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 cavities 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 for "rough casting surface".
What is Rough Casting Surface?
Rough casting surface is a phenomenon in which the entire surface of a cast product or some surfaces is not smooth compared to the surface soo degree that appears in buried materials.
Mainly due to the temperature of molten metal. Some of the castings are exposed to high temperatures, causing "rough" in parts of the product, or depressions caused by gas generation due to the mixing ratio between buried materials and water and breathability inhibition of molds.
Why Rough Casting Surface Occurs, and What are the countermeasures?
There are seven main causes of rough casting surface. We will introduce the causes and countermeasures, respectively.
１ Molten Metal Temperature is Too High
Because the molten metal temperature is too high, it is caused by a chemical reaction caused by heat between the molten metal at the time of pouring and the surface of the mold. In extreme cases, "burning" with buried materials may occur. Set the molten metal temperature low and cast.
In the wax tree shape where molten metal flows directly from the pushing water into the product part, hot spots occur around the yudo weir. In this case, please change the distance and shape of the hot spring path from the pushy water to the product part.
In addition, the place where the flow of hot water crosses in the product shape is also called "hot spot", and casting defects due to heat may occur.
２ The Casting Mold Temperature is Too High
If the temperature of the casting mold is too high, the molten metal temperature will not be lowered, and the casting surface may become rough due to the chemical reaction with the mold surface. Lower the casting temperature of the casting mold and cast it.
３ Generation of Residue Gas
Residue Roughness of casting surface due to gas is a phenomenon commonly seen mainly in pressure casting.
In casting mold calcination, if the objects in the mold are not completely burned down and the residue remains, the gas generated from the residue at the time of casting will appear as Porosity in whole or in part of the product part.
This gas can be contained in the bullion itself or due to the lack of permeability of the casting mold. The calcinate temperature gradient must be changed so that no residue remains.
４ Moisture Content of the Casting Mold
① The Amount of Water is Large
If the ad bathing ratio of the investment powder is large, the density may decrease, the gap between the particles may increase, and the mold surface may become rough.
Make sure that the amount of mixed water in the investment powder is too large.
② The Residual Moisture of the Casting Mold is Low
Excessive drying of the casting mold will cause rough casting surface.
In the case of gypsum investment powder, if the time from completion of the investment work to the casting mold to the start of calcination has passed for a long time, or if left in a very dry environment, the necessary moisture will come out of the casting mold and lead to roughness of the casting surface.
５ Flow Line
This phenomenon causes flow line on the surface of casting. It can be said that it is a state on the verge of generating casting defects mainly due to insufficient dissolution.
Increase the casting temperature of the mold or the bullion temperature at the time of casting.
６ Lack of Permeability
If the mixing ratio of gypsum investment powder is low, permeability will be impaired.
In the case of investment powder that are hardened using a binder, such as silica investment powder, gas may remain in the casting mold and depressions may occur due to the dilution rate of the binder water is too low or too dense, and the ventilation shortage of the mold that occurs because the mixing ratio of the binder aqueous solution and the investment powder is too low.
Adjust the mixing ratio of investment powder and binder to ensure permeability before casting.
In addition, the binder used in silica investment powder changes to silica at high temperatures. If this casting mold is calcinated at a maximum temperature (900 to 950 °C) for a long time, the binder will be sintered too much, and the sintering bond of silica will proceed too much, and permeability will be inhibited.
Cast with reduced sintering time during calcination.
７ Collapse of the Casting Mold
The inner wall in the casting mold is partially missing for some reason, causing local rough surface. If this is seen, be careful if there is a "chiding".
There are also rough particles (such as the size of the grains) caused by rough investment powder.
Summary of Rough Casting Surface
We have introduced the causes and countermeasures, but we will summarize what to do when rough casting surface occurs.
Check List
- Lower molten metal temperature and the casting mold temperature
- Change the distance and shape of the Sprue
- Change the calcinated temperature gradient and calcinated time of the casting mold
- Adjust the mixing ratio between the investment powder and the water or binder to change the density of the investment powder slurry
- Lower casting mold temperature and bull metal temperature during casting
- Check wax tree's dent and the casting mold strength
There is not always one cause. Please review all the items again and see how they work.
