This page contains a Flash digital edition of a book.
MARKETING MATTERS CAST TIP


Examining Mold Hardness, Strength T


AFS TECHNICAL DEPARTMENT


he control and maintenance of a green sand system is an important part of produc-


ing quality sand castings. Many tests are available to determine the critical physical properties of the sand in the system and whether the mixture is consistent. Sand tests for properties such as


moisture and compactability help con- trol water additions. Tests for methy- lene blue clay, green strength and AFS clay define bond additions. Tests for permeability, AFS grain fineness and other properties will determine grain sizing and new sand additions. Together, these tests can demonstrate if the sand mixture is consistent and indicate whether it will produce a quality casting. However, these tests are typically lab-based and do not ensure a good mold is produced on the facility floor. Testing also should quantify the effec- tiveness of moldmaking equipment to produce uniform filling and consistent compaction, density and hardness throughout the mold. Molds that are too soft can lead to:


• broken sand molds; • mold crush; • sand wash during pouring; • mold erosion; • sand inclusions; • metal penetration into the casting; • dimensional problems and over- sized molds. Excessively hard molds can produce:


• veining defects; • hot tears; • hard ram defects. Surface strength and hardness tests


can be run on the completed sand mold to help identify soft spots due to uneven compaction or squeeze and verify the hardness of the mold prior to pouring.


Mold Hardness Test The mold hardness test is similar


to the Brinnel hardness test used on metal castings, where a ball or point is pushed into the mold surface to


Table 1. Comparison of Mold Hardness and Other Sand Property Tests* Test


Compactability (%)


Bulk Density (g/cu. cm) Moisture Content (%) Specimen Weight (g) Permeability (#)


Splitting Strength (psi)


Compression Strength (psi) Mold Hardness (#) Friability (%)


30


0.98 1.9


158 228 3.9 21 92 31


Lake Sand 35


0.89 2.5


158 233 5.4 24 94 17


40


0.88 2.7


152 261 5.0 24 96 10


30


1.11 1.9


158 175 3.6 22 90 32


*This data reflects different green sand specimens (as mulled) at various compactability levels.


measure ‘pushback’ resistance. A handheld mold hardness tester is used to show the penetration of the ball or point—the softer the mold, the greater the penetration. A type B scale tester is used for


softer molds (up to 90 mold hardness reading), like those typical of squeez- ers and hand ramming. Te tester uses a larger diameter ball (about 0.5 in.) under a spring load of approximately 980g. For testing denser molds, such as those produced on automatic mold- ing equipment, the C scale hardness tester can provide more sensitive results. Te cone-shaped penetrator (about 0.375-in.) is smaller and the spring load higher (about 1,500g). Tese two instruments can be used only on flat sections of the mold, and each leaves a slight dimple on the casting if used on a mold cavity sur- face. Electronic versions of these tools also are commonly used.


Test Procedure


• Equipment: B- or C-scale mold hard- ness tester.


• Procedure: Press the base of the hard- ness tester firmly against the end of a standard 2 x 2-in. sand specimen or a flat portion of a vertical or hori- zontal mold surface. Record the test results and designate the type of mold tester used.


Mold Strength Test A mold strength tester can be


used in much the same manner as a hardness tester, but it can be more definitive on harder molds (more than 92 mold hardness) than the C hardness tester. Tis instrument has a thin, round, pin-type penetration tool (about 0.125-in.) and gives a reading comparable to the green compression test conducted in the lab. Te penetrometer is pushed into the mold surface until the indicator reads that the instrument has pen- etrated to a pre-set depth. Te reading typically gives the pounds per square inch of pressure required to reach the depth of penetration. Te advantage of the tool is that it can be used almost anywhere in the mold, regardless of geometry, and does not require a flat surface. Tis test also leaves a small impression on the mold surface and a potential dimple on the casting.


Test Procedure


• Equipment: Mold strength tester. • Procedure: Apply sufficient pressure to allow the penetrometer to move slowly into the mold surface. Record the pressure when the penetrometer has penetrated the mold surface to the depth recommended by the equip- ment manufacturer.


Te information in this article was drawn from the American Foundry Society’s Mold & Core Test Handbook and the white paper “Desirable Green Sand Properties via Aeration Sand Filling,” by Sam Ramrattan, Western Michigan Univ., Kalamazoo, Mich.


August 2011 MODERN CASTING | 51


Round Grain 35


0.98 2.3


158 193 3.9 20 94 15


40


1.05 2.1


154 201 4.3 23 93 15


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  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68