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.

It’s best if you can avoid clean-up and the cost of purges, but that depends on those prior factors. That is the best solution of all, as minimal resin is wasted and no purge material is needed. It may work for colour changes of the same resin, or changing from one viscosity to another, where the material can be ground and worked back at an appropriate percentage of virgin.

Sometimes the transition material—all right, call it scrap, but it is really $crap, as it is worth the value of the material you’ve saved by reusing it—sometimes the scrap can be used at 100%. I had a client once who paid his salesmen a bonus if they took orders for grey or black sheet, which they would make from the trim from runs of various matched colours.

Mixing and screening are especially important for such high scrap reuse. Good mixing ensures that different colours and viscosities are well homogenized, and tight screening keeps out contaminants that can affect physical properties, even if the resin molecules are essentially intact.

Lookout for part 2 where we discuss 11 principles of purging.

Source: (By Allan Griff)