid roll changes a fully automatic turret winder type DAB 1500 TR was used for the substrate. It may accommodate rolls with a diameter of up to 1,500mm. Specifically for unwinding film, it is equipped with driven lay-on rolls made of carbon (fibre reinforced plastics) that are designed to prevent shifting or telescoping of the top layers on the roll to be unwound.
A central feature of the machine is the modular application system comprising a coating unit and two different trolleys for housing the coating system. This features five rollers, which are available for coating with addition-curing silicones.
This coating system is especially suitable for achieving low coating weights at tight tolerances. In practice the process makes it possible to apply minimal coating weights of 0.5g/m2 or 0.7g/m2 to paper at tolerances of within 0.03 or 0.05 g/m2. To ensure compliance with these grammages, the applicator unit was equipped with deflection-compensated coating rollers. This is absolutely essential to ensure defined footprints that are uniform over the entire working width. At the same time all rollers except the application roller – which was executed as a sleeve – were made as double walled rolls.
Of decisive importance for trouble-free and uniform silicone application is the state of the silicone fed into the metering gap between rollers 1 and 2. The system requires a virtually constant filling level which needs to be kept as low as possible. At the same time, the quantity of silicone in the gap should be sufficient to prevent the boundary layer air that enters the metering gap due to the rotation of the rollers from being entrained through the roller nip. If the level is too high it favours the accumulation of air and hence the formation of foam. However, the use of an in-line mixing and metering system resulting in very short silicone processing times made it impossible to empty the metering gap from the edges.
Nevertheless, with the aid of boundary air layer doctors combined with automatic filling level control, a reliable production solution was found to this problem. Roller 3 transfers the silicone film to application roller 4 in the required coating width. At the same time it is necessary to ensure that the surplus silicone is returned directly to the metering gap.
Squeezing necessary between the coating rollers, especially between rollers 5 and 4 and between 4 and 3, gives rise to the formation of a silicone mist. Since it is impossible to suspend the laws of either chemistry or physics, the problem can only be solved by targeted extraction of the mist. And it is also important not to overlook the silicone mist boundary layer, which is transported on the liner web in the direction of the dryer entry after the point of application. Here too, only targeted extraction in combination with a barrier slot nozzle can improve the situation. Moreover, the diameter we selected for the coating rollers undoubtedly helped to reduce silicone mist formation.
Water based silicones
A reverse gravure coating system is available for coating with water based silicones. This system permits direct or indirect application of the silicone. An AGS 2500 S System BASF is used for coating. This is a pressure chamber doctor combined with an engraved roller. (It was used for a pressure sensitive line supplied by Pagendarm to BASF, in Ludwigshafen).
A 30m long Speed Star flotation dryer with gas heating was chosen for drying the water based silicone systems and cross linking the 100 per cent silicone systems. It is divided into five individual sections, each six metres long, and has five independent temperature zones. To cater for the widely differing drying tasks, the lay-on-air nozzles of the upper part of the dryer have been fitted with adjustable nozzle slits. An adequate nozzle slit is especially important for drying the 100 per cent silicone systems. In practice this is around 2.5 to 3mm.
However, when drying water based silicone systems the nozzle outlet speed is a crucial factor for ensuring sufficiently high drying capacity. The graphs show different drying profiles for a coating of 0.8g/m2 on a glassine paper with a substance of 63g/m2. The time required for crosslinking of the silicone inevitably results in loss of moisture from the substrate paper.
To ensure the moisture reduction is evenly distributed over the working width, the following requirements have to be met:
The drying speed in the dryer should be absolutely uniform. This requires nozzle slit settings that are 100 per cent uniform.
Uniform pressure distribution in the dryer, and also central extraction of the circulating air in the dryer.
The Speed Star flotation dryer used in this case meets all these requirements.
When it comes to humidification of the siliconized paper, the production speed of 750 m/min calls for an efficient combination of humidifying and cooling assemblies. The substrate must already have an adequate moisture level especially when hotmelts are used following siliconization. For this reason two Fluidex Plano 1.0 humidifying systems were used in combination with a water mist spray system. The humidification chamber length of three metres made it possible to ensure an adequate residence time for the paper web and additional control of plane product position by means of motorized selective blades. The downstream cooling units are each equipped with two cooling rollers of 1,200mm diameter.
Once a processable hotmelt has been obtained by extruding and melting the basic granules it is fed from an intermediate tank into the hotmelt coater. It is then applied through a roller-type die against a temperature controllable rubber coated application roller. The die is subdivided into nine zones and a total of 47 thermo blocks are used. The gap between the lips of the nozzle is also adjustable from 0.5 -1.5mm.
When designing an in-line coating line it is not only necessary to possess expert knowledge about the functioning of the individual components, but also the task of combining the components calls for the existence of comprehensive know-how within the company if such projects are to be executed to the customer’s satisfaction.
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