Pile Density Explained: What Filament Count Means for Sealing

Published 2026-10-11

Pile density weatherstrip is a brush-style seal where performance depends less on the base extrusion and more on how tightly the PP polypropylene yarn is packed. XINHAI manufactures pile/brush weatherstrip for doors and windows in Jiaxing, Zhejiang, China. The yarn is PP polypropylene. Options include silicone or non-silicone, adhesive backing or none, and plastic fin or plain pile. If you compare two strips with the same pile height and they seal or slide differently, density is often the reason.

Pile density weatherstrip: what it means

Density is the number of filaments per unit area. Think of it as how many yarn filaments occupy a given area of the base—across the width and along the length. A dense pile has more filaments packed into the same footprint. A loose pile has fewer filaments, so there is more open space between them.

Density is not the same as pile height. Pile height is the height of the yarn above the base. Two strips can both be 10 mm high, but one may have tightly packed yarn and the other a sparse yarn. Height tells you how far the pile can reach. Density tells you how much resistance that pile presents to air, dust, and sliding motion.

Density is also not the same as base width. Base width is the width of the extruded base that carries the pile. XINHAI’s base width range is 4–25 mm. Pile width is the width of the pile strip itself, and it varies by spec—for example, D pile-width codes such as 6.9 (4P): 2.9±0.3mm and 4.8 (4P): 1.9±1.5mm are referenced in the A–G spec system. A wider base does not automatically mean a denser pile. It depends on how much yarn is placed across that base.

In the A–G spec system, density is not a single letter. The system covers base width, pile height, base thickness, pile width, fin height, fin material, length, and base margin. Density is a derived property. It comes from the yarn count, yarn thickness, spacing, and the pile width. That is why you should ask the manufacturer to define density in the units they use—filaments per centimeter, yarn ends per inch, or another internal measure.

Suppliers may express density as yarn ends per inch, filaments per cm, or a yarn denier plus pile width. There is no universal density symbol in the A–G list. If your drawing has a density note, ask how it was calculated. If it has no density note, ask what yarn count the supplier will use for your base width and pile height.

How pile density weatherstrip affects wind, dust, and noise

The basic principle is simple: more filaments per unit area means smaller gaps between filaments. Smaller gaps make it harder for air to pass through the pile. That supports anti-wind and anti-dust behavior. A dense pile creates a more tortuous path for air and particles. A loose pile lets more air and dust slip through.

Dust behavior depends on particle size and the rest of the assembly. In principle, a dense pile can block larger dust particles more effectively than fine particles. For fine dust, the base contact and fin may matter more. The verified features are anti-wind, anti-noise, and anti-dust. Density is one variable that supports those features, but it is not the only one.

Noise behaves in a similar way but with limits. Sound travels through air paths. If a pile reduces air leakage around a door or window, it can reduce the noise that travels through that gap. But pile weatherstrip is not a soundproofing material. It is a gap seal. Its effect on noise depends on the assembly, the gap size, the compression, and the rest of the window or door system.

Pile height interacts with density. A taller pile can bridge a larger gap, but if it is too loose, it may not seal well against wind and dust. A shorter, denser pile may seal a small gap better, but it will not reach across a large gap. The fin—plastic or fabric—also matters. Fin height can be the same as, higher than, or lower than the pile. The fin can add a barrier, but it does not replace the pile’s density.

Silicone and non-silicone options are available. If sliding friction matters, ask how the silicone is applied and whether it is on the yarn or the base. The option list confirms silicone or non-silicone, but the exact effect depends on the construction. Do not assume silicone automatically solves friction; confirm with the supplier.

Pile width codes (D) as a density indicator

Many drawings use a pile width code, often labeled D, to describe the width of the pile across the base. It is a useful geometry reference, but it is not a direct density measurement. D tells you how wide the pile strip is. It does not tell you how many filaments are packed into that width.

Here is the practical relationship. If you increase D while keeping the same number of filaments per unit length, density can drop because the same yarn is spread over a wider area. If you increase D and also increase the yarn count proportionally, density can stay the same. If you reduce D while keeping the yarn count the same, density can rise. So D is an indicator, not a verdict.

This is why you should not specify D alone and expect a predictable seal. Ask the manufacturer: does D mean pile width or base width? Is D measured before or after the pile is brushed? What yarn count is used at that D? If your drawing uses D as a density indicator, confirm the definition before you quote.

For XINHAI, pile width varies by spec. The A–G system includes D pile-width codes such as 6.9 (4P): 2.9±0.3mm and 4.8 (4P): 1.9±1.5mm. Base thickness is 0.8 ± 0.15 mm. Base width ranges from 4 to 25 mm. Pile height ranges from 5 to 20 mm. These are the dimensions you can put on an RFQ. Density is the question you ask the manufacturer to translate into yarn count for those dimensions.

Density is not a standalone specification

Density only makes sense with the rest of the pile geometry. A 10 mm pile height with high density will feel different from a 5 mm pile height with the same density. One D pile-width code with high density will fit a different gap than another D pile-width code with the same density. The base thickness—0.8 ± 0.15 mm—and the base width—4–25 mm—also change how the strip sits in the profile. The fin can add a barrier above or below the pile. The base margin changes where the pile sits on the extrusion.

If you specify density without these dimensions, the supplier may hit the density target but miss the fit. If you specify the dimensions without density, the supplier may hit the fit but miss the sealing performance. The A–G system gives you the dimensions. The density question gives you the yarn packing. Use both.

Why denser is not always better

Denser pile is not automatically better. It trades sealing against friction and cost.

On sliding doors and windows, every filament that touches the track or frame adds drag. A denser pile has more contact points. That can make the sash harder to slide. It can also wear the yarn, the track, or the coating faster. If the door is large or the hardware is light, excess friction may be unacceptable.

Density also affects fit. A pile that is too dense may not compress into the gap. If it cannot compress, it can prevent the door from closing fully, interfere with the latch, or push the sash out of alignment. A moderate density that compresses and recovers may seal better in a real assembly than a very dense pile that blocks the gap but fights the hardware.

Cost follows material. More yarn means more PP polypropylene per meter. XINHAI’s supplier-stated capacity is 20+ production lines, 2000 sqm, and 300,000 meters per day, but density is still a cost variable. If you over-specify density, you pay for yarn and friction you may not need.

The right density depends on the application. For balcony sliding doors, low friction is usually a priority. For exterior doors exposed to wind and dust, a denser pile may be justified. For interior doors, a moderate density may be enough. For shower doors, caravan/RV, and greenhouses, the gap, movement, and exposure determine the balance. The verified applications include French patio doors, balcony sliding doors, wooden doors, interior/exterior doors, shower doors, caravan/RV, and greenhouses. Use those as a starting point, then match density to the actual gap and movement.

How to specify pile density weatherstrip on an RFQ

You can specify density on an RFQ even when the supplier’s catalog does not list a density number. The key is to give the full geometry and ask the manufacturer to translate density into yarn count for that geometry. Reference the A–G system and ask direct questions.

Send these dimensions and options:

• Base width: 4–25 mm
• Pile height: 5–20 mm
• Base thickness: 0.8 ± 0.15 mm
• Pile width / D code: for example, 6.9 (4P): 2.9±0.3mm or 4.8 (4P): 1.9±1.5mm
• Fin height: same as, higher than, or lower than the pile
• Fin material: plastic or fabric
• Length: customized
• Base margin: as required by your profile
• Silicone or non-silicone
• Adhesive backing or none
• Plastic fin or plain pile

Then ask these density questions:

• What is the filament count or yarn density per unit width? Ask for the unit used.
• How is density controlled—yarn denier, stitch density, pile width, or another method?
• Is D the pile width or the base width?
• What density do you recommend for a sliding balcony door, an exterior door, or a caravan/RV application?
• How does the silicone option change friction or wear?
• Can you provide a sample so we can test sliding force and compression in our frame?

Do not rely on a single density number without context. Density is only meaningful when paired with pile height, pile width, base width, and the gap it must seal. A dense pile in a narrow base is different from a dense pile in a wide base. A tall dense pile is different from a short dense pile.

For commercial terms, Alibaba listings show MOQ 20,000 meters and a reference price of $0.01/m. Treat those as listing references and confirm current MOQ and price with the factory. XINHAI announced ISO 9001 in company news in April 2019. If your purchasing process requires current certification, ask for the current certificate status. Do not assume a current certificate from a 2019 announcement.

How to compare samples

Ask for samples at the density you are considering. Compare them in the actual frame or a similar gap. Check how the pile compresses, how it recovers, and how the sash slides. Inspect the pile for uniform density. Look for gaps, loose filaments, or uneven pile width. These are practical checks, not lab tests. If you need lab data, ask the manufacturer what they can provide.

Next step

Next step: use the Pile Weatherstrip Selector to narrow base width, pile height, and options, or send your spec—base width, pile height, D/pile width, silicone/non-silicone, backing, and fin—to [email protected] or +86-573-86779825 for a quote. You can also review the PP pile weatherstrip range before you send the RFQ.