CO.sub.2 Nozzle and Method for Cleaning Pressure-Sensitive Surfaces

The present invention relates to an apparatus and method for creating abrasive CO.sub.2 particles and for directing the resulting CO.sub.2 particles onto a large area of contaminants to be removed from a pressure-sensitive workpiece.

The use of liquid carbon dioxide for producing CO.sub.2 particles and subsequently accelerating it to high speeds for cleaning particles from a substrate is taught by Layden in U.S. Pat. No. 4,962,891.

Lloyd, in U.S. Pat. No. 5,018,667 at columns 5 and 7, teaches the use of multiple nozzles and tapered concentric orifices for controlling the flow of the CO.sub.2 and the snow mixture. This reference seeks to disperse the snow rather than to focus it after exiting the exhaust nozzle.

Hayashi, in U.S. Pat. Nos. 4,631,250 and 4,747,421, discloses the use of liquefied nitrogen (N.sub.2) for cooling a jacket-type peripheral wall defining a sealed cavity in which a flow of CO.sub.2 gas is introduced under pressure. The cooling produced by the cooled peripheral walls causes the CO.sub.2 to change into snow within the chamber. Nitrogen gas is introduced into the chamber at high pressure in order to agitate and carry the CO.sub.2 particles from the chamber at high velocity though a jetting nozzle.

My U.S. Pat. No. 5,405,283 teaches the use of relatively low pressure shop air for transporting and agglomerating CO.sub.2 particles through a converging-diverging nozzle and onto a large workpiece to be cleaned. The larger CO.sub.2 snow/flakes, produced by the enhanced mixing action upstream from the throat, are useful for aggressively cleaning contaminants from a sturdy workpiece, but are not well suited for cleaning contaminants from pressure-sensitive surfaces because the mass and momentum of the CO.sub.2 particles will cause pitting or denting in pressure-sensitive surfaces.

My U.S. Pat. No. 5,390,450 discloses a converging-diverging nozzle for directing CO.sub.2 particles onto a workpiece. The diverging portion of the nozzle is operated in the overexpanded mode for reducing the noise produced by the shockwave by retaining it within the nozzle.

In contrast to these prior art teachings, the present invention utilizes inexpensive components and readily available low pressure shop air for accelerating relatively small CO.sub.2 particles that are injected directly adjacent to the throat section of a converging-diverging nozzle. It is therefore an object of the present invention to utilize low pressure air for transporting and directing the small CO.sub.2 particles against pressure-sensitive surfaces to be cleaned.

In an apparatus for cleaning a workpiece with abrasive CO.sub.2 particles, a CO.sub.2 nozzle is provided for receiving and expelling liquid CO.sub.2 through at least one orifice sized for at least partially converting the liquid into CO.sub.2 particles. The CO.sub.2 particles are injected into a converging-diverging nozzle at a location adjacent to the throat section thereof. Pressurized air is directed into the converging section of the nozzle upstream from the throat section. The pressurized air and CO.sub.2 particles are accelerated by the airflow through the throat section and are focused by the diverging section of the nozzle for impacting the pressure-sensitive surface of the workpiece to be cleaned.

Other objects, features and advantages of the present invention will be apparent from a study of the written descriptions and the drawings in which:

FIG. 1 is a top cross-sectioned view of the CO.sub.2 cleaning nozzle in accordance with the present invention as it operates on a workpiece, such as a pressure-sensitive printing stencil.

FIG. 2 is an end cross-sectioned view of the first preferred embodiment of the CO.sub.2 cleaning nozzle taken along section lines 2--2 in FIG. 1.

FIG. 3 is a simplified perspective view of the interior surfaces of the converging-diverging nozzle in accordance with a first preferred embodiment of the present invention. Hidden lines and cutaway sections reveal the shapes of the interior dimensions of the nozzle.

FIG. 4 is a top view of the converging section of the nozzle. FIGS. 4A, 4B, 4C and 4D are cross-sectioned views taken along the respective section lines in FIG. 4.

FIG. 5 is a frontal cross-sectioned view of a CO.sub.2 cleaning nozzle in accordance with a second preferred embodiment of the present invention.

FIG. 6 is an enthalpy diagram showing the transition or flashing of the liquid CO.sub.2 into solid CO.sub.2 particles in accordance with the operation of the method of the present invention.

A CO.sub.2 cleaning system in accordance with the present invention is illustrated generally in FIG. 1. A CO.sub.2 feed line 21 is connected to a reservoir (not shown) of liquid CO.sub.2. A source of compressed shop air (not shown) is connected through a feed line to an inlet section 11 of a converging/diverging nozzle. The solid CO.sub.2 particles 100 which are exhausted from the CO.sub.2 cleaning system are focused on the workpiece 90, shown generally as a thin (0.008 inches thick) stencil of the type used for applying solder paste to a printed circuit board. Other potential applications include the cleaning of fixtures and stencils used in the printing industry, as well as cleaning of sensitive print heads and electronic components. The size of the CO.sub.2 particles 100 and the workpiece 90 are enlarged for purposes of clarity and do not necessarily represent the size of the CO.sub.2 footprint on the stencil.

The reservoir of liquid CO.sub.2 is stored at approximately 0.degree. F. and is pumped under a pressure of approximately 200-350 psi through the feed-line 21, through a control valve (not shown) and then into a CO.sub.2 plenum chamber 22. The control valve regulates the pressure and the flow rate under which the liquid CO.sub.2 is fed into the CO.sub.2 plenum, which in turn regulates the amount of snow in the output flow.

The source of "shop air" generally comprises an air compressor and reservoir of the type normally found in a manufacturing or production environment. The air compressor is capable of pumping a large volume of air, typically 200 cfm at room temperature, through a feedline 11. A control valve (not shown) is interposed along the feedline 11 for regulating the pressure and flow rate of the air from the shop air reservoir. The use of existing shop air in the pressure range of 30 psi to 120 psi significantly reduces the capital and operating costs for the present system.

With continuing reference to FIG. 1, a converging-diverging CO.sub.2 nozzle 60 includes a body 62 defining therein a converging section 10 having an input section with a diameter of approximately 0.5 to 2 inches, with 1 inch being used in the preferred embodiment. The converging section 10 is approximately 5 inches long, and is tapered from a circular inlet to a rectangular throat section 20, as illustrated in FIGS. 4, 4A, 4B, 4C and 4D.

As illustrated in FIG. 2, the feed-line 21 carrying the liquid CO.sub.2 from the reservoir is coupled into the tapered, annular CO.sub.2 plenum or manifold 22 of decreasing cross-sectional area that provides the liquid CO.sub.2 under equal pressure into four CO.sub.2 channels 24a, b, c and d. Each of the four CO.sub.2 channels opens into a section of the converging section 10 of the nozzle 60 through a CO.sub.2 orifice 26 immediately adjacent to the throat 20. The orifices 26a, 26b, 26c and 26d are staggered generally around the circumference of the body 62. Whereas the inside dimensions of the throat 20 are approximately 0.2 inches by 1.0 inch, the CO.sub.2 orifices 26 are only approximately 0.030 inches in diameter. The CO.sub.2 channels 24 are angled at 25.degree. with respect to the longitudinal axis of the nozzle body 62 so that when the liquid CO.sub.2 is expelled through the orifices 26a, b, c and d, the resulting CO.sub.2 solid particles will have some forward velocity toward the diverging section 30. The exact angle at which the CO.sub.2 particles are expelled through the CO.sub.2 orifices 26 will vary by design, but is between approximately 10.degree. and 45.degree. with respect to the central flow axis.

If the shop air used to pressurize and move the CO.sub.2 particles through the nozzle 60 happens to contain any water vapor, then it will freeze on the inside surfaces of the converging section 10 and the throat section 20 of the body 62 defining the nozzle 60. This ice causes a disruption in the laminar flow through the nozzle. The progressive build-up of the ice can, if not corrected, eventually lead to a complete obstruction or closing of the nozzle. This problem can be corrected either by using a dryer to remove any water vapor from the shop air before it enters the shop air feedline 11, or by heating the surfaces of the body 62 where the ice tends to form.

For this purpose, the first preferred embodiment illustrated in FIG. 1 includes an electrical resistance heating surface 40 that is wrapped around the exterior circumference of the body 62. Since the flashing of the liquid CO.sub.2 into solid CO.sub.2 particles will remove heat from the brass body 62, the electrical resistance heating wrap 40 will serve to maintain the temperature of the body 62 above 32.degree. F. so that ice will not form inside the nozzle 60. A thermocouple (not shown) is attached to the body 62 for automatically controlling the temperature.

The method for generating the CO.sub.2 solid particles will now be explained with continuing reference to FIGS. 1, 2 and 3. The liquid CO.sub.2 is pumped from the reservoir through the feedline 21 under a pressure controlled by the control valve. The liquid CO.sub.2 is forced under a pressure of approximately 300 psi, a temperature of approximately 0.degree. F. and a flow rate of approximately 1.2 gallons per minute through the annular plenum 22, the channels 24 and out the orifices 26, and thereby "flashes" from the liquid state into a state that includes a solid form of CO.sub.2, which herein is referred generally as CO.sub.2 particles. The CO.sub.2 particles will be mixed with either liquid CO.sub.2 or CO.sub.2 in the gaseous form depending on the combination of temperature and pressure as illustrated in the enthalpy diagram of FIG. 6. In the preferred mode of operation, this combination of characteristics is illustrated as point 1 in the enthalpy diagram of FIG. 6. As the liquid CO.sub.2 exits the orifices 26, it will move to point 2A on the enthalpy diagram. It will be understood by one skilled in the art that point 2A may be transferred further into the area in which the exiting CO.sub.2 is in the solid and gaseous phase by increasing the pressure differential between the pressure of the liquid CO.sub.2 and the pressure of the gas within the converging section 10.

As illustrated in FIG. 6, the ratio of solid to gaseous CO.sub.2 produced by the nozzle 60 is approximately 50% to 50% for these preferred operating characteristics. As previously explained, these solid CO.sub.2 particles are quite small and have a significantly reduced mass when compared to other CO.sub.2 pellet and CO.sub.2 particles systems.

With reference to FIG. 3, the mixture of CO.sub.2 particles and gas from the orifices 26 near the throat section 20 are exhausted toward the diverging section 30. The diverging section 30 expands the stream adiabatically (and approximately isentropically) to the ambient pressure. Further conversion of any remaining liquid CO.sub.2 into CO.sub.2 particles may occur during this expansion.

As illustrated in FIG. 3, the converging section 10 includes a generally cylindrical section that ms sized for coupling with the shop air feedline 11. The cylindrical section is approximately 1.0 inches in inside diameter, and the converging section 10 tapers over a length of approximately 5 inches to the throat section 20 that has a generally rectangular cross section approximately 0.9 inches by 0.2 inches. This compound tapering shape causes a decrease in the pressure of the shop air flowing therethrough. The throat section 20 forms the narrowest section of the body 62 and opens into the diverging section 30 that defines the generally rectangular exhaust aperture 38 through which the solid CO.sub.2 particles and gases flow as they are directed toward the workpiece 90.

The converging section 10 is manufactured of brass and is designed to contain and channel the subsonic flow of the shop air flowing therethrough. The diverging section 30 is designed to accelerate the shop air to a supersonic flow through the exhaust aperture 38.

The contour or curvature of the inside surface of the subsonic converging section 10 is designed according to the matched-cubic design procedure described by Thomas Morel in "Design of 2-D Wind Tunnel Contractions", Journal of Fluids Engineering, 1977, vol. 99. According to this design, the shop air flows at subsonic speeds of approximately 10 to 1100 feet per second at ambient temperatures of from 50.degree. F. to +100.degree. F. as it converges at the throat section 20.

The diverging supersonic section 30 can operate in the overexpanded mode, which retains a normal shockwave 36 within the diverging section 30 in order to reduce the noise by about 120 to 105 db outside the nozzle 60.

The exact contour of the diverging section 30 is more particularly defined with reference to the table of dimensions as follows:

In the first preferred embodiment of the present invention, the shop air, carbon dioxide gas, and CO.sub.2 particle mixture exiting from the exhaust aperture 38 have a temperature of approximately -50.degree. F. to -200.degree. F. and a velocity of approximately as high as 2,000 feet per second. The output mixture is approximately 20-40% by mass of solid CO.sub.2 particles, which have a mean particle size of less than 0.001 inches. The diverging section 30 was designed for an inlet pressure of approximately 100 psi. The CO.sub.2 particles exit with a generally uniform distribution across the exhaust aperture 38. There is little or no coagulation and agglomeration within the throat section 20 and the diverging section 30.

The exhaust aperture 38 is designed to be approximately 0.5 to 4 inches from the workpiece 90. The angle of attack of the CO.sub.2 particles against the workpiece 90 can be varied from 0.degree. to 90.degree., with an angle of attack of approximately 30.degree. to 60.degree. being preferred for most cleaning operations.

The method of operation of the CO.sub.2 cleaning system will now be explained. Point 1 on FIG. 6 represents the state of the saturated liquid CO.sub.2 within the CO.sub.2 manifold 22 supplied at a pressure of 300 psi and a temperature of approximately 0.degree. F. Assuming a shop air pressure of approximately 100 psi and an ambient temperature of approximately 75.degree. F., Point 2A represents a CO.sub.2 partial pressure of approximately 30 psi and indicates the state of the CO.sub.2 after flashing through the orifices 26. The CO.sub.2 exiting the throat section 20 comprises CO.sub.2 in both the solid and gaseous phases having a temperature of approximately -120.degree. F. The relative levels of solid and gaseous CO.sub.2 produced can be controlled by adjusting the pressure of the shop air in the converging section 10, as well as by adjusting the ratio of CO.sub.2 and air used.

The high relative velocity between the shop air and CO.sub.2 streams causes the CO.sub.2 stream to atomize into small solid particles, thereby limiting the size of the solid--CO.sub.2 particles produced.

The CO.sub.2 particles produced by the first preferred embodiment of the present invention were directed at a solder printing stencil (8" by 14") of the type used in applying reflow solder paste for screen printing of printed circuit boards. The stencil, having a thickness of about 0.008 inches, had a coating of solder paste approximately 0.010 inches in thickness, and had been through numerous soldering application cycles in a manufacturing environment. At a shop air pressure of approximately 100 psi, the solder paste was completely cleaned from the 112 square inch surface in about 25 seconds. While commercially available CO.sub.2 pellet cleaning systems are capable of removing the accumulated solder paste, they will cause "peens" and "dimples" in the pressure sensitive surface, whereas in the preferred embodiments of the present invention no permanent deformation of the surface will result.

A second embodiment of the CO.sub.2 particle generating nozzle, illustrated generally as 160 in FIG. 5, used in conjunction with the shop air system described above. The second preferred embodiment of the converging/diverging nozzle 160 is the same in most respects as the first preferred embodiment 60, except that the CO.sub.2 plenum 122 is located more downstream so as to place the four CO.sub.2 channels 124 and four CO.sub.2 orifices 126 downstream of the throat section 120 defined within the body 162. The size of the CO.sub.2 orifices 126 has been retained at approximately 0.030 inches, but the angle to the flow axis is approximately 40.degree. but can range from 20.degree.-70.degree..

In this manner, the mean size of the CO.sub.2 particles generated by the liquid CO.sub.2 passing through the CO.sub.2 channels 124 and out the CO.sub.2 orifices 126 will be less than 0.001 inches, which is slightly smaller than the CO.sub.2 particles produced by the first preferred embodiment. These CO.sub.2 particles are slightly smaller because the liquid CO.sub.2 is injected adjacent to but downstream of the throat section 120 into a lower pressure but high velocity section of the diverging section 130 of the nozzle 160. This higher relative velocity of the CO.sub.2 particles results in better atomization of the CO.sub.2 stream, which in turn results in the smaller size of the CO.sub.2 particles. This downstream injection could create a secondary shockwave upstream of the injection plane, but does not result in any reduction in the airflow through the throat section 120 of the nozzle 160, as occurs in the first preferred embodiment of the nozzle 60.

In either preferred embodiment, the four staggered CO.sub.2 orifices 26 and 126 can be enlarged to one or more continuous slots (not illustrated) that would enable more CO.sub.2 particles to be generated. This use of slots instead of apertures would be more difficult to manufacture, and therefore this slot construction is not preferred.

In responding to the requirements of unusual cleaning applications, the diverging section 30 or 130 of the nozzle 60 or 160 may be shortened or even truncated to a point adjacent to the throat section 20 or 120 under circumstances where reduced CO.sub.2 particle speeds and a smaller CO.sub.2 particle footprint are acceptable.

While the term "shop air" has been used to described the carrier gas, gaseous Nitrogen (N.sub.2) or other equivalent gasses could also be used in place of the shop air if required by the special application.

While the present invention has been particularly described in terms of specific embodiments thereof, it will be understood that numerous variations of the invention are within the skill of the art and yet are within the teachings of the technology and the invention herein. Accordingly, the present invention is to be broadly construed and limited only by the scope and spirit of the following claims.