Every filter bag is either sewn or welded, and the right choice is a genuinely close call more often than either camp admits. We manufacture both, so this guide has no side to sell — here is how the two constructions differ, where each one wins, and the handful of cases where the choice actually decides whether your filtration works.
The Short Answer
| Sewn | Welded | |
|---|---|---|
| Seam | Thread through needle holes | Media ultrasonically fused to itself |
| Bypass at seam | Slight — needle holes are larger than most micron ratings | None — no holes, no thread |
| Silicone | Sewing lubricants may be present | Silicone-free construction |
| Media choices | All — felt, mesh, Nomex®, specialty fabrics | Thermoplastic felts (polyester, polypropylene), 1–200 µm |
| Cost | Lower | Somewhat higher |
| Best for | General-purpose filtration | Paints, coatings, inks, instrument-checked clarity |
What Sewing Does to a Filter Bag
A sewing needle punches a hole every stitch, and those holes are bigger than the micron rating of any fine felt — a 10-micron bag’s stitch line passes particles a 10-micron media face would catch. The thread itself can wick liquid along the seam and, in some materials, shed fibers. None of this matters in most applications: the seam is a tiny fraction of the bag’s area, and general-purpose duties never notice the difference. That’s why sewn bags are the industry default and the economical choice — and why we sew most of the bags we make.
What Welding Changes
Ultrasonic welding fuses the seam and bottom of the bag so the media is continuous — no needle holes, no thread, no bypass path, and a seam as strong as the parent fabric even under pressure spikes. Because there’s no sewing, there are also no sewing lubricants, which makes welded bags silicone-free by construction — the reason coatings and paint lines specify them: trace silicone is what causes fisheyes and craters in a finish.
Choose Sewn When…
- The duty is general-purpose solids removal and downstream clarity is judged by eye.
- You need media that can’t be welded — monofilament mesh, Nomex® for high temperature, or specialty fabrics like canton flannel are sewn constructions.
- Cost per bag matters more than the last fraction of a percent of efficiency.
- You need hardware variety — rings, drawstrings, tie-ons, odd dimensions — where sewing is the flexible construction.
Choose Welded When…
- Paints, coatings, and inks — silicone-free construction plus zero bypass; this is the classic welded application.
- Instrument-checked clarity — if a particle counter or gloss meter judges the result, stitch-line bypass shows up in the data.
- High-value product — when what escapes through needle holes is expensive, the welded premium pays for itself.
- Fine micron ratings — the tighter the rating, the larger a needle hole is relative to it, and the more the seam matters.
The Honest Middle Ground
Plenty of processes run sewn bags at 25–100 micron for years and should keep doing exactly that. The switch conversation starts when you tighten below ~10 micron, when a finish defect traces back to silicone or seam bypass, or when a spec citing “no bypass” arrives from a customer’s quality department. If you’re unsure which side of the line you’re on, describe the application — 800-554-8555 or the quote form. We make both constructions, so you’ll get the cheaper honest answer, not the more expensive one.