Casting defects are defects in castings that occur in the casting process. There are various defect phenomena such as cracks in castings, rough surfaces, and casting nests with porosity inside. Casting defects are classified into several types, each with different causes. In addition, since there are several pre-casting processes in the casting process, and even defects in the previous process lead to casting defects, there are complex cases where casting defects are not limited to one, and there are actually many complex causes.
Typical defects fall into six broad categories. First of all, we will look at the causes and countermeasures of "surface shrinkage and chipping".
What is Surface Shrinkage and Chipping?
Surface Shrinkage and chipping are a phenomenon in which a depression not found in the prototype or wax pattern occurs on the surface of the cast.
There are various reasons such as "chipping" and "surface shrinkage". It is a problem that is solved if you fully understand the cause and take measures.
Why do Surface Shrinkage and Chipping Occur, and What are the Countermeasures?
Surface shrinkage and chipping are caused by gas residuum and shrinkage of bullion after solidification. Here are three main causes and countermeasures.
1 Gas residuum due to lack of permeability
When pressure casting, the gas remaining in the casting mold is not efficiently discharged out of the mold, resulting in a back pressure, which causes a depression on a part of the cast surface.
When suction casting or pressure casting, the mixing ratio between gypsum investment powder and water is too low, or in the case of silica investment powder, if the concentration of the binder is too high for the amount of investment powder, or if the amount of binder aqueous solution is too large, the gap between the particles of the investment powder becomes dense. This occurs when the permeability of the casting mold is inhibited.
In addition, in order to ensure mold strength, the temperature of the casting mold should be raised to the sintering temperature of the binder (900 °C to 950 °C*), but if this sintering temperature is too high or the time is too long, sintering will become too much and the permeability of the casting mold will be impaired, so do not leave the casting mold in the furnace for a long time.
Of course, there are other possible causes. Even if the mold is well permeability, other external factors may cause gas to occur more than you can imagine. Surface Shrinkage may also be generated due to residuum remaining in the mold, gases generated from crucible, and gases inherent in bullion.
Please refer to the "Gas Porosity" section for how to remove the intrinsic gas of bullion.
* Sintering temperature of silica investment powder (binder): Yoshida cast recommends 950°C to ensure sufficient mold strength.
Cross-sectional image of surface shrinkage
The surface of the casting is often generated like "Pockmark" and the inside is rough, especially in platinum pressure casting.
The cause is gas residuum, but the first suspect cause is the dilution rate of the binder used in silica investment powder and the mixing ratio of the powder to the binder aqueous solution.
If these are appropriate, it is necessary to check whether the temperature and time specified in the mold sintering process are appropriate. In addition, we will degassing bullion to erase the factors that lead to the generation of gas.
2 Chipping
During casting, the protruding investment powder in the mold loses the momentum of the molten metal, the investment powder remains in the cast, and a part of investment powder remains in the surface layer, and artificial surface shrinkage occurs in this part.
In the chip of the investment powder, the gas porosity generated from the fragment of the investment powder is often generated around this foreign body.
Cross-sectional image of Chipping
Compared to other porosity, it has an artificial chipping shape.
It is necessary to consider chipping (protrusion of investment powder) such as wax trees and if mold strength is maintained.
3 Surface Shrinkage
Surface Shrinkage is a phenomenon in which the surface of the casting is generally recessed, such as a lump of wall thickness shape.
Since casting dissolves metal and pours it into the mold, the metal at the time of dissolution almost always expands from the state of the individual (some metals do not expand thermally). This causes shrinkage during solidification on the surface.
There are measures to prevent shrinkage such as the thickness of the "sprue gate", the mounting place, or the "reservoir", but basically the casting shrinks at the time of solidification, so it is necessary to consider the difference in shrinkage in advance.
Cross-sectional image of surface shrinkage
The thicker the casting body, the more it shrinks during solidification.
Shape in the cutting and polishing processes. In order to find dimensional accuracy, it is necessary to produce the prototype in anticipation of shrinkage.
What is Reservoir?
As a measure to prevent surface shrinkage and shrinkage porosity, a lump as shown in the figure is provided to supply an amount of molten metal commensurate with the shrinkage of the bullion. This is called "reservoir".
It is also possible to adjust directional solidification by attaching it so that it is adjacent to the thin part and the place where the solidification wants to be slowed down.
Summary of Surface Shrinkage and Chipping?
We have introduced the causes and countermeasures, but we will summarize what to do when surface shrinkage and chipping occurs.
Check List
- Adjust the particle concentration of the investment powder to ensure mold permeability
- Make sure the mold sintering process is appropriate
- Check wax tree for surface shrinkage and chipping
- Check to see if mold strength is maintained
- Making a reservoir or sprue gate
- Create prototypes in anticipation of cutting and polishing
There is not always one cause. Please review all the items again and see how they work.
