You’ve finished your run and you want to use the line with another resin. Do you:

  • Follow with the other resin right away?
  • Take the system apart and clean it all? Or just clean the die or the screw?
  • Or do you run a purge material through to make the change easier/quicker/cheaper?

There is no simple answer, as it depends on the resins involved, the needs of the new product, the time pressures on the production line, the size and complexity of the head and die, the ease and need for pulling the screw(s), the relative costs of resins and purge, and the possibility of reusing the transition material in later jobs.

If you do direct-follow, you may get a faster changeover if you change melt temperatures as needed to either flush out corners with a low-viscosity melt, or push out better with higher viscosity following melt. These represent three of the following ten (11) principles of purging: Lower viscosity to flush out corners.

  1. Lower viscosity to flush out corners.
  1. Higher viscosity to push out easy-flow melt.
  1. Change resins or temperature, or both, to get effects 1 or 2.
  1. Cycling (disco): Screw rpm is varied—slow-fast-slow-fast—to loosen and push out prior melt plus these which need an intermediary purge compound:
  1. Over-stabilized compound to allow higher melt temperature for 1, 2 and 3.
  1. Abrasive additive to scour inner walls (the kitty-litter principle).
  1. Compound with detergent action that separates melt from steel.
  1. Compound with compatibiliser that bonds with both flush-out and following material.
  1. Gas generation with foam agent or other additive decomposition, even water (Caution: steam scalds!) Cast PMMA has been used for this purpose, notably by injection moulders, as it breaks down under extrusion temperatures.
  1. Resin decomposition (dragon’s breath), where purge that contains chain-breaker fills system, screw stopped for x minutes, then started again with following resin to push out degraded mix (Caution: what comes out is particularly noxious and dangerous!).
  1. This one is good when you want to clear the feed zone to remove stuck material: Put in big chunks of the same material (such as coarse-grind sheet or thick-wall pipe), which can adhere to the hot material already in there and get it to move.

In comparing costs, remember that some purge compounds can be collected, reground and reused (like bubble-gum during World War II)—useful for $10/lb resins—but some others can’t. And make sure the purge selector knows not only what resins are involved, but their approximate viscosities.

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