Simple Riser Design Dr. Robert Tuttle Sagi Saginaw V Vall alle ey State State Unive niversi rsity ty University Center, MI Email:
[email protected]
The The fun function ion of a rise iser is to supply additio ition n molte lten metal to a casting ing to ensure a shrink inkage porosity ity fre free casti casting. Shrinkage Shri nkageporosity porosity occurs because cause of the increase in den densi sity ty from from the liquid quid to soli solid state stateof metals. To be effective ective a rise riser must sol soliidif dify after thecasti casting ng and and contain s suf uffficien cientt metal etal to fe feed the casting casting or porti portion on of a ca casting. Casting solidi soli difficati cation on ti time can can be be predi predicte cted d usi using ng Chvorinov’ hvorinov’s s Rule Rule.
(1)
Where t TS the tota total solidi soli difficati cation on time of thepart part or rise ri ser, C is a mold old consta constant, V is the the vol volume of of TS is the metal, and Asurf is the total surface area of the part or riser. Chvorinov Chvorinov’’s Rule Rule provide provides s guida guidanc nce e on why why rise risers are are typicall typically y cyli cylindri ndrica call. Thelonge ongest sol soliidifi dification cation tim time for a given given volum volume is the one where the shapeof the part has the minim nimum surface surface area. From a practical practical standp standpoi oint, nt, the cyli cylinder has the least ast surface surfacearea for for its i ts volum volume and is is easie siest to make make.. Since Since therise riser shoul should solidif soli dify y after the the ca casting, we want it’ it’s s solidi soli difficati cation on timeto belonge onger than than the the casti casting. I f we want want the rise riserr to take 20% longer than than therise ri ser then then we can write wri te the foll ollowing owing expression:
1.1.2 The The term
(2)
freque quently it i t is is given a spe specia cial name. I t is is cal called the the casting casting modul odulus. By using occurs so fre
the variabl variable e M to represent the casting casting modulus odulus and sim simpli plifying, ying, the above equation quation can be reduced reduced to.
1.2
(3)
This This expression ion is used for for the simples lest method for for desigin iging g a rise iser. It is called lled the modulus lus method. Whil hile modern odern compute computer methods make it easi asier to optim optimize the rise riser, r, an initi ini tial al guess of the correct geometry is is needed. The modulus odulus method provi provides the best best method for obtaini obtaining ng that ini initi tial al guess.
With modern CAD packages it is easy to obtain the volume and surfacearea of the part. Many packages will calculatethese values for you. If you do not have access to amodern CAD packagethen thecasting geometry is broken up into a series of cubes, cylinders, and other prismatic entities and their volumes and surface areas are added together. The modulus for a cylindrical riser is given be the following equation.
(4)
Where D is the riser diameter, and H is the riser height. Determining the final riser dimensions will require an assumption about the relationship between the riser height and diameter. Typically riser height is twice the diameter (H=2D). To ensure a riser can feed a casting or casting section, it’s maximum feeding volume should be checked against thecasting or section volume. Equation 5 provides this check for a riser without an insulating sleeve.
14 S Vmax Vriser S
(5)
Where V max is the maximum casting volume that can be fed, V riser is the volume of the riser, and S is the specific shrinkage of the alloy in percent. The specific shrinkage for a specific allow is listed in Table 1. Table 1 Typical values of volumetric solidification shrinkage1,2
Metal Pure aluminum Aluminum-Silicon alloys Aluminum-Copper alloys Grey Cast Iron White Iron Plain low carbon steel Copper Tin Bronze AluminumBronze Magnesium Zinc
Contraction (Volume %) 6.6 3.8 6.3 0-1.8 4-5.5 3.0 4.5 5.5 4 4.2 6.5
The location of a riser is very important for its performance. Riser should always be located at in-gates or in hot spots within thecasting. Hot spots are portions of a part that are thicker than thesurrounding geometry and therefore cool faster than the rest of the casting.
Example Calculations
A single shifter lever part will be placed in a mold. Determine the required riser for theshifter lever if it has a volume of 0.955 in3 and a surface area of 6.905 in2. Solution: It is best to producethis part with a single in-gate at the largeboss on the part. To create directional solidification and prevent shrinkage, the riser should directly feed the cylindrical feature by being place at thein-gate. Mriser =1.2Mcasting =1.2(0.955/6.905) =0.1660
M riser
4
D 2H
2 D 2 DH 4
0.1660
DH 4H 2D
DH 4H 2D
Assuming H/D =1 or H=D, 0.1660
DD 4D 2D
D =0.996 in H =0.996 in
Confirm the riser is large enough,
14 S 14 3.8 3 Vmax Vriser 0 . 775 2.08in S 3.8 Therefore, the riser is big enough.
The final rigging system is pictured below without a downsprue.
References 1. Groover, M.P., Fundamentals of Modern Manufacturing 3rd Edition, J ohn Wiley & Sons, 2007. 2. Flinn, R.A., Fundamentals of Metal Casting, Addison-Wesley Publishing Co., 1963.
