A pouring cup shall be introduced to reduce turbulence and to make it easy for the worker to pour the metal at the necessary pouring rate. Let us have a pouring head of 5mm. The finalized Gate is The results of the simulation leads us onto the further discussion of improving the gating system. We need to calculate the pouring time for the entire system. By taking a closer look at the ingate area, just before the entrance region we have purple color which corresponds to approximately 0.
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A pouring cup shall be introduced to reduce turbulence and to make it easy for the worker to pour the metal at the necessary pouring rate. Let us have a pouring head of 5mm. The finalized Gate is The results of the simulation leads us onto the further discussion of improving the gating system.
We need to calculate the pouring time for the entire system. By taking a closer look at the ingate area, just before the entrance region we have purple color which corresponds to approximately 0. Thus it is evident that the ingate freezes in half the time as the casting and hence prevents the backflow of metal because of graphitization.
This is the basic necessity of the gating system bringing in grey iron. It is really wonderful to have a uniform cooling rate throughout the casting and hence a uniform microstructure of grey iron. The darker areas in the plot shows the places where the temperature gradient is more, i. The lighter areas show the places of stagnation solidification and lowest thermal gradient. These areas are the places prone for centerline shrinkage in the case of pure metals, but in our case it the probable area for dispersed shrinkage.
Anyway one cannot conclude anything with temperature gradient plot and this is the main disadvantage of the method.
In addition to this, it is grey iron we are dealing with and this can compensate shrinkage. Niyama, a Japanese researcher. This is manipulated using the temperature gradient and the cooling rate. This is basically a prediction of directional solidification. He actually formulated this criterion for prediction of dispersed shrinkage in steel castings, which he managed to extend to other alloys also.
He have concluded that a value of 0 - 0. The lower the value the larger is the probability for porosity. Any way our casting has a value no way near the critical value limits. But this does not mean that we are safe as the Niyama criterion does not consider the gravity into account. For accurate plotting of results we have to switch either to the FCC Custom criterion or the Metal density plotter.
Chiesa of the Universite de Trois - Rivieres in Quebec. This is a criterion that takes the solidus velocity The velocity with which the solidus wave front moves in the molten metal pool and local solidification time. This is again a way to predict porosity. The results of this criterion is interpreted in the exact opposite way of that of the Niyama criterion.
The higher the value of the criterion, the more is the probability for the shrinkage. Once again the results are negative with no porosity. A more generalized and a more accurate method have been devised for accurate prediction of porosity termed as the "Material Density". This is the only criterion that takes the gravitational forces into account.
This is reliable enough to predict even piping of risers. It generates a value between 0 and 1 for all the points in the casting. This gives more reliable results for ferrous castings. The results when analyzed gives the presence, distribution and the amount of the porosity existing. We have severe porosity at both the ingates. The result is also fascinating as it has predicted the pipe that occurred in the sprue. Now the results concluded by analysis of the temperature gradient are found to be true.
The lightest areas in the casting had some shrinkage porosity too. Above, is a plot of regions which remain well above the critical fraction solidification time. These areas must never get isolated. If they get isolated, then no feeding can reach them. Even though graphitization compensates, the graphite formed will be very porous and localized density is lost. This again proved to be a fair reasoning for the results of the density plot.
Now our objective gets clear that we have to make changes in the gating system to acquire a shrinkage free casting. Generally this is considered as an acceptable level of porosity as far as grey iron is considered. But this is a special scenario where very huge centrifugal forces occurs because of localized micro porosity. Hence Risering becomes compulsory. We still have a little more generalized analyses which are of considerable importance..
This is simple yet effective plot that may say whether a spot may have some problems. Well this is incapable to predict the severity of the defect or the defect itself. In grey iron castings hot spots do not disturb much until they are exposed to machining. These hot spots may be of carbide and may also have shrinkage cavity.
The material density plot confirmed that we have no shrinkage cavity of that sort. But still carbide formation will be a problem as it come in the face that is machined. It also affects the service of the casting as this is a friction pressure plate and carbide causes excessive wear on the mating component and the graphite dependent lubrication is lost 18 P a g e which leads to the failure of the casting during service.
One cannot forget about the brittleness of the carbide too. Though this is not a major problem during gating. Foundry men have to adjust their sand properties to prevent this. This plot is made considering the solidification time gradient. Higher the value the more is the potential for hot tear. Now our next objective is to place a riser for our casting.
Empirically speaking, shrinkages in casting can never be removed they can only be shifted from the casting to the riser to ensure soundness of the end product. Let us try with two simple risers near the ingates. The volume of the liquid metal in the mould just after pouring is 8. Therefore volume to be fed by the riser to compensate this is 8.
We have severe shrinkage at the stem part. The volume that had the 0. Now the approximate volume of this shrinkage is 4. Now this shrinkage is not in the liquid state and hence the riser have to pipe to feed it.
The shrinkage cavity because of the first stage shrinkage resembles a cylinder, whereas the second stage shrinkage resembles a cone. Grey iron can generally be made void of shrinkage by increasing gate dimensions with would eventually feed.
Here we go for risers so that that the gate size is small enough ideal for fettling and top risers are added in the gates so that separate fettling of risers is also avoided. Height of the sprue to compensate second stage shrinkage by considering volume of cone relations.
This is the general diagram of the casting with risers This is a positive move. Now the casting is sound. The next diagram neatly visualises the piping in the riser. In order to prevent this, again we have to go for bulky risers which decreases the yield. Because of this we may have hot spots too in the casting. In order to completely eliminate the defects we have to go for a separate design of gating system. This is because we have designed the ingate to freeze before the casting and so the riser cannot communicate with hot spot.
This gate is sufficient enough to make a shrinkage free casting as we have some graphitization expansion. They can be removed only by adapting bulk risers which spoil the yield to a large extent.
Let us go for a special design that increases the yield and which increases the productivity with absolutely no hot spots in the casting.
The occurrence of the hot spot can be controlled in the casting by the addition of chills. Hot spots are reduced to a great extent. Off course the probability of hot spots occurring the surface to be machined is less. We do have cost factor involved while using these chills and we do have some hot tear problems to deal with.
In the next step let us design the down sprue and the initial gating for the predetermined tree. This time we are concentrating more on the directional solidification as our aim is to get perfect directional solidification and perfectly hot spot free.
We have traditionally gated through the stem whose modulus when approximated as a cube Lets again use a chill to regulate the directional solidification.
Riser becomes compulsory at the ingate. Therefore if we group the castings near the ingate, a single riser can be used to ingate feed all of them. Let us design with three ingates grouped together. Also by keeping the riser at the ingate, we ensure that hot metal gets into the riser. This statement is substantiated by the following figures. We can get somewhat better results with a chill.
Basic principles of gating & risering
It focuses on those aspects of the scientific element with regard to metal flow and solidification and emphasizes principles and use rather than theory, and offers functional explanations rather than formal rules. The formal aspects of principles are included where appropriate in discussing fluid flow and heat transfer with more emphasis on the development of sound design considerations. The sciences of filling and solidification were divided into separate sections, focusing on the technical principles that tend to be generic for all metals. The development of metal flow and solidification knowledge is essential to the production of high-quality castings. This text provides a fundamental understanding of the application of scientific principles to the gating and risering of castings. Other added features reflect the rapidly changing environment of the cast metals industry. Sections have been enhanced with improved graphics and photos.
Basic Principles Gating