Sofa Frames and Fillers: What a Sofa Is Made Of
Wood frame or metal, Bonnell or pocket springs, foam density 22 vs 45 kg/m³: a buyer guide to sofa construction and what to specify when ordering from China.
A sofa lasts exactly as long as its frame and fillers allow — fabric can be re-upholstered, legs replaced, but a collapsed foam core or broken spring ends the piece. When sourcing upholstered furniture from China, these hidden components are the ones most often left out of the specification — and the first ones a factory cuts corners on. This guide breaks down sofa construction from the inside: frames, spring systems, and fillers, with the technical parameters you need when ordering furniture from China.
Frame: Wood, Plywood, or Metal
The frame carries all static and dynamic load. The material determines whether the sofa wobbles in three years or holds for a decade.
Birch plywood (12–18 mm) is the industry standard for quality upholstered furniture. Its cross-laminated structure delivers equal strength in all directions. Plywood does not split under impact loads and remains dimensionally stable through humidity changes. Structural load-bearing members use 18 mm plywood; back-panel ribs use 12 mm.
Birch or beech solid timber is used at joints (mortise-and-tenon glued connections), legs, and decorative elements. Steamed beech bends easily — it is the classic material for curved chair backs. A 40×60 mm solid timber rail in the structural frame matches plywood in rigidity when properly assembled.
Steel frame (1.5–2 mm sheet) suits HORECA furniture, modular office systems, and sofa-bed conversion mechanisms. Metal does not absorb moisture, does not work loose at joints, but is heavier than wood and transmits vibration more readily.
Avoid: OSB, particleboard (chipboard), and MDF in the structural frame. These boards hold screws poorly: joints loosen within 2–3 years of sustained load, particularly at leg-attachment points and conversion mechanism brackets.
Spring Systems: Bonnell, Pocket, Sinuous
The spring system determines resilience, service life, and sleeping comfort.
Bonnell unit. The classic system of hourglass coil springs linked by helical wires into a single mat. Advantages: lower cost, uniform surface feel. Disadvantages: partner-disturbance effect (one person’s movement is felt by the other), service life 5–8 years at typical loads up to 100 kg.
Pocket springs (independently pocketed). Each spring is heat-sealed in its own fabric pocket. Standard count: 256 springs per seat section (~60×60 cm); premium pieces use 512 or more. Advantages: targeted body support, minimal partner disturbance, service life 10–15 years. Disadvantage: 30–50% more expensive than Bonnell.
Sinuous (zigzag) springs. A wavy steel strip stretched across the frame width. Used in sofa backs and narrow chair seats. Strengths: low profile, light weight, excellent durability under moderate loads. Limitation: at higher body weight produces noticeable sag — not ideal as the primary sleeping surface.
Foam-on-frame (FSS / no-spring). Solid high-density foam without springs. Used in compact sofas, children’s furniture, and modular systems. At density ≥ 40 kg/m³ provides good support; service life 7–10 years.
Fillers: PU Foam, Latex, Polyester Fibre
Polyurethane foam (PU foam) is the primary filler in 95% of sofas across all price tiers. The critical parameter is density (kg/m³):
- 22–25 kg/m³: budget grade — noticeable sag within 1–2 years of daily use.
- 28–32 kg/m³: standard domestic — performs adequately for 3–5 years.
- 35–40 kg/m³: quality grade — 7–10 years; the minimum for residential projects with regular use.
- 45+ kg/m³: contract/premium — 10–15 years; mandatory for HORECA applications.
The second parameter is firmness (ILD/IFD): soft 15–25, medium 25–35, firm 35–50. Seat cushions: medium to firm; back cushions: soft.
Natural latex. Vulcanised rubber from the Hevea tree. Maintains resilience for 10–15 years. Hypoallergenic and naturally antimicrobial. Heavy (550–700 g/L) and expensive. Typically used as a 5–8 cm top layer over foam or springs in sleep-function sofas and mattresses.
Synthetic latex (SBR). Similar feel at 40–60% lower cost; service life 7–10 years. Specify the composition in the brief — the two are visually indistinguishable.
Polyester fibre (polyfibre, hollowfibre). Used for decorative cushions and the soft fill in back pads and armrests. Not suitable as a seat core: compresses permanently and does not recover shape.
Memory foam (viscoelastic). Conforms slowly to body shape. Used as a 3–5 cm top layer in premium sofa beds. Not used as a primary filler because of heat retention and slow recovery when changing position.
Construction Comparison
| Construction | Service life | Cost | Partner disturbance | Best use |
|---|---|---|---|---|
| Bonnell + 28 kg/m³ foam | 5–8 years | $ | High | Budget domestic |
| Pocket + 35 kg/m³ foam | 10–15 years | $$$ | Low | Premium domestic |
| 40 kg/m³ foam-on-frame | 7–10 years | $$ | Medium | Compact / office |
| Pocket + latex | 15+ years | $$$$ | Minimal | Luxury / HORECA / bedroom |
| Sinuous + 32 kg/m³ foam | 8–12 years | $$ | Low | Armchairs, narrow sofas |
What to Specify When Ordering from China
At Foshan factories the cost difference between 22 kg/m³ and 40 kg/m³ foam is roughly $8–12 per linear metre of sofa. Without a spec sheet, the factory uses whichever grade costs less. Minimum technical brief for upholstered furniture:
- Frame: birch plywood 18 mm (structural) + 12 mm (secondary), joints glued and bracketed. No particleboard or OSB in the structural frame.
- Springs: type (Bonnell / pocket) and count per section (for pocket springs).
- Seat foam: density in kg/m³ (min. 35 residential, 40 HORECA) + ILD firmness.
- Back foam: density ≥ 28 kg/m³, firmness 15–25 ILD.
- Top layer: latex or memory foam with thickness in mm — only if required.
- Fabric: type + Martindale rating (see our upholstery fabric guide).
On-site verification: cut a 10×10×10 cm cube of foam at the factory and weigh it — the number in grams equals the density in kg/m³. Note that duty rates and VAT on furniture imports vary by destination (Thailand 7% VAT, UAE 5% VAT + 5% duty, EU varies by HS code); confirm landed costs before committing to an order.
Knowing what a sofa is made of is the first step. The second is finding a factory that follows the specification rather than substituting cheaper foam after the sample is approved. Dream View sources from vetted upholstered-furniture manufacturers in Foshan: we audit the production floor, verify materials at the start of the run, and inspect before shipment. Our fee is a flat 10% of order value — no markups. Learn how we work or send us your brief — we will identify suitable factories and provide a cost estimate at no charge.
Frequently asked questions
What foam density should a good sofa have?
For a living-room sofa: minimum 35 kg/m³. For heavy use (children, daily sleeping) and HORECA projects: 40–45 kg/m³. Foam below 28 kg/m³ will sag within 1–2 years of daily use.
How do pocket springs differ from a Bonnell unit?
In a pocket-spring unit each spring is sealed in its own fabric pocket and works independently: movement on one side does not transfer to the other. Bonnell springs are interconnected coils — the partner-disturbance effect is noticeable. For a sofa bed the difference is critical; for a decorative lounge sofa it matters less.
What should a quality sofa frame be made of?
Birch plywood 12–18 mm or birch timber, joints glued and reinforced with metal brackets. Plywood does not split along the grain under load and stays stable through humidity changes. Particleboard or MDF in a structural frame is a budget-production sign: joints loosen within 2–3 years.
How long do sofa springs last?
A Bonnell unit at loads up to 100 kg lasts 5–8 years. Pocket springs (256–512 per seat section) last 10–15 years under normal domestic use. Sinuous zigzag springs in the back panel have near-unlimited life when the sofa is not used as a bed.
How do I verify foam density when receiving goods from China?
Cut a 10×10×10 cm cube of foam and weigh it: the mass in grams equals the density in kg/m³. For contract projects always request a foam-batch compliance certificate — it is a mandatory line item in the pre-shipment inspection report.
Which is better for a hotel sofa: springs or solid foam?
For a hotel sofa bed: pocket springs (256+) with a top layer of latex or 40 kg/m³ foam 5–8 cm thick. For lobby seating without a sleeping function: solid 40 kg/m³ foam without springs is sufficient and reduces weight.