Abstract:
A workpiece is coated with carbonizable polymer for laser machining an aperture through the workpiece. The workpiece is protected by the polymer during laser machining so that the surface of the workpiece and areas surrounding the aperture are substantially free of debris produced by the laser machining process. The carbonized polymer is removed after laser machining. A method of forming an aperture by a laser on a thin metallic plate is also describe.

Description:
This application claims the benefits under 35 U.S.C. § 119 of provisional application S.N. 60/318,690 filed on Sep. 7, 2001, titled: “Anti-Spatter Coating for Laser Machining,” which provisional application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     A common way of machining orifice holes is punching. It is believed that smaller orifices can be formed with no loss in productivity through the use of laser machining. At least two techniques are believed to be useful for laser machining orifices. One is trepanning or helical drilling, the other is percussion drilling. Percussion drilling is believed to be less desirable due to the less controlled nature of metal heating and expulsion that increases the risk of a non-cylindrical or non-circular orifice. Trepanning, on the other hand, is believed to be more precise as a center hole is initially formed before the formation of the orifice. Helical drilling is similar to trepanning but without the initial formation of a center hole. However, it is believed that neither trepanning nor percussion drilling provides for a desired formation of entry and exit geometry in the orifices. 
     With laser machining, debris (recast material, melted material and soot from machining) can be formed both immediately at the perimeter of the orifice and on the surface some distance from it on both the entry and exit sides of the orifice. This leads to an orifice that is not uniform dimensionally with respect to, such as, the diameter, exit or entry geometry of the orifices. 
     SUMMARY OF THE INVENTION 
     The present invention provides for protection of a workpiece during laser machining of an aperture through the workpiece so that the surface of the workpiece and areas surrounding the aperture are substantially free of debris produced by the laser machining process. In particular, a preferred embodiment of the present invention provides for a workpiece. The workpiece includes first and second metallic surfaces of the workpiece being spaced at a first distance apart and a carbonizable polymeric coating of a first thickness being disposed on at least one of the first or second metallic surfaces of the workpiece. 
     In another preferred embodiment, a thin metallic plate is provided. The metallic plate comprises first and second surfaces of the metallic plate being spaced apart at a distance of approximately 50 to 300 microns. The metallic plate includes at least one orifice extending between the first and second surfaces. The at least one orifice is formed, in part, by forming a carbonizable polyimide coating on one of the first and second surfaces; drilling the orifices; and removing the coating. 
     The present invention also provides a method of forming an aperture on a thin metallic workpiece by a laser. The method can be achieved, in part, by forming a carbonizable polymer over at least one surface of the workpiece; machining at least one aperture between the at least one surface and another surface of the workpiece via a laser; and removing said polymer from the workpiece. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain features of the invention. 
     FIG. 1 illustrates an exemplary untreated workpiece. 
     FIG. 2 illustrates the exemplary workpiece with a substantially liquid coating deposited on the workpiece. 
     FIG. 3 illustrates the exemplary workpiece with a substantially solid coating formed on the workpiece. 
     FIG. 4 illustrates a cross-sectional view of the workpiece with the solid coating. 
     FIG. 5 illustrates a laser machining operation being performed on the workpiece of FIG.  3 . 
     FIG. 6 illustrates another material treatment on the exemplary workpiece. 
     FIG. 7 illustrates workpiece after removal of the coating of the workpiece. 
     FIG. 8 illustrates a workpiece with the polymeric coating before removal of the coating and subsequent to a laser machining operation of the orifice on the workpiece. 
     FIG. 9 illustrates an uncoated workpiece with an orifice formed by laser machining. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-8 illustrate preferred embodiments of the workpiece  10  and methods relating to machining of the workpiece  10 . Initially, an untreated workpiece  10  is placed in an environment so that it can be cleaned and preferably exposed to an ambient environment. The workpiece is cleaned with chemical solvent or degreaser and coated with a substantially liquid pre-polymer precursor  12 , shown here in FIG.  1 . The workpiece  10  can be of any suitable material and dimension or shapes for laser machining, including that of a thin metallic plate. Preferably, the workpiece  10  is stainless steel and generally planar in shape with a first surface  10   a  generally parallel to a second surface  10   b  at a distance (i.e. a thickness “d”) of approximately 50 to 600 microns, and more particularly of about 300 microns, see FIG.  4 . 
     The pre-polymer precursor  12  can include a suitable thermoset polymer precursor such as, for example, epoxy resins, polycarbonates, polyacrylamide or polyimide. The pre-polymer precursor  12  can be applied by transfer stamping, pouring, spraying, spreading, or being brushed on the surface of the workpiece  10 . Preferably, the precursor  12  is applied by a transfer-stamping member (not shown). The transfer-stamping member is dipped in polyamic acid and is pressed or stamped on the workpiece so that a coat of polyamic acid is distributed over the workpiece on at least one of the first and second surfaces  10   a  or  10   b . In a preferred embodiment, the polymer precursor  12  is dried in an ambient atmosphere. In another preferred embodiment, the precursor coated workpiece  10  is allowed to dry in an enclosure (not shown for clarity) containing an inert gas mixture at ambient room temperature. Regardless of the drying techniques, once the precursor coating is dried, the workpiece can be placed in an enclosure, such as, for example, a thermostatically controlled oven. The temperature of the enclosure is preferably raised to a first temperature by heating the enclosure from ambient temperature in a linear manner. Alternatively, the temperature can also be raised from ambient temperature to the first temperature in a step-wise fashion over time. As the temperature rises in the enclosure, the polymer precursor  12  proceeds through a thermosetting process. To prevent surface stress once the polymer precursor  12  has been generally converted to a thermoset polymer that forms a polymeric coating  14  over the workpiece  10  in FIG. 3, the temperature of the enclosure can be reduced in a stepwise fashion to ambient room temperature. 
     The polymer precursor is preferably a polyimide precursor  12  in liquid form. The polyimide precursor  12  can be converted to either aromatic or linear polyimide by chemical or thermal conversion of the precursor  12  to a rigid polyimide coating containing imides and carbonyl groups. Preferably, the polyimide precursor  12  is a polyamic acid coating, which may contain dissolved acids and solvents but is exclusive of surfactants. Also preferably, the polyamic acid coating  12  has a thickness of between approximately 50-500 microns prior to being polymerized at a temperature of approximately 150 degrees Celsius over a duration sufficient to polymerize the polyimide at a first level that is generally free of carbonized polymers. Preferably, the duration is between approximately 10 minutes to 30 minutes in a substantially Argon gas mixture. In preferred embodiments, the polymerized coating  16  has a thickness “t” between approximately 50-500 microns. 
     The polymer coated workpiece  10  of FIG. 2 can now be machined with a laser  18  shown schematically in FIG. 5, such as, for example, a CO 2 , copper vapor, an excimer laser or a Nd:YAG laser. Alternatively, further thermal treatments can be given to workpiece  10  instead of being laser machined at this point. Preferably, the polymer coating  14  is heated in the same enclosure to a second temperature higher than the first temperature at second duration sufficient to transform a portion of the polymer  14  to a carbonized form of polyimide polymer  16  (i.e. a polymeric substance with internal structures that are very similar to graphite) in FIG.  3 . The polymer  16  is transformed to a second level having a generally carbonized layer sufficient to resist high temperature generated during a long duration (i.e., one millisecond or greater) laser machining of the orifices. Another benefit from the curing at the second temperature is that the carbonized polymer layer and the polymer can be removed as a single layer after laser machining. Preferably, the second temperature is approximately 400 degrees Celsius at duration sufficient for the polymer to form a generally carbonized polymer layer. 
     As shown in FIG. 5, the laser  18  machines the workpiece  10  by material transformation, welding or material removal, especially drillig through the workpiece  10 . Depending on the power level of the laser, the orifice  20  can be formed by a single high powered pulse or multiple lower powered pulses over a single or multiple durations sufficient to machine through the workpiece. The duration of laser machining can be as short as a fraction of a millisecond, thirty seconds or in multiple durations of time. Preferably, the laser machine is configured to drill in the workpiece  10  circular orifices  20  extending through the workpiece  10  of approximately 20 microns to 300 microns in diameter, and particularly one or more orifices of about 160 microns in diameter over a duration of one of a fraction of a millisecond, at least one millisecond, about thirty seconds or in multiple pulses of approximately ten seconds per pulse. 
     Subsequent to the laser machining, the polymer on the workpiece  10  can be removed mechanically or chemically. In mechanical removal, physical movement of the workpiece, such as, for example, ultrasonic vibration, flexing or bending of the workpiece can remove the polymer or carbonized polymer coating in FIG.  7 . Additionally, mechanical removal can involve physical contact with another structure such as, for example, a blade, a hammer or a vibratory tool. Chemical removal can be performed by washing or soaking the workpiece in a solvent such as, for example, water or acetone. Chemical removal can also be combined with mechanical removal such as, for example, ultrasonic oscillation while immersed in water. Preferably, the coating at the second level is ultrasonically removed in a bath of water. 
     Instead of moving directly to removal from FIG. 5 to FIG. 7 (shown as dashed line  22 ), the polymerized coating can be heated (shown as solid line  24 ) to a third temperature in the enclosure over a third duration sufficient to transform the polymer in FIG. 5 to a third level containing substantially or nearly all carbonized polymer in FIGS. 6 and 8. The carbonized polymer  16  at the third level tends to be very brittle throughout its thickness, thereby facilitating mechanical removal. As used herein, the term “generally” carbonized denotes a range of between 10% to 50% of the polymer coating  14  that are carbonized, and the term “substantially” carbonized denotes a range covering between 50% to 100% of the polymer  14  that are carbonized. 
     An example of a workpiece without a coating prior to and subsequent to laser drilling is shown in FIG.  9 . In particular, soot and molten debris can be seen around the orifice after a laser machines the workpiece  10 . The unwanted debris can affect the quality of the orifice during and after the laser machining. In contrast, the exemplary intermediate product in FIG. 8 (which underwent carbonization, drilling and further carbonization but prior to removal of the polymer, as shown in FIGS. 1-6) is a workpiece with little or no debris, soot or recasts laser machining over a relatively long laser machining operation on the workpiece and around the orifice. 
     While the present invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following and equivalents thereof.