vided by the coremaking machine. The advantages and disadvantages of the different heating methods are shown in Table 1. When working with heated purge
Organic binders burn, which condensates and contaminates the mold (left). Inorganic cores leave the mold clean.
air, the heater should be installed as near as possible to the gassing plate/hood to avoid heat loss in the pipework. If this is not possible, the pipework should be insulated or trace-heated. If the machine has steam heating, a steam heated heat exchanger can be used. Due to radiation and contact heat
bowl and sand funnel of each machine also should have a cover. Even with covers over the containers, dry sand can form. Te dry sand crust should be removed at regular intervals to avoid blow tubes blocking and/or the dry sand causing core imperfections. Premature curing, sand crusting
and cleaning can be reduced by com- pensating for moisture loss. Humidi- fying via spray nozzles has proven ineffective due to water droplets washing binder away. A humidifying system can be used to add cold vapor- ized water to the sand mixture without creating droplets that can wash binder from the sand. Sand changes due to crusting
can be reduced using machines with dedicated mixers directly above the
core shooting head. The mixer also can be positioned directly on the shooter to optimize shot pressure. This system eliminates the need for the filling operation and sand deposits in the funnel.
Corebox and Coreshooter Te normal temperature of a
corebox when curing is approximately 302-428 (150-220C). Tese tempera- tures can be reached using electricity, thermo-oil or steam. Termal expan- sion should be taken into account when designing the corebox. To achieve short cycle times, sufficient vents must be used to exhaust mois- ture from all areas of the corebox. Both corebox heating and heated and dried purge air must be pro-
Table 1. Comparing Corebox Heating Methods Heating
Advantage
Electro-heating via resistance heating with heating elements
-Well known technology from the hotbox and shell process
-Easy to carry out and operate -Simple corebox design using external heating plates
Thermal oil heating
-Good heat transfer -Well known technology from the diecasting process -Easy to operate
Steam heating
-Exact temperature control -Very good heat transfer
-Minimal effort in wiring compared to thermal oil process
-High energy concentration -Exact temperature control
28 | MODERN CASTING October 2012
from the heated corebox, the core machine also warms up. Te funnel, core shooter head, plate and nozzles (if used) should be cooled to avoid pre- mature hardening of the sand mixture. Some inorganic sand systems
are particularly heat active. Cool- ing the core shooter head and plate can prevent premature hardening of the sand mixture and significantly reduce cleaning time. When cool- ing, metalcasters must ensure the temperature difference between the cooled components and ambient temperature is not too high. This can cause condensation that can wash the binder from the sand. The temperature of each water-cooled circuit can be controlled to further ensure production quality and repeatability. Depending on the binder system
used, corebox temperatures are similar to those of the warmbox and
Disadvantage -Limited heat transfer
-Efficient heat transfer depends on the quality of the corebox. -Positioning the heating elements restricts the freedom of corebox design
-The total energy input with large coreboxes is difficult
-Unusual corebox design because of the channeling for the molten material
-Complicated, costly wiring is required
-In some cases unknown technology for moldmaking -Steam is under pressure
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