What a valve bag actually does
A valve bag is filled through a small sleeve in one corner. The filling spout enters the sleeve, the bag fills under the product's own weight, and the internal pressure closes the sleeve as the spout withdraws. No sewing, no sealing, no operator standing at the mouth.
That only works if the product flows. Fine powders, free-flowing granules and uniform pellets fill a valve bag predictably. Flakes, fibres, sticky or bridging products do not: the sleeve blocks, the fill rate collapses, and the weight at the end of the cycle scatters.
What an open-mouth bag buys you
An open-mouth bag is filled across the full bag width and closed afterwards by sewing, heat sealing or gluing. It accepts coarse, lumpy, fibrous and high-entrainment products, it accepts a polyethylene liner for moisture protection, and it scales to sizes where a valve sleeve becomes unreliable.
The price is a closing station, more floor space, and one more machine that can stop the line.
Three tests that settle it in an afternoon
Pour a sample through a funnel and watch the discharge. If it arches, stalls or runs unevenly at that size, a valve sleeve will behave the same way at production speed.
Measure bulk density twice: as delivered, and after a day of settling. A product whose density moves by more than a few percent will drift on weight unless the filler weighs the product rather than dispensing a volume.
Ask the receiving end how the bag gets opened. A plant that tips bags into a hopper needs a bag that survives tipping; a retailer that puts it on a shelf needs a square, stackable shape.
The constraint people forget: the pallet
Bag dimensions are usually fixed by the pallet footprint and the customer's shelf, not by the machine. Decide the pallet pattern first, then the bag, then the filler. Reversing that order is how projects end up with a machine that fills beautifully and bags that do not stack.
Where to look next
If the product flows, the valve bag filler is the shortest route to a filled bag — the sleeve does the closing.
If it does not, start from the open-mouth baggers and budget for a closing station. Either way the line, not the machine, decides the pallet pattern: see complete packaging lines.