Excerpt from the article in Global Casting magazine's September 2018 issue
The procedure of identifying, diagnosing and minimizing or eliminating crucial casting defects is important for a foundry to run in a low-cost, and high-efficiency mode. It can disprove incorrect conventional wisdom and show that unexpected methods are actually the best way to move forward. To do this, there are three keys to remember:
1. Focus on identifying a casting defect on the basis of its appearance.
2. Be aware of the interactive nature of foundry processes and variables.
3. Use rigorous experimental design methods to study complex causes of defects.Foundry personnel can be quick to label a defect cause based on a cursory examination. Terms such as slag defect and cold shut are part of the defect process. The International Casting Defect Atlas gives a specific code/category to defects based on appearance. It also suggests foundry personnel be aware that most casting defects are due to the interaction of several process variables rather than one factor, such as temperature or gating system design. This allows the foundry engineer to design experiments to capture the complexity of the defect cause. AFS Corporate Member CWC Textron Foundry (Muskegon, Michigan) used this process of casting defect categorization, identification, cause determination and defect reduction to address an issue with camshaft castings.
The defects began occurring after an upgrade from manual pouring of molds to automatic pouring of molds. Auto pouring systems are considered to be safer reliable and efficient compared to manual systems.
However, in the early tests comparing castings from manually poured molds to castings from automatically poured molds, the casting scrap rate was always significantly higher for the automatically poured molds. Other variables, such as pouring temperature and transfer time impacted the scrap rate.
Whenever molds were poured using the automatic pouring system, the camshaft castings exhibited a high frequency of cope-side inclusion defects. These defects were small, but deep enough to cause the castings to be scrapped.
To solve the slag, pinhole, and dross defects, CWC Textron worked through a proven step-by-step approach.
Details of each of the steps followed are shared below. There may be changes in the methods and tools specific to an individual foundry.
Step 1 – Identify the Defect
Foundry personnel have a tendency to identify a defect based on cause like slag defect or sand inclusions. While this is an acceptable method, after the diagnosis is done, the International Atlas of Casting Defects recommends that unknown defects be classified based on appearance rather than cause.
Step 2 – Experimental Design
When the castings were poured with the manual ladles, the frequency of inclusion defects was very low. When the castings were poured with the automatic pouring system, the frequency of inclusion defects was very high. Most foundry casting defects are caused by interacting variables (for example, low pouring temperature + specific chemistry). Factorial design is a tool that allows experimentation on many factors simultaneously.
Step 3 – Gating Design and Filtration Review
Often, foundry personnel jump to modifying the gating system when they observe slag/dross defects. While turbulence in the gating system may be an important factor, the gating system is one of the few constants in the multi-variable production environment of the foundry.
Filters should be considered an “insurance” policy rather than the main function of keeping external slag and dross away.There are two aspects of filter sizing: the primary sizing is related to ensuring that the filter does not act as the choke. The standard rule of thumb is the cross-sectional area of the filter should be at least 4-6 times that of the choke.
Step 4 – Preliminary Trials
It is important that trials be conducted with just a handful of variables.
Step 5 – Production Trials
Production trial volumes and details are important in a quality assurance program.
During the preliminary and production trials, the pouring was done automatically but the metal transfer was manual (by forklift) and, therefore, took more time, resulting in a higher than normal temperature loss. Following the five-step process, key recommendations to reduce the defect included:
- Increase the pouring temperatures.
- Control pouring temperatures (by improving ladle insulation).
- Reduce magnesium additions.
- Increase the bismuth addition after MgFeSi treatment.
- Improve the automatic pouring ability to pour in the center of the cup.
- Reduce velocity and turbulence of the metal in the mold.
To read the full article, go to the Global Casting magazine site.
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