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Centrifugal Casting
"Technical Notes"

 

Source: A Collection of Reprints "Horizontal versus Vertical Axis"

  1. The horizontal machine is best applied to castings of greater length than the diameter.
  2. Generally speaking, when producing castings spun about a vertical axis, the length of the cylinder should not greatly exceed the diameter.
  3. Vertical centrifugal casting machines are generally used when the diameter of the casting is large as compared with the length.
  4. Castings produced in a horizontal centrifugal casting machine are usually long as compared with the diameter.
  5. A bushing (casting) whose length is less than twice its inside diameter would generally be poured in a vertical centrifugal casting machine and spun at 75G.
  6. True centrifugal castings produced in a vertical axis machine generally are held to a length not exceeding two times the inside diameter.

 

Source: A Collection of Reprints "Directional Solidification"

  1. In true centrifugal casting, without a central core, the metal solidifies from the outside toward the center (axis) of rotation in most cases considering the length to diameter ratio.
  2. In semi-centrifugal casting, cooling takes place from the periphery toward the center and from the center toward the periphery due to the center core.
  3. The true centrifugal casting is spun about its own axis, has no central core, and the cooling takes place from the periphery toward the central axis.
  4. The semi-centrifugal casting is spun about its own axis, but does have a centrally located core, and solidification proceeds from the central axis toward the periphery as well as from the periphery toward the central axis.
  5. The last part of the metal in the mold cavity to solidify should naturally be the gate. Thus is the use of the principle of directional solidification which requires that the metal in the mold cavity first solidify farthest from the gate progressively toward the gate or source of fluid metal. This fluid metal “feeds” the parts of the casting progressively as the metal cools and solidifies, producing a solid, dense casting.
  6. In true centrifugal casting, the important consideration is that the cooling takes place in only one direction, from the outside toward the center of rotation.
  7. In semi-centrifugal castings, with or without a gate, the direction of cooling is not unidirectional, as the mold surface is in contact with practically all surfaces of the castings. It is necessary to insure that directional solidification takes place from the maximum outside diameter of the casting toward the center. In order to accomplish this it is sometimes necessary to resort to the use of pads, auxiliary gates or chills. Gear blanks, flywheels, and brake drums are examples.
  8. A pure metal solidifies at constant temperature and therefore is more difficult to cast. The contraction of the cast metal after solidification and during the period of cooling to room temperature is important. The extent of this contraction determines the shrink rule to be used in making the pattern equipment and mold. The coefficient of expansion (the shrink rule) is a measure of its tendency toward cracking during the casting operation. The greater the shrink rule is the more susceptible the cast metal is to cracking.
  9. A true centrifugal casting is one essentially cylindrical in shape which is produced without a center core and in which the direction of solidification is from the outside of the casting towards the axis of rotation.
  10. As soon as the entire mold surface is covered with metal, it is desirable to slow down the pouring rate to assist the casting in the process of directional solidification. Metal molds due to their higher rate of heat extraction over sand molds require a rough practical pouring rate from 8 to 15 lbs. per second. A satisfactory procedure is to pour the first half of the metal at a high rate and then reduce the pouring rate to about half that amount for the remainder of the metal poured.
  11. Pits or shrink holes on the inside of a casting are generally indicative of an upset in the directional solidification. Metal molds, especially when used with water cooling, can sometimes be designed to promote directional solidification in otherwise difficult to cast castings. The introduction of an insulating material or “pipe eliminator” into the bore of the casting immediately after pouring is often of considerable benefit in promoting directional solidification and reducing internal shrinkage porosity. In extreme cases it is sometimes necessary to introduce an exothermic type of “pipe eliminator”.

 

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Tulsa Centrifugal Casting Machines LLC
PO Box 702751
Tulsa, Oklahoma USA 74170-2751
Phone: 918-585-1400
Phone: 918-633-7058
Fax: 918-779-6085