Mold hardness is the resistance of a mold to deformation or indentation and directly correlates to the packing density of the sand. Mold hardness/strength affects the quality of the castings produced. 

Molds with too high, too low, or inconsistent mold hardness/strength can result in a various consequences. Lack of mold hardness can lead to shrink defects, internal porosity (due to mold-wall movement), and poor surface finish. Low hardness can also be an indicator of a weak mold, leading to runouts. Harder molds result in less mold-wall movement and therefore better tolerance control. Mold hardness that’s too high can lead to gas defects (low permeability), scabs, buckles, burn-on or penetration defects.

While it is important to run primary green sand lab tests every turn of the sand system, a green sand test represents a large volume of sand, whereas within a mold, the sand can vary. This is why mold hardness/strength testing on the line is critical. It can identify variation from mold to mold and even throughout a mold. Mold hardness/strength is a valuable check to help with scrap cause identification.

Testing Hardness and Strength 

Production line supervisors and anyone working on the molding machine should have a mold hardness or mold strength tester accessible during production operations. It is a quick quality check that provides an instant assessment of the mold. It can also help troubleshoot issues that occur between the muller and the time the mold is made. 

What is the difference between mold hardness and mold strength? A mold hardness tester is used to determine the resistance of the mold surface when penetrated by a loaded plunger. It measures the depth of penetration into the mold surface. In many ways, the mold hardness is measuring the bulk density of the mold at a specific point.
A mold strength tester measures force, which also corresponds to the penetration resistance. The mold strength tester has a higher sensitivity and in addition to bulk density is a good indicator of the strength of the clay bonds. The mold strength penetrator is a thin pin/rod and the result is measured in either N/cm^3 or psi. 

In the past, mold hardness testers utilized a spring load to determine the depth of the mold penetration. The mold strength tester eliminates traditional sources of error such as non-linearity of springs and “spring-back” of highly compacted sands by utilizing a crystal oscillator sensing element. This allows the measured force to correspond to actual mold penetration resistance. 

What’s the difference between a “B” and “C” hardness tester? There are two scales for measuring mold hardness and they can be identified by the shape of the penetrator. The B-scale has a round penetrator, whereas the C-scale has a pointed penetrator. Both scales measure the surface hardness of the mold. The C-scale has a higher load on the spring and provides more resolution. It is important to note that the scales for the B and C are not the same, as it requires more pressure to move a smaller diameter. The B scale is more common in green sand foundries. The B-scale hardness tester is also commonly found in chemically bonded sand applications for measuring hardness of work and strip times. 

The hardest mold doesn’t necessarily produce the best casting. Ideally the same location on each mold from a given pattern should be consistent. The dial mold hardness testers were replaced with electronic digital testers in the early ’90s. Current versions not only provide digital results, but advanced models can download up to 900 data points with 32 pattern numbers defined by the user. Advanced models include two additional modes. The first allows the operator to measure mold hardness at several locations on the mold surface. The instrument will automatically display the number of test points and average mold hardness. The second mode allows the operator to test and store a series of points on the surface of a mold. The points can then be plotted as a hardness gradient and is useful in determining the change in mold hardness, for instance in a deep mold pocket. 

Factors Influencing Mold Hardness

Sand Composition. Sands with higher clay content (and equivalent moisture) tend to have higher hardness due to better binding properties. However, excessive clay can lead to reduced permeability and mold defects. At higher clay levels (specifically sodium bentonite) sand tends to become less flowable and without adequate aeration prior to mold filling, foundries may experience soft areas within the mold due to uneven filling of the mold. Calcium bentonite clay collapses more readily. It can be expected that molds with higher amounts of calcium bentonite than sodium bentonite will have a lower mold hardness, pending all other parameters are equal. Cushioning additives, such as wood flour, can be added to help minimize expansion defects.

Moisture Content. The moisture content in molding sand affects its compactability and, consequently, mold hardness. Optimal moisture levels are dependent on the amount of bentonite in the sand and the sand temperature. Excess moisture beyond what is needed based on temperature and clay content may lead to mold washout and defects. Insufficient moisture can lead to weak molds prone to sand inclusion defects.

Sand Compaction. Compaction methods such as ramming, squeezing, or vibration help in densifying the sand grains, enhancing mold strength and rigidity. Inadequate compaction can result in soft molds with insufficient or uniform mold hardness, leading to casting defects like sand inclusion and surface roughness.

Machine. Both too high and too low of squeeze pressure can result in a low mold hardness. The shot time in a vertically parted molding machine is determined by the shot pressure and chamber size. There is an adjustable Shot Time Correction feature if the shot time is too short or too long. A soft mold can be due to too short of a shot time. Too low of a blow pressure or squeeze pressure can also result in a low mold hardness. Settings should be maintained to provide a consistent mold hardness. It is recommended to check the mold filling process and ensure that flask or chambers have adequate volumes of sand with even fill. Machines with variable squeeze head pressure zones are designed to help with uniform mold density and ensuring that zone pressure targets are set correctly can help with tooling designs that are prone to soft areas. 

Tooling. Low mold hardness in specific areas of a mold may indicate the need to replace or fix worn tooling. Soft spots in the mold may require evaluating venting on pattern plates. “Pattern wedges” can also help direct squeezing forces to areas prone to soft molds.

Implications of Mold Hardness/Strength in Foundry Operations

Green Sand Testing. If the same sand mixture is prepared to various mold strength ranges (as mold strength increases density) green, dry, and hot compression strength increase while permeability decreases. 
Surface Finish. Mold hardness influences the surface smoothness and perceived quality of castings. Harder molds typically provide better mold cavity stability, resulting in improved surface finish and reduced surface defects such as roughness and expansion defects. 

Mold Stability. Adequate mold hardness is essential to withstand the stresses and pressures during metal pouring and solidification. Insufficient hardness can lead to mold erosion, distortion, or swell, compromising casting integrity and quality. Mold swelling can result in castings outside of dimensional tolerances.

Production Efficiency. Proper control of mold hardness optimizes foundry processes, reducing rework, scrap, minimizing runouts, and downtime associated with mold-related defects. 

Springback. Springback refers to the elasticity changes in the sand. It occurs when the sand is over-squeezed and then quickly released and can result in dimensional inaccuracies or stickers.

Poor Shakeout. Castings with mold hardness that’s too high can be difficult to shake out. 

Gas Defects. If mold hardness is too high, the permeability will decrease, and castings may have increased gas defects.

Mold hardness is a critical parameter in foundry operations, impacting casting quality, dimensional accuracy, and production efficiency. By understanding the factors affecting mold hardness, foundries can implement effective measures to control and optimize mold properties. 

Proper sand composition, compaction techniques, tooling design, and process parameters ensures consistent mold hardness. Consistent and routine testing is crucial for identifying trends. Tracking mold strength/hardness alongside other primary green sand testing helps foundries understand how their mold strength/hardness is affected by other molding sand properties.