Investment 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 gating system in casting process.
Good quality can reduce casting defects and increase yield percentage. Due to various benefits over the manufacturing procedure, the investment casting process quickly produces many complex components.
Types of gating system in casting process
There are two different types of gating system in casting process: 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 gating system in foundry 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 in foundry is used for light alloys.
Investment casting is a frequently used process to make different types of metal designs. Gating in the casting process is a kind of complex procedure to understand. In the gating system, runners and risers are essential when making different high-quality designs.
Components of gating system
The above mentions two different types of gating system in casting process. So what are the components of gating system? Generally, the components of gating system have eight significant elements.
Main elements of gating system
There are two significant steps in casting as filling process and the solidification procedure. In the filling process, the elements of gating system include 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.
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 of investment casting gating system design, 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. 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 gating system lacks a riser, Some heavy parts of 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.
Investment casting gating system design
A good investment casting gating system design can reach high-quality products with less manufacturing cost. We have learned about the components of gating systems, so what factors affect investment casting gating system design?
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 investment casting gating system design, 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.