Coating methods of hot melt coating machine
There are different methods of applying the adhesive onto the various webs:
rotogravure rotary screen
multi-roll slot die
The rotogravure system is the most commonly applied method for lamination. It is a printing system with an engraved roller, in which little cavities are engraved for transfer of the adhesive to the web substrate.
There are different patterns available, depending on the laminated product. The patterns can be regular or random round dots, pyramidal, nets, lines, etc.
The adhesive is fed in plasticized form (molten) to a coating head. From the coating head, the adhesive is taken over by the engraved roller, the blank parts cleaned off with a special doctor blade. The adhesive is then transferred onto the sub-strate passing by on the pressure counter roller.
Rotary screen system
In the rotary screen system system, the hot melt adhesives are transferred onto the substrates through a rotary screen, similar to the common textile printing method.
The hot melts are fed from the tank melter or drum melter through a heated and insulated hose to the inside of the rotary screen, just behind the doctor blade. The adhesive is distributed over the width of the fabric. The adhesive is by the doctor blade pressed through the screen onto the substrate (membrane, fabric, foam, nonwoven, etc.). Different screen patterns allow different coating. However, other than with the rotogravure method, continuous patterns like a net structure are not possible, of course.
This method is recommended only for reactive polyurethane hot melts (duro-plastic polymers), and for thermoplastic hotmelts with lower melting points, but not for coPES and coPA.
For polymers with high plasticizing temperatures, the screens are diffi-cult to clean and critical to handle, and therefore we recommend the rotogravure system for such poly-mers in case a patterned coating is desired.
The screen environment is heated with infrared heaters, closely covered against heat losses and disturbances. The doctor blade and hot melt feeding system inside the screen is heated separately.
A good and well proven method for full coating is the multi-roll system, where the molten hotmelt is transferred to the substrate by means of a coating roll with a flat surface. The coating roll is rotating together with the substrate in the same direction. It is not in reverse roll mode.
The hotmelt is fed from the melter or the extruder into the gusset between the coating roll and the dosing roll. Depend-ing on the gap between the dosing roll and the coating roll, a specified quantity of hotmelt is transferred onto the sub-strate. The dosing roll may be stationary or in reverse rotation mode to further adjust the coating weight.
The rotation speed of the coating roll can be synchronous with the rotating speed of the counter roll, i.e. with the speed of the substrate, or it can be adjusted faster or slower than the substrate. Therewith, the coating weight and also the coating pattern can be influenced.
With a speed of the coating roll slower relative to the substrate, an irregular open coating can be achieved. This can go to the extent of achieving irregular dot coatings. However, such dots cannot be controlled as with the rotogravure or screen methods.
slot die system
The hot melt is pumped from the melter or the extruder to the slot die mounted across the web substrate. Our company utilizes slot die from reputable sub-suppliers with extensive experience in hotmelt technology. A number of melt pumps distributed across the width guarantee an equal feeding of the hotmelt and therewith an even coating distribution.
Basically there are 2 applicaotn methods, with and without contact of the slot die with the substrate. For a regular closed film, the contactless method is used, positioning a film of the polymer onto the substrate on the counter roll. For open films with irregular open areas, the slot die touches the substrate in formt of the counter roll, where the substrate is free.