Wednesday, December 7, 2016

Bearing Design Guide: Chapter Twenty: Thrust Bearings or Washers


We are going to continue with our chapter, but first a little announcement...



And now onto...Bearing Design Guide: Chapter Twenty: Thrust Bearings or Washers

 


          There are three basic types of thrust washers which are defined by their mode of operation. Thrust bearings designed correctly to operate under either one of the following modes have theoretical justification for their load capacities.

          1. Flat boundary lubricated 100 PSI.
          2. Flat thermal wedge, hydrodynamically lubricated 1200 PSI.
          3. Contoured wedge, hydrodynamically lubricated 5000 PSI.

          However, in actual practice, these values are not achieved other than in theoretical design. In practice, the load capacity of each reduce to about 60% of theoretical values of 60 PSI, 700 PSI and 3000 PSI.

         For the greater majority of flat thrust washers, it is impossible to prevent some degree of hydrodynamic lubrication so that even in the worst design, the allowable loading will be greater than 60 PSI.

         In conditions of a sparse oil supply mated with steel, the high leaded tin bronze alloys (20% or greater lead content) are much more capable of satisfactory service providing their is no large amount of dirt or debris.

         If there is a fair amount of dirt present which is fairly coarse in particle size and movement or motion is infrequent or slow, the lower content leaded tin bronze alloys (20% or less) are preferred.

          For un-lubricated applications against steel, the preferred thrust washer materials are the sintered powdered bronze oil impregnated or Teflon-coated and plastic materials.

         For applications in which the non-lubricating film such as water or silicone fluids are present, the preference should be for Teflon-coated or impregnated bronzes or plastics.

         Oil Distribution Grooves: These are grooves that separate each sector of the thrust washers. The grooves must not go completely across the thrust face of the washer unless the thrust washer is completely immersed in the lubricant.

          If the lubricant is supplied at the ID of the washer, then the grooves should extend from the ID going outward covering about 80% of the distance to the outer edge.

         If the direction of rotation can be in either direction, the grooves should be radial; but if the direction of the rotation is fixed, the grooves should be slanted so that the viscous drag of the rotation will pull the lubricant into the groove in the direction of movement. The slanted angle should be between 10 to 40 degrees to the diameter.

          If the lubricant is supplied from the outer circumference (which is an undesirable condition) then the grooves must be slanted 20 to 60 degrees; but if the grooves are in the stationery member, they should be pointed in the direction of the rotation; if the grooves are on the moving member, they should be pointed against the direction of the rotation.


          Oil Collection Grooves: These grooves, like spreading grooves, should only go part way across the surface from 50 to 80% of the distance is suitable. They also should be slanted in the same direction as the oil-spreading grooves. These grooves are positioned just before the oil distribution grooves if the washer is supplied with oil from the ID.

          Oil Groove Dimension: The length of the groove is controlled by the degree of slant and should go about 80 to 90 % of the way across the annular surface.

          The groove cross-section should be in the form of a wide "V" to promote the formation of an oil film or a tear-drop design which blends into the surface.

          The thrust washer with grooves such as through grooves, tear-drop (or stopped oft) and tapered land grooves are superior to a plain washer. By adding four through grooves to the plain washer, the oil flow increases across the thrust surface and the load-carrying capacity increases by 30%.

          While restricting total oil flow through the washer with tear-drop grooves, an 85% increase can be obtained.

          With the tapered land groove which promotes an oil wedge, the increase in load-carrying capacity increases to 3 00% of a plain washer.

          According to theory, if the plain washer is perfectly flat, it would have no load capacity so surface waviness is not undesirable if it is fitted into a rigid aligned housing.

          The speed of operation of a thrust washer is not generally a problem if the surface speed is at least 25 fpm. At very high speeds - above 2000 to 4000 fpm - it may become a problem to supply ample lubricant although there is a compensating advantage of realizing a higher unit load. There also is the possibility of the oil carbonizing at this high speed, depending upon the load.

          Lubricant is important since the higher viscosity lubes such as SAE 50 offer greater chances that hydrodynamic film formation will be realized.

         The normal hydrocarbon oil lubricants are suitable for washers but the non-polymer modified oils are preferred for their higher viscosity.
 

          We are coming down to the home stretch, only three more chapters to go.  I know its a lot of information and a lot to take in, but if I have helped one person with this, I will be happy.  

That's it for today.  Until next time my metal loving friends...

Next Up: Chapter 21: Corrosion Resistance of Some Bronzes

Wednesday, November 30, 2016

Bearing Design Guide: Chapter Nineteen: Heat Treatment of Aluminum Bronze Alloys


C95400 Cast Aluminum Bronze

          The cast aluminum bronzes consist of copper and aluminum with lesser additions of iron, nickel or manganese elements, primarily to enhance the tensile strength, yield strength, hardness, wear resistance, fatigue resistance and corrosion.
     
          The more popular aluminum bronzes contain from 8 to 13 % aluminum, up to 5% iron, nickel and manganese in various combinations.

          As the aluminum content increases, the tensile strength, yield strength and hardness also increase but ductility decreases.

          The aluminum bronze alloys with 10% or more aluminum content can be heat-treated successfully to further increase the physical and mechanical properties of the alloy.

         The normal heat treatment sequence of aluminum bronze alloys with 10% or more aluminum content is to solution heat soak the alloy at 1650 degrees F for at least two hours, then quenching rapidly in water. Slow cooling results in "self-annealing" and produces a coarse structure with greatly-reduced properties.

          Following the heat-treatment sequence, a tempering treatment is required by reheating the casting to 1000 to 1150 degrees F for one hour then quenching in water.

          The heat-treatment changes the "as cast" alloy's micro-structure to a finer grain, resulting in substantial increases in tensile strength, yield strength and hardness but with a fairly high reduction in ductility.

          The wear resistance also is greatly improved by heat treatment without correlation to hardness. It does not seem to cause increased damage to steel shafts.


Solution Heat Treating
That's it for today.  Until next time my metal loving friends...

Next Up: Chapter 20: Thrust Bearings or Washers


Monday, November 14, 2016

Bearing Design Guide: Chapter Eighteen: Important Features of Aluminum Bronzes

     
         Aluminum Bronzes are a family of copper-based alloys offering a combination of mechanical and chemical properties unmatched by any other alloy series.  This feature makes the Aluminum Bronze the first choice for demanding applications.  No...I'm not on an Aluminum Bronze Campaign kick, BUT I love to learn about an alloy family that outshines the rest.  Well, at least in some areas!
So what are the attributes that make Aluminum Bronzes so hot and in demand? Check these out:

          Non-sparking: The non-sparking characteristics make the aluminum bronzes suitable for manufacturing of tools, equipment for petroleum and chemical processing, in mine service, handling explosives and in gas equipment.

           Cold Working Capability: The aluminum bronzes with less than 8% aluminum content are most suitable for cold working into tube sheets, flats, wires and other modified configurations.

           Hot Working Capability: The aluminum bronzes with 8 to 10% aluminum content result in
progressively increased mechanical strength and hardness of the alloy, requiring them to be hot worked. The hot working temperatures range from 1300 degrees F and up.

           Corrosion Resistance: The aluminum bronzes have outstanding corrosion resistance for marine, chemical and in atmospheric service because of its.rapid formation of an aluminum oxide film. If the film is damaged, scratched or abraded, this oxide film is self-healing and reforms almost immediately. The aluminum bronze alloys are resistant to exposure, to chlorides and similar other chemicals and to many acids.

          Oxidation Resistance: The aluminum bronzes are suitable for all air and steam and in temperature service up to 750 degrees F. They do not lose their mechanical strength as rapidly as the manganese bronzes or other leaded and tin bronze alloys. They are generally considered for temperature service.

          Magnetic Permeability: The aluminum bronzes have magnetic permeability often less than 1.07 at 200 oersteds. They are used in highly-stressed, rotating, non-magnetic parts.

           Electrical Conductivity: The aluminum bronzes have a relatively low value of electrical conductivity compared to copper. The lACS percentage drops as much as 75% of the original value.

          Since aluminum bronzes contain elements of aluminum, manganese, iron and nickel, the aluminum bronzes have an average of 13% lACS at 20 degrees C value of copper.

          Thermal Conductivity: Metals with high electrical conductivity usually transfer heat well, too. The aluminum bronzes, as other copper-based alloys, have better heat dissipation properties required for bearing service than steel, cast iron and other ferrous metals. The average aluminum bronze thermal conductivity value is about 15% of that of copper taken as 226 BTU/square foot/hour/degree F 68 degrees F.

          Thermal Expansion: Copper-based alloys have higher thermal expansion and contraction values than many other metals. Their approximate average expansion and contraction values are twice that of steel or cast iron. This is why it is necessary to adjust mating sizes if higher or lower than normal ambient temperature service is involved.

          The aluminum bronzes have an average coefficient of linear thermal expansion inch per inch per degree F as shown in the comparison below:

          Aluminum bronze             .000009 or 9 x 1 0 to the minus sixth power
          Manganese bronze            .000012 or 12 x 10 to the minus sixth power
          Leaded bronze                  .0000108 or 10.8 x 10 to the minus sixth power
          Tin bronzes                       .00010 or 10.0 x 10 to the minus sixth power
          Steel                                  .0000633 or 6.33 x 10 to the minus sixth power
          Cast Iron                           .0000655 or 6.55 x 10 to the minus sixth power

          Heat Treatment of Aluminum Bronzes: The aluminum bronzes with more than 9.5% aluminum content with small additions of iron, nickel or manganese which are added for specific properties (such as higher strength ,hardness, wear resistance, fatigue strength, etc.) can be heat-treated to enhance these properties more. Heat treatment increases the tensile and yield strength and hardness of the alloy but the elongation is somewhat reduced. The heat treatment of aluminum bronzes involve bringing the casting up to 1650 degrees F (900 degrees C) for two hours per inch of section thickness with a water or oil quench followed by a tempering treatment, the temperature of which must be selected in correct combination of strength hardness and ductility. This second treatment generally is done at 1000 to 1150 degrees F for one hour of section thickness.

          Stress Relieving: Stress relieving may be required when the copper-based alloy is used for bearings that are split longitudinally or when the stresses in the alloy from casting or other operation must be removed so as not to cause dimensional changes after machining the split. Such stresses may be removed or minimized by heating the casting, or semi-finished part at 1000 degrees F for an hour per inch of section thickness, followed by cooling in air (without quenching). For stress relieving leaded and tin bronzes, this temperature should be reduced to 600 degrees F.

          Wear Resistance: Tests have proven that aluminum bronzes have better wear resistance than the leaded tin bronzes, tin bronzes and manganese bronzes. (See Chapter 17)

That's it for today.  Until next time my metal loving friends...

Next Up: Chapter 19: Heat Treatment of Aluminum Bronze Alloys

Monday, November 7, 2016

Bearing Design Guide: Chapter Seventeen: Wear Resistance of Bronze Bearing Materials

    
And now back to the Bearing Design Guide...



  Although most sleeve bearings are designed to operate in a film lubricant separating the mating parts, this does not always happen. Generally, the mated surfaces will deteriorate gradually by wear of hard particles infiltrating the surface area along with breakdown of lubricant. Lubricant starvation also may occur.

          Many wear tests have been performed to determine the comparisons that various alloys exhibit under the same conditions of lubrication, shaft and material hardnesses and surface finishes.

          In comparison tests, the results were reported to be as follows:

          Aluminum bronze CDA 954 exhibited the highest wear resistance.
      
          CDA 954 performed better than 3.9 times alloy CDA 938.
      
          CDA 863 performed better than 2.5 times CDA 938.
      
          CDA 905 showed a better wear resistance over CDA 938 by 1.9 times.

          CDA 932 alloy was found to be more wear resistant than CDA 938 by 1.2 times.

This chapter is short, sweet and right to the point.  If you don't want to "wear" yourself down with machine maintenance and part replacements, be sure to choose the correct material for your project.

That's all for now, until next time my metal loving friends...

Next Up: Chapter 18:Important Features of Aluminum Bronzes

Monday, October 31, 2016

The "Other" Legend of Sleepy Hollow


Since it's Halloween, I thought I'd change things up a bit and tell a story that many people may not know.

We all know the famous telling of the tale of a Headless Horseman that terrorizes the Dutch settlement of Tarry Town, NY in a small hidden glen called Sleepy Hollow.

The Headless Horseman, is believed to be the ghost of a Hessian trooper that had his head shot off by a cannonball during a battle of the American Revolutionary War. Story has it that he "rides forth to the scene of battle in nightly quest of his head". 

Please enjoy the story below and read on to find out the "Other" Legend of Sleepy Hollow.



The Legend of Sleepy Hollow tells the tale of Ichabod Crane, a skinny, superstitious, scaredy-cat of a man who is a schoolmaster from Connecticut.  He is in love with Katrina Van Tassel, the daughter of a local wealthy farmer.  Unfortunately he is not the only one who wants Katrina's hand in marriage.  A local "hero" as some called him named Abraham "Brom Bones" Van Brunt also had his eyes on Katrina.  For whomever was to marry her, was also set to one day inherit the farmer's wealth.   


One night, Ichabod decided to attend a harvest party at the Van Tassels' home. He eats, he drinks, plays games and listens to ghost stories just like the rest of the guests, but really only has one thing on his mind and that is to propose to Katrina at the end of the night.  For one reason or another, his efforts failed and was left to go home from the feast alone and defeated.

As he traveled his sad and lonely route through the woods between Van Tassel's farm and the Sleepy Hollow settlement, he passed many of the so called haunted spots the town folk had spoken about in there stories. Spot after spot, he grows more and more frightened of the sights and sounds and just after riding under a devilishly shaped lightning charred tree supposedly haunted by the spirit of a British Spy,  Ichabod is met by a tall, dark cloaked figure on a horse.  Horribly shaken by the size and eeriness of this sight, he is more frightened to see that this ghostly figure did not have a head on his shoulders, but it sat just under his ghostly arm along side his saddle. 

In a frightful race to the bridge next to the Old Dutch Burying Ground, where the Headless Horseman is said to "vanish, according to rule, in a flash of fire and brimstone" after crossing it, Ichabod rides like the wind, pushing his sluggish plow horse down the Hollow and over the bridge. As Ichabod looks back, he is met with the horror of the ghostly figure crossing over the bridge.  The horseman stops, rears his horse, and throws his severed head into Ichabod's frightened face.

As the towns people awakened the next morning, Ichabod was no where to be found as if he just completely vanished from existence. The only evidence that he was ever there was his horse found wandering, a tattered hat, a broken saddle and the puzzling remains of a shattered pumpkin.

With never hearing from Ichabod again, Katrina took the hand of Brom and was married shortly thereafter.   Many say that Brom was behind the whole thing and that he himself was the Headless Horseman as he always has a look of uneasiness as the story of Ichabod is being told.

Many people have different opinions of the story and interpret it in their own way, but usually come to the same conclusion that the ghostly Headless Horseman was really Brom in disguise. Still no one knows where Ichabod Crane has gone and as the story goes on generation after generation the old Dutch story tellers continue to believe and preach that Ichabod was "spirited away by supernatural means," and this leaves us with the legend about his disappearance and sightings of his lost and lonely spirit.



You may think that this is the only ghostly story to come out of Sleepy Hollow, but in fact you are wrong.  There is but another one and I am here to tell you, The "Other" Legend of Sleepy Hollow.  It only seems fitting that is a story about a "bronze" statue.  Who knew bronze could be so scary?!

The Bronze Lady



Halloween night 1916, on the heels of a dare a little girl wanders into the Sleepy Hollow Cemetery. She walks slowly and cautiously around and in between the mausoleums and headstones and then out of no where, she is stopped in her tracks by a sorrowful sound.  As she listens more closely, it is the sound of a woman crying.   

Pushing her fears aside, she follows the sound and is led to a larger than life statue of a seated woman.  The crying has stopped, but she notices something strange about the woman's face.  She climbs up in to the lap of the statue, caresses its face and finds tears falling from the eyes. 

Over the years, some just hear the crying, others just see the tears as the statue sits across from the tomb of the Civil War general Samuel M. Thomas.  Some believe she cries because of a tragedy in her life.  Perhaps, the death of General Thomas. The statue was in fact ordered by his widow shortly after his death in 1903.  The artist was Andrew O'Conner Jr. and he named it Grief.  It is said that she didn't really like the statue at first because she didn't think the woman was gay enough. 

I cannot find any stories or articles about any strange occurrences documented about this statue prior to 1916.  Is it possible that Mrs. Thomas passed in 1916 and maybe it is her spirit in the lady crying over her deceased husband? 


Some say the "tears" can be explained with a scientific theory that its just the statue interacting with the weather and the environment, but the question that always remains is, where is the crying coming from? and that is one question that has yet to be answered.

Tell me...are you brave enough to go visit this statue?

I hope you enjoyed my story telling Halloween edition of our Metalchic Blog.  I hope you have safe, fun filled, Happy Halloween!



Monday, October 24, 2016

Bearing Design Guide: Chapter Sixteen: Effect of the Casting Method on Bronze Alloys




          The casting method should not be ignored but given consideration of the type of service the bronze alloy will be subjected to.

           In particular, the type of load - whether steady and continuous, intermittent or with shock impact or pounding loads - the surface speeds to be encountered and other important features required to be met.

          The casting method has a definite impact on the bronze alloy such as the resulting grain size, density, hardness, mechanical and physical properties, soundness and structure.

          In general, the slower chilled or cooled casting will give rise to coarser and larger grain size. These have a profound effect on the surface qualities, coefficient of friction, wear rate or wear resistance and loads.

          The faster cooled or chilled castings result in greater density, hardness, finer grain size, improved soundness and structure.

           Referring to the illustrations on the following page, please note the finer grain sizes developed by each method of casting.

          Sand Casting: Since molten bronze is poured into a sand mold, the sand or silica having thermal insulating characteristics, will cause slow cooling or chilling of the casting in air. This slow cooling permits the grain size to grow larger, the density, the soundness and structure to be less than by other casting methods.

          Permanent Molded or Chill Casting: The thermal insulating sand is replaced by nickel steel or cast-iron dies. The metal mold quickly chills the casting and this faster solidification results in finer grain size, no interconnected porosity, finer surface finish and improved physical and mechanical properties.

          Centrifugal Casting: Molten metal is poured into a rotating steel or cast-iron die. The centrifugal force impacts the molten metal against the inside of the die, eliminating any porosity. the rotating or spinning die is then sprayed with water coolant to obtain a faster chill than the first two methods discussed the finer grain size further improves the physical and mechanical properties still further.

         Continuous Casting: The molten metal flows by gravity through a graphite die which is chilled
immediately by the cooling jacket surrounding the die. This faster cooling further reduces the grain size and results in still higher physical and mechanical strength.

          The average increase progressively in the tensile and yield strengths is about 5000 to 10,000 PSI and hardness is increased by 10 to 20 points of Brinell hardness.

          Remarks: To further enhances the physical and mechanical properties of the bronzes, extrusion and forging operations can reduce the grain size additionally. These are special processes and the four methods of casting described earlier in this chapter cannot achieve comparable mechanical and physical strength.


Casting Effect On Grain Size and Density


          So...as you can see above, things may look the same on the outside, but they can be very different on the inside.  It is always important to really understand the final application your part will be used in so you know which casting method would be best for you.

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

Next Up: Chapter 17: Wear Resistance of Bronze Bearing Materials



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