A VFFS machine can run a trial reel satisfactorily and still produce poor packs once normal production conditions begin. Film tracking may drift, seals may weaken after a product change, or packs may lose their shelf-life performance. Understanding how to choose VFFS film specification means assessing the product, pack design and machine together, rather than selecting a film only by material type or gauge.
The correct specification must form consistently around the forming tube, seal within the available dwell time and temperature range, withstand handling, and provide the protection the product requires. A film that performs well on one VFFS line may not be suitable for another, even when the bag format appears similar.
Start with the product and its protection needs
Film selection begins with the product being packed. Its physical characteristics affect how the film runs, while its sensitivity to air, moisture, light and contamination determines the barrier properties required.
Dry products such as snacks, powders, coffee, pet food and bakery items often need protection from moisture ingress or oxygen. Products containing fat can require resistance to oil migration. Sharp, hard or irregular products may puncture a lightweight structure during filling or transit. Fine powders create a separate challenge because product contamination in the seal area can reduce seal integrity.
Establish the required shelf life before discussing film structures. This should be based on product trials, storage conditions and distribution requirements, not simply the shelf life achieved by an existing pack. If the product is packed under modified atmosphere, confirm the required gas retention performance as well as oxygen and moisture barrier.
A clear specification should define the product’s likely hazards, including:
- moisture gain or loss
- oxidation and flavour deterioration
- light exposure
- puncture, abrasion or compression damage
- contamination of the sealing surfaces
These factors determine whether a simple polypropylene film is appropriate or whether a laminated, coated or high-barrier structure is necessary.
Choose the VFFS film structure, not just the material
A VFFS web is often made from several layers, each performing a different function. The outer layer may provide print quality and stiffness, while an inner layer delivers the heat seal. Other layers can add barrier performance, puncture resistance or opacity.
Common structures include monoweb films, laminates and co-extruded films. Monomaterial structures can support recyclability objectives where the necessary barrier and sealing performance can be achieved. However, they do not automatically suit every application. A recyclable structure may have a narrower sealing window, lower stiffness or different friction characteristics than the laminate it replaces. These differences need to be tested on the actual packing equipment.
Film gauge should be selected to meet the pack’s functional demands. A thicker film can improve puncture resistance and give a more substantial pack feel, but it increases material use and may alter how the web tracks and folds. Reducing gauge may lower material cost, yet can lead to stretching, poorer registration control or failures at the gusset and seal areas if taken too far.
For products with sharp edges, assess puncture resistance rather than relying on gauge alone. Material chemistry, layer design and orientation all influence performance. A thinner engineered film can sometimes outperform a thicker standard structure.
Match the sealant layer to the pack format
VFFS bags rely on a longitudinal fin or lap seal and horizontal cross seals. The film’s sealant layer must create dependable seals at the temperatures, pressures and dwell times available on the machine.
This is particularly significant at higher line speeds. As speed rises, effective dwell time reduces. A film with a wide, forgiving seal window can help maintain seal quality when normal operating variables change slightly, such as product temperature, ambient conditions or jaw temperature. A narrow seal window may be workable during a controlled trial but less tolerant of day-to-day production variation.
The required seal performance also depends on the bag style. Pillow bags, gusseted bags, block-bottom bags and quad-seal formats place different loads on the film and sealing areas. A bag intended for heavy products or stacked retail display will need greater seal strength and resistance to creep than a lightweight single-serve pack.
Where product regularly enters the seal area, discuss contaminated-seal performance with the film supplier. Some sealant systems cope better with dust, oil or small product particles than others. This is not a substitute for good product control and dust extraction, but it can reduce the risk of leakers in difficult applications.
Confirm the web dimensions and machine running characteristics
The film specification must match the VFFS machine’s forming set and controls. Web width is calculated from the required bag width, depth or gusset, longitudinal overlap and forming tube geometry. Small changes can affect the final bag dimensions, seal placement and material usage.
Repeat length controls the bag length and print registration. For printed film, the eye mark must be clear, correctly positioned and compatible with the machine’s registration sensor. The contrast required varies by sensor type and film finish. Metallic, highly reflective or transparent films can require particular attention, as can designs with frequent colour changes around the eye mark.
Film stiffness, coefficient of friction and curl all influence handling. The web needs enough stiffness to travel through the unwind, dancer or tension system, forming collar and pull belts without wandering or creasing. At the same time, excessive friction can cause drag and inconsistent movement, while very low friction can affect pull-belt grip.
The direction in which the film is wound also matters. Confirm whether the printed side is wound in or out, the reel core size, maximum reel diameter and reel weight accepted by the machine. These are basic details, but an incorrect reel orientation or oversized reel can stop production before the first pack is made.
Specify performance at operating conditions
Film data sheets are useful, but they are not a complete operating specification. Results quoted under laboratory conditions may not reflect a production line running continuously with actual product and normal temperature variation.
Request trials that reflect the intended application. These should cover start-up, sustained running, planned speed range and expected product fill weights. Seal checks should include peel strength where relevant, burst or leak testing, and visual inspection for wrinkles, channel leaks and product trapped in seals.
For products requiring controlled atmosphere, test gas retention over the intended shelf-life period. For frozen or chilled products, assess film behaviour at low temperatures. If packs are transported in cases or handled by automated equipment, verify scuff resistance, pack compression and resistance to damage at the seal edges.
It is also sensible to define acceptable tolerances. These may include film width, repeat length, gauge, coefficient of friction, reel splice method and print registration position. Agreeing these parameters helps purchasing, production and the film converter identify variation before it becomes a line issue.
Account for equipment changes and production flexibility
A specification that is right for one SKU may not suit a wider product range. If a VFFS line will run several bag lengths, products or pack weights, the film needs to support the planned operating envelope.
For example, a film selected for a low-speed coffee pack may struggle when the same line changes to a fast-running snack format. Equally, a structure developed for a narrow bag may behave differently on a wider forming set because web tension and material draw change. Specify the full SKU range where possible, including planned future formats.
Changes elsewhere in the line can also affect the decision. A new multihead weigher may increase product drop speed. A different checkweigher or case packing arrangement may introduce more handling. These are reasons to review puncture resistance and seal strength rather than treating film as an isolated purchasing item.
How to choose VFFS film specification with suppliers and engineers
The most reliable approach is to provide the film converter and packaging machinery supplier with a concise application brief. Include the product, fill weight range, required shelf life, bag style, target output, pack dimensions, storage conditions and any sustainability requirements. State the VFFS machine model, forming set details, sealing arrangement, reel limits and whether printed registration is required.
This information allows practical trade-offs to be identified early. Higher barrier can add cost and reduce recyclability options. A thicker structure can protect the product better but increase material consumption. A lower-seal-initiation-temperature film can support speed, but it still needs to perform across the machine’s full working temperature range.
Pac-right would normally consider these interactions when assessing a VFFS application, particularly where film choice, dosing equipment and downstream handling need to operate as one system.
The best film specification is therefore the one that protects the product, produces repeatable seals and runs predictably at the required output. Validate it under production-like conditions, document the agreed tolerances, and treat any new product, pack format or machine setting as a reason to check the specification again.