Patent Publication Number: US-6659844-B2

Title: Pliant coating stripping

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to repair processes, and, more specifically, to surface stripping. 
     The manufacture of typical products occurs in various steps from raw material to finished article. Various coatings may be applied to the external surface of the finished product for various reasons. For example, the product may be coated with paint for durability and aesthetic reasons. Or, such coatings may be pliant in the form of various synthetic rubber. 
     Such coatings may be found in consumer and industrial products, manufacturing equipment or machinery, and commercial or military aircraft and aircraft engines for various purposes. In many of these typical applications, it is desirable to remove the original coating after extended time and service and reapply a new coating for further extending service. 
     Pliant coatings are particularly difficult to remove in view of the flexibility and resilience thereof. Abrasive grit blasting is inefficient since the small particles of airborne girt dissipate their kinetic energy as the pliant coating resiliently deforms under impact. And, hot knife removal of the pliant coating is labor intensive. 
     Accordingly, it is desired to provide a new method of stripping pliant coatings with increased efficacy. 
     BRIEF SUMMARY OF THE INVENTION 
     Abrasive shot is injected into a carrier stream of compressed gas. The shot and gas stream are directed against a pliant coating. The compressed gas expands at the coating for cooling thereof which decreases coating resiliency for enhancing stripping thereof by the impinging abrasive shot. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, in accordance with preferred and exemplary embodiments, together with further objects and advantages thereof, is more particularly described in the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic representation of an apparatus and associated method for stripping a pliant coating from a workpiece in accordance with an exemplary embodiment of the present invention. 
     FIG. 2 is a partly sectional elevational view of the nozzle illustrated in FIG. 1 for discharging abrasive shot in a compressed gas against the pliant coating for stripping thereof in an exemplary embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in FIG. 1 is an apparatus  10  for stripping a pliant coating  12  from the external surface of a workpiece  14  in accordance with an exemplary embodiment of the present invention. The workpiece may have any suitable form such as a part in commercial or industrial products, manufacturing equipment and machinery, or in military or commercial aircraft or gas turbine engines thereof. 
     The pliant coating  12  may have any suitable form as desired for these products, and is typically in the form of a thin coating which is flexible and pliable for various aesthetic or functional reasons. 
     For example, the pliant coating may be a synthetic rubber, like silicone, having a smooth and continuous outer surface when applied. And the coating is resilient under contact and readily returns to its original shape. However, after extended use in service, the pliant coating may degrade due to wear or other reasons, and the removal and replacement by a new coating is desired. 
     Accordingly, the apparatus  10  illustrated in FIG. 1 is specifically configured for efficiently stripping the pliant coating  12  from the workpiece notwithstanding the inherent flexibility of the coating being removed. Means are provided for supplying abrasive shot  16  in a compressed gas  18  through a common discharge nozzle  20  tar blasting against the pliant coating  12  for stripping or removal thereof from the underlying workpiece  14 . 
     The shot is initially stored in a suitable hopper  22  having a discharge conduit or hose  24  joined to the nozzle  20 . A suitable gas supply  26 , such as a bottle or canister of compressed gas, is joined by another conduit or hose  28  to the common nozzle  20 . 
     The shot may be gravity or force fed through the supply hose  24  to the nozzle, with the compressed gas  18  being suitably regulated in flowrate to the nozzle. In this way, a stream of the abrasive shot may be injected into a carrier stream of the compressed gas inside the nozzle  20  for discharge therefrom against the pliant coating. 
     The nozzle  20  may then be manually or automatically carried in a translating carriage over the workpiece for directing or impinging the shot and gas stream against the coating. The initially compressed gas being discharged from the nozzle expands to ambient pressure at the surface of the coating for significantly cooling that coating to decrease its resilience and permit enhanced stripping thereof by the abrasive shot carried in the gas stream. 
     FIG. 2 illustrates in more particularity the nozzle  20  suitably positioned over the workpiece, with the abrasive shot  16  being shown magnified in part for clarity of presentation. The shot preferably comprises a multitude of individual pellets each having a plurality of abrasive particles  16   a  imbedded therein. The shot pellets are preferably dense plastic of any suitable composition, with the abrasive particles having any suitable material composition, such as various minerals, which are imbedded in the pellets for exposure around the surrounding surface thereof. 
     The carrier gas  18  has two fundamental purposes for carrying the abrasive shot  16  in impingement against the pliant coating while simultaneously cooling the pliant coating as the gas expands during impingement thereof. Expansion of compressed gas removes heat from the pliant coating and reduces its temperature significantly for correspondingly reducing the resilience and flexibility thereof. In this way, as the abrasive shot impinges the cooled coating, the coating is less susceptible to elastic deformation and the kinetic energy of the shot is more effective for abrading and stripping the stiffened or hardened coating from the workpiece surface. 
     The carrier gas introduces kinetic energy into the abrasive shot as it is accelerated through the nozzle to a high velocity for impinging the coating. Kinetic energy of the abrasive particles is substantially increased by the larger plastic pellets in which they are imbedded. Since the coating is cooled by the expanding gas its flexibility is substantially reduced, and the stream of relatively large pellets carrying particles of abrasive have enhanced kinetic energy for abrading and stripping the pliant coating. This combination of features is referred to as Kinetic Energy Enhancement for Pliant Coating Stripping (KEEPCS). 
     In the preferred embodiment, the compressed gas  18  is carbon dioxide for its substantial cooling ability under expansion to ambient pressure, and since it is relatively inexpensive. Other suitable gases, such as compressed nitrogen, could also be used for their ability to effectively cool the pliant coating. However, compressed nitrogen is more expensive to use than compressed carbon dioxide. 
     In the preferred method, the compressed gas expands as it impinges against the pliant coating  12  to substantially harden that coating by cooling thereof for stripping the coating from the workpiece surface by the impinging abrasive shot carried by the gas. Although air may be compressed and used as the carrier gas, compressed air has little efficacy in reducing the temperature of the pliant coating for decreasing its resiliency. 
     Accordingly, compressed air is not preferred in practicing the stripping process, with carbon dioxide being preferred instead for its substantial cooling capability upon expansion from its initial storage pressure. And, suitable canisters of compressed carbon dioxide are commercially available and are readily joined to the nozzle in a simple configuration for use in stripping the pliant coating. 
     As shown in FIG. 2 the nozzle  20  is located sufficiently close to the surface of the workpiece so that the abrasive shot and gas stream discharged therefrom can impinge obliquely against the coating for stripping thereof. Substantially normal or perpendicular impingement of the abrasive shot maximizes the transfer of kinetic energy from the shot into the temporarily hardened pliant coating for abrasion and stripping thereof from the underlying workpiece surface. However, the nozzle may be inclined at acute angles of incidence where desired for also stripping the pliant coating with less efficacy. 
     In the preferred embodiment illustrated in FIG. 2, the compressed gas  18  is discharged through an eductor nozzle in which the abrasive shot  16  may be entrained by vacuum formed in the nozzle. The eductor nozzle  20  is a tubular member having a center venturi  30  therein which converges in flow area from a circular gas inlet  32  at the proximal end of the nozzle to a throat  34  of minimum flow area in the middle region of the nozzle and diverges in flow area to a circular outlet  36  at the opposite, distal end of the nozzle. 
     A side inlet  38  is joined to the shot hose  24  for receiving the stream of abrasive shot. The shot inlet  38  is preferably located downstream of the throat  34  in the diverging portion of the venturi so that as the compressed gas is channeled through the nozzle and expands through the venturi, vacuum is created at the shot inlet  38  for entraining the abrasive shot therein. 
     In this way, the abrasive shot is drawn into the nozzle by the compressed carrier gas being discharged therethrough, with the shot and gas stream then being directed in impingement against the pliant coating  12  which is cooled by the expanding gas and abraded by the impinging abrasive shot carried therein. 
     The pliant coating  12  may vary in thickness from relatively thin to relatively thick, yet is readily stripped by the abrasive action of the high kinetic energy plastic pellets having the abrasive particles carried therein. As the pliant coating is cooled and abrasively stripped from the workpiece surface, the nozzle may be moved laterally across the workpiece for correspondingly stripping the coating from the entire intended region thereof. 
     Stripping of the pliant coating may therefore be effected with a substantial increase in efficiency over hot knife removal of the coating. And, the cooling capability of the compressed carbon dioxide carrier gas in conjunction with the plastic carrier pellets and abrasive imbedded therein substantially increases the kinetic energy of the abrasive particles and efficacy thereof in stripping the coating as compared with conventional abrasive grit blasting in which small particles of abrasive are carried in a stream of air. 
     The plastic pellets may be varied in size as desired for carrying a suitable number of abrasive particles in each pellet to increase the collective kinetic energy thereof. And, the self-cooling capability of the compressed carrier gas simultaneously pretreats the pliant coating for reducing its resilience and correspondingly increasing the abrasion thereof by the abrasive pellets. This self-cooling, multi-particle shot blasting process may be used to remove pliant coatings in various parts and products where economically feasible. 
     While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.