Sand casting is mainly associated with making different metal items with complex geometries in the metal casting field. Various factors affect yield percentage and cast quality, including the design of the sand casting gating system and the riser.
If it has a good quality, it reduces the casting defects and increases yield percentage. Due to various benefits over the manufacturing procedure, the sand casting process quickly produces many complex components.
Types of gating system in casting
There are two types of gating system in casting: pressurized and unpressurized gating system.
1) Pressurised gating system
Here the complete cross-sectional area reduces towards the mould cavity. The restrictions in the metal flow will maintain the backpressure. It’s similar to the flow of liquid (volume) from all gates. Especially the backpressure is essential to decrease the aspiration. The reason is when sprue every time runs entirely.
Due to the restrictions, the metal flows at increased velocity cause a lot of turbulence and chances of mould erosion. Usually, casting yield is given since the volume of metal used up in the runners and gates is reduced.
Due to the turbulence, this kind of sand casting gating system is not generally used for light alloys but may be frequently utilized for ferrous castings.
2) Un-Pressurized Gating System
Here, a complete cross-sectional area develops towards the mould cavity. There is a restriction only at the lower of sprue. Liquid volume flow is changeable from gate to gate. Aspiration in the gating system as the system never runs entirely. Comparably, there is low turbulence. Due to the turbulence occurrence, this type of gating system is used for light alloys.
Sand casting gating system is a frequently used process to make different types of metal designs. Gating in casting is a kind of complex procedure to understand. In casting, runner and riser are essential when making different designs with high quality in sand casting gating systems. Since there are many functions of the gating system, people use it to make metal products.
Gating design in casting
It is crucial to make high-quality products with a low budget within a short period to survive in the competitive market. Most of the time, beginners have to face many problems because of the poor quality of products. A well-designed gating system can reach high-quality products with less manufacturing cost. The same is true for the sand casting gating system.
We can call a gating system a basic design. It’s essential to build a smooth and proper filling of the mould cavity of the casting. There should not be any discontinuity or solid inclusions. Properly made gating system and riser confirmed the uniform and finished filling of clean molten metal.
Gating system in sand casting
So what parts does the sand casting gating system include? Generally, the sand casting gating system has eight major components.
Main Elements of gating system
There are two significant steps in casting as filling process and the solidification procedure. When considering the filling process, that gating system has a pouring cup, spare base, runner, sprue, gate, and riser.
In shape, it is rectangular or circular. It adds the molten metal, which is poured, from the ladle. The mainly pouring cup is used for decreasing the momentum of the liquid flowing into the mould by settling first into it.
The pouring basin must be deep as well as the entrance into the sprue is a smooth radius of at least 25mm. According to the recommendations, the pouring cup depth should be 2.5 times the sprue entrance diameter. Mainly it is beneficial for smoothing metal flow and protect from vortex formation.
In the cross-section, it is circular. It causes the molten metal to fall from the pouring cup to the sprue properly. The sprue should be a vertical taper passage through the cope to increase the velocity of the metal. The reason is it flows down, decreasing the air aspiration. It connects the pouring basin to the runner also. The taper can obtain by the continuity equation in the sand casting gating system.
It changes the direction of the molten metal flow to the perpendicular angle and passes it to the runner. At the bottom of a sprue well is essential to decrease the velocity of the entered metals and the mould erosion.
The usual rule for the sprue well is five times that of the sprue choke area. According to the recommendations in the sand casting gating system, the excellent depth should be precisely similar to that of the runner.
This part is beneficial to take the molten metal from the sprue to the casting. We can consider the last step where the molten metal goes from the runner to the mould cavity.
Gates are the molten metal openings into the mould cavity in the sand casting gating system. The most important thing is it can be easily broken off after casting solidification.
Different types of gates are used in the casting procedure, depending on the use.
This is the source of extra metal. It flows from the riser to the mould cavity to compensate for shrinkage. This procedure takes place in the casting when it begins solidifying. If the sand casting gating system lacks a riser, Some heavy parts of the sand casting gating system will get shrinkage abnormalities. That can be on the surface as well as internally.
The riser should be created to freeze after finishing the central casting to satisfy its function. The riser should remain molten until after the casting solidification.
There are two main functions of the riser. It compensates for solidification shrinkage, and the second one is the heat source. Various types of risers can be introduced according to the requirement in the sand casting gating system.
Design of gating system in casting process
1. Riser placement: the spacing of risers in the gating system in the casting process should be considered by accurately calculating the feeding distance of the risers.
2. Riser shape: The recommended riser shapes are cylindrical. Though it’s usually used most of the time, the casting procedure is complicated with it. To develop the volume/surface area ratio, the lower surface of the riser can be shaped like a hemisphere. The essential thing is the riser solidification time should be longer than the casting solidification time.
Types of Riser
In the sand casting gating system, we can categorize this according to the three facts.
1. Where it is situated
2. Whether it is open to the air
3. How it is filled.
If the riser is situated on the casting, it is known as a top riser, but if it is located next to the casting, it is known as a side riser. Top risers are advantageous because they take up less space in the flask than a side riser, plus they have a shorter feeding distance.
When the riser is open to the atmosphere, it is known as an open riser, but if the riser fully has in the mould, it is known as a blind riser. An open riser is usually more significant than a blind because the open riser loses more heat to mould through the top of the riser.
Lastly, if the riser gets material from the gating system and is complete before the mould cavity, we can call it a live riser or hot riser. If the riser fills with material that has already flowed through the mould cavity, we can call it a dead or cold riser. Live risers usually are smaller than dead risers.
Gating system design
The efficiency or yield of casting is defined as the weight of the casting divided by the weight of the total amount of metal poured. Risers can add a lot to run weight, so optimizing their size and shape is crucial.
Because risers exist only to ensure the integrity of the casting, they are removed after the part has cooled, and their metal is remelted to be used again. As a result, riser size, number, and placement should be carefully planned to reduce waste while filling all the shrinkage in the casting.
The liquid metal that runs through the various channels in the mould obeys Bernoulli’s theorem, which states that the total head remains constant at any section.
Optimal Filling Time
A casting that fills too slowly can have discontinuities such as cold shuts and misruns. Too fast filling leads to solid and gaseous inclusions. The higher limit of filling time (slowest filling) is governed to avoid premature freezing in thin sections before complete filling. The onset of surface turbulence controls the lower limit of the filling time (fastest packing).
The correct filling time lies between fast and slow filling. It is a function of cast metal, weight, minimum section thickness, and pouring temperature. Several empirical equations to determine the correct filling time for metals are developed by casting researchers based on experimental investigations.
The suitable filling time is decided such that gating channels could be designed to prevent surface turbulence and reduce the bulk turbulence within the gating channels and the mould cavity. In the sand casting gating system, it mainly depends on the velocity of the molten metal, which differ widely within the gating channels and inside the mould cavity. For a given location in the casting, the velocity changes with time, from the beginning to the last of filling.