The science of making molds and cores has advanced considerably in the past decade, and the efforts to improve metal quality are constant. But, what can be done to improve the result when the metal meets the mold?
Earlier this year Sheffield Forgemasters International Ltd. reported it is the first foundry in the U.K. to adopt a state-of-the-art mold and core coating technology, improving the quality of its large-dimension, high-integrity castings, and establishing more efficient production processes in the bargain. Forgemasters is a multi-faceted engineering group with specialties in several critical energy markets, including nuclear power plants and offshore drilling platforms. The group’s cast products represent huge investments of time and capital, and its customers have high stakes riding on the performance of these parts.
The coating process Forgemasters adopted is called Coating Preparation Plant (CPP), developed specifically for metalcasting operations and distributed exclusively by Foseco International Ltd.
Coating the mold is a common practice for foundries, to prevent interaction between the metal and mold, and to promote better surface finish and reduce cleaning and re-work requirements after casting. Forgemasters uses a two-part coating: Foseco’s SEMCO Coating 9223 as an undercoat and SEMCO Zir 7300SF as a topcoat. “The undercoat is designed to seal the surface and prevent metal from penetrating into the voids between sand grains,” explained Tim Birch, Foseco’s International Marketing Manager - Mold & Core. “The topcoat acts as a highly refractory protective layer on top of the sand preventing metal/mold interaction.
“The coatings are applied one after the other with no need to dry the undercoat prior to the application of the top-coat,” he continued, making a point about the efficiency of the CPP system in operation. He noted also that CPP is not restricted to these particular coatings: it works effectively with the full range of coating products that Foseco has developed.
“The need for such equipment is based on foundries’ ongoing need to consistently apply the correct layer thickness of coating,” Birch said, “as variations in applied layer thickness can lead to poor cast surface finish due to interaction between the molten metal and the mold if the coating layer is too thin, or the risk of dimensional inaccuracies, coating scabbing or poor surface finish due to runs or drips if the coating layer is too thick.”
Forgemasters’ manufacturing complex at Sheffield, England, includes electric arc furnace melting and casting for ingots, foundry and forging operations, as well as large-dimension machining capabilities.
The 10,000-metric tons/year foundry pours ladles of carbon or stainless steel up 100 metric tons, for products with maximum dimensions of 16×7.6×4.6 meters. It is able to produce castings weighing up to 350 metric tons, and engineering, patternmaking, ultrasonic testing, and heat-treatment of finished products are done on site.
As Tim Birch recalled, the foundry had been using an “undercoat/topcoat concept” for its molds, but continued to have a high degree of mold penetration. That resulted in excessive production delays, and increased fettling costs for finished castings. Foseco’s advice was sought to reduce the metal penetration, and to advise a best practice for mold coating at Sheffield Forgemasters.
“The principle change was to convert the accepted practice of undercoat spray application to flow-coating,” Birch explained. “With the correct dilution of SEMCO Coating 9223, flow-coating allowed deeper penetration of the coating into the core/mold surface, thus reducing the possibility of metal being forced into the substrate.”
Once Forgemasters had adopted flow coating for both applications, Foseco’s challenge was to devise a method for delivering the coating in “a consistent reproducible dilution, regardless of other influencing factors,” he noted.