Sunday, January 17, 2010

Extrusion of PETG - more thoughts

Skippy - hey Buzz - we never did actually give a "heat profile" to start up the PETG in the last discussion -

Buzz - yes, correct. Why? Well, mostly to see if these comments were helpful in the event that they did in fact have a start profile capable of

a) getting melt to the tooling and
b) keeping a steady state melt coming

No mention of the current melt temperature or current heat profiles or equipment, rates, tooling etc was given, (we are also assuming that adequate drying was in fact occuring) and PETG doesn't have too much in the way of "hot guts" anyway -

Skippy: so we pulled up a little short in the actual "heat profile" part of the answer on purpose since they didn't mention the equipment, screw/barrel etc being used. Suffice it to say that achieving the recommended melt temperature on heated (and controlled heating) screws and tooling is necessary; more information is required to make a more specific recommendation.

Buzz: Yes, much will depend on the screw length/diameter and screw design and intended rates and draw-down. The first couple of zones can be used to keep the melt cooler (but this again depends on screw design) and it is generally wise to keep barrel cooling OFF during start up to avoid freezing material along the screw and creating melt blockages.

Skippy: Typically, processors successful at extruding tubing and profiles from PETG use an "efficient" screw design to keep the melt and unmelt seperate until near the end of the transition area - particularly with thin re-granulated clean dry scrap. And generally (as indicated by Eastman) high compression screws like those designed for HDPE will generally not be suitable for PETG copolyester since excessive melt temperature and poor melt strength usually result -

Just our two cents
Skippy and Buzz