INTRODUCTION
The slot die coating positioner. That’s not even a real word! If you type in the word positioner, the word processing program complains that you didn’t spell positioned correctly. Well, the software developers didn’t work in the coating and converting industry. For those of us who do, the positioner, or mounting equipment for the slot die is critical to fluid coating application.
In standard proximity coating, a slot die is held the wet coating distance away from the substrate, which is supported by a precision backing roll. But what is holding the slot die in place? The mounting system that holds the slot die within microns of the proper coating position is referred to as a positioner in coating manufacturing circles. As products are coated thinner and more precisely, the potential variation in the slot die to substrate gap can be the difference in precision coated products and scrap.
The positioner has some key features that make it work properly. Mechanically, the slot die mounting system should be designed to not twist or torque under the movement and pressures associated with the slot die interaction. Finite element analysis of the mechanical structure is recommended during the design phase. Movement along linear bearings allow for precision control and smooth transition from the off-coat to the on-coat position. Pneumatic actuation is common, but servo motors can allow for improved repeatability and less settling time, although the strength of the motor versus the back pressure in the slot die to roll operation should be considered.
Setting the gap initially is important to flow control at the exit of the slot die, but repeatability is also important. If the substrate requires a splice to pass, the positioner is the equipment that jumps the splice with a micro movement. Having the positioner return to the coating position quickly and repeatably allows the coating to continue with very little interruption. If the coating line needs to stop for an extended period of time, the positioner allows the slot die to be retracted a distance that allows the operator to clean the front face with no potential of interfering with the substrate or precision roll. This coarse adjustment is typically 6 inches (152 mm) or more. The splice jump movement is enough to allow the splice to pass without keeping the slot die disengaged too long. Repeatability is typically 0.0001 inch (2.54 microns).
A positioner is not only critical in proximity coating. In tensioned web over slot die (TWOSD) coating technique, a uniform placement of the slot die in relation to the web tensioned over the slot die is paramount. Skew in the slot die to substrate position could lead to cross web coating non-uniformity, wrinkling or coating defects.
In hot melt adhesive coating, the positioner has the added function of controlling the attack angle of the slot die to the backing roll. Because of the stronger elastic characteristic of hot melt adhesives, the variation of the attack angle plays an important role for successful coating. The rotation of the slot die should be around the lower corner of the upper lip (coating in a horizontal position). Rotating in another location of the slot die will lead to improper geometric changes and having to shift the die to coat correctly.
There are 2 critical gaps in slot die coating technology, the slot die opening itself and the gap between the slot die and the substrate. While the slot die opening is more critical and controlling a cubic volumetric flow value, the positioner controls the linear relationship of flow control at the exit point of the slot die. If the coating bead at the dynamic wetting line is not stable, neither will the flow control.
The positioner controls the coating bead development which keeps out air and develops the meniscus across the length of the slot die lip face. The variations in pressure from the upstream to the downstream portion of the fluid flow can be successfully managed through this gap control and proper lip offset. In the common coating defects seen in fluid applications to substrates, some of the most common are related to the slot die to substrate gap which the positioner controls.
Position your slot die properly and enjoy coating success! Regardless of whether a positioner is a real word or not.
