fi nish at all levels. Sand grains that are packed tightly produce a more uniform surface over which molten metal will eventually fl ow, mimick- ing the appearance of the sand itself. Hand ramming makes for the worst compaction. Blowing produces a smoother surface than hand ram- ming, while the jolt-squeeze method provides an even smoother, more compact mold than blowing. Auto-


matic molding machines also gener- ate superior surfaces. Sand makes up a large majority


of the casting mold, and it has an equally signifi cant impact on the casting’s eventual surface fi nish. In all of the bonded sand pro-


cesses, the fi ner the sand grains in use at your supplier’s metalcasting facility, the smoother the surface fi nish you will see. Larger grains


produce larger peaks and valleys as molten metal fl ows into the spaces between them. T ose peaks and val- leys, generally measured in microns, produce a roughness value—root mean squared (RMS)—where small numbers mean a slick fi nish. Smaller grains sit more closely


to one another to produce that smooth surface fi nish, but they also will produce a less permeable mold,


Table 1. Surface Finish Capabilities by Process (in RMS, with extreme low values in parentheses)


Sand Processes Shell


Nobake Coldbox


Lost Foam


75-150 (40) 150-600 (40) 150-600 (40) 125-175 (100)


Horizontal Green Sand 250-900 (100) Vertical Green Sand Vacuum Casting


250-900 (100) 150-200 (125)


Metal Mold Processes Diecasting


Squeeze Casting Centrifugal Continuous


90-200 (20) 90-200 (20)


450-500 (100) 450-500 (100)


Low Pressure Perm. Mold 250-400 (160) Permanent Mold


250-420 (180)


Ceramic Mold Processes Plaster


Ceramic Investment


40-125 (25) 60-175 (25) 50-125 (32)


1000


800


600


400


200


0


The chart shows ranges of RMS values each process is capable of producing. Extreme low values are indicated below the range. Jan/Feb 2016 | METAL CASTING DESIGN & PURCHASING | 43


Shell Nobake Coldbox


Lost Foam Horizontal


Green Sand Vertical


Green Sand


Vacuum Casting


Diecasting


Squeeze Casting


Centrifugal Continuous


Low Pressure Perm. Mold


Permanent Mold


Plaster Ceramic Investment


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