Breaker cores are available with a variety of openings shapes and locations.
A 50% round breaker core opening (right), and star breaker core opening (left).
Foundries are continuing to see pressure from their customers to provide better castings at lower costs. In order to help meets these demanded requirements, foundries need to continue to work on improving their casting yield and reduce the casting processing. To achieve this, foundries need to reexamine their overall practices for “methoding” castings.
There have been a lot of improvements in the science of methoding castings. The direct-pour technique has proven to be extremely effective at improving the yield of many castings. Any time you can reduce the amount of gating required to get a sound, defect-free, casting you are saving money. However, this practice is limited and after the casting is shaken out there is still the matter of cleaning the casting. One of the big costs is in removing and cleaning the riser contact area(s) on the casting. Initially full-contact risers were used to feed castings.
In 1908 Edwin C. Washburn took a significant step toward reducing the costs associated with removing the riser and cleaning its contact area. He patented the use of a core that reduced the contact area between the riser and the casting and provided a sharp angle that made it easier to knock off the riser. Using Washburn’s invention, many risers could be removed by impacting the riser with a hammer rather than by cutting. The “Washburn core” has become known more commonly as the breaker core.
Since that time, breaker cores have taken on various shapes and sizes but the standard breaker cores have 40%, 50%, and 70% round openings, depending upon the alloy involved. This is where the diameter of the opening is 40%, 50% or 70% of the diameter of the riser diameter.
Other options include oval and kidney shaped openings. These are commonly used where the casting design does not have room for a round contact, such as on a long thin section of the casting. Normally the openings in a breaker core are located in the center of the riser. However, sometimes it is necessary to move the contact to an offset position under the riser. The main reason for offsetting the opening is to make sure that the contact is located over the heaviest section of the casting in a specific area. Core prints on the casting or bars in the cope flask both can create obstacles for locating the riser contact directly over the heavy section of the casting if the hole is located in the center of the riser. By offsetting the contact opening, the riser can be placed so that its opening is located properly over the heavy casting section.
Varying the size of the contact area provides several functions. Small openings make the riser neck solidify faster, which is sometimes needed in ductile iron castings, to trap the graphite expansion in the casting. Larger openings are more beneficial for steel castings where the demand on the riser to feed the casting is high, and in some steel alloys the need to prevent alloy segregation is met better by a larger opening. But, when considering the removal of the riser and cleaning of the casting, the shape of the opening can have a big impact, especially as the size of the riser and its contact area increases.
Though they have been around for years, star-shaped breaker core openings have not been much used. This may show a lack of understanding of how effective the star breaker shape can be, or simply that it has not been readily available. A recent study shows several potential advantages to a star-shaped breaker core opening. The original intent of the star shape was to make it easier to break off. This is true. A mathematical analysis of the contact area of a star breaker core opening compared to a round opening having the same cross-sectional area showed that the star breaker core opening takes about 21.5% less energy to break off the riser.
In addition, tests reveal that the star shape also offers a slight improvement in the feeding of the casting and a slight reduction in alloy segregation when comparing the star breaker core to a round breaker core contact having the same cross-sectional area. This might be explained by the difference in the surface-to-volume ratio of the two different shapes and the possibility that the sand breaker cores are not “transparent or as thermally neutral” as previously believed. The size of the sand breaker core opening, its overall thickness and the shape of its opening will affect its overall performance. A star shape has more surface area compared to the round opening so it will heat up faster. The star’s sharp points are thinner than the rest of the breaker core so they will heat up much faster and require less energy to heat up their thinner cross-sections. The net result is a slightly faster overall heating of the breaker core, which results in a slightly better feeding.
Shown here are two castings made using two different breaker core openings. The casting on the left was made using a traditional round 50% opening. The casting on the right was made using a star-shaped breaker core having the same cross-sectional opening. After etching these castings you can see the alloy segregation in the casting under the breaker core contact of the 50% opening. The casting made using the star-shaped breaker core has the alloy segregation moved up into the riser above the breaker core.
Both castings were poured in low alloy steel from the same ladle. In steel alloy castings that are prone to alloy segregation the old practice was to go to a larger breaker core opening but with the use of a star-shaped breaker core opening this may not be necessary. Larger openings are harder to break off and have been known to impart more stress on the casting, which can cause cracks. And, the larger cross-sectional area requires more time in the cleaning room to grind off the contact area, so being able to use a smaller star breaker core opening, time and money can be saved in the cleaning room
The use of Ashland’s new star breaker cores can provide improvements in the overall feeding efficiency and performance of a riser and at the same time offer potential cost reductions in the cleaning room. Tests in both the lab and in the foundry confirm the overall performance advantages when EXACTCAST® Star Breaker Cores are used compared to the use of traditional round breaker core openings. Coupled with the use of Ashland’s EXACTCAST® or STA-HOT® Riser Sleeves, star-shaped breaker cores can save both time and money.
Ron Aufderheide is a senior product manager with Ashland Casting Solutions. Visit www.ashland.com.