The present invention relates to a method for applying a coating to a part, such as an anti-friction coating to a piston skirt, using a stencil to apply a predetermined thickness of the coating to a pad for subsequent application to the part surface.
Current methods of coating the outer circumference of pistons include conventional pad printing, rotary pad printing, and direct screen printing. The purpose of the coating in the case of automotive skirt coatings is to eliminate scuffing in the skirt area of engine pistons while improving engine performance and reducing hydrocarbon emissions. Additionally, some pistons, such as air conditioner compressor pistons, use the anti-friction coating as a fluid seal. Bearings and shafts can also benefit from a coating as a result of the enhanced anti-friction and reduced wear characteristics imparted to a coated part.
Direct screen printing is widely used to coat pistons in the 5 micron to 40 micron range of thickness. The drawback in direct screen printing is that the sharp edges of the piston cut through the screen after approximately 5,000 to 10,000 impressions. Direct screen printing of the piston above approximately 45 microns results in an uneven texture produced by the necessarily wide diameter threads of the screen fabric.
Conventional pad printing and rotary pad printing, as disclosed in U.S. Pat. No. 5,266,142, are severely limited with respect to the thickness of coating that can be applied, typically less than 15 microns per pass, or requiring two passes for approximately 25 microns of thickness, due to the use of an etched clichxc3xa9. The etched clichxc3xa9 does not transfer film thicknesses over 15 microns due to the high surface area and cohesion of the coating to the interior surface of the recess in the clichxc3xa9. Additional passes of the pad increase coating time and slow down the coating process.
It is desirable in the present invention to provide a method and apparatus for applying a wide range of coating thicknesses from 5 microns to over 100 microns. It is desirable to provide a method and apparatus which can eliminate the problem associated with printing screens wearing out as a result of sharp edges of the part to be coated cutting through the screen. It is desirable in the present invention to provide precise control of the coating thickness being applied to the part. Increased speed of application, particularly of relatively thick coatings is also desirable. It is desirable in the present invention to reduce or eliminate the deformation associated with screens leading to inaccurate or unacceptable coating applications. It is desirable in the present invention to provide relatively thick coats without uneven or textured coating surfaces.
While the present invention will be described and illustrated with respect to a piston, it should be recognized that many parts can be advantageously coated with the apparatus and method according to the present invention including, but not limited to automotive and small engine pistons, two-cycle engine pistons, air compressor pistons, air conditioner compressor pistons, pump pistons, bearings, shafts, inside diameter of bearings, and parts having surfaces to be coated with geometry that is flat, conical, spherical sections, cylindrical sections, and both inside and outside diameters of rings. Generally, for illustration purposes of the present invention, a piston is described as having an aluminum casting with a machined outer diameter, however alternative metals may be used for the part to be coated without departing from the disclosure of the invention.
An apparatus according to the present invention applies a coating of material to a predefined area on a surface of a part. The apparatus comprises a movable stencil having a wall or surface with a predefined aperture corresponding to a pattern of the predefined area to be coated, a doctor blade or squeegee, generically referred to here as a wiper, fills the aperture through the stencil with the coating of material to be applied to the part, and a pad receives the coating of material passing through the aperture of the stencil for applying the coating of material to the part in response to relative movement between the part and the pad. A method according to the present invention is for applying a coating of material to a predefined area on a surface of a part and includes the steps of moving a stencil having a wall or surface with a predefined aperture corresponding to a pattern of the predefined area to be coated with respect to a pad, filling the aperture with the coating of material to be applied to the part with a doctor blade or squeegee, generically referred to here as a wiper, and receiving the coating of material passing through the aperture of the stencil on the pad and applying the coating of material to the part in response to relative movement between the part and the pad.