MARKETING MATTERS CAST TIP
Designing a Runner System T
KEVIN FLEISCHMANN, AFS AND THE INSTITUTE
he gating system, also known as the runner system, consists of several passageways for
metal to travel from the ladle to the casting cavity. In a horizontally parted mold, the passageway connecting the downsprue to the gate is commonly referred to as the runner. Once the metal has fallen down the sprue, metalcasters are presented with op- portunities, and threats, to the metal quality. Tere may be multiple sprues, runners and gates, but the following design suggestions still apply. Venting is a good practice throughout
the entire mold, so remember to vent the end of the runners. In nearly all gating system designs, the runner is the first place metal can be slowed down after gravity has vigorously pulled it from the ladle, through the pouring basin and down the sprue. Tis first metal through the gating system, once clean and now damaged, should not be allowed to enter the casting cavity. Given sufficient time and mold contact area, accumulated slag, sand and air bubbles can and will float up in the runner and stick to the mold before making their way through a gate and into the casting cavity. Improper runner-gate interchanges can further damage the quality of the melt and intro- duce turbulence, which can lead to lower quality metal and, ultimately, lower me- chanical properties. Sharp turns also can induce turbulence, once again damaging the metal, potentially eroding a sand mold and entraining loose sand into the metal. Where high pressure areas exist against a mold, low pressures are close by, where air aspiration can take place. A small sacrifice in yield can save a
large amount of scrap. Te runner should not end at the last gate it is supplying. By extending the runner past the last gate, with proper taper, the initial dirty metal going through the gating system will be trapped at the end of the runner. If the mold does not have room for such an extension, a sump or well can be utilized. Once the first metal drops into the well, the following, higher quality metal will pass through the gates. Any loose sand, slag, dross or other impurities picked up
This is an example of what happens when oxides are trapped in the runner and in casting cavities, a bad gating practice.
along the way or produced in the initial filling stages of the gating system should not be allowed in the casting cavity. An oversized or insufficiently tapered runner can cause a back splashing effect of the initial stream of metal filling the runner. Tis now turbulent, damaged metal will be pushed through the closest gate, further reducing the metallurgical quality of the casting. In a sprue-choked gating system,
the runner should be placed entirely in the drag. Te gates are connected to the top surface of the runner. In the initial stages of pouring, the sprue quickly fills and floods the runner. It is not until the runner is completely full that metal passes through the gates. A reduction in cross section is needed as the runner passes by a gate. Tis reduction is equal to the cross sectional area of the gate passed. Smoothly tapering the runner past each gate can minimize turbulence and help avoid momentum effects. Te amount of taper should reduce the cross section of the runner at each gate by the area of that gate, in order to provide equal flow through all subsequent gates. Te first gate should be placed as far as allowable from the sprue to allow maximum time and space for any less dense impurities to float up out of the metal stream, sticking to the cope surface of the runner. A commonly accepted distance for the first gate from the sprue exit is four times the
thickness of the runner. Sprue-choked runners are typically short and wide, giving impurities less travel distance to exit the metal stream. If the molding process does not allow for a tapered sprue, the choke can be placed im- mediately in the runner following the sprue. Te remainder of the gating system remains unchanged. In the gate-choked gating sys-
tem, runners are more commonly tall and narrow. In this system, gates are connected toward the bottom of the runner. Te runner and gates all can be located in the cope for easy mold- ing. If drag gates are required, the runner should split the parting line, with equal thickness of gates in the drag. Te gate-runner profile remains unchanged, regardless of exactly where the parting line is chosen. Continuous taper to the runner is not as critical in this system, since the flow of metal quickly backs up from the gates. Te taller profile to this runner type al- lows for floating impurities to remain at a higher level in the metal stream and ultimately is desired to stick to the cope, with cleaner metal flowing underneath and into the casting cavity. A visual examination of a cleaned cast- ing with its gating system in tact can complete the picture of how the metal acts while flowing through the mold. With the primary goal being defect- free castings, a dirty runner will prove to be doing its job.
November 2012 MODERN CASTING | 59
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