September 28, 2008

Why Plastic Flows Better In Aluminum Molds

By Anonymous

It has been reported that aluminum injection molds provide faster cycles than P20 steel. One explanation has been that aluminum's greater thermal conductivity causes the mold surface to heat up rapidly from contact with the melt, so there is less solidification near the mold wall and faster mold filling. This theory was debunked in a technical paper at April's MoldMaking Expo 2008 in Novi, Mich., sponsored by our sister publication, MOLDMAKING TECHNOLOGY magazine. The paper was written by David Bank, president of Aluminum Injection Mold Co., Rochester, N.Y., along with processing consultant Dave Klafehn and Ron Smierciak, market development manager of Alcoa Forged Products, Cleveland. They molded a variety of resins in identical spiral-flow molds of P20 steel and Alcoa's QC-10 aluminum mold plate. To their surprise, there was "no noticeable difference in flow length under identical processing conditions."

To simulate real-world processing, the team conducted more trials in which the spiral-flow mold was completely filled and packed. PS and nylon were run near the high and low end of their recommended melt-temperature ranges, and cycle times were optimized. Temperature sensors showed sharp upward spikes in mold-surface temperature when melt was injected, followed by slower cooldown. Cooling took longer with P20 steel, and the baseline temperature of the tool rose gradually over repeated cycles, while the aluminum mold showed less temperature rise. As a result, PS cycles were 30% to 44% shorter in the aluminum mold, and nylon cycles were 8% to 12.5% shorter.

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Copyright Gardner Publications, Inc. Sep 2008

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