How to make collation shrink film?

Shrink film, bundling and overwrap film refer to polyethylene films used to wrap individual products such as paper, books, diapers, frozen pizza, toys, board games, etc. to be displayed at the point of sale or to be combined several items in packages, example soaps, soft drinks, directories and books.

Collation Shrink is a flexible packaging material and can be used in various forms: transparent or colored, without our printing and with different film thicknesses ranging from 20 to 150 μm. High quality shrink strips offer a significant shelf feel.

Mainly produce on a 3 layer blown film , although still could on mono to 9 layer structure.

A little more than 50% are made of conventional high pressure LDPE. LDPE dominates the case overwrap and paper shrink market segments. LLDPE, blended with LDPE is used in paper shrink overwrap applications. LDPE, LDPE/EVA and LLDPE are used in the crosslinked film market. In addition to LDPE, two other non polyethylene resins serve in the crosslinked shrink film segment. They are PVC shrink film and shrink film made from biaxially oriented polypropylene (BOPP) and I will write another article about them in future.

These applications require higher forces and rates of contraction in the machine direction. because the film is usually available in single sheet and it will be wrapped around the package along the shorter dimension.

This condition can be accomplished using low BUR (1.6 - 2.0) if the film is produced only with LDPE or with higher BUR (2.8 - 3.5) if LLDPE is included in the structure. Additionally, larger die gaps and higher output rates will produce higher orientation in the MD.

Film width and TD's shrink rate must be carefully selected to prevent the sides of the packaging from becoming unprotected. It could allow the contents to be extracted or the packaging to be unstable.

The most common types of films used for these applications are single sheets, side seals sleeves, side seals bags or center folded sheets. Some machines are able to seal the film and produce pouches or bags inline.

Tubular sleeves are very rare because they require very high BUR to accomplish the same shrink rates and are limited by the size of the equipment to produce the film.

Normally these films shrink in small tunnels in line with the packaging machine. H0t air and fans to distribute it throughout the chamber. Speed of the belt and the temperatures in the tunnel are crucial for shrinkage wanted to achieve. These parameters depend on the thickness and the type of polymer.

Process requirements

The extrusion process is one of the most important parameters influencing properties of the shrink film. These films are generally produced by extrusion of blown films.

This method is preferred because of the properties due to the biaxial orientation of the polymer molecules in the bubble and the flexibility of the process to change these properties.

Variables such as like BUR, draw ratio and cooling rate or height of frost line affects the structure of the polymer and therefore the final properties of the film. An important factor in the final properties of the film which is unrelated to film shrinkage is coefficient of friction. With modern coextrusion technology, it is possible to produce shrink film with strong adhesion on one side of the film and high slip on the other. High grip side allows the film to adhere to the wrapped product while the top slip side allows it adjacent packages should be stored close together without disturbing the other product 's packaging.

Typical slip values ​​would be 2.0 to 4.0 (slip = mutual coefficient for friction), while high levels (slip value> 4.0) are sometimes used products that are easy to place in shrink bags or tubes, etc.

Blow up rate (BUR)

This setting affects the actual amount of residual orientation frozen in the film and degree of shrinkage that can be achieved in MD and TD. BUR is the variable that can they have the greatest influence on the final shrinkage properties. Keeping constant the melt temperature, output rate, frost line height and thickness, if BUR is increased, TD shrinkage increases with only a small decrease in MD shrinkage.

The BUR is closely related to the degree of molecular deformation and the stress levels induced in the film, especially in TD. In addition, BUR changes the level of crystallinity in the film. Higher BUR means lower crystallinity and lower BUR means higher crystallinity due to the molecular orientation in one direction.

The stability of bubbles is affected by BUR, if values ​​greater than 3.0: 1 are used, it may lead to instability in the form of lateral oscillations. These instabilities affect the thickness uniformity and all the properties that depend on it. In many cases this kind of process issue can be reduced or eliminated by appropriate selection of the specifications of equipment.

Film Thickness and Drawdown

At higher thickness values such as 125 - 200 microns, the shrink percentage and shrink force are lower than at lower thickness values like 25-75 microns. Lower draw down means lower shrink rates in the MD.

Frost line heights or cooling rate

Low frost line means a quick cooling of the orientation conditions in the film and that is allowing very little polymer relaxation. High frost lines permit higher polymer relaxation, providing higher shrink rate in TD. These concepts can be used to set the processing conditions in order to accomplish the properties that the final application requires. The dimensions of the final package must always be taken into consideration, to select the best way to produce the film in terms of width, depth of the gussets and/or bag length. The selection of the material and process both affect the shrink forces, and by choosing the correct dimensions for the bag or sheet these forces can tremendously improve the stability of the package.

Die Gap

The die gap is closely related to the final thickness of the film and the draw down that can be produced. If the film thickness is small, example 25-75 microns and the die gap used is larger than > 1.2 mm, the draw down that bring the melt to the final thickness will be very high.

This stretching effect produces a residual stress in the polymer molecules that is “frozen in” once the material is cooled. Faster cooling means higher stress and deformation in the film. This stress is relaxed when the film is re-heated and the molecules try to return to the original state due to the “Elastic Memory”.

If a higher shrinkage value is required in MD, a larger die gap should be used, but it has some limitations due to the stability of the process.

Some times if the difference between die gap and final thickness is high and LDPE resins with low MFI (< 0.5 MFI) are used, some problems can be present. These problems are loss of the bubble or lower output rates due to the difficulty of stretching these resins.

Air Ring

At higher cooling rates, higher output and thus line speed will be accomplished. The higher haul off speeds allows less time for the polymer to relax below the frost line thus gives greater MD orientation, which reduces the ability to achieve balanced films.

Basic equations:

Total orientation = Die Gap / Film Thickness = Drawdown (DD)

TD Orientation = Bubble Diameter / Die Diameter = Blow-up-Ratio (BUR)

MD Orientation = Total Orientation / TD Orientation = DD / BUR

= Die Gap x Die Diameter / Film Thickness x Bubble Diameter

Film Structures

– Monolayer Structure

• LDPE rich shrink engine

• HDPE or LLDPE added for modulus or toughness

– Three-Layer Structures (10-20% skin layer thickness)

• Skins: LLDPE / LDPE blend for clarity, gloss and toughness

• Core: LDPE rich shrink engine with HDPE or LLDPE added for modulus or toughness

3 Layer Formulation

Outside layer (A): 10%

100% Butene LLDPE (PF-Y818-FX)

Core layer (B): 80%

50% LDPE (LF-Y819-A) + 50% Butene LLDPE (PF-Y827-FP)

Inside layer (C): 10%

100% Butene LLDPE (PF-Y818-FX)

(Materials from Nova Chemical)

Mono layer Formulation

Mono layer (A): 100%

80% (Lupolen 3010D) + 20% (LD 150)

(Materials from LyondellBasell and Exxonmobil)

Above are basic formulation and for more complicated formulations, always you can contact me and I can cooperate with you for development.