My first job after graduating was at a polypropylene (PP) film plant (one that I've discussed many times in the past) so even though I haven't made any film in 20 years, I still am curious as to what is happening in the industry. Plastics News has a report
on new tenter film equipment being shown at this weeks Chinaplas show. This is obviously a big show for the equipment vendors since the film market in SE Asia is growing hugely.
The point of discussion is not the extruders, which are available from an endless number of suppliers, but on the orientation equipment - the roll stacks and the tenters. For background, the PP is first extruded as a wide sheet and cooled. The sheet passes to a stack of heated rolls which (re)heat it from both sides. At a certain point in the stack, there is a gap between the rolls. All the rolls before the gap are turning at one speed, those after the gap are going 5 times faster. This speed difference then draws the film thickness down in one direction (the machine direction).
This film then enters the tenter, a large heated oven. In the oven are ever widening tracks which have clamps moving in the machine direction at the same speed as the film. These clamps grab the edge of the film and then stretch in to a width 10 times that of the original width. At the far end of the oven, the film is cooled and the clamps loop back around to the front. The edges of the film are trimmed off removing the polymer that was in and around the clamps, and then everything is wound up.
One new supplier had this to say about his unique approach:
"The newcomer to the BOPP arena goes by the acronym ESOPP, for energy saving orientation production process...ESOPP's most recent developments focus on energy conservation. "The processing of film is energy-intensive and we have developed a patented system to recover and reuse heat from the TDO section back into the MDO section," says (owner Jean-Pierre) Darlet. ESOPP has also developed new clips for stretching PET that can realize a BOPET film with zero shrinkage in both directions. "This is particularly important for BOPET film used in solar applications," says Darlet."
The acronyms came pretty in there, so let me translate and explain. BOPP is "biaxially oriented polypropylene" - PP film made with a tenter, as opposed to OPP (same decoding pattern) which is PP film made in a blown bubble. TDO/MDO is transdirection orientation/machine direction orientation, which are the roll stacks and the tenter. BOPET you should be able to figure out. As far as I am aware, PET film is never blown, only made on a tenter.
I guess I never thought of the rolls as being an energy intensive operation. I can see that in spades in the tenter oven, and given that the energy for the rolls is comingfrom
the oven and going to
the rolls, I think I am correct, but regardless, it is an interesting approach to consider when making this investment in a world with ever more expensive energy costs.
I also am curious as to why the clips
would play a role in making non-shrinking BOPET film. Shrinkage can be reduced by bringing the tracks in the oven in slightly at the very end, but that's the tracks and not the clips. Something to puzzle over, I guess.