Choosing the right filter.

The primary objective of metal filtration is to remove from the metal stream non-metallic inclusions, which are a common cause of casting defects and scrap castings. Removing inclusions improves mechanical properties, casting soundness, machining and surface finish, and reduces overall scrap rates.

Filters work by physically stopping large particles from entering the casting cavity. Viscous oxides stick to the filter, and the metal flow becomes more uniform in direction, thereby reducing the potential for oxides to form due to turbulent flow.

The initial decision to use filters in metalcasting resulted in improved runner system design to exploit the cleaning power of filters. The next step in the process of continual improvement is proper filter selection. Every filter provides a combination of characteristics, process parameters and costs, which the metalcasting engineer should assess in the design process. Each job has specific requirements for yield, process time, costs, inclusion control and final machining. In addition, filters have an optimal filter print design for each application. It is up to the metalcasting engineer to make sure that the final design takes advantage of that print pattern.

Types of Filters

The key obstacle to filtration when it was first used more than 25 years ago was finding materials that could withstand the molten metal environment. These materials had to stand up to extreme temperature changes while still providing slag retention. They also had to be manufactured in configurations and shapes that could be applied in metalcasting facilities to retain slag in practice. Various manufacturers took different approaches and traded benefits in one area for another to develop filtration products.

Today, the main products available for metal filtration are foam, cellular, pressed, and screens or cloth. Each filter type offers trade-offs among inclusion removal, cost and performance parameters. Further, three main types of filtration occur with a filter--sieving or screening, filter cake, and internal or deep bed. The various products provide different levels of each type of filtration, depending on the total design.

Sieving--All filtration devices produce some level of sieving. This is controlled by the hole size compared to the slag particle size. A particle that is too large for the hole will not travel into the casting cavity. Smaller holes capture smaller particles of slag on their front face. The tradeoff for small holes is the amount of ceramic material necessary to produce the passageways. Generally, parts with smaller holes contain more material, which is likely to restrict the flow and lead to slower pour times.

Filter Cake--Filter cake occurs after the first slag hits the filter. When even a small amount of slag sticks to the filter material, it will hold more and finer slag, as similar materials stick together. Subsequent and finer slag continues to build on the earlier slag throughout the filling process.

Internal--Internal or deep bed filtration can occur under two conditions--mechanical or chemical. Mechanical filtration is set up by the configuration of the filter. Chemical filtration is set up by the chemistry of the filter material.

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The internal mechanical mechanism of a filter employs a structure that splits and breaks up the metal stream as it passes. Inside the filter, the walls and strands initiate turbulence in the liquid metal, forming areas where slag can separate. After separation, the slag is retained in these areas by the general metal flow. Since turbulence and separation of the stream is taking place inside a full filter, oxygen is not available to cause slag by re-oxidation. On the outlet side of the filter, the metal recombines to form a coherent stream with low turbulence.

The internal chemical mechanism relies on the design of the material used to make the filter. By choosing the correct combination of ceramics, the filter material can enhance slag movement to the walls of the filter and increase the adhesion of the slag once it is near the wall. The goal of filter material selection is to create a condition where the metal does not "wet" the filter material, but the slag does. The slag in molten metal will easily wet a filter composed of materials similar to it, producing a condition where small, fine slag particles are captured and held within the filter.

Finding the Balance

All filtration and runner system designs must be judged by the results that you obtain from the complete package.

In choosing the best total system for a casting, all of the requirements of the end use application should be included. Final customers often dictate that meeting shape and physical property requirements are just the beginning, and castings must be easy to machine and free of defects.

Principal Author: Andy Adams, Foseco, Cleveland AFS Cast Iron Div. Gating and Risering (5-G) Committee, Schaumburg, Ill.