Examining Mold Hardness, Strength
The control and maintenance of a green sand system is an important part of producing quality sand castings. Many tests are available to determine the critical physical properties of the sand in
the system and whether the mixture
Sand tests for properties such as moisture and compactability help control water additions. Tests for methylene 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 effectiveness 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 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 squeezers and hand ramming. The 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 molding equipment, the C scale hardness tester can provide more sensitive results. The cone-shaped penetrator (about 0.375-in.) is smaller and the spring load higher (about 1,500g). These 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 surface. Electronic versions of these tools also are commonly used.
- Equipment: B- or C-scale mold hardness tester.
- Procedure: Press the base of the hardness tester firmly against the end of a standard 2 x 2-in. sand specimen or a flat portion of a vertical or horizontal mold surface. Record the test results and designate the type of mold
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. This 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.
The penetrometer is pushed into the mold surface until the indicator reads that the instrument has penetrated to a pre-set depth. The reading typically gives the pounds per square inch of pressure required to reach the depth of penetration. The 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. This test also leaves a small impression on the mold surface and a potential dimple on the casting.
- 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 equipment manufacturer.
The 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.