used to develop 15 speci- mens for each green sand systems studied were: A. 100% Western bentonite. B. 100% Southern bentonite. C. 75% Western bentonite, 25% Southern bentonite. D. 50% Western bentonite, 50% Southern bentonite. E. 25% Western bentonite, 75% Southern bentonite. AFS sand tests measured
properties of each green sand system (Table 1). Te key variable was compactability, a mechanical property related to the percentage decrease in height during compaction at the molding machine. Moisture and compactability are the key tests for controlling water additions. Density, specimen weight, perme-
Fig 3. The green sand samples subtly change color after TET for various temperatures.
ability and grain fineness are consid- erations when determining new sand additions. Before proceeding with the other tests that require a standard test specimen, the specimen weight needed to be determined. Compacted density can be determined simultaneously. It is important to record the specimen weight because the weight provides useful information regarding changes in sand composition. If the speci- men weight increases, the sand’s silica content has increased, because silica is the heaviest component of the sand. If it decreases, either the additives have increased or the amount of dead clay and ash has increased. In this way, it can be used as a guide for determining the need for new sand additions.
Procedures & Results Te erosive flow of molten metal is
a major cause of casting defects. Te brittleness of green sand will only serve to augment this issue. Four green sand tests are indicative of green sand brittle- ness. Friability and wet tensile strength tests are currently used in the industry, the modified cone jolt and thermal erosion tests may allow metalcasters to improve the control of their green sand systems, in a more timely manner. Wet Tensile Strength (WTS)
Test: Tis is useful for determining the quality of incoming bentonite consignments. A higher WTS value means the sand blend will resist scab-
bing. During casting, water from the sand adjacent to the molten metal is driven back, creating a condensation zone between the dry and wet sand. Te strength of the sand in this layer is considered the wet tensile strength. As compactability increases (increas- ing moisture) for the same level of clay in the green sand system, WTS increases until 39% compactability, after which, the data indicates a decline (Fig. 1). Te green sand system A (100% West- ern bentonite) had the higher WTS compared to the green sand system B (100% Southern bentonite) but a more dramatic decline was observed in system A. Te point of inflection corresponds to the thermal erosion test data. Friability Test: Friability measures the sand’s surface brittleness and abra-
sion resistance on mold edges, corners, parting lines and abraded mold surfaces devel- oped during molding, core set- ting and handling. Loose sand can result in sand inclusion defects on casting surfaces. In general, friability is inversely related to compactability. A small drop in compactability, or a brief air-drying period, will produce a large increase in friability. Higher clay levels will reduce friability. As expected, compactability and friability have an inverse
relationship. As moisture content in the green sand increases, surface brittleness decreases in the green sand for the same level of bentonite (Fig. 2). As friability increases, the ability to
draw deep pockets decreases, causing the sand to fall away resulting in defects such as sand inclusions. Te AFS fri- ability test is run at room temperature and does not add heat or pressure to the sand specimen, which doesn’t mirror a green sand mold experiencing heat and erosive flow from molten metal. When clay in the sand is heated at the mold metal interface, it loses moisture and may go through a phase change. When organic additives to green sand, such as starch, dextrin and cereals, are heated at the mold metal interface, they burn-off. Te current test does not recognize that
Table 1. Properties of the Green Sand Systems A
Compactability (%) Moisture (%)
2" x 2" Specimen Weight (g) Permeability Number Mold Hardness (B scale)
Green Compression Strength (PSI) Dry Compression Strength (PSI) Wet Tensile Strength (N/cm²)
Weight of Compactability Specimen (g) Working Bond (%) Available Bond (%)
Muller Effectiveness (%)
Hot Compression Strength (PSI) 1,650F 1,850F 2000F
Methylene Blue Active Clay (%) AFS Grain Fineness Number AFS Clay Content (% <20 µm)
37 B 37
96 91 27 74
C 38
96 93 23 59
D 38
96 93 22 51
E 38
2.43 2.69 2.65 2.65 2.84 170 166 168 168 165 84 90 27 61
94 90 20 48
0.499 0.256 0.458 0.407 0.288 254.6 243.4 226.5 241.6 226.8 4.4 6.1 72
4.4 6.4 68
3.7 5.9 64
3.6 5.8 62
3.2 5.8 56
450 300 515 430 400 190 105 175 115 115 62
42
7.8 62
8.0
7.9 61
7.8 85
7.2 62
8.0
65 105 6.9 62
6.9 61
7.9 7.8 September 2015 MODERN CASTING | 37
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60