Simple Solutions That Work! Issue 12

60 Part where shrinkage occurred Two H13 core pins Reinforced cooling by three pins DHA-­‐Thermo pins Dramatically improved yield rate biscuits being measured by radiant thermograph after unclamping and just before lubricating, the biscuit off the Thermo sprue core was 120 C cooler in the same time frame. Figure 2 shows the improved cast structure of the aluminum in the biscuit off the Thermo as a result of the more rapid cooling. A die or insert made from one of the high thermal conductive steels will typically have an in-service surface temperature 50 C-to 90 C cooler than an H13 component with the same cooling. One benefit of the lower die temperature and improved heat transfer is reduced porosity. Figure 3 shows a typical comparison of the improved part quality due to reduced porosity. Figure 4 is a piston for an Italian made motor scooter. The dies for these pistons were formerly made of Din 1.2343 ESR (H11). Switching to a high thermal conductivity steel yielded several benefits. First, they were able to shorten the die build time from 4 weeks to 3 weeks due to the elimination of the need to heat treat. Toolox 44 is supplied pre hardened to HRC45. Second, scrap rate due to porosity was substantially reduced. Third, the cast structure and mechanical properties of the piston were improved due to the more rapid solidification. An unanticipated benefit was that heat checking was slower to develop. It was quickly ascertained that this was due to the lower operating temperature of die surface, 50 C to 90 C cooler – there was a reduction of thermal stress. How Thermal Conductivity Affects Heat Checking Resistance Thermal stress applied on die surface σ = C x E x a x ∆ T C : Constant including Poisson ratio E : Young’s modulus a : Thermal expansion coefficient ∆ T: Temperature difference between surface and inside HIGHER THERMAL CONDUCTIVITY = reduced ∆ T = reduced σ Heat checking on moving die com- ponents as of 10,000 shots Resolving Shrinkage Issues to Improve Yield Rates Figure 6 is a motorcycle cylinder head cover. The original design called for two cooled core pins in the high lighted area. Insufficient cooling resulted in serious porosity due to shrinkage problem and a high scrap rate. A third H13 cooled core pin was added, but the high scrap rate due to shrinkage persisted. The H13 cooled pins were changed out for Thermo cooled core pins. The scrap rate due to shrinkage went to zero. Soldering is another issue that can be improved by using a high thermal conduc- tivity die mate- rial. Soldering is a chemical combination of the alu- minum alloy and the iron in the die steel. Lowering the surface temperature of the die, core pin, or insert that is sol- dering is one of the ways to improve the situation. Tungsten alloys have proven effective in lowering the surface temperature, but their high cost and low mechanical properties make them a last resort. The new, higher thermal conductive steels offer a new opportunity to reduce soldering issues. Using the same cooling as an H13 core pin or insert in a Toolox 44 or Thermo component will reduce the operating tempera- ture of the surface of the component by 50 C minimum and help reduce instances of soldering. By taking advantage of the high thermal conductivity steels, die casters are reducing scrap loss, improving the quality of the castings, and reducing cycle times. Figure 4 Figure 5 Figure 6 Figure 7 Contact: TOM SCHADE tos@imsteel.com