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MARKETING MATTERS CAST TIP


Sewer Gating: Garbage In, Garbage Out H


KEVIN FLEISCHMANN, AFS INSTITUTE


ave you ever struggled to keep the downsprue full while pouring? Have you


increased the size of a gating com- ponent just to be safe, only to see an increase in scrap? Erring on the side of “bigger is better” when design- ing or modifying a gating system is exactly that—an error. Assuming your metal handling and pouring equipment is adequate, let’s look at the results of unnecessarily large gating system components. We’ll start with what we can


see during pouring: the downsprue. Because the amount of liquid metal in the downsprue varies during the pour, the metallostatic pressure on the rest of the metal flowing down- stream also varies. Inconsistent flow means inconsistent thermal gradients and chaotically meshing streams of metal inside the mold cavity. A sprue that’s not kept full will increase slag, dross and air ingestion and produce inconsistent mold filling times. So you’re one of the lucky ones with a basin or cup? A full pouring cup or basin without a full sprue underneath presents a false sense of security. Vortexing, splashing and air aspiration could be occurring right under your nose because of an insufficiently tapered sprue or when employing a sprue choked system in an automatic molding machine with a reverse tapered sprue and lack of runner bar choke.


films, sand and refractory are prone to float out in the run- ner. Impurities also can enter into the mold cavity because of improperly placed gates, but we will focus on the runner for now. In a gate choked system, metal backfills the runner and downsprue. If the runner is too large or too tall, buoyant impuri- ties float along the surface of the metal and eventually get en- trained back into the stream and casting cavity instead of adher- ing to the mold surface. The lack of a tapered runner extension or sump in a sprue choked system is another method for delivering defective metal into the mold cavity. At this point in the gating


An inspection of the running system at shakeout will re- veal the casting’s worth. Notice the very slim downsprue.


When it comes to a well, you can take it or leave it. Many have gotten away without using one at all. Wells traditionally have been used to clean up fallen metal from the downsprue, as if they were a pouring basin in and of themselves at the parting line of the mold, but an improperly designed well can damage the metal. An oversized runner can be as


costly to casting quality as an over- sized downsprue. Buoyant impurities including slag, air bubbles, oxide


Inconsistent flow means inconsistent thermal gradients and chaotically


meshing streams of metal. A sprue that’s not full will increase slag,


dross and air ingestion and produce inconsistent mold filling times.


system, all that is left is to pass metal through the gates and into the mold cavity in the hope the pattern is as well oriented as pos-


sible. You may have come across a rigging that began as a gate choked system and was later opened up, probably in an attempt to fix other casting issues. Tese gates—now much larger—no longer back up the metal flow and leave the runner incompletely full, causing the prob- lems previously discussed. Dispro- portionally large gates in a sprue choked system might not entirely fill their cross sections, producing erratic mold cavity filling. A gating system, no matter how


poorly designed, will fill entirely as the metal in the mold cavity rises appropriately. At shakeout, it will appear as though the gating sys- tem performed flawlessly and each component is full of solid metal. But close examination of the casting will prove its true worth. A metalcaster can take infinite paths to move liquid metal from the pouring ladle to the mold cavity. By understanding fluid flow, you can design a slim rigging system with good yield that will pro- duce high quality castings.


November 2013 MODERN CASTING | 65


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