VFFS and Film Performance
A VFFS system consists of two components: ď‚ˇ The Filler or Dosing, which takes unmeasured quantities of product and rearranges them into measured, repeated doses. ď‚ˇ The Wrapper, which forms a bag from one or more rolls of material and, once the product is inserted onto the bag, seals the bag so the product is safely packaged within. Intermittent Film Belt Driven VFFS Principle
The spring-mounted dancer arm keeps the film web under the proper tension
The two edges of the film are sealed as they pass along the metal tube.
A first transverse seal forms the bottom of the bag that is them able to be filled through the hollow tube.
Product coming from Dispenser
Coming from the roll and passing through a series of rollers to a bag forming collar, the film is wrapped around a metallic tube that gives it a cylindrical form.
The package film then advances a predetermined distance that equals the desired bag length.
o The cylindrical film gets another transverse seal to close the filled bag.
Filling Tube Forming Shoulder
Vertical Seal bar Horizontal Seal bar Eye mark register A printed rectangular mark often at the edge of web stock that can be detected by a photo cell and assures the transport of one bag length per machine cycle.
The time and steps that are needed to make one bag are collectively called a machine cycle. Evolution of VFFS Machines
Before the invention of plastics in the early 50’s, paper and the cellulose based product cellophane where the only available flexible packaging materials. Compared with plastic films, this cellulose based products: Withstand pulling forces without tearing apart or stretching. o VFFS of the time were designed to pull the web film through the feed and forming sections, applying high stress to the films running through. Support a broader temperature seal range without burning, and withstand higher heat sealing bar temperature without softening or sticking to the sealing bars. o VFSS of the time operated with unregulated on/off temperature controls within a broad temperature range Plastic films wouldn’t work in VFFS machines designed to work with cellophane. The plastic film will elongate, varying the length of the bag and creating misaligned packages, or will break due to the stress applied to the web. The plastic film will soften under the high temperature of the sealing bar; it would deform and/or stick to the bar. The sealant layer under high bar temperature will produce burned seams, compromising the integrity of the package. Plastic films were cheaper than cellophane, and between then, OPP was the frontrunner to replace Cellophane. OPP is one of the stiffest plastics and is able to withstand high temperature, only second to PET in both characteristics. But packagers preferred OPP due to its better yield, transparency and gloss over PET. Packagers and machine manufacturers were forced to modify their equipment to match the requirements of the new OPP film. OPP film manufacturers facilitated the revolution by retrofitting packaging machines with no cost to the customer. Retrofitted VFFS machines and new machines designed to run OPP films were the speedlimiting factor; their operation was intermittent and applied to much stress to the film running through. Improvements in film coextrusion technology, additive technology formulation, and sealant polymers optimized efficiency, but the machines could only go so fast. In the 1980’s VFFS machines experienced revolutionary improvements: film belt drives, film feeding/measuring, and combination net weigh scales. Baggers still operated Intermittently but productivity increased 50% or more. The 80’s improvements made newer machines more film forgiving. Seal range and strength grew less critical. A low outside COF was no longer as important because film was “fed” to the forming collar rather than being dragged over it. The 1990’s saw the introduction of servo-drives into belt-driven machines which increases productivity over 30% to approximately 120 ppm (for 6-8 inch cut-off length). Continuous motion VFFS machines also came out that increased production rates again, to about 150 ppm
Older packaging machines and even older film technology are still in use today. In this variety on plant floors, a film that runs great on a new high-speed, servo-driven machine may jam and miss-feed at another customer running a 30-year old draw-bar machine.
Film requirements Machinability is the ability of a film to run, form and seal on packaging equipment. To achieve machinability, the packaging film is designed with certain properties. Stiffness and Heat Stability o The incorporation of an external layer of PET or OPP film in a multilayer film structure suffices to comply with these two requirements. Low COF o COF is the measurement of surface slip. Additives are included during the formulation of the layer which would slip through the forming collar- the sealant layer- to achieve the desired high slip. Wide Seal range o A wide seal range compensates temperature variations at the sealant layer. Is difficult to maintain precise temperature control in the seal area: Heat seals are made very rapidly during a very short dwelling time. Sealing bars constant movement causes cooling. The temperature at the sealing bar has to propagate through different film layers to reach the sealant layer. Hot Tack Is the capability of a heat seal to remain sealed when it is stressed while still hot. Measures the seal strength of the seam immediately after the product is dumped on a VFFS packaging process. The weight of the product has to be supported by the still hot seal without presenting deformations – mooning- of the sealing area after the jaws release.
In processes where the seam is not stabilized quickly enough, (poor hot-tack), compressed air is used for cooling immediately after sealing, which adds an extra processing step.
Caulkability Is the ability of the sealant resin to flow into voids, channels or wrinkles in the seal area during the short dwelling time, in order to provide hermetic seals. Sealants before the development of metallocene plastomers in the late 80’s couldn’t adequately caulk when heat-sealed at or below the distortion temperature of OPP (145oC/295oF). Metallocene plastomers has the ability to flow when heat sealed at temperatures below 295 0F.
VFFS Film Transport There are two techniques to transport the web of film through the machine.
Uses conveyor belts with which the packaging material is pushed down along the forming tube. Two types of belts are used, the friction and the vacuum belts. Good tracking behavior of the web film: the side o Needs correction of the tracking behavior to movement left/right of the web running through the avoid problems with forming the longitudinal VFFS machine is minimal. seam. Advanced machines automatically correct The sealing jaws move along with the falling the course of the web with ultrasonic and photo product to ensure that there is a soft landing of the edge detection. product in the bag, which is especially important for products susceptible to breakage. No bag with a block bottom can be made. Able to o Allows for the production of all conceivable bag produce only pillow bags and gusset bags shapes. o Friction belts requires that the friction between the conveyor belts and the packaging material been greater than the friction between the packaging material and forming tube. The external layer of the packaging film has to have a higher COF than the one of the sealant layer. With vacuum belts, this added requirement in the formulation of the packaging film is avoided. o
Simplest technique uses the sealing jaws that grip the web of film like the draw bar and pull the material downwards. o o
Longitudinal Seam The length of the sealing unit (in an intermittent VFFS unit) will usually be equal to the maximum bag length that can be made on the machine. The longitudinal seam is sealed multiple times in shorter bags. Overlap seam Less packaging material is used than in the fold-over seam.
Fold-over seam One side of the web of film is folded over by the forming shoulder to obtain an interior of film on interior of film seal. Generally used for film laminates where the sealing medium is located on the inside of the material. Overlap Seal Exterior of film on interior of film
Fold Over, Fin Seal Interior of film on interior of film
Film width â€“ longitudinal seam width = bag width 2 INTERMITTENT AND CONTINUOUS MACHINES In an intermittent machine, the packaging material is stopped as the longitudinal seam is sealed (once per machine cycle). With a continuous machine, the material is transported continuously.
Benefits of Continuous Over Intermittent Machines Higher output. 30 to 50% higher
Lower film speed The packaging material moves much more slowly in a continuous machine than in an intermittent machine. The continuous machine has the complete machine cycle to transport a bag length, while an intermittent machine has only one third of the cycle. The intermittent machine must speed the packaging material to 3 times the speed of the continuous machine to maintain the same rate of bags per minute.
More rest, less disruptions A lower film speed creates fewer disruptions. The material progresses much more regularly because the web of film does not have to start and stop during every cycle; it does not “skip” as occurs in intermittent machines. Also the chance of sideways shifting is minimized.
Sealing Process Variables Temperature. - Limited by the nature of the packaging material. The temperature window indicates the minimum and maximum temperature at which the material melts but does not burn. Sealing pressure. - Has limits of a mechanical nature. o The thicker and stiffer the packaging material, the higher the pressure needs to be in order to seal the film. Sealing time. - Depends on the hot tack and SIT properties of the sealant resin. PACKAGING SPEED The speed with which bags can be made is mainly determined by the sealing time of the packaging material and the length of the product stream. In addition, the bag length and – sometimes – the falling time of the product play a roll. Length of product stream The product stream is the distance between the first and the last product from one batch, or the distance between the first and last peanut of the contents of one bag. The length of the product stream is much more important for the packaging speed. The bag can only be closed once the complete batch (portion of product) is in it. The length of the product stream is determined partly by the volume of the product and the opening of the forming tube. A compact product in a short, wide bag yields the fastest machine speed. For more voluminous products that are packaged in a long, narrow bag, the machine speed is dramatically reduced. Falling time Often it is thought that the falling time of the product also determines speed, but that is a misunderstanding – with some exceptions. If there are high speeds, multiple batches can
be underway simultaneously. If the product is interrupted while falling, for example by catch plates, then the falling time is important: the product must get back up to speed after each interruption. Variables of Sealing Time Seal initiation Temperature SIT Is the minimum temperature required by the resin to obtain an acceptable seal. The lower the SIT, the faster the resin will melt to wet the sealing area.
Lower SIT Resin
Shorter Dwelling time on
Higher Resin HT Force
The sealing time is directly related to the machine speed; a longer sealing time usually requires a slower machine speed. With an intermittent machine, sealing (which includes the jaw close and open time) take up more than 1/3 of the machine cycle. As an example, let measure the impact of a new film with a sealant layer that reduces sealing time by 20% -from 500ms to 400ms- in an intermittent VFFS
Sources 1. 2.
Guide to VFFS Baggers www.boschpackaging.com Film Properties For Good Performance On Vertical Form/Fill/Seal Packaging Machines T A. Clark, J R. Wagner, Jr Crescent Associates, Inc. Rochester, Ny
Plastic Film Echnology and Packaging Applications Jenkis Osborne
Seal Through Contamination Performance of Metallocene Plastomers P. Mesnil, R. Halle, N. Rohse Exxon Mobil ; J. Arnauts DexPlastomers 2000 The Use of mPE in Coextruded Lamination Films Simone Vigano Exxon Mobil 2007