posted June 04, 2004 07:49 AM               

I'm a new process engineer in a blown film plant. They are making a lot of colored plastic films in small batch. So they have to change the color a lot of time. I would like to know the best way to change the color without stopping the blown film line.

posted June 04, 2004 08:26 AM               

Another method is to stop the color addition and the line will eventually purge it's self to natural uncolored material. Then you can start adding the next color. The purging time can be very long sometimes, so this may be less economical than breaking down the equipment and cleaning.

In the case of running transparent materials after opaque material, it is almost impossible to purge out the color. The pigments in opaque colors tend to stick and hold to meltal surfaces. An equipment breakdown is required.

To facilitate purging, a flow analysis of the adaptor and die can be performed to highlight slow purging areas. These areas can be modified so that the adaptor and die will stay clean and moving all the time. This type of modification has the potential to cut purging time to 1/10 the original. The side benefit will be a reduction in gels and burnt particles in the product.

posted June 04, 2004 09:09 AM               

Do you recommend the use of a purge resin. I have looked for this and I'm wondering if it's gone a do a good job.

posted June 04, 2004 01:57 PM               

If you want to purge, pick a lower MI resin of the same base type, and vary the RPMs while you purge.

I have run other processes with similar color change issues. The best results come through proper scheduling, and strategic clean-outs.

There are many styles and forms of purging. There is no univeral favorite, nor are there any purging techniques without problems and risks associated.

Tom C