CASTING  (RISER  DESIGN)  

Assist.  Prof.  Dr.  İlkay  KALAY  Materials  Science  and  Engineering  Department  

Çankaya  University    

MSE  324    CASTING  and  SOLIDIFICATION  

Thermocouples

Casting

A metal casting may be defined as a metal object produced by pouring molten metal into mold containing a cavity which has the desired shape of casting, allowing the molten metal to solidify in the cavity, and then removing the casting.

Casting Advantages of casting processes

•  Casting can produce complex shapes and can incorporate

internal cavities or hollow sections.

•  Very large parts can be produced in one piece.

•  Casting can utilize materials that are difficult or uneconomical to process by other means.

•  The casting process can be economically competitive with other manufacturing processes.

Casting

The important factors in casting process are: 1.  The flow of molten metal into mold cavity 2.  Solidification of metal from its molten state 3.  Heat transfer during solidification and cooling of the metal

in the mold 4.  Influence of type of the mold

Casting Requirements:

-  Mold cavity with desired shape and size -  Melting process to provide molten metal -  Pouring process to introduce the metal into the

mold -  Solidification process controlled to prevent defects -  Ability to remove the casting from the mold -  Cleaning, finishing and inspection operations

Typical Casting Nomenclature

Typical Casting Nomenclature

Typical Casting Nomenclature Mold Cavity: The hollow mold area in which metal solidifies into the part Riser: An extra cavity to store additional metal to prevent shrinkage Gating System: Channels used to deliver metal into the mold cavity Pouring Cup: The part of the gating system that receives poured metal Downsprue: Vertical channel Runners: Horizontal channels

Typical Casting Nomenclature Flask: The box containing the mold Cope: The top half of any part of a 2-part mold Drag: The bottom half of any part of a 2-part mold Core: A shape inserted into the mold to form internal cavities Core Print: A region used to support the core

Fluid Flow Metal is poured through a pouring cup Risers hold and supply metal to prevent shrinking during solidification Gates are designed to prevent contaminants from reaching the mold cavity

Fluidity of Molten Metal

Fluidity: The capability of a molten metal to fill mold cavities Viscosity: Higher viscosity decreases fluidity Surface tension: Decreases fluidity; often caused by oxide film Inclusions: Insoluble particles can increase viscosity, reducing fluidity Solidification pattern: Fluidity is inversely proportional to the freezing

temperature range

Fluidity of Molten Metal Mold design: The design and size of the downsprue, runners, and risers

affect fluidity Mold material and surface: Thermal conductivity and roughness decrease

fluidity Superheating: The temperature increment above the melting point

increases fluidity Pouring: Lower pouring rates decrease fluidity because of faster cooling Heat transfer: Affects the viscosity of the metal

Casting Good casting requires a design effort to:

•  Create a gating system (which consist from: pouring basin,

downsprue, and runner) to bring molten metal into the mold cavity

Casting Good casting requires a design effort to:

•  Provide a riser (feeder) which is a reservoir to feed molten metal to

the casting as it solidifies to prevent internal and external shrinkage in the casting. The riser may have to provide up to 5-7% by volume for the casting as it solidifies.

The risers should be designed keeping the following in mind 1.  the metal in the riser should solidify in the end of the process 2.  the riser volume should be sufficient for compensating the

shrinkage in the casting.

Casting Good and sound casting requires a design effort to:

•  Control heat flow , in order to make the last liquid to solidify is in

the riser

•  Control the rate of heat flow so as to control the nature of the solidified product (microstructure , mechanical properties)

Fluidity of Molten Metal

Fluidity of molten metal helps in producing sound casting with fewer defects. •  It fills not only the mold cavity completely and rapidly but

does not allow also any casting defect like “misrun” to occur in the cast object.

•  Pouring of molten metal properly at correct temperature plays a significant role in producing sound castings.

•  The gating system performs the function to introduce clean metal into mold cavity in a manner as free of turbulence as possible.

Fluidity of Molten Metal Prevent casting defect such as misruns without use of excessively high pouring temperatures is still largely a matter of experience. •  To fill the complicated castings sections completely, flow

rates must be high but not so high as to cause turbulence.

•  It is noted that metal temperature may affect the ability of molten alloy to fill the mold, this effect is metal fluidity.

•  Often, it is desirable to check metal fluidity before pouring using fluidity test.

Fluidity of Molten Metal •  Often, it is desirable to check metal fluidity before

pouring using fluidity test.

Fluidity tests, in which metal from the furnace is poured by controlled vacuum into a flow channel of suitable size, are very useful, since temperature (super-heat) is the most significant single variable influencing the ability of molten metal to fill mold. This test is an accurate indicator of temperature.

Heat Transfer •  The metal that solidifies first is at the wall of the mold; this solid layer

thickens as time passes •  Shrinkage during cooling can change the part dimensions and

sometimes cause cracking; it is caused by the metal’s thermal expansion properties and the phase change between liquid and solid.

Common Defects in Castings

Shrinkage

Solidification Time

Heat Transfer

Common Defects in Castings

Typical Casting Nomenclature

Riser Design- Feeding Distance

Two criteria determine whether or not a riser is adequate: 1.  the solidification time of the riser relative to that of the

casting, and

2.  the feeding distance of the riser.

Riser Design- Feeding Distance

Two criteria determine whether or not a riser is adequate: 1.  the solidification time of the riser relative to that of the

casting.

To  be  effec(ve,  a  riser  should  con(nue  to  feed  liquid  metal  to  the  cas(ng  un(l  the  cas(ng  has  completely  solidified.  Thus,  the  riser  must  have  a  longer  solidificaQon  Qme  than  the  cas(ng.  For  a  riser  of  fixed  volume,  a  minimum  amount  of  heat  loss  will  occur  when  the  riser  geometry  has  the  smallest  possible  surface  area.

Riser Design- Feeding Distance

Two criteria determine whether or not a riser is adequate: 2. the feeding distance of the riser.

•  The feeding distance (FD) is the maximum distance over which a riser can supply feed metal such that the casting section remains relatively free of internal porosity.

•  Hence, the feeding distance determines the number of risers

needed. •  The feeding distance is always measured from the edge of the

riser to the furthest point in the casting section to be fed by that riser.

Centerline Feeding Resistance (CFR)

Caine’s Curve

Caine’s Equation

Naval Research Lab. Method

Naval Research Lab. Method

Naval Research Lab. Method

Naval Research Lab. Method