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 porosities 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 "fin / flash / burr".
What is Fin / Flash / Burr?
Burr is a phenomenon in which extra protrusions that do not exist in the prototype appear in the casting.
Burrs are mainly caused by cracks that occur in casting mold, but the way of dealing with casting mold cracks changes depending on why they occur. In addition, there may be other causes besides cracks in the casting mold.
Why Burr Occur and What are the countermeasures?
Burr is caused by casting mold cracks and wax burr in the pre-casting process. Here are two main causes and countermeasures.
１ Cracks in the Casting Mold
Cracks in the casting mold may cause burr. There are four possible causes of the casting mold cracking. Review these to address the applicable issues.
① External Factors
Physical impacts, such as falling casting mold, can break the mold, causing burr.
② Thermal Crack
Thermal crack means that the casting mold cracks due to a sudden temperature change during heat rise. Metal may enter the cracked part and occur.
③ Partial Deletion of Investment Powder
Molten metal cast into the casting mold during casting may break the casting mold and become burr. Two or more product parts on the wax tree may get too close together and destroy the walls with the force of the cast metal.
In addition, since it may occur when the strength of the mold is insufficient, such as when the mixing ratio of the investment powder is too high or the casting mold is insufficient sintered in calcination, it is necessary to review the mixing ratio of the investment and the calcination curve of the casting mold.
③ Dehydration Conditions of Silica Investment Powder
If the dehydration conditions of silica investment powder are inappropriate, cracks may occur in the casting mold during the dehydration and calcination processes, and burr may occur in the cast.
④ Expansion of Resin in 3D Printers
When calcinated molded products made with a 3D printer, the expansion of the resin (a mixture of acrylates and waxes) can destroy the inner walls of the casting mold and cause burrs.
In addition, if the material of the machined molded material such as CNC is something other than for lost wax, the same thing may occur as if the 3D printer is resin even if the material is excessively inflated during the de-waxing process of mold calcination.
The mold used in lost wax casting is assumed to be a material that "burns" with heat, but choosing a "melting" material is a more favorable condition. Increase the mold strength or reexamine the burn-out curve gradient of the casting mold.
Protrusions may occur in areas partially exposed to high temperatures due to molten metal. Instead of actually protruding, the overall point where the gas is generated is recessed, and the metal is partially filled by pushing off the back pressure of the gas.
It is caused by excess gas, please check the following.
In addition, it may occur because the molten metal temperature is too high or the molten metal temperature in the vicinity where the protrusion occurred is too high. In that case, check the casting temperature, the location of the sprue, etc.
Summary of Burr
We have introduced the causes and countermeasures, but we will summarize what to do when burr protrusions occur.
- Check for missing molds or cracks
- Review of dehydration conditions for silica investment powder
- Check the wax model for burrs
- Check the intrinsic gases and residues of bullion
- Check the mold's permeability
- Check the casting temperature and where to install the sprue
There is not always one cause. Please review all the items again and see how they work.