Mold Maintenance & Repair

OCT 2015

Mold Maintenance & Repair

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F E AT U R E 8 Mold Maintenance & Repair FIGURE 4: Illustrating a micrographic examination of the metallurgical structure of martensitic stainless steel (440C) after dry ice cleaning, the images on the left show unaltered carbon particles at the core before (top) and after (bottom) dry ice cleaning, while the images on the right show the same particles at the surface. particles at the core and at the surface, before and after dry ice cleaning. A third study, "Dry Ice Blasting for the Conservation Cleaning of Metals" by Rozemarijn van der Molen, Ineke Joosten, Tonny Beentjes and Luc Me- gens, revealed that cleaning with dry ice does not damage most industrial substrates because the particles are relatively soft. The hardness of dry ice pellets was found to be 1.5-2.0 Mohs, which is softer than other forms of blast media (see Figure 5), therefore dry ice particles are non-abrasive to any substrate harder than dry ice. One of the reasons dry ice cleaning is so successful on more delicate mold substrates and complex geometries is its process fexi- bility. The kinetic energy can be controlled by changing the size of the dry ice particles and the pressure/velocity settings on the machine. For example, gentler cleaning can be achieved by using 0.3-mm dry ice microparticles in lieu of the traditional 3.0-mm pellets (see Figure 6). The smaller-sized microparticles of dry ice require less air to attain full acceleration, bring- ing gentle but effective cleaning to delicate substrate metallurgies or mold features. The decreased air movement also results in much quieter cleaning operation. The thermal effect of dry ice cleaning also improves its effectiveness. The dry ice's inherently low temperature of -109°F (-79.5°C) creates a unique, thermodynamically induced surface mechanism that affects the adhesion of contaminants to the substrate. The process shrinks the contaminant, creating rapid micro- cracking and causing the bond between the contaminant and the mold to fail, enabling the contaminant to be easily removed from the mold. This thermal effect rapidly disappears once the dry ice strikes the contaminant, so the mold itself is not damaged. Another aspect of dry ice cleaning is the sublimation that occurs when the dry ice makes contact with the contaminants on the mold. Volumetrically, the dry ice will expand around 800 times its size during sublimation, which essentially enables the contaminant to be blown off the mold from the inside out. "C0 2 Pellet Cleaning–A Preliminary Evalua- tion," a study conducted by James Snide and published by Materials & Process Associates Inc. in October 1992, measured the thermal stress that occurs during dry ice cleaning. This study showed that the temperature decrease occurs on the surface of the mold only, so there is no thermal stress to the substrate metal. To FIGURE 5: This chart shows the Mohs hardness scale for minerals commonly used as blast media. 1 – Talc 2 – Gypsum, Dry Ice (fngernail, baking soda ~ 2.5) 3 – Calcite (penny) 4 – Fluorite (corn cob ~ 4.5) 5 – Apatite (glass beads, nut shells ~ 5.5) 6 – Orthoclase, Feldspar, Spectrolite (steel fle ~ 6.5-7.5) 7 – Quartz, Amethyst, Citrine, Agate (garnet ~ 7.5) 8 – Topaz, Beryl, Emerald, Aquamarine 9 – Corundum, Ruby, Sapphire (aluminum oxide ~ 8.5) 10- Diamond

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