The disclosed methods and systems relate to aerodynamic polymer spray deposition techniques and, more specifically, to using electrical fields and charged polymer droplets to assist in polymer spray deposition.
Many manufacturing and industrial applications benefit from fluid atomization to create a fine vapor mist or aerosol, such as ink print heads, three-dimensional (3D) part manufacturing, fuel/air mixing used in combustion applications, atomizing air-paint mixtures for spray painting, applying coatings to pharmaceuticals, applying adhesives to various objects and surfaces, and the like. Once a component solution is atomized it can be readily processed to coat virtually any shaped surface.
Regardless of the application, most spray deposition systems create droplets at a nozzle tip that have inherent directionality. The conventional spray systems use airblast, shear atomizers, upstream atomizers, and a variety of collimation methods (i.e., virtual impactors and sheath flow control) to focus and direct the spray into a nozzle and then to targeted deposition. Generally, in the context of a print head, for example, the aerodynamic, airflow velocities required to deposit droplets of a small size, in the order of 1 micron, is high, about 30-50 m/s for droplet throw distances on the order of a millimeter (mm).
One conventional spray deposition method uses lateral cross-flows in a shared manifold of multiple fluid ejectors. The lateral cross-flows likely generate flow instabilities and secondary flows at the cross-flow velocities required to successfully deposit the small droplets with high spatial precision for a 3D printing application. Alternatively, some other conventional spray deposition systems use multiple, dedicated feed lines to each ejector or jet and a specialized inlet design that requires dedicated, miniature aerosol jet arrays. However, such a dedicated, specialized system is complex and the manifold inlet design is vulnerable to clogging, especially with fluids having non-Newtonian properties like high-viscosity solutions commonly used in 3D part fabrication and polymer melts used in fused deposition modeling (FDM) systems. Many of the drawbacks of the current systems and methods are amplified in a system with multiple nozzles.
To help with directing and focusing droplets, some droplet deposition systems, such as spray or powder coating painting systems used to apply metallic paint to vehicles, use rotary atomizers coupled with external corona generators and electrically grounded parts to achieve an electrostatically-assisted spray process having a highly-efficient deposition of material and uniform coating. Similar corona charging systems are used with polymer powder coating devices. However, such systems suffer from charge build-up and require the parts to be grounded. The high surface voltage build-up leads to electrical breakdown across the coating and coating thicknesses must be limited to 10's to 100's of microns, depending on the system. Coating dielectric (plastic) parts remains difficult because of the lack of an available grounding path in the dielectric material. Electrostatic directing and focusing strategies are simply not suitable for and present too many challenges to be efficient for many applications, including 3D part fabrication and other printing applications.
Therefore, the spray deposition art would greatly benefit from systems and methods that can direct and focus spray droplets and facilitate aerodynamic spray deposition.
A polymer spray deposition system has a charging system, focusing electrodes and a charge removal system to help focus and direct droplets during deposition of the droplets on a substrate. A spray director can also be used, either before or after the droplets are charged, to further tightly collimate the droplets. The droplets can be formed by a spray generator, for example, having a fluid source. In some examples, steering electrodes can help steer the droplets into the charge removal system. Optionally, deposition airflow can be used to further focus the neutralized droplet spray onto the target substrate.
In other embodiments, a method of depositing polymer spray on a substrate includes depositing a charge on fluid droplets. The fluid droplets can be generated by a spray generator. The charged fluid droplets are focused into a tight fluid droplet stream. The charged droplets in the tight fluid droplet stream are deposited, on-demand, onto a substrate. The charge on the deposited fluid droplets is neutralized.
The disclosed polymer spray deposition systems and methods use spray charging and discharging techniques to assist with digital deposition of polymer spray droplets on a substrate. The disclosed example systems and methods employ electrical fields in conjunction with charged polymer droplets to tightly collimate and assist deposition of polymer sprays without using high-velocity lateral air flows that are often required in conventional grounded, electrostatic spray deposition systems. Further, the disclosed systems and methods for polymer spray deposition use electrostatic assist to direct and focus the polymer spray droplets by using aerodynamic impact and field steering rather than the conventional use of grounded target surfaces in other electrostatic assist particle deposition techniques.
The disclosed example systems and methods work well in depositing low-velocity sprays. Specifically, the disclosed systems and methods are useful for depositing spray in additive manufacturing applications like 3D part fabrication and in other coating and plastics applications. For example, 3D printers often use high-density polymer melts to deposit layers of the molten melt onto a substrate according to a digital model. Conventional droplet deposition or coating methods are limited because they either require high-velocity lateral air flow and/or grounding to efficiently deposit the droplets without clogging the jets depositing the droplets. The new designs disclosed here can be applied to 3D printing systems and print heads, among other applications, to improve the efficiency and capabilities of 3D part manufacturing by providing an aerodynamic system that does not require grounding and by not requiring high-velocity air flow that disrupts the deposition stream and creates secondary flows at the nozzles of the jets.
I. The Charging and Discharging Polymer Spray Deposition Systems
The disclosed polymer spray deposition methods and systems have a droplets charger, such as a corona charging system that charges the fluid droplets. The fluid droplets can be generated by a spray generator. Some of the examples create fluid droplets from fluids with non-Newtonian properties and might benefit from a fluid atomizer that uses fluid stretching techniques to overcome strong extensional thickening characteristic of non-Newtonian fluids. The fluid stretching technique stretches fluid into filaments between two diverging surfaces, such as a pair of counter-rotating rollers or between surfaces of two diverging pistons. When the stretched fluid filament reaches a point of the liquid bridge becoming unstable, which is also the capillary break-up point for the fluid filament, the fluid filament breaks up into several droplets leaving some excess fluid behind on the diverging surface(s). The formed droplets then enter the charging/discharging portion of the polymer spray deposition system.
The block diagrams shown in
The block diagram of the polymer spray depositions systems shown in
In the example with the counter-rotating roller pair, two droplet spray streams generally form during the spray generating process—the first stream at the thin boundary layer and the second stream at the downstream side of the nip. Both of the streams can be captured for charging in the charging system or can be captured by a spray director and then charged. The spray director is optional, although it is helpful especially in embodiments with multiple streams of formed droplets, such as the pair of counter-rotating rollers and/or systems with multiple pairs of counter-rotating rollers.
The droplets are charged by a charging system 108, such as a corona charging system, either after or before the optional spray director 106, 107, as shown in
Referring again to
The charged, collimated, and now accelerated droplets are then neutralized by a charge removal system 114, as shown in
The polymer spray deposition system 100 shown in the block diagrams of
The fluid filaments extend between the diverging surfaces of the rollers 304, 306. As the rollers 304, 306 counter rotate, the fluid filaments are stretched until they exceed their capillary break-up point and break into droplets on the downstream side of the nip 308. Some fluid retracts back onto the surface of the rollers 304, 306 so it can pool on the upstream side 310 of the nip 308 and can then be drawn in through the upstream side 310 of the nip 308 when the process repeats. The rollers 304, 306 are coated with the fluid in any suitable manner including pan coating, drip coating, slot bead coating, curtain coating, or any other fluid coating technique that coats one or both of the rollers with the fluid.
The arrows 314 show the direction of the droplet movement away from the downstream side 312 of the nip 308 and towards the spray director 316. The arrows 314 can represent multiple streams of the formed droplets, such as the stream formed on the thin boundary of the counter-rotating rollers 304, 306 and the stream formed by the breaking of the fluid filaments on the downstream side 312 of the nip 308 in a direction away from the nip 308. The arrows 314 can represent any number of streams of formed droplets and the number and type of streams vary depending on the type of fluid atomizer used to generate the droplets, the fluid being atomized, and other system variations.
The spray director 316 collects the each of the streams of droplets and directs the droplets away from the spray generator 302, as shown by the arrow 318 through the spray director 316. The spray director 316 is funnel-shaped in this example and also directs the droplets towards the charging system 320 to charge the droplets. The charging system 320 in
The positively charged droplets then flow between three pairs of focusing electrodes 324 to form a tightly collimated and focused droplet stream 326. The two electrodes in each pair of electrodes 324 are spaced apart from each other across the flow pathway of the droplets 324. The pairs of spaced apart electrodes are positioned next to each other to form a tunnel-like pathway through which the positively charged droplets flow. Any suitable number of electrodes and electrode pairs can be used. The electrodes apply an electric field to the droplet stream. In some examples, the droplet stream flow shown by arrow 318 is helped by airflow from an air director 317 applied to the droplets to move the droplets along the flow pathway between the focusing electrodes. The airflow can help to move the droplets along, to steer the droplets in a particular direction to help in the collimation process, to adjust or maintain the velocity of the droplets, etc. The combination of the applied airflow and the compression forces of the applied electric fields causes the droplets to tightly pack together in the resulting collimated droplet stream.
The collimated droplet stream droplets 326 are still positively charged. The collimated droplet stream exits the focusing electrodes and flows through a pair of baffles 328. The baffles 328 are spaced apart from each other and taper in the direction of the droplet pathway. The tapering of the baffles 328 helps further focus the collimated droplet stream. After the charged, collimated droplet stream exits the baffles 328, deposition airflow 332 directs the droplet stream to turn about 90° in a clockwise direction towards the substrate 338 onto which the fluid droplets are to be deposited. The droplet stream travels by an opening between two walls 330. The deposition airflow 332 flows through the opening between the walls 330 and controls the direction of the fluid stream by changing its direction from generally horizontal to approximately vertical, which is an approximately 90° change in direction. The new, approximately vertical direction of the fluid stream 334 directs the fluid stream 334 towards the substrate 338 onto which the fluid droplets are deposited.
The polymer spray deposition system 300 shown in
a. The Charging System
In the Pauthenier equation, q is the charge acquired by a spherical dielectric particle having a relative radius, a, and relative permittivity, εr, when exposed to an ion flux in a field, E. The ion flux is implicit in the charging time constant q, which is given by:
Where J is the current density of the ion flux.
The Pauthenier equation shows the relationship between charge acquired by a dielectric polymer droplet as a function of the corona parameters. If the droplets are exposed to field charging for a time, t, significantly longer than τ, then charging is maximized.
Droplets that generally have residence times in the corona charge on the order of 0.001 seconds (s) are sufficient to reach the maximum charge level for the droplets, which is known as the Pauthenier limit, calculated by the equation shown below:
qmax=12πa2ε0E
The resulting maximum charge is then known.
b. The Focusing Electrodes & the Steering Electrodes
The RF ion guides are ion funnels typically used in mass spectrometry instruments and are an exemplary realization of charged particle collimation using time- and space-varying electrical fields. The ion funnels use closely spaced electrodes 602 and RF potentials 604 that confine a dispersed cloud of ions and that focus the ions into a tight beam.
In the ion funnel, alternating potential fields help to collimate the droplets together by applying a net potential across the flight tube, the length of the stacked rings of electrodes 602 shown in
Optionally, steering electrodes help to steer the droplets onto the substrate. Electrical fields can be used to induce droplet motion away from the main droplet direction flow and towards the substrate. The electrical fields can vary to allow some droplets through, but not others, which helps control the volume of droplets deposited on the substrate. For example, electrical steering techniques are used in inkjet print heads to dump the spray into a recycle trap until droplets from the jet need to be deposited on the substrate. The steering electrodes are turned off or the electric field is reduced when a jet of droplets is desired, which is known as “off-mode” steering of inkjet streams.
The steering electrodes can direct any portion, including all, of the droplets towards the recycle trap and can operate in a range of applied voltages. The range of applied voltages can vary the electric field being applied to the droplets, which in turn varies the volume and direction of those droplets that are redirected from the droplet pathway towards the recycle trap. For example, in an inkjet print head the steering electrodes can be turned on for a particular jet until that jet is supposed to deposit droplets onto the paper or other substrate. When turned off, the jet deposits droplets onto the substrate. The captured droplets in the recycle trap can be recycled back into the fluid source that feeds the spray generator, in some examples, or can be discarded or recycled to a different portion of the system in other examples.
Some example spray deposition systems have either the focusing electrodes or the steering electrodes. Other alternative systems have both the focusing electrodes and the steering electrodes. Still in other systems, the focusing electrodes and the steering electrodes can be integrated into a single focusing/steering element that both focus the droplets into a tightly collimated stream and steer the formed droplets in a desired direction.
c. The Charge Removal System
A charge removal system removes the charge from the droplets either during the droplet deposition process or after the droplets are deposited on a substrate. For example, after aerodynamic deposition, the charge removal system neutralizes the charge of the droplets. For example,
The charge removal system 900 shown in
II. The Charging and Discharging Polymer Spray Deposition Methods
Methods of charging and discharging polymer spray deposition are also disclosed. The spray deposition methods include depositing a corona charge on fluid droplets generated by a spray generator and then focusing the charged fluid droplets into a tight fluid droplet stream. The tight fluid droplet stream is deposited, on-demand, onto a substrate. The charged droplets are neutralized either while they are being deposited on the substrate or after they are deposited on the substrate.
The corona charge is deposited using any of the above disclosed methods or other known methods to charge droplets. The droplets are charged downstream of the spray generator 1000, as discussed above and as shown in
The tight fluid droplet stream is deposited onto a substrate. In some examples, the deposition of the droplets happens on demand, either with a manually controlled or an automatically controlled system. For example, synchronized steering electrodes, such as the steering electrodes discussed above, and air pulses are used to deposit material on-demand. The combination of the steering electrodes and the air pulse can be used to achieve droplet deposition of a specific volume and density at a specific flow rate on a specific location.
For example, a 3D printer has digital data depicting a particular part to be printed. A controller controls individual jets of the 3D printer according to the digital data for the part, layer by layer. When a particular jet needs to deposit droplets, the 3D printer sends that jet a signal to trigger the jet to deposit droplets, which can sometimes be an instruction to deposit a specific volume, density, and/or flow rate of the droplets. Multiple jets deposit droplets in unison. Any suitable on-demand system can be used.
Either during deposition of the droplets or after the droplets are deposited, the charge on the droplets is removed by a charge removal system. For example, the charge removal system can be an oppositely charged plasma system that is charged opposite of the charge deposited on the droplets by the charging system. In the example shown in
III. Examples of Molten Polymer Melt Systems & Methods
In an example, a dielectric, molten polymer melt, such as those used in 3D printers, is atomized by a spray generator, such as the spray generator examples discussed above having two counter-rotating rollers that stretch fluid filaments that break into droplets of the molten polymer melt. For the 3D printers, a molten polymer melt droplet is can be 1 μm in diameter. The molten polymer melt droplets are charged using the corona charging systems discussed above to achieve a charge of about −1e−12C, calculated by the Pauthenier equation discussed above in regards to
Because of the 1 μm diameter and a charge of about −1e−12C on each droplet, each droplet's trajectory is a function of the Coulombic force on the droplet, as calculated by the following equation:
FC=qE
The Colombic force, FC, variables include q, which is the electric charge on the droplet (C), and E, which is the electric field (V/m).
The drag force on each droplet is:
FD=½ρCCDA|u−v|(u−v)
The drag force variables include ρC, which is the density of the conveying phase (air); CD, which is a drag coefficient; A, which is the area of the droplet; and (u−v), which is the slip velocity or the distance between drop and carrier phase velocity.
In this example, the dielectric polymer melt droplets have persistent surface charge deposited by the upstream corona charger. Using electric fields, the charged droplets are steered and collimated prior to aerodynamic deposition on a substrate. To achieve droplet steering, the electrical forces must be comparable to or greater than the aerodynamic drag force. In aerodynamic deposition, the drag force is on the order of 1×10−9 to 1×10−12 Newtons (N), depending on the velocity of the droplets. To generate a Coulomb force of comparable magnitude requires an electric field strength of around 1×103−1×104 volts/meter (V/m). This magnitude of electric field strength can be achieved in small gaps without high voltage fields and is less than the breakdown strength of air. Air's breakdown strength is about 1000 times higher, around 3×106 V/m. For example, a lateral electrical field of 5000 V/m is applied to the 1 μm droplets using a 4V potential to steer and collimate them into a tight droplet stream.
A specific example molten polymer melt print head system schematic 1100 is shown in
The steering electrodes include two pairs of electrodes 1110, 1112, as shown in
An exit orifice 1612 extends through the bottom manifold layer 1604 through which the droplets exit the jet to be deposited onto the substrate (not shown). An exit orifice electrode 1614 is positioned within and is integral with the exit orifice 1612. The example print head shown in
It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
This application is a divisional of and claims priority to U.S. patent application Ser. No. 14/573,602, filed Dec. 17, 2014, which is incorporated herein in its entirety.
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20180111312 A1 | Apr 2018 | US |
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Parent | 14573602 | Dec 2014 | US |
Child | 15848378 | US |