A flow pack is a pillow-shaped bag formed around a product from a single roll of film: one seal runs along the length of the pack where the two film edges meet, and a crimped seal closes each end. The product is inside the film before any seal is made, so the pack takes the product’s shape instead of the product being dropped into a bag that was formed first. The machine that makes it is a horizontal flow wrapper, also called an HFFS machine, and the pack itself is sold under the names flow wrap and pillow pack.
Nothing in the process stops: the product moves, the film moves with it, and all three seals are made while both are moving. Making every seal on a moving product is what gives the format its speed, and it is also why the product itself — its shape, its firmness, its length — decides the speed the machine will actually hold.
A flow pack machine wraps a product that never stops moving: an infeed conveyor spaces the products, a forming box folds the film into a tube around them, heated wheels seal the tube shut along the bottom, and a pair of jaws seals and cuts between one product and the next.
The infeed conveyor carries lugs at a fixed spacing, and each lug pushes one product forward so the products reach the film at an even pitch. That pitch is the bag length, because the end seals will land in the gap between one product and the next; a longer product means a longer pitch and fewer packs per minute at the same conveyor speed.
The infeed is also where speed is lost first. A lug can push a product only as fast as the product will stay upright and in place, and a soft cake, a cookie standing on edge or a tray with loose contents tips or slides long before the sealing stations reach their limit. A rated speed on a data sheet assumes a firm, flat-bottomed product; a soft or tall one will not reach it.
The film comes off the roll over tension rollers so it reaches the forming box flat and at constant tension. With printed film, a sensor reads a registration mark on every print repeat and corrects the film feed so the print lands in the same place on every pack; plain film runs by length alone.
The film passes over a forming box (some builders call it a forming plow or collar) that folds it down around the product as it passes beneath, bringing the two film edges together under the product. The box is shaped to the product’s cross-section, so a change in product height or width is a forming-box change, not a setting. A wide or irregular product needs a looser box and more film.
Pairs of heated wheels pinch the two film edges together and seal them while the tube keeps moving, so the seal along the bottom is continuous and is never the station that limits speed.
The edges are either sealed face to face so they stand out as a fin — the fin seal — or one edge is laid over the other and sealed flat as a lap seal. A lap seal uses less film and lies flat but needs a film that seals on both faces; a fin seal works with any film that seals on its inside face, which is why it is the default.
A pair of jaws closes on the film in the gap between two products, presses the top and bottom layers together into a crimped seal, and a knife in the jaw cuts through the middle of that seal. One closure therefore finishes the trailing end of one pack and the leading end of the next.
On most flow wrappers the jaws are mounted on rotating shafts and close once per revolution per pair. Rotary jaws give the film only the moment the jaw faces are in contact, which is enough for a thin heat-seal film on a short pack.
A thicker film, a film that needs more heat to seal, or a pack that has to be fully airtight needs longer dwell. For that the jaws travel with the film while closed and then return — box-motion sealing. Box motion seals better and runs slower, which is the second reason a rated speed does not apply to a different product and film.
The cut pack leaves on a discharge conveyor to whatever follows on the line: a checkweigher, a metal detector, a cartoner or a case packer. The wrapper runs continuously, so packs leave at the infeed pitch, and the next station has to accept them at that rate or the wrapper stops.
Flow packs suit any product that a lug can push along a conveyor and that keeps the same cross-section from one piece to the next, which covers most solid food and a large share of non-food retail items.
What does not flow pack is anything that will not hold its shape on the infeed. Powders, granules, liquids and loose snacks go into a vertical bagger, where the bag is formed first and the product is dropped in. Small pieces sold by count — a stack of biscuits, a set of screws — can flow pack, but only after a counter or collator upstream has built the group into one unit.
No. A flow pack is heat-sealed loosely around the product and stays loose; a shrink wrap is a heat-shrinkable film that a heat tunnel pulls tight onto the product after it has been sealed. The two share the sealing step and differ in what comes after it and in the film that goes in.
In a flow pack the film is ordinary heat-seal film — printed, laminated, perforated if the product needs to breathe — and the pack leaves the wrapper finished. Heat touches the film only along the three seal lines, so the product is not warmed and the printed face stays flat and readable. The air inside the pillow is not a defect; a flow pack is meant to sit loose.
In shrink wrapping a sealer makes a loose sleeve, and the pack then passes through a heat tunnel where the film shrinks onto the product’s contours. The result is tight, clear and tamper-evident, which is why shrink wrap dominates multipacks, bundled trays and irregular non-food items. The cost is a second station, heat applied to the product, and print that distorts as the film shrinks.
Because a flow wrapper can be the sealer in front of a shrink tunnel, the two are not rival machines; the choice is whether the film should end up tight on the product. Single retail items in printed film, heat-sensitive products such as chocolate and bakery, and lines that need the wrapper’s speed take a flow pack. Bundles, tray overwraps and irregular items that have to be held together take shrink wrap.
