Sand Core Making – Is It Time to Vent?

Sand cores are a crucial element in the casting process because they are used to create complex interior cavities. For example, sand cores are used to create passages for water cooling, oil lubrication, and air flow in typical V8 engine casting. Ever wonder how a sand core is made? How can a material that works so well for making sandcastles on the beach be made into complex forms able to withstand the brutal conditions of hot metal flowing and solidifying around them? In this blog I will walk you through the process of how sand cores are made and describe the modeling tools in FLOW-3D CAST v5.1 that help engineers design their manufacturing processes.

The Sand Core Making Process Workspace

Choosing the correct physics models for such complex flow dynamics to model sand core making can be daunting. The Sand Core Making Workspace addresses this challenge by providing automated settings for numerical techniques and activating the appropriate physics models. Sub workspaces for cold box, hot box, and inorganic processes guide the user through the setup process with ease.

Sand Shooting

The starting point with all sand cores is the shooting process. In the shooting process, a mixture of air, sand, and binder is “shot” under high pressure into a core box with air vents placed strategically around the cavity to allow air to be displaced by sand.
Water jacket sand core
Simulation of a water jacket sand core. The sand/binder mixture is shot into the core box through the 8 inlets at the top. Air vents of varying size are placed around the sand core to allow air to escape.

The primary goal of a sand core shooting is to create a sand core with uniform density. Two design factors play important roles in achieving this goal — the location of sand inlets and the location and size of the air vents. Simulating the flow of the sand mixture using FLOW-3D CAST allows us to study different inlet and air vent configurations.

This video shows the filling pattern of H32 sand with a 2% binder additive being shot to produce a water jacket sand core. Notice that some of the regions are underfilled.

To address underfilling, air vents can be easily and accurately placed at the problem area using our interactive geometry placement tool. Here, a 6 mm air vent (see red arrow) is placed at a location where incomplete filling was observed.

This video shows a comparison of the filling in the region where the air vent has been added compared with the original result. The filling is now more complete in the region where the air vent was added. More vents can be added to address other underfilled regions.

Core Hardening

Once the air vent configurations have been placed and the shooting provides a uniform sand distribution, the sand core needs to be hardened. Three different hardening methods can be simulated in FLOW-3D CAST: cold box, hot box, and inorganic.

Drying Sand Cores in an Inorganic Process

The sand/binder mixtures used to produce inorganic cores are water based. To harden them, energy from the hot core box along with a hot air purge evaporate the water and carry it out of the core through the air vents. In this video, an intake manifold sand core shot with a sand/binder mixture containing 2% water by weight is dried by a hot (180 C) air purge. The blue region represents the water remaining in the sand core. The air vents are shown in gray. After 150 seconds of drying, the moisture continues to be pushed to the area where the most venting occurs.

Hardening Cores in a Hot Box Process

Sand cores shot in a hot box process are hardened using energy from the core box to cure the binder. This video shows the temperature distribution in the sand core as it is heated by the hot core box.

Simulating the hardening step allows us to determine the temperature distribution in the shot sand core and identify the time required to ensure that all regions of the core are sufficiently heated to harden it.

Gassing Sand Cores in a Cold Box Process

The binder used to produce sand cores shot in a cold box process contains a phenolic urethane resin. To harden these cores and give them the strength required to withstand flowing hot metal in the casting process, hot air carrying a catalyst (amine gas in this case) is used to purge the core. The hot air/amine gas mixture is introduced through the inlets and leaves the core box through the air vents that were used in the shooting step.

This video shows the evolution of amine gas through the porous shot sand core, which is a water jacket for an internal combustion engine.

With FLOW-3D CAST v5.1, sand core manufactures have the tools they need to model their sand core making processes to optimize the quality of their cores. Learn more about the Sand Core Making Workspace.

John Ditter

John Ditter

Principal CFD Engineer at Flow Science