Heat Transfer Coefficients are values that indicate how well or how poorly heat is transferred across a SURFACE, as opposed to heat flow WITHIN a material.
For most sand and investment castings, we are concerned primarily with how well heat is transferred from the exterior surface of the mold. The internal HTC’s are masked by the low thermal conductivity of the mold material. In permanent mold applications, we want to model the effect of mold coatings on the die surfaces, so we TURN ON internal HTC’s and enter appropriate values in the HTC table.
To set Heat Transfer Coefficients, from the menu bar select Model… Materials List and then click on the tab labeled HT Coefficients. The following shows a screen that would be typical for a sand casting simulation:
Here there is no check mark in the box labeled Use Internal HT Coefficients, so the system is considering only external surface HTC’s.
Suggested values for the external HTC for sand casting would be about
1.5 BTU/hr-sqft-F, and for investment casting would be around 8-20 BTU/hr-sqft-F.
For permanent mold applications, we would click on the box labeled use Internal HT Coefficients, and then set HTC’s between different materials as shown here:
When “Use Internal HT Coefficients” is selected, is necessary to specify an HTC between ALL materials in the model that are in contact. To do this, select the first material in the left-hand window, select the second material in the middle window, and then enter the HTC in the right-hand window.
Some suggested values for use in typical permanent mold applications are:
Mold to Casting 200 1135
Mold to Riser 125 710
External Mold Surface 6-8 35-45
Air Cooling Channel 25 140
Cores to Mold 150 850
Cores to Casting 150 850
The HTC Calculator utility can be used to estimate HTC values for a number of situations, such as air and water cooling channels, convection, radiation and insulating coatings. Full instructions for this utility are given in Unit 46 of this workbook.