A typical construction of upholstered furniture intended for sleeping and/or relaxing consists of (Fig. 4.79):
• a set of foams glued to the frame,
• a frame,
• a mattress assemblage or seat made of layers:
– support and base,
– spring (elastic) and
• a cover being the tapestry layer of the mattress or seat.
Fig. 4.80 Construction of seat backrest assemblage of a single-person armchair:
(1, 2) side, 3 transverse joining skirt, 4 dowel connectors, 5 supporting block, 6 upholstery belts,
7 longitudinal joining skirt and 8 upholstery cardboard
The frame is an essential construction assemblage of upholstered furniture, and it is responsible for its durability, strength and stability. Mostly, furniture frames are made of chipboards, OSB boards, wooden strips (mainly deciduous—beech), hard fibreboard and upholstery cardboard. Figures 4.80 and 4.81 show the frame structure of a seat backrest assemblage and side of a single-person armchair. As it can be seen, some of the construction elements have been connected using dowel joints. In industrial practice, however, inserted staple-type connections are mostly used.
The support and base layer is a part of the upholstery assemblage, on which the remaining layers of the mattress or seat pillow are rested. It has the form of a frame, which gives the desired stiffness and constitutes a bearing part of the product. The base in such a construction can be:
hard made for example of wooden strips, chipboards, hard fibreboards
flexible made of rubber belts, textile rubber tapes, sackcloth belts (Fig. 4.82a); and spring made of flattened spiral sinusoidal springs and springing grids
The strength of the upholstery frames is connected with the cross-sectional dimensions of rails and type of the material used. In these constructions, it is recommended to use rails with the cross section 38 x 43 mm. Research confirms that not only traditional frames made from solid wood of different species, but also frames made of elements glued from layers, even bases made of wood-based materials meet the strength expectations, often increasing the stiffness of an upholstery base and equalling the strength level of the whole base structure (Kapica et al. 1983, 1985).
Spring layers in upholstered furniture are an essential part of the product both in terms of volume, as well as usage. In furniture for sleeping and/or relaxing, they may constitute the fillings:
spring made using spring units: bonnell, schlarafia, pocket springs
without made from flexible elastomers, latex foam, coconut mat, seagrass,
springs etc. (Fig. 4.83b)
Fig. 4.82 Types of support layer bases: a sackcloth belts, b sinusoidal springs, c springing grid, d flattened springs and e hard fibreboard
Fig. 4.83 Construction of upholstery system: a spring, b without springs: 1 upholstery fabrics, 2 wadding, 3 highly flexible foam, 4 soft polyurethane foam, 5 coconut mat (stubble and latex mat), 6 springs (biconical, cylindrical, barrel, etc.), 7 covering fabrics or felt, 8 base layer, 9 upholstery frame, 10 underlay fabrics and 11 hard polyurethane foam
Spring layer gives the upholstery system appropriate elastic properties, which in effect leads to a dynamic adaptation of the base shape to the silhouette of the user, and at the same affects in a significant way the comfort and ergonomics of use of the product. Construction of the spring layer should be dependent on the individual needs of the user, and above all on his weight, age and preferences (most commonly adopted position during sleeping and relaxing). The stiffness of spring systems (and thus the feeling of comfort of use) is affected by the use of springs with different internal diameters. In addition, the softness of upholstery units (depending on the purpose of furniture) can be easily modelled by the appropriate selection of inside diameter of springs, their number and spacing (Dzi^gielewski and Smardzewski 1995a). The softness of biconical springs, and at the same time of bonnell type spring units, depends on the diameter of the smallest coil (Kapica and Smardzewski 1994), and these units, despite their widespread use, do little to comply with ergonomic requirements for lounge furniture (Smardzewski 1993). According to Kapica (1988), upholstery systems without springs made as a layer system show generally greater hardness than upholstery systems with bonnell spring unit. The most widely used spring materials include:
• bonnell spring units, made with biconical springs with five coils and wire diameter 2.0-2.2 mm, connected at the top and bottom part with spiral springs of wire diameter 1.3—1.4 mm. Mostly, springs have dimensions: external diameter 80-85 mm, height of the springs 120-130 mm and distance between the springs in the unit 20-40 mm;
• pocket spring units, made of cylindrical springs of wire diameter 1.2-2.5 mm, placed additionally in bags made of canvas or other upholstery cloth and woven
upholstery belts, joined with upholstery cord (Dzi^gielewski and Smardzewski 1995b);
• polyurethane foams, belonging to the group of elastomers, are characterised by high durability, chemical and physical resistance, and most of all resistance to abrasion;
• latex foams are made in the process of vulcanization of foamed natural rubber milk. Due to low natural ventilation of the material, in latex boards special ventilation channels are made;
• coconut mats are made in two technologies as needle coconut mats and vulcanised mats; and
• seagrass—combed and dried leaves of palm trees, additionally can be subject to needling on a base fabric.
Properties of polyurethane foams primarily depend on the structure that is the size of the cells and their shape (density), the construction of the cells (open or closed), as well as material constant values (Saha et al. 2005). Brandel and Lakes (2001) have showed that there is a possibility of thermomechanical modification of the polyurethane foams structure in which there is a change of Poisson ratio—from positive to negative. This positively affects the physical properties of these materials and causes almost equalling the value of linear pliability module with the value of figural pliability module. In mattresses, foams with a density from 14.5 to 65.0 kg/m3, stiffness from 1.0 kPa to approximately 7.0 kPa, permanent pliability from 4 to 20 % and resiliency of 37-80 % are used. Figure 4.84 shows the order of glueing foams on the seat frame. Foams most widely used in furniture industry include:
• classical (standard), e. g. type T;
• classical flame resistant, e. g. type C;
• highly elastic (highly resilient), e. g. type HR;
• highly elastic—flame resistant (highly resilient flame resistant), e. g. type
• low elasticity (thermoelastic).
Thermoelastic foams have so-called memory, which is the ability to slowly return to the original shape after strains. In addition, they are characterised by the ability to absorb vibrations and shocks, which has been confirmed by research: 20 % effectiveness of rebounding a ball and at 50-60 % effectiveness for regular polyurethane foams. The foam has the ability to adapt to the contours of the body, which causes that its surface gently affects the human body, especially in such sensitive areas as heels, hips, elbows and head. Foam properties largely depend on ambient temperature: in low temperature—e. g. about -1 °C—the foam becomes very stiff, while optimal stiffness is observed at 13-29 °C. Susceptibility to strains increases along with increased relative humidity of air.
The lining layer in furniture functions as insulation (heat shield) and affects the softness of the system, increasing the comfort of use. Usually, fabrics weighing 50-400 g/m2, as well as canvas, packing fabrics, wadding and sheep’s wool are used.
Tapestry layer is the last, outer layer (cover), fulfilling the function of a tensioner of the upholstery system and raising the aesthetic appearance of the product. This layer is typically made from fabrics such as terry, stretch, jacquard, chenille, velour, nubuck, cotton, as well as knitted fabrics, natural and ecological leathers.