★This column is a commentary on lost wax casting with a block molding method
This column has been presented by Yoshida Cast
Since attaching gate sprues and runner sprues are directly linked to the quality of products, it is necessary to carefully consider the attaching location and thickness.
In addition, it is seen many times that unnecessarily extra sprues are added to several casts because of the fear of casting failure. Increasing the number of attaching sprues more than necessary will cause turbulence, which is a negative effect.
In this column, we will discuss the prerequisites discuss the prerequisites for attaching gate sprues and potential for turbulence due to shapes of ingates.
What is ingate
The definitions of "gate sprue" and "ingate" are slightly different depending on the casting technique such as sand casting and ceramic shell casting.
In our column, "ingate" is defined as the part of gate sprue where it in connected a gate sprue and a cast (product).
Essential conditions for gate sprue
Normally, there is only one gate sprue to one cast (product), but depending on cast shape, multiple gate sprue may be attached, or thickness and angle of gate sprue may be changed.
Thickness of gate sprue should be equal to or at least 2/3 of the cross-sectional area of the thick wall of product.
If there is resistance to flow of melting metal at ingate, turbulent flow may be created, and since this is where gas and melting metal collide, there is a high risk of gas porosities and shrinkage porosities.
To avoid causing casting defects, the following 3 basic conditions should be secured.
|ESSENTIAL CONDITIONS for GATE SPRUE|
|1||Being able to supply melting metal in short time||Prevention of temperature drop down|
|2||Being able to cast quietly||Prevention of turbulence|
|3||Being able to supply melting metal until solidification of casts (products) is completed||Ensuring directive solidification|
Time of metal in-flow should be short.
Longer metal in-flow time lowers temperature of molten metal and increases probability of casting defects originated from insufficient metal temperature, such as dendrite porosities and even misrun.
The 3 methods summarized below are used to reduce metal in-flow time.
|1||Increase the thickness of gate sprue and ingate|
|2||Shorten the length of gate sprue|
|3||Increase the number of gate sprue|
In case if thickness and length of gate sprue is unnecessarily thicker and shorter may create other casting defects.
In case if the number of gate sprue is too many, casting defects related to turbulence may be created.
To ensure that molten metal flows into the product section with as little resistance as possible, attach gate sprues by adjusting location and in-flow angle.
Failure to take this into account will result in turbulence and a high likelihood of gas entrainment.
Turbulence occurs when there is resistance to the flow rate of molten metal in a gate sprue, gas porosities occur because gas and the molten metal collide, and shrinkage porosities or cracking problems will occur depending on thickness of the attached sprue.
The risk for those defects will be higher if theory for sprue system is ignored.
Sufficient supply of melting metal until complete solidification
Directive solidification must be ensured by allowing coagulation to begin at the furthest point from an ingate. This means basically attaching a gate sprue(s) to the thickest part of a cast (product).
If necessary, additionally install a reservoir near location where is required to slow speed of solidification down.
■ How to install a "reservoir is explained in detail in another column "reservoir" to prevent shrinkage porosities"
Considerations of cross-sectional shapes of gate sprue
Effect changes depending on A cross-sectional shape of gate sprue.
Generally, it is round, but it may also be square.
In casting with the ceramic shell method, cross section of gate sprues is added quadrangle instead of round. This is because the molten metal in the cavity is said to have a circular cross section due to tension, and spaces are created at the four corners of the square cavity, and the gas inside of a casting mold can easily escapes through these gaps.
NOTE : ■RED ARROW / Inflow direction of melting metal ■BLUE ARROW / Outflow direction of internal gas
It is basic to attach thicker gate sprues for larger products. As a result, the flow rate of the molten metal in second increases, and it becomes possible to fill up cavity inside a mold with molten metal before solidification begins.
If the diameter of sprue is doubled, the cross-sectional area will be quadrupled, so the filling time of molten metal will be 1/4 when compared with the same product shape.
However, there is a limit to the thickness of sprues for product. Turbulence may occur depending on the shape of products at the ingate.
Also, depending on the air permeability of investment powder, if the flow rate in second is too high, the gas in cavity inside a flask will be pushed by molten metal and become back pressure. Since this back pressure is relatively high, there is actually a limit to how long casting time can be shortened, and it does not go as calculated.
|CHANGES IN MELTED METAL FLOW RATE|
|When thickness of sprue is doubled, the cross section will be quadrupled||Velocity for melting metal pare second becomes quadruple|
Changes in molten metal flow due to difference in sprue shape
Flowing velocity of molten metal within a unit time changes depending on difference in area between molten metal supply side and product side.
Let “A” be the supply side and “B” be the product side and divide the difference in area between “A” and “B” into four patterns.
Then, difference of flow rate and the occurrence of turbulence will be explained in those four patterns
In case “A” and “B” have the same thickness
In a case when thickness of “A” and “B” is the same, there is no change in the flow rate of the molten metal flowing through the gate sprue, but turbulence is likely to occur when it flows into product part.
Turbulence tends to subside if angles where located ingate of gate sprue at the corner of product part are rounded.
In case “B” is narrower than “A”
In a case thickness or size of “B” is smaller or narrower than that of “A”, the flow rate of the molten metal flowing through the gate sprue becomes faster as it approaches product part. As the result of this, the molten metal diffuses so as to be scattered the moment when it flows into the product part.
If flow rate exceeds flow velocity, the molten metal will stagnate at ingate area.
In addition. If the thickness of the product part is thinner than the gate sprue, directive solidification cannot be ensured.
In case “B” is wider than “A”
In a case “B” is wider than “A”, the flow rate of the molten metal flowing through the sprue will become slower as it approaches product part.
Thus, turbulence will be less likely to occur when the molten metal flows into the product part.
Also, if thickness of product part is approximately the same as the gate sprue or thinner, directive solidification is likely to be ensured.
However, if a product is annular and the area of ingate is larger than the cross-sectional area of the product, cracks may occur at the product near ingate.
In this case, use extreme cautions for the thickness of ingate.
In case shape of “B” is oval and area of cross section is same as “A”
In case thickness of a product part is thinner than thickness of gate sprue, make the gate shape be oval to eliminate level difference between a gate sprue and a product. In this case, make sure that the cross section of “gate sprue", “ingate” and "product " should be the same area.
If those areas of cross sections are the same, there will be no difference in the flow rate of molten metal passing through it. And then, turbulence in the product part can be reduced to some extent.
On the other hand, if the thickness of the ellipse becomes too thin, it will be difficult to secure directional solidification.
Large surface area accelerates cooling rate of the molten metal and it may create casting defects related to low temperature such as shrinkage porosities, misrun and etc.
Author : M. Yoshida