Mold Maintenance & Repair

OCT 2014

Mold Maintenance & Repair

Issue link: https://mmr.epubxp.com/i/389776

Contents of this Issue

Navigation

Page 10 of 27

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).

Articles in this issue

Archives of this issue

view archives of Mold Maintenance & Repair - OCT 2014