7D Fiberfill Specifications Guide: Understanding Polyester Fiber Ball Filling for B2B Buyers

If you're sourcing polyester fiberfill for pillows, cushions, stuffed toys, garments, or furniture, you've probably come across terms like "7D," "hollow conjugated," and "siliconized." What do they actually mean, and how do these specifications affect the feel, durability, and price of your final product? In this technical guide, we break down everything a B2B buyer needs to know about 7D fiberfill — from denier basics and fiber structure to rebound testing standards, fill weight calculations, and how to specify your requirements correctly to Chinese suppliers.

1. What Does "7D" Mean in Fiberfill? Understanding Denier

The "D" in 7D stands for denier — a unit of measurement for the linear density of textile fibers. Specifically, denier is the weight in grams of 9,000 meters of fiber. A higher denier means a thicker fiber; a lower denier means a finer fiber.

Why 7D Is the Most Common Fiberfill Grade

7D is the workhorse denier for fiberfill because it strikes the perfect balance between softness and support. It's thick enough to provide good loft and resilience, yet fine enough to feel comfortable against the skin. This makes it ideal for the widest range of applications — from pillows and cushions to stuffed toys and garment padding.

Denier	Fiber Thickness	Feel	Typical Applications
1D - 2D	Very fine	Ultra-soft, down-like	Premium pillows, sleeping bags, high-loft jackets
3D	Fine	Soft, plush	Soft toys, cushions, garment lining
7D ⭐	Medium	Balanced softness & support	Pillows, cushions, furniture, general filling
15D	Thick	Firm, supportive	High-density cushions, furniture cushions, pet beds
20D+	Very thick	Rigid, structural	Heavy-duty applications, foam replacement

Fiber Length (Cut Length)

In addition to denier, fiberfill is also specified by cut length — the length of each individual fiber strand. Common cut lengths for 7D fiberfill are 32mm, 51mm, and 64mm. Longer fibers generally produce better resilience and less fiber migration, while shorter fibers feel softer but may shift more over time.

• 32mm: Short fiber — best for open blow filling, very soft, but more prone to clumping
• 51mm: Standard length — most common, good balance of softness and resilience
• 64mm: Long fiber — best resilience, less migration, ideal for fiber ball production

2. Hollow vs. Solid vs. Conjugated Fiber: What's the Difference?

These terms describe the internal structure and shape of the fiber, and they have a major impact on performance and price.

Solid Fiber

Solid polyester fiber is just what it sounds like — a solid filament throughout. It's the cheapest type but also the heaviest and least resilient. Solid fiber tends to flatten quickly and doesn't bounce back well. You'll mostly find it in very low-cost applications where performance doesn't matter much.

Hollow Fiber

Hollow fiber has a single hollow channel running through the center of each filament. This trapped air is what gives hollow fiber its loft and insulating properties. Hollow fiber is lighter per gram than solid fiber and has better resilience. There are also multi-hole fibers (4-hole, 7-hole) with even more air channels for superior loft.

Conjugated Fiber (Crimped / 3D Crimp)

Conjugated fiber has a natural curl or crimp built into the fiber structure, like a spring. This crimp is what makes the fiber "bounce back" after being compressed. Conjugated fibers are made by extruding two different polyester polymers side-by-side that shrink at different rates, creating a permanent helical crimp.

The Gold Standard: 7D Hollow Conjugated Siliconized (7D HCS)

When people say "7D fiberfill" in the context of premium filling, they almost always mean 7D hollow conjugated siliconized fiber — often abbreviated as 7D HCS. This is the highest-performing and most widely used specification for quality fiberfill products.

Property	Solid	Hollow	Hollow Conjugated	Hollow Conjugated Siliconized
Resilience	★☆☆☆☆	★★★☆☆	★★★★☆	★★★★★
Loft / Volume per kg	Low	Medium	High	Very High
Softness	Stiff	Medium	Soft	Velvety soft
Fiber migration	High	Medium	Low	Very Low
Relative price	1x	1.3x	1.6x	1.8x
Best for	Low-cost fill	Budget products	Mid-range products	Premium products

3. Siliconized vs. Non-Siliconized Fiberfill

Siliconization is a surface treatment that coats each fiber with a thin layer of silicone. This is one of the most important quality differentiators in fiberfill, yet many buyers don't fully understand what it does.

What Siliconization Does

• Reduces friction between fibers — fibers slide past each other more easily, giving a silkier, down-like feel
• Improves resilience — fibers spring back faster after compression because they don't stick together
• Prevents fiber clumping — the slippery surface prevents fibers from matting together over time
• Reduces fiber migration — the even surface helps fibers stay distributed evenly within the casing
• Improves washability — siliconized fibers recover their loft better after washing and drying

Types of Silicone Treatment

Not all siliconization is equal. There are different levels of silicone coating that affect both performance and price:

• Light silicone (0.3-0.5%): Basic treatment, slight softness improvement, low cost
• Standard silicone (0.8-1.2%): Most common, good softness and resilience for general use
• Heavy silicone (1.5-2.0%): Premium treatment, maximum softness and slippery feel, used in high-end pillows
• Non-silicone "slick" treatment: Alternative for products that can't use silicone (e.g., some medical applications)

Pro tip: Always specify the silicone percentage when ordering. Two suppliers both offering "siliconized" fiber may be using very different coating levels, resulting in different performance and pricing.

4. Fiber Ball vs. Flat Fiberfill: Which Should You Use?

Fiberfill comes in two primary forms: raw loose fiber (flat fiberfill) and fiber balls (ball fiber / pearl fiber). The form you choose depends on your filling method and product requirements.

Flat Fiberfill (Loose Fiber)

Raw, loose polyester fiber that comes in bales. It's the raw material form and requires an opening machine to fluff before use.

• Pros: Lower cost per kg, can be custom-blended, works with blow-filling machines
• Cons: Requires opening equipment, more labor-intensive, may have uneven filling
• Best for: High-volume factories with automated filling lines, custom density products

Fiber Balls (Ball Fiber / Pearl Fiber)

Fiber that has been processed into small, round balls about 5-15mm in diameter. Fiber balls are ready to use straight out of the bag — just pour or blow them into the casing.

• Pros: Ready to use, very easy to fill, excellent resilience, minimal clumping, consistent fill density
• Cons: Slightly higher cost per kg, requires ball-forming equipment to produce
• Best for: Pillows, cushions, stuffed toys, products sold with refillable filling

MULIN's Fiber Ball Advantage

At MULIN, we produce fiber balls using 7D hollow conjugated siliconized fiber processed through specialized ball-forming equipment. Our fiber balls are uniform in size, have excellent roundness, and maintain their shape and resilience even after multiple compression cycles. We supply fiber balls in 20kg compressed bales that expand to 4-5x their compressed volume when opened.

5. Denier Comparison: 3D vs. 7D vs. 15D — Applications & Feel

Choosing the right denier depends on your product's intended use, target price point, and desired feel. Here's how the three most common deniers compare:

Property	3D Hollow Conjugated	7D Hollow Conjugated	15D Hollow Conjugated
Feel	Very soft, plush, down-like	Balanced soft & supportive	Firm, supportive, structured
Loft height	Medium-high	High	Very high (firmer)
Support	Low-Medium	Medium	High
Compression set (5000 cycles)	~15-20%	~10-15%	~5-10%
Fill weight for standard pillow	800-1000g	600-800g	500-700g
Price per kg (relative)	1.1x	1.0x (baseline)	0.95x
Best applications	Soft pillows, plush toys, garment padding	Standard pillows, cushions, furniture, general use	Seat cushions, pet beds, high-support products

Blending Deniers for Custom Feel

Many premium products use a blend of different deniers to achieve a specific feel. For example:

• 70% 7D + 30% 3D: Soft but supportive — a popular pillow blend that feels luxurious while maintaining shape
• 50% 7D + 50% 15D: Extra firm — for high-support applications like orthopedic pillows
• 60% 3D + 40% 7D: Down-soft feel — for ultra-plush toys and decorative cushions

6. Rebound Resilience Testing: How to Measure Fiber Quality

The single most important quality metric for fiberfill is rebound resilience — how well it bounces back after being compressed. Poor resilience means your pillows and cushions will go flat quickly, leading to customer complaints and returns.

Standard Rebound Test Methods

1. Ball Drop Test (ASTM D3574)

A steel ball of specified weight is dropped from a known height onto the fiber sample. The rebound height is measured and expressed as a percentage of the drop height. Higher percentage = better resilience.

• Good quality 7D HCS fiber: ≥ 60% rebound
• Premium 7D HCS fiber: ≥ 70% rebound
• Low quality / solid fiber: ≤ 40% rebound

2. Compression Set Test

A fiber sample is compressed to 50% of its original height for a specified time (usually 22 hours at 70°C), then released. The residual thickness loss is measured as compression set percentage.

• Good quality 7D HCS: ≤ 15% compression set after 22h at 70°C
• Premium quality: ≤ 10% compression set
• Acceptable minimum: ≤ 20% compression set

3. Cyclic Compression Test

The sample is repeatedly compressed and released (typically 5,000 or 10,000 cycles) and the loft loss is measured. This simulates real-world use over time.

• After 5,000 cycles: ≥ 80% loft retention = good quality
• After 10,000 cycles: ≥ 70% loft retention = premium quality

How to Run a Quick In-House Quality Check

You don't need lab equipment to do a basic quality check:

1. Take a handful of fiber and fluff it up
2. Compress it firmly into a ball with both hands for 10 seconds
3. Release and observe how quickly and completely it expands back
4. Repeat 10 times — if it still recovers to nearly full volume, it's good quality
5. If it stays clumped or flattened after a few compressions, quality is poor

7. Fill Weight & Loft Calculation: Getting the Right Firmness

One of the most common questions from buyers is: "How much fiberfill do I need for my pillow/cushion?" The answer depends on the size, shape, desired firmness, and fiber quality.

Standard Pillow Fill Weights (7D HCS Fiber Balls)

Pillow Size	Soft Fill	Medium Fill	Firm Fill
Standard (48×74cm)	500-600g	700-800g	900-1000g
Queen (50×76cm)	600-700g	800-900g	1000-1100g
King (51×91cm)	700-800g	900-1100g	1200-1400g
Travel (30×40cm)	150-200g	250-300g	350-400g
Body pillow (35×120cm)	1000-1200g	1400-1600g	1800-2000g

Cushion Insert Fill Weights

Cushion Size	Soft / Decorative	Medium	Firm / Seat Cushion
30×30cm	100-120g	150-180g	200-250g
40×40cm	180-220g	250-300g	350-400g
45×45cm	230-280g	320-380g	450-500g
50×50cm	280-350g	400-450g	550-600g
60×60cm	400-500g	550-650g	750-850g

Quick Fill Calculation Formula

As a rough rule of thumb for 7D HCS fiber balls in a rectangular cushion:

Fill weight (grams) = Length(cm) × Width(cm) × Height(cm) ÷ 200 (medium firmness)

Adjust for firmness: ÷ 250 for soft, ÷ 150 for firm.

Note: Always order samples and test fill weights before committing to bulk production. Different fiber qualities and casing fabrics can significantly affect the final feel.

8. Full Quality Specification Sheet for 7D Fiberfill

Use this specification sheet as a template when requesting quotes from suppliers. Providing a complete spec sheet ensures you get apples-to-apples pricing and avoid quality surprises.

Parameter	Specification (Premium 7D HCS)	Test Method
Fiber type	Polyester staple fiber, virgin	Visual + chemical test
Denier	7.0 D ± 0.5 D	ASTM D1577
Cut length	64mm ± 3mm (for fiber balls); 51mm ± 2mm (standard)	Manual measurement
Fiber structure	Hollow, 4-hole (or 7-hole for premium)	Microscope
Crimp type	Conjugated (3D helical crimp)	Visual
Crimp count	12-16 crimps per 25mm	ASTM D3937
Silicone coating	1.0% ± 0.2% (premium silicone oil)	Solvent extraction
Tensile strength	≥ 3.0 cN/dtex	ASTM D3822
Elongation at break	≥ 50%	ASTM D3822
Rebound resilience	≥ 65%	ASTM D3574 Ball Drop
Compression set (22h, 70°C)	≤ 12%	ASTM D3574
Moisture regain	≤ 0.4%	Oven drying
Color	Raw white (or custom dope-dyed)	Visual + colorimeter
Flammability	California TB117 / BS5852 / CFR1633 (as required)	Specified standard
Food contact safety	Not applicable (not for food use)	—
Packaging	20kg compressed bales, PE bag inner + woven bag outer	Visual

9. Sourcing Tips & Common Pitfalls When Buying from China

9.1 Always Request Samples First

Never place a bulk order based on photos or specifications alone. Always ask for physical samples — at least 500g of fiber — so you can test the feel, resilience, and quality yourself. Reputable suppliers will send samples for a small fee (usually refundable against your first order).

9.2 Watch Out for "Regenerated" Fiber

Some suppliers offer very low prices because they're selling regenerated polyester fiber made from recycled PET bottles or production waste. Regenerated fiber is legal and has its uses, but:

• It has lower tensile strength and resilience than virgin fiber
• Color may be inconsistent (off-white or yellowish)
• May contain impurities or mixed deniers
• Often fails flammability and safety standards

If you need virgin fiber, explicitly state "100% virgin polyester" in your spec and verify with samples. If regenerated fiber is acceptable for your application, you can save 20-30% on cost.

9.3 Understand Packaging & Volume

Fiberfill is extremely lightweight and voluminous, which means shipping cost is usually a significant portion of total cost. Fiber is typically supplied in highly compressed bales to minimize shipping volume.

• Compressed bale density: ~250-300 kg/m³
• Fluffed density: ~20-30 kg/m³ (about 10x expansion)
• 20ft container capacity: ~5,000-6,000 kg of compressed fiber bales
• 40HQ container capacity: ~12,000-14,000 kg of compressed fiber bales

9.4 Confirm MOQ and Lead Time

• Standard 7D HCS fiber: MOQ 500kg, lead time 3-7 days (if in stock)
• Custom specifications: MOQ 3,000-5,000kg, lead time 15-25 days
• Fiber balls (standard): MOQ 200kg, lead time 5-10 days
• Custom colors: MOQ 5,000kg, lead time 20-30 days

9.5 Ask About Certifications

Depending on your market and product, you may need:

• OEKO-TEX Standard 100 — for textiles that contact skin
• REACH compliance — for EU market
• California TB117 / BS5852 / CFR1633 — flammability standards for furniture/mattresses
• ISO 9001 — factory quality management

10. Frequently Asked Questions

This article was written by the MULIN Lingerie technical team. With over 12 years of thermal foam shaping and fiber product experience, MULIN manufactures premium 7D fiberfill, fiber balls, molded bra cups, and 3D eye masks for brands worldwide. For OEM inquiries, contact our sales team.