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feed path or direct solidification. Inserts made of materials such as titanium can slow cooling rates and increase the effectiveness of the feeder.

Cooling Jackets Cooling jackets control the direc-

tion of the flow of water for cool- ing. In most cases, the area must be flat. Jackets are suited for small and thin-wall castings and frequent mold changes. Tey aid in achieving a more uniform thermal profile, shorter pro- cess times, and repeatable ramping. Jackets also work to reduce hot and cold spots on the mold.

Fig. 1. Two spout ladles are used to fill a permanent mold-cast piston.

lines, mold coatings, and changes to cycle times. Each casting or mold cycle has an

operating window in which to achieve a quality part. In this window, the molten metal has the chance to flow through the gating to completely fill the mold without leaving voids behind because of insufficient metal feeding due to metal in the flow path that solidified too quickly. Te goal of a permanent mold

casting operation is to regulate the solidification cycle, and controlling and monitoring the metal mold’s temperature is one facet to achieving that goal.

Cooling Lines

Permanent molds can be air- or water-cooled. Air-cooling the dies is the simpler method and produces less thermal shock, resulting in the lon- gest mold life, but the capabilities for cooling are limited. In air-cooling, an evaporator removes heat from the mold and a condenser removes heat from the evaporator. Cooling fins and com- pressed air can enhance cooling rates. In water-cooled molds, passage-

ways for the lines are drilled in for precise application of high flow-rate coolant. Cooling lines are effective for extracting the casting thermal energy in localized or bulk regions. Te heat flux of cooling lines depends on the Reynolds number as well as length,

28 | MODERN CASTING February 2017

diameter, junction design and cleanli- ness. Depending on the specific region requirements, lines can be on continu- ously or controlled to turn off and on by time or mold thermocouple logic. Water contained in cooling lines

is chemically treated to prevent mold and bacterial growth or con- tamination of waterlines and water stone buildup.

Mold Coatings

Two categories of mold coatings are used in permanent mold: refractory for altering the rate of heat flow from the metal into the mold, and lubricating for releasing the solidi- fied casting from the mold. Coatings often are applied to pre- heated mold pieces. Mold coatings can

prevent soldering of the solidifying metal to the mold, minimize thermal shock and control the rate and direction of solidification.

Inserts The use of

inserts placed in a mold can help improve a poor

Fig. 2. Casting process simulation proved that gating into the thin skirt wall would feed the hottest metal into the thinnest walls.

Spray Cooling Vertical passageways can be

sprayed with water from nozzles. Te water transforms from liquid to steam and cools the mold, however the cooling lacks uniformity com- pared to other cooling methods. Another drawback to this method, unless distilled water is used is the buildup of minerals from the evapo- rated water.

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