Solution to Assignment 2:
Question 1 (10 marks):
a) To determine the geometry of the riser. (4 marks)
TSTcast=3.5 min.
TSTriser=3.5*1.5=5.25min.
Vcast 2
)
Acast
Vcast = 4лr3/3=4*3.14*0.53/3=0.523m3
Acast =4лr2=4*3.14*0.52=3.14m2
TSTcast  Cm * (
So,
Cm=3.5/(0.523/3.14)2=126 min/m2
The same:
TSTriser  Cm * (
Vriser 2
)
Ariser
Assume that the riser’s diameter is D, height is H, and known D=H.
Vriser =(лD2/4)*H=л*D3/4=0.25лD3
Ariser=лD*H+2*лD2/4=1.5лD2
So,
5.25=126*(0.25лD3/1.5лD2)2=126*(0.167D)2
D=1.22m
H=D=1.22m
b) Design a pattern (3 marks)
The allowance is 0.8%-1.0% based on the length. Take an average value 0.9%,
Dpattern=Dcast+Dcast*0.9%
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For the large casting, the allowance should have extra value 1%-5%. Take an average value
2.5%,
Dpattern= Dcast + Dcast *0.9%+ Dcast *2.5%= Dcast (1+0.009+0.025)=1.034 Dcast
Remark 1. Please read my handouts on the topic of casting.
Remark 2. The allowance is not based on the volume but based on the linear dimension. In
our case, this is along the radius of the sphere.
c) Design a pouring system. (3 marks)
One of the design rules for the sprue is: MFT<TST (for casting or mould cavity).
Take MFT=3.5 min or any smaller value such as 3.0min. You can have more options that you
would like, but all these options should be subject to the constraint of MFT < TST.
MFT=V/Q,
so:
Q=Vcast/MFT=0.523/3.0=0.174m3/min=0.0029m3/s
Q  A2  v2  A2  2  gL  r 2  2  gL
L —the length of the sprue (m)
A2—the bottom area of the sprue (m2)
v2 — the velocity at the bottom of the sprue
Here, we have two variables, A2 and L, but one equation. Therefore we have to assume either A2
or L. Let us assume L.
First, L should be better larger than the radius of the casting sphere to avoid some problem in the
casting, e.g., hot spot. Second, L should be better higher than the riser. Therefore, we take L=1.5
m. From the above equation, we can then find
r2=0.0029/(3.14*3.28)
r=0.013 m
d=2.6 cm
Here, we need to check if the diameter designed, i.e., 2.6 cm, is reasonable. For example, it
cannot be larger than the diameter of the sphere.
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Question 2 (10 marks):
Parting line or surface is a cut surface on the casting pattern to separate the cope and drag. The
draft is a geometrical feature that facilitates the drawing of a casting out of the cope and drag.
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