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Spiral dies were the answer to removing knit lines from spider dies. Spiders required melt to flow around legs that held the center mandrel. Spiral dies were first developed by Egan (now part of Davis-Standard) for pipe in the early 1960s. They were center-fed with the flow split among radial feed ports leading into spiral channels milled into the die surface.
In the early ’60s Egan adapted the new spirals to blown film, a logical step because blown film grew out of experiments on inflating pipe in the late 1940s.
Within a few years, the spiral die became the most widely used die for pipe and blown film. Egan, however, never applied for a patent on its invention because Egan mistakenly thought it fell under an earlier Davis-Standard patent on a wire-coating die with a single spiral channel. Before long, many competitors were making spiral dies, too.
In the 1960s and ’70s, blown film dies had relatively few spirals. An 8- to 12-in. die might have had four. By the 1980s, die designers were learning to use flow analysis and had learned that roughly six overlaps per spiral start obtained good throughput.
In the 1990s, spiral designs became shorter and more streamlined to reduce residence time, especially for coex dies with new heat-sensitive barrier and tie-layer materials.
Very few readers of this issue can remember, or even imagine, what it was like when an injection mol...