From: Dow Corning Corporation
MOLYKOTE(R) D-10 ANTI-FRICTION COATING
PISTON COATING PROCESS RECOMMENDATIONS
Version 1.9, Effective: 1/22/98
Before coating, the pistons should be thoroughly cleaned to remove any soap or phosphate residue, water spots, machining oil, dirt, etc. picked up during the manufacturing process. For optimum results a mechanical type cleaner (e.g., spray) is recommended. Ultrasonic baths, vapor degreasers, etc. may not provide sufficient cleaning action to effectively remove contaminants from piston surfaces. A typical cleaning process would consist of:
After cleaning, surfaces to be coated should not be touched by hand prior to the coating operation, since small amounts of oil from the skin can affect adhesion.
NOTE: If pistons are to be tin-plated, an evaluation of the tin plating process with test pistons is recommended. Tin plating before applying the D-10 coating has been shown to reduce the adhesion properties of the cured film. And, in a few instances, tin plating after the D-10 coating has been applied has caused bubbling of the coating. Generally, it is recommended that pistons not be tin plated, but if it is required, it should be done AFTER coating the piston with the D-10 material.
*** Crystalline zinc phosphate precoats are often used to inhibit corrosion and increase the adhesion properties of anti-friction coatings. On aluminum pistons, phosphating may provide some slight improvement to coating adhesion if the piston skirt has a smooth finish, but in most instances this additional processing step is not needed to obtain good adhesion of the D-10 coating.
Recommended methods for applying MOLYKOTE D-10 AF Coating to pistons are either screen print or spray techniques. For screen printing, the coating can be used directly from the container, but it must be stirred well prior to using. Because of the high viscosity, the solids do not settle out quickly, but the coating should be stirred a minimum of once per 8-hour shift. New containers of material should be mixed (e.g., drum rolled, propeller style agitator, etc.) a minimum of 12 hours before being used in production. For spray applications, the coating should be diluted 20-30% (by volume) with n-Methylpyrrolidone. Spray cups or tanks should be equipped with agitators to keep the solids suspended after the material has been diluted.
Commercially available spray equipment is generally sufficient for applying this coating. Recommended spray nozzle diameter is 0.8 mm with a working pressure of 40-70 psig. Distance between the part and the spray head should be set so that the coating reaches the part in the wet state. To great a distance will allow the coating to dry before it reaches the part, resulting in poor uniformity and adhesion. For optimum uniformity, film thickness per pass should not exceed 5 microns. Greater film thickness can be achieved by using multiple passes or additional spray heads.
Several manufacturers of screen printing equipment have also adapted their equipment for applying the D-10 coating to pistons. When designing a production line to use a screen printer, there are several important criteria that must be considered. These are:
Murakami Screen Company, Ltd.
A second source of completed screens that use a proprietary emulsion coating process is:
Sawyer and Smith, Corporation
NOTE: Sawyer and Smith also has the capability of supplying a full, turnkey piston coating production line that includes washing equipment, screen printing machine(s), curing oven, and associated parts handling equipment capable of achieving output rates as high as 1500 pistons/hour.
NOTE: Estimated screen life is approximately 10K printings. On average, the squeegee should be replaced with every 3rd screen change.
In order to achieve the optimum degree of effectiveness and adhesion, the MOLYKOTE D-10 AF Coating should be applied with a minimum thickness of 5 microns and a maximum of 25 microns. Generally, a coating thickness in the range of 10-12 microns is needed to reduce cold scuff while 18-20 microns is effective in reducing both scuff and noise (i.e., piston slap). The thickness of the cured film can be determined by electronic gauging of the coated piston, or by measuring the thickness of the coating directly. Instruments available for this latter method are the eddy-current type (for non-ferrous parts) or magnetic field strength meters (for ferrous parts). A supplier of both types of these instruments is:
Fischer Technology Inc
Fischer Isoscope MP3 (eddy-current type) , Fischer Permascope ES (magnetic type)
After the coating has been applied, the pistons should be placed in (or conveyed through) a circulating air oven in such a manner that the specified cure time and temperature is maintained. Some recommended cure conditions are shown below. These times and temperatures may have to be adjusted depending upon piston size and materials of construction.
When screen printed, the wet volume of the coating is approximately 5X the cured volume, with the major portion of material being removed being the n-Methylpyrollidone solvent.
Suggested cure times:
Quality Control Methods:
After curing, the coating on a representative sample of production parts should be checked for:
Sufficient cure of the coating can be determined using the Solvent Rub Test (ASTM D-4752), with the exception that n-Methylpyrollidone is substituted for the Methyl Ethyl Ketone (MEK) specified by the procedure.
Proper adhesion of the coating can be checked using the Cross-Hatch Adhesion Test (ASTM D-3359). The test procedure requires the use of a special pressure-sensitive tape (outlined in the Note below) with a known minimum adhesive strength. A more severe test of coating adhesion often used to identify improperly cleaned pistons or pistons with excessive metal porosity is the boiling-water test.
This test requires the pistons to be placed in a 100C water bath for a minimum of 1/2 hour. After removal from the bath, the pistons are cooled to ambient temperature and then examined for any visual signs of flaking or blistering of the coating. Successful completion of the Cross-Hatch Adhesion test at this point will ensure optimum coating adhesion (See Note #1).
Coating thickness/uniformity can be checked using one of the measuring devices listed above and taking several readings across the coated area.
Removal of MOLYKOTE(R) D-10 from Coated Pistons:
Prior to oven curing, the wet or flash dried film can be easily removed using the n-Methylpyrrolidone solvent. After oven curing, the bonded film can only be removed by abrasive blasting (e.g., sand, glass bead, etc.)
NOTE: Dow Corning believes that the information and recommendations in this document are accurate as of the effective date listed. Contact Dow Corning for any information published after that date. It is the purchaser's responsibility to provide a final system design that is in accordance with all applicable engineering codes and standards. Dow Corning expressly disclaims any liability for incidental or consequential damages. Nothing contained herein should be taken as an inducement to infringe any patents.
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