Flexible Packaging Laminations
Laminations are two or more most often dissimilar films which are married together with adhesive. Laminations differ from coextruded films in that they are married together in a secondary process vis-a-vis substrates extruding out of a die simultaneously.
There are three usual scenarios in which laminations are needed:
1. Print must be "trap printed" or sandwiched, between the layers to prevent abrasion of print.
2. Oils in the product migrate and push the ink off the film.
3. A differential in melting points is needed to seal on a packaging machine with bare metal seal bars or rollers. This is usually referred to as "resistance" seals.
Through the additional mass of sealant layer, adhesive and outer layer, better barrier properties will be achieved. Whether it is sufficient depends on the application. For a serious barrier defined as an OTR of 0.6 maximum, a barrier layer of EVOH or PVDC a/k/a " Saran " is necessary.
The achilles heel of EVOH is adequate thickness of the layers around the EVOH layer. As you may know, EVOH is always the core layer of at least a five layer coextrusion. Think sandwich. EVOH would be the meat. Tie layer is analogous to the mayonnaise. The outer layers are LDPE. If the layers on either side are too thin, the hydroscopic EVOH takes on water through osmotic transfer thereby compromising the barrier properties and massive recalls.
( " Barrier " is a relative term - a continuum. In the healthcare or surgical drape realm, any film is considered a barrier to fluids. In fresh cut produce packaging the game is a balancing act to match the respiration rate of the vegetable to the OTR and MVTR of the vegetable. Too much oxygen results in shorter shelf life; too little results in anaerobic conditions which are conducive to pathogens. For meats and cheese, locking out oxygen is the main objective. )
DId you know ? Rice must be packaged in a lamination of two layers of identical low density polyethylene (LDPE) to both prevent abrasion of print during handling and print deterioration from oils in the rice. Every few years somebody tries an overlay with dismal results.
This bag is a lamination of LDPE to LDPE with trap print, overlap seals made on impulse end seal jaws.
If you're just starting out, an outer layer of polyester ( PET ) is best because of it's melt temperature which is over 400 degrees F.
The coefficient of friction ( COF ), or slipperiness, of the sealant layer is important so the film will slide easily on the forming cone of VFFS or the former on the flow wrapper. The outer layer COF is not customizable. A high slip / low COF outer layer runs great on the VFFS or flow wrapper. The trade off is palletization for the product manufacturer.
Our seal layer of LDPE polyethylene has low seal initiation temperatures and fast tack. This translates to seals through contaminants, no channel leakers and maximum cycle speeds.
If you require a pinhole-free package, a fin seal with multiple layers is a must-have for what is known as an " integral seal ."