The Design of Gating System 4. Design of gating system elements 1

Transcription

1 MME 345 Lecture 17 The Design of Gating System 4. Design of gating system elements 1 Ref: [1] P. Beeley, Foundry Technology, Butterworth-Heinemann, 2001 [2] J. Campbell, Castings, Butterworth-Heinemann, 2001 Topics to discuss Introduction 2. Design of pouring basin 3. Design of sprue 4. Design of sprue base

2 1. Introduction Reminding the essential functions of a good running system 1. Economy of size 2. The filling of mould at the required speed for most castings, this roughly equals 0.5 m/s 3. The delivery of only liquid metal into the mould cavity no other phases such as slag, oxide, sand, air or other gases 4. The elimination of surface turbulence preferably at an early state in the running system; should gather the fragmented stream due to the long fall through the sprue together again 5. Establish proper temperature gradient 6. Ease of removal 3/24 Two other points to be understood while designing a gating system 1. Top pouring is prone to produce turbulence; bottom pouring is always preferred 2. Although a pressurised system produces high yield and reduces pouring time, unpressurised system produces more sound casting because of minimised turbulence and air entrapment 4/24

3 2. Design of Pouring Basin Principal objective: Make it easy for the pourer to maintain a full system and to provide the required flow of liquid metal The pouring cup needs to be kept full of metal during the whole duration of pour A slow pour, or interrupted pour, or simply poured down the centre without touching the sides of the sprue, and without filling the basin at all would spoil the casting Unfortunately, even keeping the pouring cup full during the pour is no guarantee of good castings if the cup exit and the sprue entrance are not well matched. 5/24 Conical Cup Easy and economical to make Tolerable for small casting (weighing only a few grams) if pour using hand ladle from about 50 mm high (speed ~1 m/s) 6/24

4 Bad effects of using conical cup 1. Metal enters at unknown velocity making designing of the remainder of the gating system difficult 2. Metal enters at high, unchecked velocity adds difficulty to reduce turbulence 3. Any contaminants (dross, slag) that enters with the melt are necessarily taken directly down the sprue 4. Works as an air pump sucking air into the liquid flow the most severe disadvantage, as air is the single most important contaminant in the gating system 7/24 5. Makes it difficult for the pourer to keep full even for a small volume of basin air is automatically entrained as the cup becomes partially empty from time to time during pouring 6. Mould cavity fills differently depending on precisely where in the cup the pourer directs the pouring stream casting is not reproducible 7. Most susceptible to form vortex any slight off-axis direction will tend to start a rotation of the pool 8/24

5 Offset Basin / Pouring Bush Volume of basin, m 3 V r = Q / t r Q = volume flow rate, m 3 /s t r = response time, s Figure 2.9 Offset pouring basins (a) without step (definitely not recommended); (b) Sharp step (not recommended); (c) residual step (recommended) 9/24 1. The offset blind end brings the vertical downward velocity to a stop It also avoids the incoming liquid goes straight down the sprue with unchecked velocity and taking with it unwanted components such as air and dross, etc. 2. The step (or weir) (a few mm to 20 mm high) is essential to eliminate the fast horizontal component of flow over the top of the sprue It also reverses the downward velocity to make an upward flow, giving some opportunity for lighter phases such as slag and bubbles to separate prior to entering the sprue. 3. The provision of a generous radius over the top of the step smoothing the entrance into the sprue, further aids the smooth, laminar flow of metal. 10/24

6 Criticism of pouring basin Low yield This can be overcome by (1) raising the height of sprue, so that the basin drains very quickly and as completely as possible, and/or (2) making the shape slimmer Mismatching the basin outlet and the sprue entrance this can be overcome by completely removing the bottom of the basin on the sprue side Side and plan views of offset basins: (a) conventional rectangular; (b) slimmed shape to streamline flow and improve metal yield Offset step basin (with open delivery side) 11/24 Use of a stopper Placing a small sand core in the entrance to the sprue Alternatively, a wire attached to the core or a long stopper rod lifted by hand accomplishes the same task It ensures that only clean metal is allowed to enter the sprue It greatly improved the filling of the sprue and reduced the fill time by 60 per cent. This is further proof that the system runs much fuller. (a) Baffle core, (b) Ball plug Despite the inconvenience, when the best quality castings are required, a stopper is advisable 12/24

7 2. Design of Sprue The sprue has the difficult job of getting the melt down to the lowest level of the mould while introducing a minimum of defects despite the high velocity of the stream The fundamental problem with the design of sprue is that the length of fall down the sprue greatly exceeds the critical fall height (13 mm for Al, 8 mm for iron) How then is it possible to prevent damage to the liquid? 13/24 The secret of designing a good sprue is to make it as narrow as possible so that the metal has minimal opportunity to break and entrain its surface during the fall The concept on protecting the liquid from damage is either (i) (ii) to prevent it from going over its critical velocity, or if the critical velocity has to be exceeded, to protect it by constraining its flow in channel as narrow a possible so that it is not able to jump and splash 14/24

8 Many sprues are oversized. This is bad for metallic yield and the economy. But it is much worse for the metal quality. it takes more time to fill, takes air with the melt, causing severe turbulence and oxide formation suffers severe erosion damage In a correctly sized sprue, metal fills quickly, excluding air before any substantial oxidation to occur. 15/24 How then is it possible to be sure that the sprue is exactly the right size? The sprue is designed to mimic the taper that the falling stream adopts naturally as a result of its acceleration due to gravity The shape is a hyperbola The effect of the gap between the straight taper sprue and the stream can be corrected by making sprue entrance 20% larger 16/24

9 The taper has to be correct (within the 20 % outlined above) Too little or too much taper both lead to damage of the melt The attempt to provide gating or feeding off various parts of the sprue at various heights is almost always a mistake, and is to be avoided. 17/24 To summarize: For ease and safety of design, the sprue should be a single, smooth nearly vertical, tapered channel, containing no connections or interruptions of any kind. The rate of filling of the mould cavity should be under the absolute control of its cross-section area. If, therefore, the casting is found in practice to be filling a little too fast or too slow, then the rate can be modified without difficulty by slight adjustment of the size of the sprue. 18/24

10 3. Design of Sprue Base The point at which the falling liquid emerges from the exit of the sprue and executes a right-angle tum along the runner requires special attention. 19/24 The Well to avoid initial splash and suppress formation of vena contracta. may require a choke if a pressurised system is used. the resistance and the delay in the well assist in the back-filling of the sprue at the earliest possible moment. The optimized size of a well is double the diameter of the sprue exit and double the depth of the runner. 20/24

11 But recent X-ray radiographic studies revealed that if a well of any kind was provided, the additional volume creates an opportunity for additional surface turbulence to damage the melt after the well was filled, the rotation of the liquid in the well acts like a ball bearing, reducing the friction on the stream at the turn. In this way the velocity in the runner was increased. Thus the sprue/runner junction was best designed as a simple turn, provided that the channels were of minimum area the high surface tension of the liquid metal assists in retaining the integrity of a compact liquid front constraining the melt the velocity of the metal in the runner is decreased by about 20 per cent due to additional friction from the wall 21/24 Radius of bend If no radius is provided, the melt cannot follow the bend so that a vena contracta is created, where copious volume of air is sucked into the liquid This is particularly severe in sand moulds In contrast, in the bend with a sufficiently large radius, the melt will turn the corner without cavitation or turbulence To be effective, the radius needs to be at least equal to the diameter of the sprue exit, and possibly twice this amount. D R = 2D 22/24

12 Concept of choke Previously a choke of various kinds at the entrance to the runner was used to arrest the surging liquid front and to reduce the initial metal velocity Recent research demonstrated that any such constriction merely results in the jetting of the flow into the more distant expanded part of the runner 23/24 Next Class MME 345, Lecture 18 The Design of Gating System 5. Design of gating system elements 2