record mold temperature and help alert the metalcaster when a mold is too hot or too cool and may lead to casting defects or misruns. Tis data is used to monitor the preheat, heater and cooling line timing and can be useful for defining the defect-free process window. Metalcasters may automate their
temperature control of permanent molds. For instance, using PLC logic control, a cell can be set up so that when a thermocouple located at key points registers a temperature that is outside the defined parameter, the part will be automatically rejected, or at a minimum an alarm signal is generated.
Piston Case Study
As the tooling for a part is mocked up in the drafting room, thoughts are initially on the design of the part itself. Part designer, mold designer and cast- ing engineers all must consider the feed path, gating and risering for a quality part that is manufactured efficiently and effectively on the shop floor. How- ever, in many situations, because of the way the part geometry works and other requirements to achieve the neces- sary mechanical properties, additional mold methods such as forced cooling, coatings, etc., are used for directional solidification. Te ultimate goal in casting and tool design is to prevent turbulence in the flow of the metal and avoid leaving places where the solidify-
ing metal is separated from the liquid metal in the feeder. Two spout ladles are used to fill the
permanent mold-cast piston in Figure 1. Casting process simulation proved that gating into the thin skirt wall would feed the hottest metal into the thinnest walls (Fig. 2). No cold shuts were predicted with an initial 65.6F (150C) mold temperature. Te compact design of the gat-
ing resulted in a relatively high metal velocity that may result in trapped air defects. Additional top wall stock was considered in the design, as well as cross hatching to aid air evacuation. With the gating finalized, the mold design must be engineered. In
the piston example, an optimized process would achieve a 60-second cycle time, no predicted porosity and a warm-up of four start-up castings before steady production. A center core and slide water
lines were added to the mold to realize the short cycle time through fast cooling (Fig. 3). Multiple angles of the side slide lines were needed to cool the in-gates, as well as at the top crown region. Extraction tabs were added to the
side for a stiff feature to secure the part when extracting it from the mold. Te feeder was still in the mushy zone, which is the range between the liquidus and solidus temperatures of the alloy, at extraction and could be stressed at that time or its dimension would be deformed. The cooling curves in the
riser and extraction tabs in Fig- ure 5 show a near-steady state behavior after four castings. Two small regions of 2% porosity were predicted that required further refinement of the water lines and their timing. This included the use of bubblers for targeted cooling in tight regions and high-conductivity mold inserts to act as heat sinks cooling a large mass region.
This piston design has as-cast features which prohibit a center feeder. Feeders on both sides of the piston are required, with additional thermal management methods.
30 | MODERN CASTING February 2017
This article is abstracted from a section of the AFS Institute course, Permanent Mold Thermal Management, which will be held March 28 in Schaumburg, Illi- nois. Register at
www.afsinc.org/courses.
Fig. 5. The cooling curves in the riser and extraction tabs show a near steady state behavior after four castings.
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