Woven fabric, LDPE film, kraft paper. Each ply has one job, and understanding which ply does what makes it much easier to specify the right build for a moisture-sensitive product.
Cement, milk powder, guar gum and polymer compounds have little in common commercially. They share one packaging problem: they are ruined by moisture, and a single-layer sack will not stop it.
The paper-laminated bag answers that by doing three jobs in three separate plies instead of asking one material to do all three badly.
Layer one: woven fabric, inside
The innermost ply is woven HDPE or polypropylene fabric, and it carries the load. This is what gives the bag its tensile strength and its resistance to puncture and tearing — the properties that make a sack viable for 50 kg of cement.
Without it, the paper outer would split the first time a bag was dropped.
Layer two: PE film, middle
Between the fabric and the paper sits an LDPE extrusion film. It does two jobs: it bonds the plies to each other, and it is the water barrier that stops liquid water reaching the contents.
It is a genuinely separate function from the layer above it, which is why the two are not interchangeable.
Layer three: kraft paper, outside
Kraft paper forms the outer surface and acts as the moisture barrier, handling humidity rather than liquid water. It is available in brown and white kraft.
It also happens to be an excellent print surface, which is why paper-laminated bags can carry high-quality graphics without a separate label.
When three layers are not enough
For expensive or particularly sensitive products the same principle extends. A metallised film ply on the face adds a light, aroma and vapour barrier that paper cannot give you. An HM or LDPE liner inside adds a further contamination barrier. And where you want an all-paper sack, multiwall is built up to four plies rather than three.
If your product has a documented shelf-life requirement, bring it to the conversation. The number of plies is a decision that follows from it, not the other way round.
Formats
Paper-laminated bags are offered as Centre Sealed (back seam), 'L'-type stitched and box-type block bottom, and in sewn open mouth, self-opening square, pinched bottom open mouth, pasted open mouth and pasted valve styles.
Which one suits you depends mostly on your filling equipment — which is the subject of the next article.
