Curtain Coater

The casting process comes from the surface coating of wood. In this process (Fig. 11-9) a casting head creates a curtain of liquid. The horizontal leather passes through this verticaUy descending curtain. Casting finds appHcation in particular in 

Binders are among the most important components of a finish system, whether it is a pigment finish or a top coat. They bind the color-conferring pigments, which have no inherent affinity for leather, and protect the leather surface through their fihn-forming property. Aqueous finishes generally utilize aqueous polymer dispersions. 

As well as from binders and pigment preparations, finishes are prepared from rheological additives (thickeners, flow control agents, solvents in appropriate cases), matting agent, crossHnker and handle modifier. The following product classes are available as polymer dispersions  

---

In the automated lay-up system, each layer of veneer (with exception of the top surface veneer) passes under an automatic adhesive appHcation system. This may be a spray appHcation, a curtain coater, or an extmder, each of which is designed to apply a uniform adhesive spread on the upper face of each veneer. After all except the top veneer have been spread with adhesive and laid together, the top veneer is added. The me of the veneer—adhesive assembly at this point should be about 8%. 

In most coatiag operatioas a single layer is coated. Whea more than one layer must be appHed one can make multiple passes, or use tandem coaters where the next layer is appHed at another coatiag statioa ioimediately foUowiag the dryer sectioa for the previous layer, or a multilayer coatiag statioa can be used. Slot, extmsion, sHde, and curtain coaters are used to apply multiple layers simultaneously. SHde and curtain coaters can apply an unlimited number of layers simultaneously, whereas slot and extmsion coaters are limited by the complexity of the die internals. 

A basecoat is applied to the panel after the filling operation. This is normally a pigmented lacquer or vinyl based material whose function is to hide the surface and provide a uniform color on which the grain will be printed. The basecoat is commonly applied by a curtain coater or more commonly with a roller coater. Obviously, skips in the basecoat, whether the result of deformed application rolls or panel unevenness, cannot be tolerated. Surface depressions not completely filled during the filling operation will not be basecoated and will appear as small white dots on the panel. 

In hardwood plywood, almost all applicators are roll coater types. In softwood plywood, the large production mills used roll coaters for 50 years or more, but are now moving more and more toward spray applicators and curtain coater applicators. In the southern section of our country where 57 softwood plywood mills are now located and have been built since 1963, approximately one-third of the mills use curtain coaters, one-third use spray applicators and the other third use roll coater applicators. 

The curtain coater and spray applicator are innovations which can conserve adhesives in the range of 20 percent and permit recycling of glue which is not applied to the veneer. 

With curtain coaters glue is forced from a reservoir through a narrow elongated gap or slit and falls as a continuous thin curtain across the entire width of the veneer... 

Figure 11.9. Various glue spreaders (Sellers, 1985). (a) Roller applicator, (b) Curtain coater. (c) Foam extrusion with in-line roll prepress.

Curtain coaters have recently appeared on the market that offer the user more control over the operation. These so-called Omega curtain coaters promise a more effective use of the process and a wider application (3). 

Feed liquid is spread across die coaters, slot and extrusion coaters, slide and many curtain coaters by chamber-and-slot distribution. The liquid is introduced into a chamber or manifold as long as the coating is to be wide, which delivers the liquid in turn to a narrow slot through which it flows to an exit aperture where it may be applied to a substrate, turn and run down an apron or slide, or fall free as a curtain. 

When the flow per unit width across the exit aperture of a chamber-and-slot arrangement can be made uniform enough, distribution doubles as metering. Indeed, the distribution and metering functions are inextricably combined in this manner in slot, extrusion, slide, and curtain coaters that employ chamber and slot and operate in the pre-metered feed mode. The combination is ubiquitous in multilayer coating. 

Whereas fillers are applied using a roller machine, putties are usually applied by reverse roller, spray gun or curtain coater. Depending on the substrate, the application rate is between 50 and 200 g/m. Fillers are mainly formulated with paraffin-type UP resins or air-drying types which are UV-cured. For fillers cured with hy-droperoxide/cobalt, air-drying UP resins are normally used in combination with flexibilising resins. 

Low-viscosity soybean briquetting adhesives are generally applied by spray. Paper and softboard laminating glues are usually applied by curtain coater, knife, or indirect roller. High-viscosity plywood and lumber assembly formulations are fairly well limited to application by spreader roll or extrusion. For very small assembly jobs, soybean glue can easily be applied by brush. 

Uses Defoamer for acid-cured and NC lacquers applied by curtain coaters, as well as fiberglass-reinforced polyesters and gel coats Features Silicone-free EFKA -2021 [EFKAAdditives BV]... 

Nevertheless, this is the most widely preferred of all methods. In these days, when so much furniture is made from pre-finished flat boards, the basecoat can be applied accurately in thin coats by roller coater and thus can contain higher solids. The finish is then applied by a curtain coater, which is positioned immediately after the roller coater on the production line. 

Curtain coaters Entrained air web speed too high/coat too thin and Reynolds number too low. 

The selection of mix ingredients may depend heavily on the type of adhesive application equipment used. The main types of equipment for plywood and LVL are roll coaters, spray systems, curtain coaters, and foam extruders. When foam extruders are used, animal blood and surfactants are usually added to the mill mix. For other application systems, resin producers typically supply the non-filler additives as part of the resin composition. 

Adhesives of the aminoplastic (see Step polymerization) and phenol formaldehyde (see Phenolic adhesives single-stage resoles and Phenolic adhesives two-stage novolacs) types are most widely used. Although basically similar, an adhesive for plywood manufacture will require a different formulation to one for particle board, or medium-density fibre board (MDF) since methods of application and processing differ. Thus, in plywood, large sheets of veneer must be uniformly coated with adhesive, usually by a roller or curtain coater in particle board, chips or wafers must be coated with very fine adhesive droplets, while small bundles of wet fibres must be sprayed with adhesive in the manufacture of MDF. Hence, formulation and production of resins has become a mixture of art and science, with resin manufacturers able to produce resins tailored for use in a particular board-manufacturing plant, or with a particular species of timber. 

Curtain coating is a method of application that is similar to flow coating. Instead of a stream of product, a thin sheet or film of the liquid is formed to fall on the substrate being transported through it. Curtain coaters are of two general types—weir and orifice. 

Key to type of equipment manufactured 1. air spray 2. airless spray 3. electrostatic liquid spray 4. electrostatic powder spray 5. pumps 6. fluid handling 7. multicomponent 8. automatic curve following 9. hot air ovens 10. infrared ovens 11. infrared sources 12. electrical induction generators 13. roll coaters 14. curtain coaters 15. dispensers for anaerobics 16. cleaning materials. 

In some modem curtain coaters, the top tank is a small volume chamber equipped with a rotating double knife. It may apply a thin film of adhesive, with reduced entrapment of air and an electronic control of thickness. With this closed application system there is no evaporation of solvents, it may work with both solvent based or water based adhesives and even the moisture curing polyurethanes. 

Uses Only thin and flat parts may be coated with curtain coaters as it is shown on Fig. 68. 

It is possible to use a curtain coater with two troughs and two pumps, which gives two curtains ... 

Curtain coaters are used for applying adhesives over wide areas by passing the web below the nozzle of the tank (Fig. 7). Highly viscous hot-melt adhesives and thermoplastic polymers are applied from wide-slot nozzles to form a weblike film on the substrate. 

---