A properly designed machine has no unnecessary parts. Anybody who makes coextruded multilayer plastic films will tell you a simple monolayer plastic film is sooo 1970 ! Getting coex specified is a barrier to entry to monolayer film producers. Complex is sexy. Simple is boring. Problem: There is an interdependency between the layers. If something goes awry with one layer, it affects the entire structure.
In general, it's a good idea to ask somebody what they do for a living before you ask them for advice. The questions you need to ask proponents of coextruded blown film:
Q. Why does the structure absolutely have to be coex ?
A. When each layer has a specific job to do. Hosokawa Alpine now has an 11 layer line. Seriously ?
is 11 enough ?
Examples of must-have coex:
differential COF - high slip / low slip ( mattress bags )
light blockout ( white / black election signs )
differential melt temps ( vertical form/fill/seal )
A common example of 2 layer coex can be found in a cereal or crouton box. The verbal shorthand is "chip film" or cereal liner. This 2 layer structure is the low, mature end of coex.
The outer layer is HDPE for barrier. The LDPE is for sealability. See our blog post on PE
Election signs and overnight envelopes must be 100% opaque and require a "core" layer of black.
A more complex example is the category of meat films. They commonly have a core layer of EVOH. Since the viscosity of the core layer is so much different than the outer layer usually of PE, a "tie" layer which takes up the slack in viscosity between the layers is necessary. So just like that, you're up to 5 layers. The Achilles heal of EVOH is the tie layers. If they are off, it's a nightmare scenario. EVOH is very hydroscopic. If the EVOH layer is not isolated from the ambient, it will degrade.
A big selling point with die manufacturers is enabling the extruder to bury scrap that would otherwise be discarded in one or more core layers to keep costs down.
Here are a few examples of films which absolutely do not need to be coex:
Blue / white surgical drape films. Over 50 years ago, somebody made the case for a 2 layer laminate as a pinhole-free film. The theory is that the chances of 2 pinholes lining up is infinitesimal. True for a lamination, but not for coex. Coex film can create voids as it exits the die just like monolayer film. When coex came along, the film was made in 2 colors to emulate the lamination. Albeit slowly, drapes such as mayo stand covers and nurse's back table drapes transitioned to single layer.
Frozen food IQF packaging. Barrier films promoted for frozen food have a feelgood security blanket factor, but are unnecessary. We have made monolayer film for IQF frozen film for decades (both form/fill/seal and thermoformed bone guard ). By the way, barrier won't prevent freezer burn.
Coex layers are described by letters, each designating a resin type. An " A/B/C/B/A " structure is a symmetrical structure. Going back to the example of meat film, the A layer is LDPE, B is the "tie" layer and "C" is the nylon or EVOH "core" layer. C does not stand for core.
drawbacks to coex:
scheduling with ramifications for longer lead times and higher minimums
Minimizing scrap is even more important in the coex realm. Instead of taking less than an hour to change over with a single layer film, it can take literally all day to set up a coex structure. After a coex order is set up, staying in that structure as long as possible makes common sense. It is very expensive to break in to accomodate a customer who calls with a hot fill-in requirement. All the time an order is being set up, the extruder is running scrap. It takes as long as it takes for transition materials to clear out and for the new structure to do what operators call "settle down." There are no short cuts. The higher setup costs must be amortized with high minimums. Some coextruders such as CharterNex won't talk to you about running a trial even if you offer to pay for the scrap and machine time. It's 10,000 pounds minimum.
did you notice ? coex films are very rarely an even number of layers, they are almost always an odd number of layers