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Tobin's antec â??nuggetsâ??, part ii: conformal cooling of mold cavities - Injection Molding Machin by airbru airbru
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Tobin's antec â??nuggetsâ??, part ii: conformal cooling of mold cavities - Injection Molding Machin by AIRBRU AIRBRU
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| Article Posted: 08/19/2013 |
| Article Views: 92 |
| Articles Written: 1137 - MORE ARTICLES FROM THIS AUTHOR |
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Tobin's antec â??nuggetsâ??, part ii: conformal cooling of mold cavities - Injection Molding Machin |
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Business,Business News,Business Opportunities
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finishes. Here conformal cooling again raised its head allowingthis particular product design to achieve high cosmetic finishes byinjecting high pressure steam into the cooling matrix therebyachieving the cosmetic finish via a 180° plus mold; and oncethe cavity was filled, the steam was chased out by flooding thecavity with cooling fluid. The cycles went from 18 seconds to 7.5seconds. Wow! Laser sintering by definition is done in layers. Unfortunately theprocess is still not that well controlled to make a homogeneouscore or cavity.
It looks kinda like wood, where you can see thegrain. Sintering is also porous and since the matrix is quicklymelted with the intense heat of the laser and then air quenched,these sintered parts are necessarily brittle. With the LCD TV bezels the combination of thermal shock and thecompression of the sintered cavities normally associated withmolding, the life span of this particular technology is around20,000 closures before the inserts start to crack. Sintered parts cannot be machined or welded. Thus if you smash apart into a cavity that is laser sintered, or it cracks due tothermal cycling and normal wear, it must be totally replaced.
In conventional molding (not steam injection) it would appear thesintered inserts can survive up to 50,000 closures before theycrack. This is because of two factors: (1) the closer yourwaterline is to the plastic surface, the higher the probability itwill crack in production. (2) Sintered metal doesn't compress verywell and still hold its structure. The LCD TV guys have the volumes of their product so high, andsince the savings using the steam/cool sintered cavity withconformal cooling are also financially attractive, they can affordto have about 10 molding machines producing these bezels and twolaser sintering machines constantly producing new cavities toreplace the ones that break in production. The nugget we take away here is that this technology, with both itsbenefits and its faults, can very adequately service a need in aspecific niche.
The economics of it doesn't justify it as the"cure-all" to every cooling problem; but in its place; it can't bebeat. Who needs laser-sintered cooling channels when you have ballbearings Another nugget was presented in a paper with faux-conformalcooling. The product was more like a box. We all know cooling thecore of a box is usually bad, and that's why the walls of the boxroutinely warp inward. In this case, the engineer simply machinedout the interior of the core steel.
He then filled this pocket withstainless steel ball bearings and attached a water-tight cap to theback of the core with one 'in' at one end and one 'out' at theother. The presence of the ball bearings created a matrix flow (turbulent)in the entire area of the core, enhancing the cooling ability. Noneed to balance the flow, the balls gave enough resistance tospread the flow of water/pressure evenly. With the ball bearingspacked into the core, it also gave structure to withstand therigors of the compressive force of clamping and injection. It's an interesting technique that takes advantage of conformalcooling without the problems of laser sintering.
About the author : Bill Tobin's injection molding career began in 1968 at GeneralMotors and has included stops at Ford, Mattel, Hewlett-Packard, andseveral custom injection molders along the way. In 1975, he founded WJT Associates , which provides design and project engineering consultation onmechanical designs, specification of tolerances, partmanufacturability and machine costing, governmental and agencyregulatory compliance, budgets and profitability improvement, andexpert witness testimony. Editor's note : In part III, Bill swaps "war stories" with fellow ANTEC delegateson abusive supplier/OEM relationships and calls for an end to thedrama. In part I, he discussed the capabilities, and limitations,of simulation software.
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