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From: Dow Corning Corporation

MOLYKOTE(R) D-10 ANTI-FRICTION COATING

 PISTON COATING PROCESS RECOMMENDATIONS  

Version 1.9, Effective: 1/22/98

Cleaning Recommendations:

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:     

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1) Hot alkaline wash (120-140 F)

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2) Deionized water rinse

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3) Hot air blow-off (140 F)

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. 

 Coating Methods:

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:

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1) For optimum coating application, the coating equipment should be maintained in a temperature and humidity controlled environment. Recommended air temperature is 20 C +/- 4 C (68 F +/- 7 F) with a relative humidity below 50%. Rapid drying of the coating on the screen can be slowed by diluting the D-10 with a small amount (e.g., 1-ounce per quart) of the n-Methylpyrollidone solvent.

bullet2) To improve coating flow-out, it may be necessary to add a SMALL amount of DOW CORNING(R) 29 Additive.  A suggested use level for this material  is 5-6 drops per  5-kg. container of D-10.  The 29 additive can be ordered from Dow Corning by    calling 1-800-248-2481.
bullet3) Pick-and-place parts handling equipment must be designed such that the pistons do not become nicked, scratched or contaminated. 
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4) Printing screens must have the proper mesh for the desired coating thickness. Some suggested mesh sizes for typical coating thicknesses are 85 (18-20 microns) and 120 mesh (10-12 microns)  

bullet5) Printing screens should be coated with a protective emulsion coating that is compatible with the n-Methylpyrrolidone solvent used in the D-10 product. One material that has shown good solvent compatibility is SP-9701 diazo-type emulsion. This material is available from:

Murakami Screen Company, Ltd.
 953 South Meridian Avenue
Alhambra, CA 91803
Phone: 818-284-4596 

 A second source of completed screens that use a proprietary emulsion coating process is:    

Sawyer and Smith, Corporation
2615 St. Clair Avenue
Cleveland, OH  44114-4014
Phone: 216-861-7300
FAX: 216-687-8094

               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.

bullet 6) Operating life of the printing screen can be maximized by keeping the printed area on the piston away from any sharp edges and by using the minimum squeegee pressure necessary to obtain a uniform coating thickness.

               NOTE: Estimated screen life is approximately 10K printings.  On average, the squeegee should be replaced with every 3rd screen change.

a) 5-10 minute delay    insert piece of paper between piston and screen and print on  "Manual" setting until image  is uniform and fully-filled

b) 15-30 minute delay   remove screen and thoroughly clean with solvent  (n-Methylpyrrolidone)

 7) When screen printing operations are interrupted, the screen should be cleaned based on the following guidelines:

Film Thickness:

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
750 Marshall Phelps Road
Windsor, CT  06095
Phone: (203) 683-0781

Equipment:                                                         

 Fischer Isoscope MP3 (eddy-current type)  , Fischer Permascope ES (magnetic type) 

 

Curing Operation:                                                         

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:

150 C (302 F) for 60-120 minutes,  180 C (356 F) for 30 minutes,   (or 20 minutes @ 180 C part temperature) 

NOTE: To prevent damage to the coating, part temperatures should not exceed 200 C for more than 30 minutes

Quality Control Methods:

After  curing, the  coating on a representative sample of production parts should be checked for:

1) complete cure 2) adhesion  3) coating thickness/uniformity

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.  

NOTE: 
1) The boiling-water test for coating adhesion is a severe test and may  not produce repeatable results if used as an on-line Quality Assurance measure. Minor changes in the piston cleaning process and/or increased metal porosity may result in failures when tested by this procedure. 

An alternative test for coating adhesion that is not as severe as immersion in boiling water uses a heating bath filled with standard engine oil.  An oil temperature of 325 F with immersion times as long as 96 hours have been evaluated with no noticeable loss of coating adhesion when the material was properly applied.                                        

2) The pressure-sensitive tape used in the Cross-Hatch Adhesion test is Permacel #P-99, with a minimum adhesive strength of 40 ounces per inch (as measured by ASTM D-1000, Method A).                                                                                             

3) The above referenced ASTM procedures for Solvent Rub and Cross-Hatch Adhesion are attached to this document. 

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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