NADCA Die Materials Committee Evaluation of Bubblers and Baffles for Cooling of Die Casting Dies John F. Wallace David Schwam Sun Feng Sebastian Birceanu Case Western Reserve University Cleveland, OH - March 5, 2003
Introduction Background: Thermal fatigue cracking of die inserts is a constant problem in die casting industry. High temperature of die surface is the most frequent cause of thermal fatigue cracking, because it promotes higher thermal stress and accelerates softening. A temperature threshold exists, below which the thermal fatigue damage is minimal. Bubblers and baffles are typical water cooling devices for die inserts, slides and cores.
Effect of Overheating to H13 Die Steel Specimen: Ø1 0.5, Premium Grade H13 Steel, quenched and tempered to 46 HRC. 1000 F 1050 F 1100 F 1150 F 1200 F 1 hour No.1 No.5 No.9 No.13 No.17 10 hours No.2 No.6 No.10 No.14 No.18 50 hours No.3 No.7 No.11 No.15 No.19 (15h) 100 hours No.4 No.8 No.12 No.16 No.20(20h)
Temperature-Time Effect on Softening of H13 50 1000 F 45 Hardness [HRC] 40 35 1050 F 1100 F 30 1200 F 1150 F 25 0 20 40 60 80 100 120 Tempering Time [hour] Discussion: Critical temperature is between 1050 o F and 1100 o F. If the die insert surface temperature can be kept below 1050 o F, the service life of the die insert will be prolonged.
The Effect of Temperature on Thermal Fatigue Cracking Different Immersion Times 200 180 15, 000 cycles 12 sec Total Crack Area [x10 6 m 2 ] 160 140 120 100 80 60 40 9 sec 20 5 sec 7 sec The Effect of Immersion Time 0 900 950 1000 1050 1100 1150 1200 Temperature [F]
The Effect of Thermal Cycling on Thermal Fatigue Cracking Different Immersion Times 180 160 5 sec 7 sec The Effect of Immersion Time 12 sec Total Crack Area [x 10 6 m 2 ] 140 120 100 80 60 40 9 sec 12 sec 9 sec 20 0 7 sec all below 0.2 5 sec 5000 10000 15000 Number of Cycles
The Effect of Temperature on Thermal Fatigue Cracking Different Cooling Line Diameters 120 15,000 cycles 1.5" 100 Total Crack Area [x10 6 m 2 ] 80 60 40 20 1.8" 1.7" 1.6" The Effect of Cooling Line Diameter 0 900 950 1000 1050 1100 Temperature [F]
Illustration of Bubblers and Baffles Used in the Experiment Baffle Bubbler
Schematic of Water Flows Directed by Bubbler and Baffle Water Inlet Bubbler Water Inlet Water Flow Water Outlet Water Flow Water Outlet Specimen Baffle Specimen T/C Hole T/C Hole Water Flow Directed by Bubbler Water Flow Directed by Baffle
Geometry Details of Bubbler Cooled Specimen f1.5 f 9/16 Bubbler A-A Section A A Gap Between Bubbler and Specimen Specimen I.D.. O.D. 0.06 1 0.2 Bottom T/C
Geometry Details of Bubbler Cooled Specimen (continued) Inner Diameter of Bubbler [inch] 0.170 0.214 0.307 Outer Diameter of Bubbler [inch] 0.250 0.312 0.437 Wall Thickness of Bubbler [inch] 0.040 0.049 0.065 Inner Section Area of Bubbler [inch^2] 0.023 0.036 0.074 Bubbler/Specimen Gap [inch] 0.156 0.125 0.063 Bubbler/Specimen Annular Area [inch^2] 0.199 0.172 0.098
Geometry Details of Baffle Cooled Specimen f 1.5 f 9/16 A-A Section Baffle A A Specimen Controlled Gap Size 0.06 H 1
Cooling Efficiency Evaluation of Bubblers and Baffles in Molten Aluminum Dip In-Dip Out Experimental Setup Water Outlet Water Inlet Flow Meter Specimen (Cycle time: 25s in/25s out) Thermocouple Molten A356:1350 ºF (600lb) Furnace
The Specimen Used in Dip In- Dip Out Experiment 3/8 NPT 0.4 3/8 NPT 0.06 0.075 8.75 10 f 9/16 3/8
Data Processing for Dip In-Dip Out Experiment Bottom T/C Temperature [F] 1100 1000 900 800 700 600 T1 T2 T3 T4 T5 T = 1 n n i= 1 T i T6 T7 T8 T9 T10 T11 T12 T18 T19 T13 T14 T15 T17 T16 500 400 0 200 400 600 800 Time [second] 1000
Difference of Specimen Surface Temperatures with/without Water Cooling (Dip In-Dip Out Experiment; 1 Immersion) 1400 Average Peak Temperature [F] 1300 1200 1100 1000 900 1312 915 900 892 882 875 874 800 0 0.41 0.83 1.38 2.15 3 3.7 Flow Rate of Cooling Water through 0.17 Bubbler [gallon/minute]
Cooling Effect of Bubblers and Baffles on Hot Spot (Dip In-Dip Out Experiment; 1 Immersion) 1020 1000 Baffle: Gap Area=0.17 inch^2 Average Peak Temperature [F] 980 960 940 920 900 880 860 Baffle: Gap Area=0.09 inch^2 0.307 Bubbler 0.214 Bubbler 0.17 Bubbler 0 1 2 3 4 5 Flow Rate of Cooling Water [gallon/minute]
Effect of Bubbler Size and Water Jet Velocity on the Efficiency of Hot Spot Cooling (Dip In-Dip Out Experiment; 1 Immersion) Average Peak Temperature [F] 930 920 910 900 890 880 0.307 Bubbler 0.214 Bubbler Water Jet Velocity where F A d V Flow Rate = Section Area F A I.D. of Bubbler = 0.17 Bubbler π 4 d F 2 870 0 10 20 30 40 50 60 Water Jet Velocity [feet/second]
Effect of Water Velocity on Baffles Cooling Efficiency on Hot Spot (Dip In-Dip Out Experiment; 1 Immersion) 1030 Average Peak Temperature [F] 1020 1010 1000 990 980 970 960 950 940 Gap Area=0.17 inch^2 Velocity of Cooling Water: F 0 5 10 15 20 A V = where: Flow rate; Gap Area Velocity through Gap [feet/second] F A Gap Area=0.09 inch^2
0.06 Position of Thermocouples for 4.5 Immersion Experiment 0.23 T3, T3 0.23 T2, T2 1 3.5 T1, T1
Cooling Efficiencies of a Bubbler and a Baffle for a Large Area (Dip In-Dip Out Experiment; 4.5 Immersion) 1100 Average Peak Temperature [F] 1050 1000 950 900 850 800 T1 T2 T3 T1' T2' Immersion Depth: 4.5 : Bubbler; I.D=0.17 : Baffle; Gap Area=0.09 inch^2 750 T3' 700 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Flow Rate of Cooling Water [gallon/minute]
Conclusions The surface temperature of a die casting die plays an important role in the failure of the die. Overheating to die steel results in degradation of the microstructure and softening of the steel. The use of a bubbler or a baffle can be very effective in lowering the surface temperature of a die insert, core or slide. A baffle is more suitable to cool a certain area, while a bubbler is better for cooling a specific hot spot. For a given insert and cooling mode, higher flow rates improve cooling leading to lower surface temperature.
Conclusions (continued) For a given cooling line and flow rate a bubbler with a smaller inner diameter has a better cooling efficiency at the hot spot than a bubbler with a larger inner diameter. At same flow rate, high velocity of cooling water enhances the heat transfer between the cooling water and the sidewall of a cooling line. For design and application purposes it is preferable to set up a high velocity of cooling water in a cooling line in order to lower surface temperatures.