Expected life of wooden pattern for gray iron castings 4

[tamal1475]

We are trying to estimate cost of large iron castings. One major factor is the pattern cost and its life. Can any foundry professional suggest how many minimum cycles a Teak wood pattern would normally last before replacement? The castings are for large diameter conical castings (Yoke for PF Mills) weighing upto around 25000 kg. The pattern itself would weigh few hundred kgs. What measures can we take to improve the life?

 

[EdStainless]

There are coatings (resins such as epoxy) that are used to coat patterns to make them more durable and slicker. Then you can periodically repair and re-coat the pattern. This maintenance is an ongoing process.
It is critical to make sure that the patterns are handled and stored correctly.

I find it hard to believe that the pattern cost is significant compared to the casting value. If you were only making a few parts maybe but if you are making a few hundred parts the pattern in a minor item.

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Plymouth Tube

 

[arunmrao]

With good seasoned teak wood patterns, you can expect about 100 castings, before it needs a major repair.

The frequency or life cycle of the pattern depends on its original construction, usage and preventive maintenance after each usage. The way you store the patterns is also important.

As suggested by Ed, you can extend life by providing epoxy coatings,which also improve the surface finish of the castings. You may also clad the wooden pattern with aluminium sheet .

 

[tbuelna]

Besides the good advice provided in the previous posts, I would add that the processes employed in preparation of the sand molds can have a significant effect on pattern wear. For example, it is common to use pneumatic hammers to ram large molds. Unless the operator of the pneumatic hammer is very careful, it is quite easy to abrade the surface of a wooden pattern when ramming the mold.

If you know how many parts you will need to produce from your wood pattern, you can consult with your pattern maker on modifications to the pattern design/construction. For a little bit of added expense up front, the pattern can be constructed in ways that make refurbishment of wear-prone areas (core prints, small radius external corners, etc.) easier and less costly. You might also look at relaxing the dimensional/geometry tolerance requirements of the finished casting in areas that may be prone to pattern wear. This would reduce the non-recurring cost associated with tool maintenance/rework, and also reduce the recurring cost due to scrap rates.

I've attached a table that provides some guidance on pattern wear vs material. The term "hardwood pattern" in this table would mean something like laminated mahogany construction.

Hope that helps.
Terry

 

[arunmrao]

Terry, a star for providing an useful table. All your other comments are commendable .

 

[tamal1475]

Terry,
Thanks for the table. That's very specific. I am going to use the other suggestions also. We were estimating a life of 50. With the suggested actions we should be able to go atleast to 200.

 

[tbuelna]

tamal1475-

I don't know too many pattern makers that still use a lot of wood since there are now some excellent synthetic (polyurethane) pattern materials such as OBOModulan or RenShape. These synthetic materials are much more expensive than any wood material, but they can save money overall. They are very resistant to moisture, they are dimensionally stable, they are easy to machine, and they are very durable.

You can get an idea of the relative difference in service life between polyurethane and wood pattern materials from the table I posted. While the raw material cost for polyurethane is probably 5 to 10 times that of quality hardwood, the rest of the costs for producing the pattern should be similar.

Good luck to you.
Terry