generic pq2 diagram
DESCRIPTION
die casting pq2 diagramTRANSCRIPT
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GENERIC PQ2 DIAGRAM
1
A completed PQ
2 diagram is shown bellow. Noticed that the gole is to predict where a
given die will operate when mounted on a machine with known performance. Also shown
is an operating window that developed to match the needs of the casting quality
requirements. The object is to make a design that will have the die and the machine
operate within the range of the developed operating window.
1.Plotting machine power The first step in constructing the PQ
2 diagram is to plot the machine power line. The
machine power line is a straight line with one end represented by the “static” metal
pressure and the other end represented by the maximum machine flow rate performance
(dry shot speed) as it is calculated with a know shot sleeve diameter.
The “static” metal pressure is calculated from hydraulic pressure on the cylinder that is
transferred to the metal cylinder that transferred to the metal through the plunger tip.
The hydraulic pressure used is the shot bottle pressure or the hydraulic pressure used
during the cavity-filling portion of the cycle, not the final intensified pressure. The
formula used is:
Pm = ph *(dh2/dp
2)…………………………Eq. 1
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Where:
Pm –metal pressure in kg/cm2
ph –hydraulic pressure in kg/cm2
dh2- effective hydraulic cylinder diameter in cm
dp2 – plunger diameter in cm
Pm
ph
Picture 1. Hydraulic cylinder with Tailrod.
Substitute for dh2 in Eq.1
dh2 = dc
2 – dr
2
Where:
dh2- Effective hydraulic cylinder diameter in cm
dc2 – Main hydraulic cylinder diameter in cm
dr2 – Tailrod diameter in cm
Plunger
Hydraulic cylinder piston
Tailrod
Pm
kg/cm2
Q (cm3/sec)
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The other end of the machine line is defined as the maximum machine performance.
It is calculated by multiplying the maximum dry shot speed(at the hydraulic ‘shot’
pressure used above) time the area of the plunger.
Qmax = vp * (п dp2/4) ………………………………. Eq. 2
Where.
Qmax = maximum fill rate in cm3/sec
vp = max. Dry shot speed in cm/sec
п = 3.142-constant
Dry shot speed-if there is no molten metal available in shot sleeve, there is no
resistance to the motion of the shot cylinder piston other than the resistance of
machine elements, in that apt the plunger velocity will be maximum and this velocity
is called as dry shot speed.
Pm
kg/cm2
Q (cm3/sec)
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Pm
kg/cm2
Q (cm3/sec)
Higher hydraulic pressure
Lower hydraulic pressure
Q (cm3/sec)
Pm
kg/cm2
Smaller plunger
Larger plunger
Smaller plunger Larger plunger
Effect of Plunger size
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2.Plotting Theoretical Filling Time.
Once filling time is identified for the volume of the casting being produced, a fill rate is
calculated. This fill rate is considered the theoretical minimum fill rate that can be used to
produce the highest quality casting. Filling time is identified as ‘t’ sec, and is calculated.
t = k *T { (Ti - Tf + SZ) / (Tf - Td )} ………………..Eq. 3
Where:
t = The ideal filling time in sec.
K = Empricalcally derived constant, sec/ cm.
T = casting thinness in mm.
Ti = Temperature of the molten metal as it enter the die in 0C.
Tf = Minimum flow temperature of the metal as enter in 0C.
Td = Temperature of the die cavity surface just metal enter in 0C.
S = Percentage solid fraction allowable in the metal at the end of the filling in %.
Z = Unit conversion factor in 0C / % .
Filing time can also be chosen by experience or general practice as shown in bellow
table.
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Cavity volume is the amount of metal that flows through the gate during the cavity fill.
Volume are calculated by .
Vcav = W /density (kg/cm3)……………………Eq. 4
Where :
Vcav = volume of the metal passing through the gates in cm3.
W = weight of the metal passing through the gates in kg.
Once the volume, and filling time is known the filling rate Q can be calculated by
Qth = Vcav/ t…………………………..Eq.5
Where :
Qth = Theoretical filling rate calculated in cm3/sec.
Vcav= casting and overflow volume in cm3
t = theoretically fill time in sec
Theoretical fill rate is plotted as a vertical line on the PQ2 diagram as shown bellow
Q (cm3/sec)
Pm kg/cm2
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3.Metal Pressure To further identify a proper die cast process gate area and the velocities through gate
play a major role. The next step is identify the metal pressure required to move metal
through the gate at different values of gate velocity.
Bernoulli’s Equations Gate velocity is the speed at which molten metal moves through an orifice or gate.
The load on the die casting machine’s injection system comes from the force
required to push the molten metal through the die orifice or gate and can be
represented as the discharge pressure that is developed when the metal is forced
through the gate at the desire gate velocity.
pm = ( rho * 2g) * ( vg/cd )2 …………………Eq. 6
Where :
p = metal pressure in kg/cm2
g = 981 cm/sec2
vg = gate velocity cm/sec
cd = coefficient of discharge
cd for Al - 0.5 to o.6
Zinc – 0.6 to 0.7
Mg – 0.6 to 0.7 Velocity of the gate is calculated by using
Vg = Q/ Ag …………………………….. Eq. 7
Gate velocity can be chosen from the reformation
m/sec
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4.Die line (Gate Area) Any point within the process window is an acceptable process that can be run on
the diecast machine for the specific casting to be produced. The best starting point is to
pick a process that is in the center of the process window. This allows the process very
within the process window due to normal variations within the die casting process.
Q (cm3/sec)
Pm kg/cm2
Q (cm3/sec)
Pm kg/cm2
Prepared by
Maheshwar N Morab