Mold Maintenance & Repair

OCT 2014

Mold Maintenance & Repair

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October 2014 9 FIGURE 2: This illustration depicts the difference between 3-mm dry ice pellets (top) and shaved micro-particles of dry ice (bottom). The micro-particles are more effective than larger pellets for cleaning delicate substrates with intricate geometries or tiny openings. cleaning method is similar to sand blasting, but this media is non-abrasive and does not create secondary waste. All the dry ice used in the process is recycled CO 2 from other manu- facturing processes. The dry ice cleaning process is based on three principles: the pellet kinetic energy ef- fect, thermal effect and gas expansion effect. Pellet kinetic energy effect. Similar to cleaning with blast media, the kinetic energy associated with dry ice cleaning is a function of particle mass density and impact velocity. To achieve the optimal kinetic energy effect and therefore optimal cleaning, dry ice parti- cles (which have a hardness of only 1.5 to 2.0 Mohs) are fully accelerated in a pressurized air stream to supersonic velocity at speeds of 600 to 1,000 feet per second. A change in dry ice particle size and velocity will impact the amount of cleaning accomplished. Figure 2 compares the greater fux density of the dry ice micro-particles that come from shaving a block of dry ice versus typical 3-mm pellets. The micro-particles are more effec- tive than larger pellets for cleaning delicate substrates with intricate geometries or tiny openings. They also produce 1,000 times more surface strikes for a given volume of ice, which provides more thorough surface coverage and faster removal rates on the thinner resin con- taminants that are commonly found on molds. Thermal effect. An inherently unique dry ice characteristic is its extreme temperature of -79.5°C (-109.3°F). This low temperature causes the resin contaminant to embrittle and shrink, creating rapid micro-cracking and causing the bond between the contaminant and the substrate to fail. This is the effect of the coeff- cient of thermal expansion and contraction of dissimilar materials. The hotter the mold, the greater the temperature differential between substrate and dry ice, and therefore the great- er the contribution of this effect, resulting in easier, faster mold cleaning. Gas expansion effect. Another unique dry ice characteristic is its ability to sublimate, or change from a solid to its natural gas state. This eliminates the secondary waste or grit entrapment that is often associated with other blast media types. Upon impact, the CO 2 par- ticles expand to as much as 800 times their original size, lifting the resin contaminant off the mold substrate from the inside out (see Figure 3).

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