The present invention relates generally to non-contact dispensing systems for dispensing flowable material onto a substrate and, more particularly, to an electrostatic dispensing system for electrostatically dispensing flowable material onto a moving substrate.
In known electrostatic dispensing systems and processes for electrostatically dispensing flowable liquid materials onto a substrate, the flowable material is typically dispensed from one or more dispensing outlets and directed toward a target surface of the substrate. The dispensing outlet of the electrostatic dispenser may comprise either a series of closely spaced nozzles or an elongated slot which receives flowable liquid material at a controlled flow rate and hydrostatic pressure from a fluid delivery system. Where spray nozzles are used as dispensing outlets, the flowable liquid material is typically atomized into a fine particle spray for providing a uniform coating on a surface of the substrate. A dispensing nozzle having an elongated slot outlet, on the other hand, typically provides either a series of spaced, continuous beads of flowable material or a series of spaced, discontinuous streams of droplets which are applied to the surface of the substrate.
The flowable liquid material in an electrostatic dispensing process is electrically biased relative to the target substrate to cause an electrostatic force attraction between the dispensed material and the substrate. The electrostatic force is created by electrically charging components of the dispensing nozzle in contact the flowable material, while the electrically conductive substrate is simultaneously grounded. The required charging voltage for the dispensing nozzle is provided by coupling a high voltage power supply, generally having an output voltage range between 10-50 kV, to conductive components of the dispensing nozzle in contact with the flowable material. In this way, the voltage potential created between the charged dispensing nozzle and the grounded substrate creates an electrostatic force which causes the charged continuous beads or discontinuous droplets of liquid material to be attracted to the grounded substrate.
Electrostatic dispensing systems having charged dispensing nozzles are generally not well suited for dispensing applications which require heating of the nozzle to melt the flowable liquid material prior to dispensing. In these applications, the heating element mounted within the dispensing nozzle must be electrically isolated from the charged components of the nozzle through the use of nonconductive materials, such as plastic. However, the nonconductive materials typically employed are not good thermal conductors and therefore make heating of the dispenser nozzle difficult. Moreover, the known alternative of electrically isolating high voltage electrodes within an electrically conductive dispensing nozzle body requires complex internal charging and isolating devices to be incorporated into the nozzle. Additionally, in applications that require electrostatic dispensing of flowable materials on non-conductive substrates, it is not possible to ground the substrate to create the necessary electrostatic attraction between the flowable material and the substrate.
The electrostatic dispensing system disclosed in U.S. Ser. No. 08/977,796, now abandoned previously incorporated herein by reference and owned by the common assignee, solves these shortcomings and drawbacks by providing an elongated dispensing nozzle supported in spaced relationship relative to an electrostatic field generator. The dispensing nozzle is supported in spaced, non-contacting relationship on one side of the moving substrate, and the electric field generator is supported in spaced, non-contacting relationship on the other side of the moving substrate. The spacing between the dispensing nozzle and electrostatic field generator defines a space for receiving the moving substrate.
Flowable liquid material, such as pressure sensitive hot melt adhesive, is supplied to the dispensing nozzle at a controlled rate and low hydraulic pressure from a material delivery system. The electrostatic field generator is operable to generate an electrostatic field through the moving substrate to attract flowable liquid material from the dispensing nozzle in a series of uniformly spaced, continuous beads or streams. The continuous beads of flowable liquid material are intercepted by the moving substrate and carried away as parallel beads on the surface of the moving substrate facing the dispensing nozzle.
The dispensing nozzle disclosed in U.S. Ser. No. 08/977,796, now abandoned comprises a pair of mating die bodies which include an internal shim and a grounded distribution plate to define an elongated dispensing slot along a lower edge of the nozzle. The distribution plate has a series of uniformly spaced teeth extending slightly beyond the lower edge of the dispensing nozzle which define the even spacing of the continuous material beads deposited on the moving substrate when the longitudinal axis of the dispensing nozzle is arranged perpendicularly to the direction of travel of the moving substrate. As the dispensing nozzle of U.S. Ser. No. 08/977,796 is grounded rather than being charged by a high voltage power supply, the dispensing nozzle may be made of metal or any other suitable material having good thermal conductivity for improved heating of the nozzle. Moreover, the grounded dispensing nozzle eliminates the need for incorporating any complex internal charging or isolating devices in the nozzle.
Notwithstanding the advances made by the electrostatic dispensing system disclosed in the present assignee""s U.S. Ser. No. 08/977,796, there is still a need for an electrostatic dispensing system that reduces the volume of liquid material within the dispensing head of the liquid dispenser to improve shut-off capability of the dispenser. There is still also a need for an electrostatic dispensing system that provides the ability to mix and match different liquid dispensing technologies and patterns across the width of the substrate.
To these ends, an electrostatic dispensing system is provided for electrostatically dispensing flowable material onto a moving substrate. The electrostatic dispensing system includes multiple liquid dispensers that are aligned along a common axis and supported in spaced, non-contacting relationship on one side of the moving substrate. An electrostatic field generator is supported in spaced, non-contacting relationship on the opposite side of the moving substrate to generate an electrostatic field through the moving substrate which may be substantially non-conductive.
Flowable liquid material, such as pressure sensitive hot melt adhesive, is supplied to the liquid dispensers at a controlled rate and low hydraulic pressure from one or more metered fluid supplies. The electrostatic field generator is operable to generate an electrostatic field through the moving substrate to attract the flowable liquid material from the liquid dispensers in a series of uniformly spaced, continuous beads or streams. The continuous beads of flowable liquid material are intercepted by the moving substrate and carried away as parallel beads on the surface of the moving substrate facing the dispensing nozzle.
In one aspect of the present invention, each of the liquid dispensers includes a shim having multiple elongated fluid passageways formed through the thickness of the shim. The elongated fluid passageways are generally parallel and define multiple dispensing outlets along an elongated, interrupted edge of the shim. Each of the liquid dispensers further includes a distribution plate mounted adjacent the shim that terminates in either a serrated or scalloped edge proximate the dispensing outlets formed in the shim. The elongated fluid passageways of the shim are aligned with teeth formed on the edge of the distribution plate so that the spacing between the teeth generally defines the spacing between the continuous beads dispensed onto the substrate. Each of the liquid dispensers includes a valve to control the flow of the liquid material through the dispensing outlets.
The multiple liquid dispensers and electrostatic field generator of the present invention are operable to produce controlled patterns of flowable material on a moving substrate with low add-on weight, accurate bead placement and high pattern repeatability. The provision of multiple liquid dispensers along a common axis, with each dispenser having a selectively operable valve, minimizes the volume of liquid material in the dispenser to improve cut-off of the dispensed bead pattern over larger dispensing nozzles. Further, the multiple liquid dispensers of the present invention provide the unique ability to mix and match different liquid dispensing technologies and patterns, such as electrostatically applied continuous parallel beads with non-electrostatically applied swirl patterns, flat ribbons, or fibrous webs of liquid material, across the width of the substrate.
The above features and advantages of the present invention will be better understood with reference to the accompanying figures and detailed description.