The basic principal of the Continuous Press is that endless belts float on a cushion of air provided by the air plenums. This flotation principal is used in air tables in the garment industry to float large heavy blocks of material on a cutting table and by heavy equipment movement pads. Hover craft also use the same air floatation principal. The air is not sealed between the belt and the plenum and continually flows out of the sides of the belt. High pressure blowers are available that can produce 0.5 to 10 PSI of high volume air flow. The more normal combining pressures would be in the 0.5 to 1 PSI range. The total combining pressure on a 10’ wide by 10’ long unit would be about 7 tons.
The general method used in industry is to apply adhesives to the materials and then place them into a stationary press. These presses may have heated surfaces to speed the bonding process. The most efficient of these presses have a belted conveyor running through the press opening. The products are placed on the conveyor and automatically cycled into and out of the press. The open times on the adhesives must be longer to accommodate the stop and go nature of the stationary press operation. Open time is defined as the time allowed from the time the glue is applied to the time it will not longer form an adequate bond. In case of an interruption to the process all parts between the adhesive application and the press are scrapped.
The Continuous Press is designed to provide a continuous pressure to laminate two or more materials that require time under pressure in order to bond. The open time required for the adhesive can be adjusted to a matter of seconds due to the continuous nature of the process. One example would be using water or solvent based adhesive. The adhesive is sprayed or coated on one part and the other is placed on top of it on the infeed conveyor. The parts enter the Continuous Press and are conveyed under pressure for the time required for the adhesive to reach a safe bond level. Hot air can be used in the laminator to speed up the process. A second method is to use hot melt adhesives that bond by cooling or reaction to moisture. The produce is then conveyed thru the Continuous Press under pressure while the adhesive cools and the bond is formed. A third method is to provide heating and cooling under pressure to activate, bond, and cool thermoplastic materials including powdered adhesives. There exists equipment called flat bed laminators that are currently being sold for this function. They are much slower and smaller because they are dragging the conveyor belt over platens under pressure. The drag generated, limits the pressure and maximum size of these units. The pressure generated is not uniform or continuous in flat bed laminators.
The Continuous Press process is to introduce two or more layers of material into the press. The upper plenum is height adjustable to provide the proper gap between the belts to accommodate the material to be laminated. The plenums have a large number of small holes to introduce the air between the belt and the plenum, thus floating the belt on a cushion of air. The pressure exerted between the belts is adjusted by changing the height of the upper plenum. If the materials to be laminated have low porosity the use of the belts is optional, as the air pressure can be exerted directly on the materials without the use of a belt. The use of air floatation provides a cushion between the plenum and the belt. The belt does not have to be rubber to provide a resilient surface. The belt can even be metal, thus increasing the heat transfer from the floatation air and the product.
When using a cold set or cross linking adhesive this is all that is required. If using thermoplastic bonding with hot melts or materials having thermoplastic properties, heat and cooling are required. The laminator is divided into two sections with the first one having a heating chamber to heat the air used to float the belt or materials. The second section will have a chiller to cool the air. In this type of equipment it is important to enclose the equipment and recover the air. The fan suction is connected to an enclosure around the laminator to recover the air after it exits the floatation zone. Radiant heaters can be built into the surface of the plenums as an option for use where the belt is not present. A working model is needed to determine the temperature distribution across the width of the material. There may be differences in the air flow from the center to the sides of the material that would result in an unacceptable temperature profile. Since this principal has not been used for heat lamination the temperature distribution will have to be determined experimentally. If there are static zones causing a temperature differential, the problem can be overcome by allowing some of the air in the center of the floatation zone to escape. It may even be possible to use the beltless version as a dryer. Air floatation dryers do exist, but they use an air foil principal the uses high velocity air to strip moisture from the surface of a continuous web. They can not be used at all for non continuous product.
There is currently no equipment on the market designed to do time under pressure lamination on a continuous basis. There are presses that will fill the function on a static basis and over and under conveyors that do not provide an even pressure to achieve the goal. The pressure provided by the paired conveyors is very uneven and results in an uneven bond between the materials. The current flat bed laminators without the heat and cooling would be able to do this kind of lamination, but they can not generate significant pressures and are very slow. The requirements of a given process dictate the temperature and dwell required to achieve the lamination. One high speed Continuous Press line will replace a number of the current stationary press lines. This would result in a significant reduction in the capital requirements to produce laminates.
There is an area of development in the production of durable Nonwovens that will be of interest to a number of nonwoven roll goods producers. The use of a class of PUR adhesives that are durable can produce materials that can be washed and dried. It is not currently possible to make these materials because of the time under pressure requirements of the PUR adhesives. This technology though simple and could have been invented anytime over the last 50 years will make major changes to the way products are made and make possible the cost effective production of products that currently can not be marketed.