Tuesday, October 18, 2016

Bearing Design Guide: Chapter Fifteen: Comparative Casting Methods


      When I first starting working here and learning the industry, whenever I would think about molds and castings I would automatically picture Play-Doh in my head.  I mean its a little messy, but seriously, who doesn't like playing with it?  I'm a grown woman and still can't keep my hands off it when my kids have it out.  You can basically manipulate that stuff into whatever shape or form you want and how cool is that?!   Here at Atlas, it just puts that end result on a bigger scale and there are so many more methods of casting your material to get to the desired finished product.

         I love learning about new things and teaching others too whenever I can.  About four years ago I had the opportunity to go to a local elementary school and help the 2nd grade class learn about mass, matter, solids and liquids and just how material can go from solids, to liquids, and right back to solids again.  So instead of Play-Doh...I had a better idea.

          When I was explaining to them that a customer can come to us and ask us to make something for them in the exact shape and size that they want, I decided to show them what I meant.  In order to give the kids a visualization, I decided to bring in some candy melts and candy molds.  As the candy melted in the pot the kids were amazed just how quickly the candy melted.  Once it was ready I showed them the candy mold and started to fill them with the melted chocolate just as if we were pouring molten metal into a mold.  And, just like that with in seconds it began to harden and take a solid form again in the exact shape of the candy mold.


          Needless to say they were pretty amazed and of course a little more excited about the candy treat they were about to have.

          There are various casting methods available for casting ferrous and non-ferrous metals. A brief
description of each follows with a listing of advantages and disadvantages as well as other pertinent data.

         
Sand Casting: Moist bonded sand or resin coated sand is packed around a wood or metal pattern of the item or items to be cast. The pattern is removed and the cavity or cavities are filled with the molten bronze.

          Following the air cooling of the mold, the casting or castings are removed to be cut or sheared off from the gate and runner as individual castings. Watch the video below.

         

          Advantages: Any metal can be cast -ferrous or non-ferrous- without limitations to size, weight or shape. It is one of the most versatile and low-cost methods available including tooling costs. This method is economical and suitable for low to unlimited quantities.

          Disadvantages: Close tolerances are difficult to achieve and some machining may always be necessary. Interconnected porosity is generally inherent to this process and a fairly rough surface finish averaging 1000 RMS is obtained. The typical tolerances range from plus-or-minus 1/32 to as much as plus-or-minus .090 and greater across parting lines.


     Permanent Mold Casting: The mold cavities are machined out of a nickel steel or cast-iron die blocks since they are designed for repetitive use. Generally, steel cores are used although sand cores of intricate design can be used. Because of the casting heat, the sand cores are expended while the steel cores can be expected to give reasonable life before they are replaced. The mold halves are clamped together and the molten bronze poured into the cavity by gravity without turbulence or under a low-vacuum pressure.
    

          The mold is opened within a few seconds following approximately a 50-degree drop from casting, temperature with aluminum bronze or manganese bronze alloys. The casting with gate and riser is ejected immediately.


          Advantages: Good dimensional accuracy is obtained, good grain size and structure results from the rapid chill. Casting tolerances possible range from plus-or-minus .010 to plus-or-minus .015 per side or surface and parting lines can beheld to about plus-or-minus .030.

Casting variations from casting are rarely existent except after tooling begins to show signs of wear.

          Disadvantages: This method is normally limited to non-ferrous alloys. Size, shape and intricacies also are somewhat limited, although many sections can be cast thinner than sand castings. To justify this method, a moderate volume of 1,000 through 50,000 pieces yearly would be necessary to offset expensive tooling costs. Each individual casting must have a gate and riser which reduces the effectiveness of the yield.


          Centrifugal Casting: In this process of casting, steel or cast-iron dies are used and the molten metal is poured into the rotating or spinning die. After pouring, a water spray is directed onto the rotating die, cooling it more rapidly.

 



           Advantages: Since the molten metal is forced by centrifugal action of the rotating die, the metal thus centrifuged is free of porosity, more dense with a structure designed to carry heavy loads with impacts. The alloy cast in this method can withstand substantial hydraulic pressures without leaking. This method is suitable for ferrous and non-ferrous alloys.

          Disadvantages: Although a controlled stock allowance is set by the die, a machining operation is generally required to remove the rough surface finish and excess stock.



         Continuous Cast Method: In this process, the die is made out of carbon graphite which is surrounded by a cooling jacket through which water flows to chill and solidify the cast tube, bar or shape. As it exits from the furnace proper by gravity, the casting solidifies. It is pulled out slowly by pull rolls or pinch rolls. This rapid cooling reduces the grain size and as the casting exits from the lower section of the holding furnace, a homogeneous micro-structure is obtained.





 
           Advantages: A minimum of stock allowance can be controlled to plus-or-minus . 015 reducing the amount of machining necessary as in other methods. Various shapes are cast reasonably to size without need for precision machining. The resulting structure is generally suitable for acceptance by radiographic tests and will withstand a substantial hydraulic pressure without leaking.

           Disadvantages: Initial high unit cost investment and space; graphite dies must be replaced after each run and each size requires a cooling jacket.



     
Die Casting: Molten metal is forced into closed steel dies at high velocities by application of pressure.




    



          Advantages: Excellent dimensional accuracy is obtained across parting lines plus-or-minus .005 and plus-or-minus .001 to plus-or-minus .003 across extremities and surface finishes 100 RMS or less.

          Disadvantages: This process requires high volumes of20,000 to a million pieces or more since the relative die cast is extensively high. It also is limited to non-ferrous metals and porosity may be encountered as a result of entrapped air in the die. Size is limited to 3 feet square and under 15.0 pounds.



           Investment Casting: Various ferrous and non-ferrous materials are used to make a wax or thermoplastic pattern which is expendable in the process. Hot wax or plastic is injected to make a pattern under pressure into the die and multiple patterns are mounted on a common sprull made of the same material. The assembly, called a tree, is dipped into a liquid surry followed by several immersions in dry fluidized bed of fine sand. Each dipping operation requires drying time. As many as five to eight clippings are required to build a shell around the tree. For wax removal, the tree is placed into a steam autoclave. Before pouring, the molds are kiln-dried and tongued from the furnace to the pouring box and poured while cherry red.



          Advantages: There is no parting line and no draft. The surface finishes are less than 125 RMS and shapes are cast which couldn't be produced by other methods. This process becomes most economical when two or three machining operations can be eliminated. The typical tolerances are usually plus-or-minus .005 and high volume is not a criterion. Tooling is less costly than pressure-die casting.

          Disadvantages: Although this method has the fewest design limitations of shapes, size or design, pound for pound the cost of this process is comparatively high.

          There are several other methods of casting which include shell molding as a modified sand casting which offers closer tolerances as plus-or-minus .007 to .015. The surface finish is much better than sand casting and there is better definition of details such as lettering, etc. The cost of pattern equipment is higher than for sand casting and the process necessitates higher quantities.




  Plaster molding and ceramic mold casting are similar to investment casting. But the molding material is more expensive and the processes have never been suitably automated to reduce the labor intensity of making the molds. The casting tolerances are reasonably close to investment casting.







          I have to tell you this was the best post I have done so far!  Watching all the videos was so much fun.  I hope you enjoyed learning about the different options of casting and watching how all of the processes are done.

Well...that's it for today.  I say goodbye for now.  Until next time my metal loving friends...

Next Up: Chapter 16:Effect of the Casting Method on Bronze Alloys






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