|hints & tips|
Hints and tips for coating
Hot Melt Systems
In hot melt systems bars may be used to imprint a ridge pattern or give a flat film, depending upon the temperature profile achieved in combination with the use or none use of a chill roll.
Generally bars are used singly. Multi-bar coating heads employing two or more bars are in commercial use. Compound heads can give excellent coating quality and greater flexibility. Very rapid trial runs are achieved. Flying changes are made saving time and materials.
Metering Bar Diameters
The core diameter of metering bars varies over approximately 4 mm to 50 mm, but the range most commonly employed runs from 5 mm to 12 mm diameter. Clearly bar life increases with increasing diameter, but handling difficulties and increased costs associated with large diameters result in the 6 mm to 10 mm range proving very popular. Even smaller diameters can be advantageous in certain circumstances. When used with a heated bearing, small diameter bars readily reach operating temperature after bar change over. Larger diameters give greater 'beam strength' enabling them to be used without supporting bearings.
Metering System Station Arrangements
Nearly all coating stations consist of an applicator stage followed by a metering stage. Surplus mix removed by the metering bar is returned to a receiver for re-use. In some situations, it is advantageous to employ continuous filtration to remove foreign particles and coarse aggregates from the mix.
The applicator can be a simple applicator roll, a reverse roll, a fountain jet. Use of an immersion bath is another form of applicator. Other systems use a metering bar to apply coating to a covered roll. The covered roll is used to transfer coating to the web. This system can be used as a two-side system at the size press. This last system is useful in cases where the web is very fragile or not fully formed to transfer mix to filter paper or filter pad material.
The bar bearing is an important component in the system and a simple 'V' form will often be employed using a wide range of materials from mild steel, Cu-based compounds and polyurethane through to PTFE. In some designs, there are facilities for the adjusting the degree of bow in the bar holder across the web and many bearings employ flushing (solvent or water) to keep the bar wet and the coating mix liquid during machine stops, also to help dislodge particles and fibres from the bar or bearing.
Most bearing systems employ relatively small contact areas between the bearing and the bar in order to obtain a good wiping action. In cases where the bar and bearing assembly acts directly on to a backing roll, various forms of floating bar bearing assembly are used to enable the bar to conform to the roll form.
If a wide range of coating mixes are to be employed, consideration must be given to ensure that the bearing material is compatible with the coating solvents and any cleaning solutions or solvents that might be used. Of the non-metallic materials, PTFE. has the widest range of solvent resistance but is expensive and requires careful machining.
If possible, control rolls should be placed reasonably close to the metering bar to ensure that even feed and web tension occurs across the bar and reduces the effect of slack edges or centres. In addition, compensation rolls may be used and applied to the web just before the coating head assembly. On many installations a distance of 120 - 200mm between rolls and holder has ensured that the web is presented as evenly as possible, some systems employing additional 'back side' floating pressure bar blades.
Direction and Speed of Rotation
Bars are generally, rotated slowly (very often against the web flow direction) to produce even wear and facilitate dislodging of trapped fibres. Speeds of zero to 100 R.P.M. are in common use and the direction of rotation may also be changed. Some users favour increasing the speed of rotation as the speed increases, e.g. 1 R.P.M. / 50 metres per metres per minute, 10 R.P.M. / 300 metres per minute, 20 R.P.M. / 600 metres per metres per minute. In fact, the speed of the bar circumference can be as high as the web speed in certain installations.
At high web speeds, a low pressure air knife can be used to advantage, to obtain additional smoothing of the coating after leaving the metering station. The aim is to flatten the coat without any loss of coating mix. This has advantages over the use of the high pressure air knife which at high speed causes spray problems. The integrity of the resulting film using the metering bar and low pressure air knife can give excellent results when applying non - permeable coatings to plastic packaging films for use in the food industry.
Filtration and De-Aeration of Coating Mixes
The majority of systems do not use in line conditioning of the coating mix but continuous treatment is always advantageous and is essential for technically demanding applications. Straining or filtration of the coating mix is particularly important when making high grade coatings, such as are used in photo-sensitisation and colour filtration applications. If solvents are employed, a filter press used in conjunction with the mix feed is helpful. Although the metering bar has a high degree of self cleaning action (unlike the blade system), particles can lodge under certain circumstances and straining or filtration can help to eliminate the problem. De-aeration is essential on certain types of application to present a bubble free coating mix to the metering point.
Coating mixes which do not readily re-dissolve
Care is required, particularly with water based mixes that do not readily re-dissolve after drying. Most users simply remove the bar from the machine and place in water to prevent drying out on the smaller 'off machine' coaters. The larger 'on machine' coaters employ a water flush bearing and rotate the bar during machine stops
As a general guide, chromium plated plain bars can give 3-30 days continuous production and even wire wound bars can be run for 2-7 days under similar conditions. Life is governed by the abrasiveness of the mix, the pressures employed and the web speed. Obviously, in 'off-machine' applications where abrasion is low, much longer life can be obtained.
Engineering grade Hard Chromium plating
Chromium plating gives extended wear life and protects the highly accurate metering bar surface from casual damage. Various thicknesses can be achieved for differing environments.
The price of metering bars is low in relation to other items of equipment and the accuracy of the coating weight is very high. This can be illustrated when considering reverse roll coaters which when new may have a 0.005mm run out which is equivalent to a 0.05mm difference in the wire diameter for a wire wound metering bar. After use, a run out of 0.013-0.026mm or more may develop which is equal to a 0.13-0.26mm variation in wire bar diameter.
The wire wound bar therefore is inherently a very precise system due to the 10:1 leverage and the very high standard of precision attainable in producing wire to special tolerances. The relative effect on film thickness can be ascertained from Table 1 on p.6 of our brochure. In some cases, bars wound with un-plated stainless or high carbon steel wire can produce a corresponding saving in unit cost and speed of supply. Un-plated wire wound bars are normally specified for the most sensitive applications such as soft highly polished film.
This is often achieved by doctoring the applicator roll to restrict the width of the wet, un-metered-coating presented to the metering bar. On 'off-machine 'coaters, a piece of thin film can be placed between the web and the metering bar to separate the un-metered coating from the metering bar. Bars with restricted length windings can also be used.
Coating Mix Formulations
It is not intended to discuss the extensive field of coating mix formulation in these notes but important factors are the end viscosity and the dilatent flow characteristics of the film after leaving the metering station. Stocking a range of metering bars can save much time in eliminating the need to modify the formulation of mixes as the bar can be quickly changed instead of re-formulating the mix.
Laboratory draw down sets are an invaluable tool for assessing coating mixes and are in widespread use for obtaining objective and subjective data on mix formulation and making simultaneous comparisons between colours. They are also used to test printing inks, paints, adhesives, and in-numerable substances where objective comparisons are required.