【Beginner's Edition】SPRUE SYSTEM for better results in JEWELRY CASTING / How CASTING METAL FLOWS

★This column is a commentary on lost wax casting with a block molding method
This column has been presented by Yoshida Cast

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The idea of molten metal flow and metal solidification is very important in casting.

A good product can be made with fully understanding the characteristics such as “flow of melting metal during casting”, “solidification time of alloys”, “viscosity of molten metal” and “surface tension” and taking countermeasures.

Proper temperature for casting after considering the flow of molten metal and the basics of metal characteristics are introduced in this chapter.

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How melting metal flows

The basics of how the molten metal flows during casting are considered in this section.

Normal flow of molten metal is thought to flow in layers as shown in Figure 1.
(1) in Fig. 1, coefficient of friction due to contact with the inner wall of a casting mold increases.
As it is shown in blow figure (2) and (3) get closer to the center, melting metal flows faster.

 fig. 1

Viscosity and surface tension

In addition to friction, the flow of melting metal is also related to viscosity and tension of metal.

Viscosity of metal

 
The table below quantifies viscosity of metals compared to water as a basis.
Viscosity varies depending on fluid, and it increases in order of gas → liquid → solid. It can be seen that the viscosity decreases as the temperature of molten metal increases.

Kinematic viscosity coefficient is the value obtained with dividing static viscosity coefficient by specific gravity and the amount of metal is less than water.
At first glance, molten metal that flows into a casting mold has a value that makes it easier to flow than water. However, actual ease of flow of liquid is affected by surface tension as well as viscosity.

Surface tension of metal

Atoms inside liquid are attracted to surrounding atoms and kept in a state where energy is balanced. But when there are other substances outside, attractive force becomes unbalanced and atoms on surface tend to shrink.
This is called surface tension and always works on surface of the liquid.

Surface tension varies depending on type of substance and also changes with temperature.

* 1 Dyne is a force that gives an acceleration of 1 centimeter (cm) per second squared (cm /s2)　in that direction when acting on an object with a mass of 1 gram (g).

Generally, Tension is lower with higher temperature.
However, in accordance with above chart, surface tension of metal is much higher than water even at high temperatures.
In addition, it is said that the tension becomes even higher when a substance with a large heat of vaporization or an oxide film is formed on the surface of the molten metal.

In other words, even if viscosity is less than that of water, tension works more.
Therefore, it can be said that melting metal is harder to flow than water.

Temperature of molten metal in casting should be higher

Figure 2 shows basic flow of melting metal when temperature of molten metal is low.

When temperature of (1) in Figure 2 drops due to contact with wall of casting mold, viscosity and tension increase. And when temperature of molten metal falls below solid phase temperature (solidus), solidification begins.
As temperature of layer (2) is slightly higher than (1), it is advanced forward. But when it touches inner wall of casting mold, temperature drops, and it solidifies. This is repeated and finally central part of ③ is solidified with the same process.

Surface solidified with this process has a texture similar to ripples created on coastline.
This surface is called "flow lines" and is a type of casting defect.
In addition, a boundary created at the place where two flows of molten metal meet on verge of solidification collide is called “cold shut”. This is also one of casting defects related to lower temperature.

As it is understood, molten metal is cooled as it passed through cavity inside mold. This is resulting in increased viscosity and tension that creates poor fluidity.
Especially when a cavity is thin or/and narrow, it cools rapidly and becomes more viscous.

When metal is cast, as it is generally known, pressure is required to overcome this problem, and so set casting temperature higher than melting point of alloys use for casting.
Generally, for metals with a melting point of around 1000 ° C, the casting temperature should be set 50 to 100 °C (120 - 210 °F) higher than melting point.
And for metals with a melting point of 1600 ° C or higher, proper casting temperature should be set 150 to 200 °C (300 - 390 °F) higher than melting point.

Author : M. Yoshida