Sachet Sealing Types Explained: 4-Side, 3-Side & Back Seal
Pick the right seal construction for your single-serve sachet — flatness, fill speed and film cost compared side by side.
Seal construction is one of the first decisions in any single-serve sachet project. It drives flatness, fill speed, film cost and how the pack presents on the shelf. The diagram below compares all three on the same film and the same flat size — only the seal placement changes.
All four edges heat-sealed
Every edge of the film is closed by its own seal bar, so the sachet lies perfectly flat with no weak point anywhere. The trade-off is more seal area: it uses the most film and needs four seal stations, so it runs a little slower than the other two. It is the construction of choice when the pack must be dead flat and fully tamper-evident — single-dose powders, samples and very small portions.
Left, right and bottom sealed — top is a fold
The left, right and bottom edges are heat-sealed; the top edge is a fold of the film, not a seal. Dropping that one seal line means one fewer seal station, faster filling and lower cost — while the pack stays fully airtight and flat. This is the standard construction for 45–100 g flat sachets such as a chocolate bar wrap.
Centre fin seal on the back + top & bottom seals
The film is formed into a tube and sealed down the centre of the back (a "fin" seal), then the top and bottom are crimped shut. This is the classic pillow wrap for chocolate bars. It gives the fastest machine speed and the least film waste of the three, but the centre fin stands proud on the back, so artwork must be split either side of it and the front takes on a slightly rounded "pillow" shape.
Quick comparison
| Seal construction | Sealed edges | Shape | Fill speed | Film use | Best for |
|---|---|---|---|---|---|
| Four-Side Seal | 4 | Flattest | Medium | Highest | Tiny single-serve & samples |
| Three-Side Seal | 3 (top folds) | Flat | Fast | Low–medium | 45–100 g flat sachets |
| Back Seal | Back fin + 2 | Pillow | Fastest | Lowest | Powders, chocolate bars |
Frequently asked
What is a sachet seal construction?+
It is how the flat roll of film is folded and heat-sealed into a closed single-serve pack. The placement of the seal lines decides whether the sachet is flat or pillow-shaped, how fast it fills, and how much film it uses.
What is the difference between a 4-side and a 3-side seal?+
A four-side seal closes all four edges; a three-side seal seals the left, right and bottom and leaves the top as a fold instead of a seal. The three-side version drops one seal line, so it fills faster and costs less while staying flat and airtight.
What is a back seal (fin seal) sachet?+
The film is wrapped into a tube and sealed down the centre of the back, then crimped at the top and bottom. It is the classic pillow-shaped wrap for chocolate bars — the fastest and most film-efficient of the three.
Which sachet seal keeps the product freshest?+
All three are fully airtight when sealed correctly. Freshness depends more on the barrier film (foil, metallised or mono-PE) than on the seal style. Four-side and three-side give the flattest, most tamper-evident pack; back seal is equally protective but pillow-shaped.
Which sachet seal is cheapest to produce?+
Back seal is usually the lowest-cost because it uses the least film and runs at the highest machine speed. Three-side seal is close behind and stays flat; four-side seal costs a bit more due to the extra seal and film.
Can you print on all three sachet types?+
Yes. All three accept custom printed film — full-colour flexo or digital print on the front panel. On a back-seal sachet, keep the centre fin area clear of critical artwork because the fin stands proud on the back.