Hiixocastiiig
Chapter 4 Results and Discussion of Results
4.2 Initial design of Component die and Initial castings produced
4.2.1 Die Design
The design of die die as discussed in Chapter 3 enables modifications to be done at a later stage. The modifications expected are overflows, gate geometry changes and cooling and or heating channels.
Drawing 4-2 shows die original die back plate (normally called the ejector half). The die is cut from a Plastic Mould Steel called M201. The plate size is 180mm wide by 320 in height and 50mm tiiick. The component geometry is cut into the die half with an increase of 1% on all dimensions. The increase of 1 % so as to compensate for cooling of the part once it has been ejected from die die1271. There is a recess at die bottom to form the biscuit. This recess is placed here to collect any oxide layer on die end of die billet1541. Leading from die biscuit section is die gate this is conical in profile and ellipsoid in section. From die modular study conducted in the Mediods section it is clear that diere could be a possible problem widi directional solidification between the hour glass section and die 23mm diameter cylindrical section. For this reason channels were drilled dirough die die which could be used as cooling or heating channels in later experiments.
Diagram 4-3: Original Die Back Plate (See Diagram 4-39 for larger modified view)
Short Arm Die Backplate
Material: Plastic Mould Steel M201 Quantity: 1 off
The conical gate was made so as to shape the metal to a similar cross sectional section as the
ellipsoid end cap which the metal will come into contact with first. This gate is an experimental gate and the fact that it is smaller than the thick 25mm diameter cylinder, which according to literature'71
the gate should be of similar thickness, means that it can be milled out to the same size for later experiments.
The front plate of the die mates with the Shot Sleeve through which the metal enters the die. A recess is milled on the external face to envelope the shot sleeve. The recess is 75mm in diameter.
The depth of the recess is critical since the end of the shot sleeve must be in contact with the external face of the die and there must be less than 0.1 clearance. If there is more clearance than this the metal will force a passage between the face of the recess and the end of the sleeve and then continue out to atmosphere. This is undesirable for two reasons, the metal pressure will be lost and
metal will be lost out of the die and there could be a shortage of metal left to fill the component cavity in the die. The depth of the recess suits the shoulder of the shot sleeve which is 15.3mm.
Drawing 4-4: Original Die Front Plate (See Diagram 4-40 for larger modified view)
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Shoi^t Arm Die Frontplate
Material: Plastic Mould Steel M201 Quantity: 1 off
The gate is where the metal enters the component. As discussed in the Methods section, two gate geometries were experimented with. The first gate's geometry is ellipsoid in section. This gate is shown in diagram 4-3 and 4-4. The other gate to be experimented with is cylindrical and is 25mm in diameter. Once the experiments have been done with the ellipsoid gate the gate will be milled out to the 25mm diameter size. The 30mm major axis will protrude from the cylindrical gate which will give the cylindrical gate more volume and slightly more surface area. This is not seen as a problem to the casting. This cylindrical gate is not shown in diagram 4-3 or 4-4.
The other machining work on the die, worth mentioning, are the air vents. These are machined on only one half of the die, the back plate half. The depth of these is 0.2mm. From experience in industry this is the maximum depth through which the molten Aluminum, under pressure will not escape. The requirement in the methods section that the air flow through this vent should not choke, means that the maximum airflow must be calculated.
The top three vents will be considered since the bottom two will be blocked by Aluminum in the last moments of the die being filled. The condition when the two side vents are blocked and only the top vent open and all the flow of air is escaping through this vent is neglected. The reason for neglecting this condition is that this only happens during filling of the very top layer of the ellipsoid hemisphere, which is a non critical portion of the casting.
When the air can only escape through the top three air vents, the metal flow rate will still be at its highest value. The maximum flow rate used in the experiments will be obtained when the injection plunger travels at its highest velocity, which from Chapter 3 is given as, 0.85ms"1. The
corresponding fluid flow rate for this injection plunger velocity is:
Qdm = up Acs Eqn. MF
0.85 [ms1] R m//4 0.85 [ms1] JI (47.76[mm])2 / 4 0.001523 m V
Where Qdm is the flow rate of the metal up is velocity of the injection plunger
A„is the cross sectional area of injection plunger mj is the diameter of the metal injection plunger
The flow rate of air out of the die through the vents must equal the flow of metal into the die. Since there is no other passage through which the air may escape and for negligible increase in air pressure Eqn. AF must be satisfied.
Qdm = Qda Eqn. A F (Acsa)( ua)
0.001523 m V = (0.2 [mm] * 4 [mm] * 2 [off] + 0.2 [mm] * 0.010 [mm]) ua Solving for ua,
ua = 585.685 ms"1 Where Qdm is the flow rate of the metal
Qia is the air flow rate
Vd is the depth of the air vent channel VH. is the width of the air vetn channel
V„ is the number of effective air vent channels ua is the velocity of the traveling through the vents
From B.S. Massey I4fi| the air escaping experiences choking at the speed of sound. We are assuming the passage of the flow of air is very close to isentropic (adiabatic and frictionless).
a=J(JRT) Eqn. SS
For the air escaping we can safely assume it is at, at least the same temperature as that of the die, 250°C at the time of metal injection into the die. For air of moderate humidity, y = 1.4 and R = 287 J/(kg K)
a = 7(1.4)(287)(273 + 250) a = 458.4 ms"1
Now the required air velocity is 585.685ms"1 however the maximum speed that the air will rise to through the vent [modeled as a simple nozzle] is 458.4 ms"1. This means there will be an increase in air pressure because of the differing speeds. It is not necessary here to calculate what pressure will be reached. It is necessary to take note of this and consider that due to the increased air pressure it more likely that air will be entrapped inside the metal during the filling of the die when an injection speed of 0.85 ms"1 is used as opposed to 0.3ms"1. In the analysis of the cast component it will be necessary to check well for any entrapped gas (which will cause spherically shaped porosity).
At the lower injection plunger velocity, 0.3 m s"1 the required flow rate, from Eqn. MF is, 0.000537 mY1 and the velocity of the air mrough the vents required to satisfy Eqn. AF, is 206.7 ms"1. For this case it is clear that there is a very low likelihood of any air pressure build up inside the die during filling since this is well below the choking speed of 458.4 ms'1.
4.2.2 First Run of Component and Results