North American Die Casting Association – 114 th Metalcasting Congress CastExpo’10, March 20-23,...

AFS NADCANorth American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA

Diamond Polishing to Extend Cavity Life

Gary Lockwitz

Harley-Davidson Motor Co


A great deal of research has gone into the die steel used for a cavity:

• The chemical composition and grain size of the steel for a cavity

• The effect of the specific heat treat process on the life of the cavity

• The effect of compressive stresses induced into the cavity by outside forces

• The surface coatings applied to the cavity

Very little research has gone into the surface preparations of the cavity itself.


Background

•The cover cavities for the part in question run more than 400,000 pieces per year.

•The guaranteed cover cavity life was 40,000 pieces (also, actual cavity life was very close to 40,000 pieces).

•There has been scope creep on what is cosmetically acceptable causing tools to be retired earlier then they have been in the past.

•We have worked with the part design, casting process, cavity steel, Metallife, and coatings in an effort to improve the tool life and have gained the improvements we could from those areas. We were looking for other options to extend cavity life. The next progressive step we reviewed was the polishing – or more correctly, the effect of the scratches left by the polishing process.


A cavity (cavity 94) for a cosmetic part was run and observed with 20X magnification regularly during its life. At this magnification die fatigue could be seen as early as 1425 shots. These heat checks seemed to be directly related to the polishing scratches left in the die during the tool build.

The heat check was monitored through the life of this cavity.


Shown are the polish scratches on cavity 94 and the propagation of the die fatigue.

Above picture shows stress cracks at shot 1425 shots on Cover Cavity 94

Above picture shows polish scratches on sample at 263shots on Cover Cavity 94


The cavity 94 die fatigue becomes more obvious as die life continues.

Rocker Cover C94 - Shot 2597 Rocker Cover C94 - Shot 6498


A new process (diamond polishing the fillet radii) was implemented on cavities based on the cavity 94 information, and the tests displayed the positive results. The supplier was confident enough with the results that they have increased the guaranteed tool life by 25%.

Below is a comparison of 2 cavities: 1 with diamond polishing in the fillet radii and 1 with a 300 stone finish.

Cavity 102 (Diamond Polished)5157 Shots

Cavity 98 (300 Stone Finish)5770 Shots


The results on the same 2 cavities as the previous slide at approximately 17,500 shots:

Cavity 102 (Diamond Polished)18241 Shots

Cavity 98 (300 Stone Finish)16714 Shots


Two sample cross sections are shown here at 500x magnification 1 is the normal stone finish and the other is diamond polished. Notice the propagation points for fatigue on the stoned sample that are missing on the diamond polished sample.

Diamond Polished 500x Stone Finish 500x


Armed with this information all cover cavities for this part starting with cavity 99 were diamond polished in the areas highlighted in red.

The shot-life on cavity 99 at retirement was 98062 shots.


Some Comparisons

Cavity 94 was retired at 40,771 shots. This tool was not diamond polished.


Starting with cavity 99 all tools have been using diamond polishing. This cavity had a life of 98,082 shots. While there wasn’t a picture at 98,082 shots the picture below was the review at 46,222 shots. At this point there was very little die fatigue.


Cavity 103 shown at 65,342 shots was retired at 67,739 shots. There was a little die fatigue and a small amount of cavitation damage.


Cavity 104 pictured at 61,605 shots was retired at 61,869 shots. There was a little die fatigue and a small amount of cavitation damage.


Cavity 111 pictured at 59,825 shots was retired at 61,742 shots. There was the start of die fatigue and a very small amount of cavitation damage.


Cavity 110 was pictured at 70,022 shots and is currently at 70,831 shots. Since this cavity has very little die fatigue and a little cavitation it is still in production.


The average shot-life on the 10 cavities produced using this diamond polishing technique was 61,588. Before using this process the shot-life was guaranteed (and averaged) 40,000 shots. The supplier’s guaranteed shot-life has been raised to 50,000 shots.

Diamond polishing has been implemented on all of our cosmetic casting, cover cavities at this supplier.

Diamond polishing is also being pursued on non-cosmetic castings to extend the cavity shot-life.

With diamond polishing additional care must be taken through out the life of the cavity. If the diamond polish is removed while removing carbon or solder it must be reinstalled or the benefits will be removed.


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North American Die Casting Association – 114 th Metalcasting Congress CastExpo’10, March 20-23,...

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