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Predicting Binder Gas Pressures and Blow Locations in Chemically Bonded Sand Cores

This Development Note highlights the Sand Core Gas model which will be a new feature for FLOW-3D metal casting customers to be released in FLOW-3D version 9.4.

Castings with internal cavities (valve bodies, engine blocks and heads, etc.) can only be made with multi-part mold assemblies. In addition to the drag and the cope, chemically bonded sand cores are “printed” into the mold halves to shape the internal cavities. During the pour of the metal and throughout solidification, the core binders thermally degrade—enough for the core sand to be simply shaken out from the casting cavities.

The insufficient mechanical strength associated with fine core features is typically compensated by chemically bonding core sand. Some of the commonly used binders are polyurethane cold-box (PUCB) and a shell sand binder. The higher than normal heating rates in small cores and the associated higher decomposition rates of the binder are accommodated through the use of specialty sands with high gas permeability and through the use of special venting techniques. For example, in some castings, one drills through the mold to core prints. In others, the core is "shelled" by not letting the binder cure to full depth during the core manufacture.

Monitoring Binder Degradation

Figure 1. Evolution of the binder degradation zone. A small 2 x 1.12 inches in diameter PUCB bound core is held in an insulated steel holder and immersed in iron. (Metal flow field is not shown.)

A new model is being developed for FLOW-3D that will allow casting users to monitor binder degradation in cores and realize optimal core venting strategies. The model will predict core pressures, surface locations of binder gas loss into the metal, binder degradation zone evolution and binder gas flow fields. The development is undertaken in collaboration with experimentalists who are providing calibration data for commonly used commercial binders.

Figures 1 and 2 show the results of a binder degradation simulation in one such calibration sample. A small cylindrical, 2 inches by 1.125 inches in diameter core bound with a PUCB binder is held in an insulated steel holder and immersed in iron at 2600 Fº. The binder degrades quickly at the core surface which gives a peak in collected gas volume at about 2 seconds. The complete degradation of the core takes much longer. The binder degradation model captures the timing and the magnitude of the first volume rate peak, suggesting that during the critical part of degradation the model results for peak pressures will be accurate.

PUCB binder calibration data fit with the FLOW-3D core gas model
Figure 2. Calibration data for PUCB binder and fit with the FLOW-3D core gas model. The real data can be captured to allow simulations of PUCB degradation in more complex geometries and under different immersion/flow conditions.
FLOW-3D simulation of steel pressure and core gas pressure
Figure 3a. Core gas pressures and metallostatic pressure in steel. At the top of the core, near the holder, the gas can vent into the metal.

 

FLOW-3D simulation of mass flux of core gas at core surface
Figure 3b. Mass flux of core gas at core surface. In addition to the desirable venting into the gas collector at the top of the core, the gas is escaping into the steel along the upper side of the core.

To take the modeling one step further, the calibrated model is used to simulate the degradation of binder in a longer, 4 inch cylindrical core immersed at 2900 Fº in steel. Here the data shows that almost half of binder gas is not collected at the top of the core. The simulation predicts (Figs. 3a, 3b, and 4) that for shallow immersion depths a significant amount of core gas is vented into the metal along the upper side of the core. However, at larger immersion depths the core is shown to be sealed by the pressure of the surrounding steel.

Simulated total core surface gas mass flux
Figure 4. Simulated total core surface gas mass flux (black line) and the mass flux into the metal (red line). It is evident that a large portion of binder gas escapes into the metal. At about 40 seconds, the core is sealed again since a sufficient amount of binder degraded and pressure in the core decreased.