Gating and Risering Systems for Steel Casting and Aluminium Alloy Casting

Introduction

To be able to obtain a sound casting, the gating and riser design plays a pivotal role in the sand casting process. The gating system of sand casting is designed to direct the liquid to the mould cavity metal for filling. The riser system is direct to counterbalance shrinkage cause by casting solidification. Proper gating and risering design is important for producing sound castings and to improve the yield of the casting. To achieving this the engineers to design filling and feeding system with very accurate performance.

The pattern allowances are also affects the quality of the casting. Solidification is the important factor considering for the reason of defects as the selection of correct allowances helps to reduce the rejections.

Gating system

Gating system is known as the simple design that is important to be able to construct a smooth and appropriate filling of the mold cavity of the casting without any discontinuity, voids or solid inclusions. A proper method of gating system formed are able to ensure that it leads the pure molten metal to flow through a ladle to the casting cavity, which ensures proper and smooth filling of the cavity.

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From here, there are different types of gating systems for different application which includes the horizontal, vertical, bottom, and middle gating system. For horizontal gating system which is used most widely. This type is normally applied in ferrous metal's sand casting and gravity die-casting of non-ferrous metals. They are used for flat casting, which are filled under gravity.

Next, for vertical gating system that is applied in tall castings were high -pressure sand mold, shell mold and die -casting processes are done.

Components of gating system

The main element needed for the gating system includes pouring basin, sprue, runner, gates and risers.

Pouring basins

From here, pouring basin which also known as bush or cup that are rectangular in shape collects molten metal that is poured from the ladle that direct the flow of metal to sprue. Moreover, it helps to maintain the required rate of liquid metal flow. While doing that, it is able to reduce the turbulence at the sprue entrance and helps separating dross, slag and other unwanted stuff from metal before it enters the sprue. If pouring basins are made large, the dross and slag formation will tend to float on the surface of the metal and may be stopped from entering the sprue and hence the mould. These large basins may be filled quickly without overflowing and act as reservoir to compensate metal shrinkage or contraction.

Sprue

Next, the sprue is circular in cross section where it leads the molten metal from the pouring basin to the sprue well. Other than that, the runner takes the molten metal from sprue to the casting as it is located in the horizontal plane which connects the sprue to its ingates. In other words, the sprue is tapered with its bigger end at top to receive the liquid metal. The smaller end is connected to runner.

Gates

The gates plays a roles of a channel where it connects runner with the mould cavity and through which molten metal flows to fill the mould cavity. From here, a small gate is used for a casting which solidifies slowly and nice versa. Other than that, a gate should not have sharp edges as the may break during the pouring and sand pieces may be carried with molten metal in the mould cavity. The different types of gates include top gate, bottom gate and parting line side gate.

• Top gate. The top gate is sometimes called the drop gate as the molten metal just drops on the sand in the bottom of the mould.
• Bottom gate. The bottom gate is made in the drag portion of the mould. The liquid metal fills rapidly the bottom portion of the mould cavity and rises steadily and gently up the mould walls.
• Parting line side gate. This gating system combine the characteristics of top and bottom gating system. In this technique, gate is provided along the parting line such that some portion of the mould cavity will be below the parting line and some portion will be above the parting line.

Riser system

Risers are known as the channels that allows continuous flow of molten metal which appears to be vertical. From here, it is able to eliminate shrinkage as solidification which occurs during the casting process. In general, metals are generally less dense as liquids than solids which results in casting shrinkage during cooling process. Thus, risers plays a pivotal role on preventing this to happen by feeding the molten metal to the casting as it solidifies.

Types of riser

Open riser

Open risers are the ones that are exposed to the atmosphere and are easy to be moulded. This risers allows to know whether the mould is completely filled or not. Open riser includes top risers or side risers. Open riser must be large in size. The advantage of top riser is that the pressure occurred due to the height of the metal causes feeding through thin sections and is preferred for light metals such as aluminium. And in the case of side riser it should be placed at a higher level for proper feeding which help the riser to receive hot metal.

Blind riser

Blind risers, on the other hand, are completely enclosed in the mould. Blind risers must be smaller in size than open risers as it loses heat slowly. If a riser solidifies before the cavity it is to feed, it is useless and produces unsound casting. A blind riser which is in contact with the mould on all surfaces. Thus a blind riser may be made smaller. Blind riser reduces the energy and time required in removing the riser from the casting.

Types of gating systems

In general, different materials such as ferrous (steel) and non-ferrous (aluminium alloy) requires different process of casting which therefore leads to two types of gating systems:

• Pressurized gating system
• Un -pressurized gating system

Pressurized Gating System

This type of gating system are known as Gate Control System and is used for steel casting which are ferrous metals. The total cross-sectional area decreases towards the mold cavity. Moreover, the back pressure is maintained by the restrictions in the metal flow. With that, the flow of liquid (volume) is almost equal from all gates. Other than that, the backpressure helps in reducing the aspiration as the sprue always runs full. More turbulence will happen and higher chances of mold erosion due to the restrictions of metal flows at high velocity. This type of gating system normally provide casting yield since the volume of metal used up in the runners and gates are reduced. Due to the turbulence of this type of gating system is not used for light alloys but can be advantageously used for ferrous castings. The gating system of a typical pressurized gating system is sprue are: Runner area: Ingate area 1:2:1 or 1:0.75:0.5.

Non-Pressurized Gating System

For this gating system, known as Choke Control System are used for aluminium alloy casting which are for non -ferrous metals. The total cross sectional area increases towards the mold cavity. With that, the restriction is only at the bottom of the sprue.

Furthermore, the flow of liquid is different from all gates. Thus, the aspiration in the gating system as system never runs full. With that, there will be less turbulence. Because of the turbulence this type of gating system is used for light alloys such as aluminium and magnesium alloys. This type of gating system have tapered sprues, sprue well and pouring cup. On top of that, the gating system of a typical un-pressurized gating system is sprue are: Runner area: Ingate area of 1:2:2 or 1:3:3, 1:1:3.

Comparison table

No Pressurized gating systems vs. Unpressurized gating systems

1. Gating ratio may be of the order of 1:2:1 or 1:0.75:0.5. Gating ratio may be of the order of 1:2:2 or 1:3:3, 1:1 :3.
2. Air aspiration effect is minimum. Air aspiration effect is maximum.
3. Volume flow of liquid from every ingate is almost equal. Volume flow of liquid from every ingate is different.
4. They are smaller in volume for a given flow rate of metal. There fore the casting yield is higher. They are larger in volume because they involve large runners and gates as compared to pressurized system and thus lower cast yield
5. Velocity is high, severe turbulence may occur at corners. Velocity is low and turbulen ce is reduced.

Recommendation

Precautions for gating system

Moving on, any gating system designed should be able to provide a defect-free casting. There are various steps to be done on making a perfect casting. The steps are as shown below.

•  A gating system must avoid of having sudden or right-angle changes in direction.
• A gating system should fill the mould cavity before freezing.
• All undesirable materials such as slag, dross and other mould materials should not be allowed to enter the mould cavity.
• The metal entry into the mould cavity should be properly controlled in such a way that aspiration of the atmospheric air is prevented.
• A proper thermal gradient should be maintained so that the casting is cooled without any shrinkage cavities or distortions.
• The metal flow should be maintained in such a way that no gating or mould erosion takes place.
• The gating system should ensure that enough molten metal reaches the mold cavity.

Conclusion

In conclusions, it cannot be denied that each materials require s different gating system to obtain a sound and clean casting for different applications. From here, the pressurized and non-pressurized gating system both supply different structure and purposes. The structure of the gating systems are pivotal to ensure the mold are completely filled with metals without having turbulence and mould erosion in casting. Moreover, it is important to have defect-free casting using the correct gating system.

References

1. HAWORTH CASTINGS. (21 January, 2016). Retrieved from Risers and runners: -and -runners

2. J.Yokesh Kumar, M. S. (October, 2016). Volume 2 Issue 10. Retrieved from Review on DEsign and Optimization of Gating and Risering System for Sand Casting Process:

3. Levture 13&14. (n.d.). Retrieved from Gating system: - contents/IIT -ROORKEE/MANUFACTURING -PROCESSES/metalcasting/lecture13&14.htm

4. Nandagopal M, S. K. (November, 2017 ). International Journal of Recent Engineering Research and Development (IJRERD). Retrieved from Study of Sand Casting Gating System: jrerd.com/papers/v2 -i11/5 -IJRERD -B555.pdf

5. Tyagi, V. (16 December, 2014). Slideshare. Retrieved from Elements of gating system: -system -in-casting