The present invention relates generally to abrasive blast coating removal methods, and more particularly to nozzles for abrasive blast removal of coatings from delicate substrates without damaging the substrate. 
Paint coatings, photoresist coatings and other protective and decorative coatings frequently must be removed from their underlying substrates.  A common method for removing these coatings is abrasive blasting.  Abrasive blasting shoots a stream of abrasive particles, or media, through a nozzle at the coating to be removed.  The abrasive particles may be carried by a stream of a compressible vapor or gas, generally air, or a stream of an incompressible liquid, generally water.  The most commonly used blasting media in the past was sand.  Unfortunately, sand blasting, while very effective at removing paints and other coatings, often imparts unacceptable levels of damage to thin or delicate substrate materials.  Printed circuit boards are an example of such a substrate material.  So too are newer aircraft outer skin materials such as fiber reinforced plastic composites and very thin aluminum. 
To deal with these problems, the prior art has tried softer media, such as plastic beads, sodium bicarbonate, ice crystals, wheat starch, walnut shells, glass beads and the like.  Even with such softer media, however, there has been a concern in the aerospace industry about the residual effects of abrasive blasting on aerospace structures.  For example, thin aluminum structures of 0.080 inch and less are susceptible to fatigue life reduction from the abrasive blasting process.  To reduce this problem, acceptable blasting parameters for plastic media blasting in the aerospace industry using conventional compressible gas type round-conical blasting nozzles have been limited to air pressures of 40 psig, and less, at a 12 to 24 inch standoff distance.  These same lowered abrasive blasting parameters have also been used to remove paint, called depainting, from composite substrates without imparting significant, or at least unacceptable, substrate damage. 
As mentioned, round-conical blast nozzles are conventionally used with plastic media entrained in a compressible gas stream to remove coatings from delicate substrates.  A primary problem with such round blast nozzles is their nonuniform velocity distribution.  The center of the blast stream constitutes a "hotspot" of higher velocity particles, surrounded by a greater area of slower, and less effective, media.  Further, the outside area of slower media is ineffective, overblasts the substrate, and can add an undesirable peening effect to delicate substrates which results in unnecessary residual stresses. 
Another problem with conventional round-conical blast nozzles is the small size of the blast footprint projected onto the coated substrate.  Not only is it slow to remove surface coatings by sweeping a 0.5 to 1.5 inch round spot, it results in a mottled surface appearance, especially on composites.  Also, much of the residual primer left over after outer paint removal is nonuniform and unsightly, requiring further treatment to level the finish or resulting in local gouging or fiber damage to composite matrix material. 
A further problem with conventional round-conical blast nozzles is that the maximum stripping rate that can be maintained by an operator is very limited.  The physical traverse rate that can be finely controlled and is repeatable is only a few inches per second.  Also, the uneven distribution of blast media makes overlapping necessary, further reducing efficiency, and does not provide a sharp edge for careful work.  Only a large stripping path, with more even distribution of both blast energy and media, will increase production rates. 
The prior art has also tried fan nozzles to achieve at least a wider swath, and hopefully a more even distribution of media particles and a flatter velocity profile at the nozzle exit.  These prior art fan nozzles, while an improvement, unfortunately have not been uniformly successful.  They generally still suffer from a nonuniform distribution of abrasive media particles and an insufficient particle velocity to preferentially blast the coating desired to be removed instead of the underlying substrate. 
The prior art has also tried supersonic fan nozzles where the throat of the nozzle opens to a largely unwalled open divergence volume, apparently because of concerns with wear.  These nozzles have also met with limited success and are not currently used in production.  The unconstrained divergence area may contribute to a lack of uniform particle distribution and velocities at the somewhat undefined nozzle exit. 
The prior art has also used supersonic fan nozzles with cryogenic particles, such as frozen carbon dioxide.  The dimensions of these fan nozzles, sized for use with very high pressure ratios and with pressurized air supplies capable of delivering 350 CFM, unfortunately also suffer from a nonuniform distribution of abrasive media particles.  Moreover, such nozzles using CO.sub.2 pellets as the abrasive media have, in paint removal from aircraft tests, too severely damaged underlying aircraft skins made of composite material or thin aluminum to be usable for such tasks. 
Thus it is seen that there is a need for a nozzle for abrasive blast removal of coatings, particularly for use with softer media, that will better remove coatings from delicate metal and composite substrates faster, with less substrate damage, and with more precise edge control. 
It is, therefore, a principal object of the present invention to provide an improved nozzle for abrasive blast removal of coatings that provides very high abrasive media particle velocities with a very flat particle velocity profile. 
It is another principal object of the present invention to provide an improved nozzle for abrasive blast removal of coatings that provides a very uniform distribution of abrasive media particles. 
It is an object of the present invention to provide an improved nozzle for abrasive blast removal of coatings that accomplishes the principal objects of the invention with conventional shop pressurized air supplies such as are used with prior art 1/2 inch nozzles. 
It is a feature of the present invention that it can also work with much higher gas pressures than prior art nozzles designed for conventional shop pressurized air supplies, thus producing even higher, and more efficient, blast energies while preserving the objects of uniform particle distribution and velocity profiles. 
It is another feature of the present invention that it can remove tough urethane topcoats from fiberglass aircraft components without removing all of the underlying gel coat primer. 
It is an advantage of the present invention that it causes less residual stress damage to substrates than do prior art nozzles. 
It is another advantage of the present invention that its efficiency and removal rate is far greater than prior art nozzles. 
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.