EP1957741B1

EP1957741B1 - A frame structure and a method for manufacturing such a frame structure

Info

Publication number: EP1957741B1
Application number: EP06805606A
Authority: EP
Inventors: Henrik Danielsen
Original assignee: VKR Holding AS
Current assignee: VKR Holding AS
Priority date: 2005-11-21
Filing date: 2006-11-21
Publication date: 2013-02-20
Anticipated expiration: 2026-11-21
Also published as: WO2007057029A1; EP1957741A1; CN101313122B; PL1957741T3; CN101313122A

Description

The present invention relates to a method for manufacturing a frame structure and to a frame structure, such as a window sash or a frame for a window or door, including side, top and bottom pieces, said frame structure comprising a core including a wooden core member and a plastic covering encasing the wooden core member.
There is a growing interest for robust and aesthetically pleasing frame structures, especially caused by positive economical development in the western world in recent years, increased building activity and increased building renovation. Further, legislation regarding energy conservation is becoming more and more strict, and frame structures for buildings should meet requirements regarding U-value to thereby reduce the energy consumption in houses.
Applicant's patent EP-B1-0 251 804 discloses a frame structure of the kind mentioned in the introduction. The frame structure is in the form of a window structure, for instance a window frame or sash. The plastic covering of the frame structure disclosed in this document is obtained by encapsulating the core at least partly by moulding a layer of for instance polyurethane on the core. The core may be made of wood or wood-based material, such as pine, chipboard or plywood. This construction is a success and provides a number of advantages, especially with regard to weatherproofness. With regard to insulating properties of the construction, there is, however, room for improvement.
AU-A-87 071/82 relates to a structural element with a core of timber and a covering of polyurethane at least partly around the core, thereby improving the dimensional stability and durability of the timber structural element. This structural element is not optimum with regard to insulation properties.
It is an object of the invention to provide a frame structure of the above kind having improved insulating properties, while still providing a weatherproof construction.
This is achieved by a frame structure according to claim 6.
Heat-treated wood has been found to possess improved insulating property in comparison to untreated wood, and hereby is achieved that the frame structure provides improved insulating properties in comparison with the prior art. Until now there has been a reluctance or even prejudice to use a relatively expensive and high-class material as a core member in such constructions. However, despite the higher price of heat-treated wood compared to ordinary, untreated wood, the overall price of the resulting frame structure is surprisingly not significantly raised when using heat-treated wood. This is partly due to high dimensional stability of heat-treated wood, so only a comparably very small percentage of the wood needs to be scrapped. Further, heat-treated wood is relatively easy to machine.
When moulding a plastic material around wood, it is often experienced that resin of the wood spoils the result and may even necessitate a cleaning of the mould. Surprisingly, with the frame structure according to the invention this is not an issue. During heat treatment of the wood of the wooden core member the resin flows out of the wood and/or crystallizes and is hence rendered passive at the elevated temperature during the heat treatment. Heat treatment also lowers the moisture content of the wood, which is found to greatly facilitate the process of encapsulating the wood, and further the risk of deterioration of the encapsulated wood is reduced. A description of a heat treatment process can be found in a publication entitled "Heat treatment of wood in Finland - state of the art" by the authors Tuula Syrjänen, Saila Jämsä and Pertti Viitanierni.
In order to facilitate the manufacture and make it possible to optimize the properties, the core may include a plurality of core pieces, each said core piece including at least one core member of heat-treated wood.
Conveniently, said plurality of core pieces may total four core pieces corresponding to the side, top and bottom pieces of the frame structure.
In a further development of this embodiment, at least one of said core pieces includes a plurality of core members. Each core piece may have its individual configuration of core members in dependence of the need for strength, weatherproofing properties and/or aesthetical appearance.
The configuration of each individual core piece may vary. For instance, a first core member and a second core member of said core piece may be connected with each other at a first set of mutually facing contact surfaces, said first or second core member being formed from heat treated wood having a density lower than the density of the second or first core member and better thermal insulating properties than the second or first core member. -
In order to obtain further advantages during manufacture and/or in use of the frame structure the core piece may comprise a third core member connected with the first and/or the second core member at a second set of mutually facing contact surfaces. In one further development of this embodiment, the third core member is made from the same material as the first core member. This makes it possible to obtain similar properties and appearance on several faces of the frame structure.
To obtain further flexibility with respect to the configuration of the core pieces, the third core member may be connected with the second core member by allowing the second set of contact surfaces to extend substantially in parallel with said first set of contact surfaces, or, alternatively, substantially perpendicularly to said first set of contact surfaces.
Even further advantages may be achieved by forming the core piece with a fourth core member connected with the first, second and/or third core member at a third set of mutually facing contact surfaces. In one advantageous further development of this embodiment, said fourth core member is made from the same material as the first core member.
The third core member may be connected with the second core member, said second set of contact surfaces extending substantially in parallel with said first set of contact surfaces, and the fourth core member may be connected with at least the second core member, said third set of contact surfaces extending substantially perpendicularly to said first and second sets of contact surfaces.
In an advantageous further development of the embodiment relating to core pieces formed by a plurality of core members, the first core member of each core piece faces inwards with respect to the frame opening. This makes it possible to achieve a harmonized appearance on the side which is visible to the user in the mounted state of the frame structure.
Depending on the intended field of application of the frame structure, the plastic covering may in cross section cover only part of the surfaces of the wooden core member. This makes it possible to optimize the use of the plastic material, which is relatively costly.
Preferably, the plastic covering in cross section comprises two groups of each at least two parts provide on substantially opposite surfaces of said core member.
The plastic covering may cover parts of the exterior surfaces of the wooden core member visible to a user in a mounted state of the frame structure. The plastic covering provides protection and a smooth surface, which in addition to being aesthetically pleasing is advantageous from a practical point of view. Such a plastic covering provides for a surface which is easy to clean and demands almost no maintenance in the form of e.g. painting, lacquering, oil treatment etc..
The heat-treated wooden core member preferably has a density between 350 and 500 kg/m³. A density within this range provides a good compromise between strength and heat conductivity. A higher density provides for better strength characteristics, but higher heat conductivity.
The wooden core member may be shaped by any suitable process, but according to an embodiment the wooden core member is chamfered to the desired shape. The resulting outline or cross section of the core member is relatively smooth with rounded edges, whereby flow of the plastic material is facilitated, the production time reduced, and a high quality coated frame structure may be achieved.
Jointing of wooden core members of different pieces of the frame structure may be accomplished by any suitable joining means, such as nails, screws, fittings, glue etc. In an embodiment of the invention the wooden core members are assembled with finger joints with a plurality of fingers, of which one finger joint is an interference fit, whereas the rest of the finger joints are loose fits. Hereby the frame structure may be assembled quickly and with relative ease without need for special tools. The interference fit holds the members together during moulding of the plastic encasing, and the loose fit allows for the plastic material to flow into the gaps between the fingers during manufacture, and when the plastic is set, the joint is effectively fixed. As the joint is completely encased in plastic material, the surface of the frame structure is closed and impervious making the frame structure weatherproof and robust.
The thickness of the plastic material encasing the wooden core member is a balance of integrity of the resulting frame structure, material cost, insulation properties etc. A suitable interval of the average thickness of the plastic material is 1 to 8 mm, preferably at least 2 mm and maximum 5 mm. The plastic material is relatively expensive, and hence the thickness should be kept at a minimum, but practical problems relating to moulding of the plastic sets a lower value of about 1 mm.
Another aspect of the invention according to claim 1 relates to a method for manufacturing a frame structure comprising the steps of: providing a core member of heat-treated wood, which has been treated at a temperature of 150-240°C for 0.5-4 hours, pre-treating the surface of the core member to facilitate adherence, assembling a core piece of a plurality of core members, placing a set of core members in a mould, injecting a setting plastic material in the mould, and removing the finished frame structure from the mould. This method can be performed with a limited degree of manual labour, thereby resulting in a relatively efficient and low cost method for manufacture when producing such frame structures in large numbers.
The core member may be shaped in any suitable way, such as by sawing, planing and milling or the like, however according to an embodiment the core member is machined in the appropriate shape by moulding, chamfer, or the like. Hereby the resulting core member has an outline with a relatively smooth contour, which is advantageous with regard to moulding of the plastic around the core member, as the flow of plastic in the mould is facilitated.
The core members may be joined at corners by any suitable means, such as screws, nails, fittings etc., but according to an embodiment ends of the core members are machined to provide engaging finger joints with a plurality of fingers of which one set of fingers are adapted to provide an interference fit, whereas the rest of the finger joints are loose fits. Hereby jointing is facilitated, and there is no need for special tools are separate elements. Further metal elements will have the disadvantage that due to the thermal conductivity of metal there is a risk of cold bridges in the construction and poor insulation property of the construction. The interference fit of some fingers will hold the construction assembled in the mould, whereas the loose fit providing gaps between other fingers will allow plastic material to flow into the gaps during moulding and securely fix the joint when the plastic sets.
Pre-treating of the surface of the core member to facilitate adherence may be accomplished in a number of ways, such as by etching the surface, applying a suitable primer or the like. According to an embodiment the step of pre-treating the surface of the core member is a step of roughening the surface, to provide a non-smooth surface having improved adherence characteristics. The roughening entails an increase in the area of the contact surface between the core and the plastic covering. Such roughening may also include e.g. a chamfering or grooving operation, in which the surface is provided with a number of grooves of identical or varying depth and width. This operation is carried out by an appropriate tool, such as a gouge or moulding iron, which may have e.g. a zigzag pattern of valleys and troughs to form grooves of a depth of e.g. 1 mm in the surface of the core member. The advantageous effect may be due to facilitated wetting of the surface, so the plastic material will get in intimate contact with the surface during moulding, and hence form a strong bond between the core member and the plastic material.
According to an embodiment, the setting plastic material is polyurethane, which is a well-known material with suitable properties with regard to weatherproofness, insulation, mouldability etc.
In the following the invention will be described in more detail by way of example and with reference to the drawing, in which:

Fig. 1 is a perspective view showing a frame structure in a first embodiment of the invention;
Fig. 2 is a cross-sectional view along the line II-II in Fig. 1;
Fig. 3 is a photo of a cross-sectional slice of a window frame prototype, said photo corresponding to the cross-sectional view of Fig. 2 of the first embodiment of the frame structure shown;
Fig. 4 is a cross-sectional view corresponding to Fig. 2 of a second embodiment of the frame structure according to the invention;
Fig. 5 is a cross-sectional view of a third embodiment of the frame structure according to the invention;
Fig. 6 is a perspective view showing a frame structure in a fourth embodiment of the invention;
Figs 7 to 9 are cross-sectional views along the lines VII-VII, VIII-VIII and IX-IX, respectively, in Fig. 6;
Figs 10 to 12 are cross-sectional views corresponding to Figs 7 to 9 of a frame structure in a fifth embodiment of the invention;
Fig. 13 is a photo of a corner joint in the window frame prototype shown in Fig. 2;
Fig. 14 is a front view photo of a core of a window frame prototype corresponding to a sixth embodiment of the frame structure according to the invention;
Fig. 15 is a side angle view photo of the prototype window frame core of Fig. 14;
Fig. 16 is a front view photo of a core of a window sash prototype corresponding to a seventh embodiment of the frame structure according to the invention; and
Fig. 17 is a side angle view photo of the prototype window sash core of Fig. 16.

In the first embodiment of the frame structure shown in Fig. 1, the frame structure is a window frame having two side pieces 1 and 1a, a bottom piece 1b and a top piece 1c. As will be described in further detail below, the structure of the individual pieces may vary within the general principle underlying the invention, i.e. that a core including a core member comprising heat-treated wood is encased by a plastic covering. However, in the following reference will be made only to the right-hand side piece 1.
In the drawing of Fig. 2 and the photo of Fig. 3 a cross-sectional view of a window frame side piece according to the invention can be seen. The side piece 1 comprises a core consisting of a core member 2 of heat-treated wood encapsulated in a plastic covering in the form of a surface layer 3 of plastic material. The surface layer 3 is formed of foamed polyurethane. The wooden core is machined into shape by moulding or chamfer in long units, and can even make up an entire side piece of the window if considered advantageous. Hence the cross-section shown comprises recesses 4 and 5, which are not used at this position, but has a function elsewhere along the frame side piece. The recess 6 is adapted for taking up a lining. With reference to Fig. 3, the growth rings of the heat-treated wood can be seen, and it is also seen that the heat-treated wood is relatively dark in colour. Research work on heat treatment of wood has been performed e.g. in Finland for some years, and reference is made to the previously mentioned publication. In this context heat treatment processes are processes in which the physical properties of the wood are changed permanently, i.e. moisture content, but also strength, insulation property etc. Such changes are effected when wood is subject to temperatures above 150°C. The heat-treated wooden core member preferably has a density between 350 and 500 kg/m³.
The wood species of the heat-treated part of the core may be any species suitable to the purpose, examples being softwood species such as pine or spruce.
The effect of heat treatment of wood depends on a number of parameters, such as

the maximum temperature and the length of the actual heat treatment period
the temperature gradient
the length of the entire heat treatment
the use and amount of water vapour
the kiln drying process before the actual heat treatment
the wood species and its characteristic properties.

As mentioned temperatures over 150°C alter the physical and chemical properties of wood permanently, for example the colour of the wood darkens, the shrinkage and swelling of the wood is reduced, and the equilibrium moisture content of the wood is improved. It is also found that the strength is reduced, but the insulation property is enhanced. The heat treatment process is normally divided in a preheating period, an actual heat treatment period and a cooling period. During preheating and cooling the temperature gradient is controlled to avoid excessive splitting or cracking of the wood.
Heat treatment reduces and lowers water uptake, which is advantageous with regard to durability of the wood and resistance to deterioration. However, this effect also lowers the adherence of the plastic material during moulding, and hence it has till now been considered very difficult to encapsulate heat-treated wood in plastic.
Swelling and shrinking of heat-treated wood is reduced, so the risk of cracking of the plastic covering is reduced.
A disadvantage of heat-treated wood is that the strength decrease by 10-30% in bending and pulling, and the wood becomes more brittle, but it is surprisingly found that the reduction in strength is not a problem in the frame structure according to the invention.
Figs 4 and 5 schematically show alternative embodiments of the frame structure according to the invention. Fig. 4 shows a cross sectional view of a window frame side piece, and Fig. 5 correspondingly shows a cross sectional view of a window sash side piece. In Figs 4 and 5 a plastic covering in the form of a surface layer 103, 203 of plastic material covers only part of the surfaces of a wooden core member 102, 202 seen in cross section. The surface layer 103, 203 comprises two groups of each at least two parts provided on substantially opposite surfaces of said core member. As previously described large binding forces are achieved between the surface layer 103, 203 and the core member 102, 202 because the plastic material adheres well to heat-treated wood. However, additional safety from separation of plastic from heat-treated wood is achieved by this particular provision of surface layer parts.
In the embodiment of Fig. 4 the first group comprises parts 103a - 103i, parts 103a - 103c being disposed on surfaces of the core member 102 facing a window sash of the window, and parts 103d, 103e being disposed on opposite surfaces of the core member, i.e. the surface of the core member, which in a mounted state of the window is facing a roof into which the window has been integrated. The second group comprises parts 103f - 103i, parts 103f - 103h being disposed on surfaces of the core member 102 in a mounted state of the window facing the inside, i.e. a room of a building into which the window has been built, and part 103i being disposed on an opposite surface of the core member 103, i.e. the surface of the core member 103, which in a mounted state of the window is facing the outside.
Correspondingly, in the window sash of Fig. 5 the first group comprises parts 203a - 203c, part 203a being disposed on a surface of the core member 202 facing a windowpane of the window sash, and parts 203b, 203c being disposed on opposite surfaces of the core, member 202, i.e. surfaces of the core member, which in a mounted state of the window are facing a window frame of the window, e.g. the window frame of Fig. 4. The second group comprises parts 203d - 203f, part 203d being disposed on a surface of the core member 202 in a mounted state of the window facing the inside, and parts 203e, 203f being disposed on opposite surfaces of the core member 203, i.e. the surfaces of the core member 203, which in a mounted state of the window are facing the outside.
In the embodiment of Fig. 4 the parts 103d, 103e are separate parts disposed on a mutual surface of the core member 102, thus forming a discontinuation of the surface layer 103 on the surface of the core member 102, which surface in a mounted state of the window is facing and hidden by the roof. Since this surface is hidden, it serves no aesthetic purpose to let the surface layer 103 cover this surface. Similarly, in the embodiment of Fig. 5 the surface layer 203 does not cover the surfaces of the core member 202 hidden, during normal use in a mounted state of the window, to a user inside the building. This saves plastic material, which is relatively costly in comparison with the material of the core. Moisture protection and weather resistance of naked heat-treated wood is typically sufficient, even on surfaces of heat-treated wood facing the outside. Even other materials utilized as core members may have sufficient properties to face the outside.
In alternative embodiments the core of the side pieces, the top piece and/or the bottom piece comprises further core members of heat-treated wood, non-heat-treated wood or any other suitable material such as an insulating foamed material. The strength requirements of the top and bottom pieces are typically lower such that these pieces may comprise core members with improved insulating properties, but poorer strength properties, such as an insulating foam material. Such core members are preferably completely encased by more durable core members such as wooden core members.
While core members visible from the inside of a building into which the frame structure has been built, as mentioned in the above, are preferably covered by a surface layer, core members only visible from the outside of the building can be manufactured from another material such as not encased heat-treated hardwood to provide a compromise between appearance and costs. Heat-treated hardwood has a lighter colour than heat-treated softwood. The hardwood may be in the form of birch or European aspen.
Figs 6 to 9 show an embodiment, in which the pieces 301, 301a, 301b, 301c constituting the core of the frame comprises a number of core members manufactured from different materials. In the embodiment shown, the core may form the core of a frame structure of a window frame, but the principle is fully adaptable to other frame structures, such as a window sash or a frame of a door. It is even possible to let the core form a substantial part of the frame structure.
In Figs 7 and 9 a first core member 302 and a second core member 303 of the side core piece 301 are connected with each other at a first set of mutually facing contact surfaces 311, the second core member 303 being formed from heat treated wood having a density lower than the density of the first core member 302 and better thermal insulating properties than the first core member. The material of the first core member 302 may be any material fulfilling the aesthetically and practically convenient demands. For instance, the material may be lacquered pine.
As shown in Fig. 8, the top core piece 301c comprises a third core member 304c connected with the first and second core members 302c, 303c at a second set of mutually facing contact surfaces 312c. In the embodiment shown, the third core member 304c is made from the same material as the first core member, viz. a wooden material in the embodiment shown. The third core member 304c is connected with at least the second core member 303c, said second set of contact surfaces 312c extending substantially perpendicularly to said first set of contact surfaces 311c.
The bottom core piece 301b shown in Fig. 9 has substantially the same configuration as the side core piece 301, and comprises a first core member 302b and a second core member 303b.
Figs 10 to 12 shows a further development of the principle underlying the embodiment of Figs 6 to 9, and only differences with respect to this embodiment will be described in detail. The side core piece 401 comprises a third core member 404 and a fourth core member 405. The third core member 404 being connected with the second core member 403 and the fourth core member 404 in a second set of contact surfaces 412 extending substantially perpendicularly to the first set of contact surfaces 411, whereas the fourth core member 405 is connected with the second core member 403 in a third set of contact surfaces 413 substantially in parallel with the first set of contact surfaces 411.
The top core piece 401c shown in Fig. 11 is of a similar configuration.
The bottom core piece 401 has a third core member 404b and a fourth core member 405b connected with the second core member 403b along a respective set of contact surfaces 412b and 413b.
In the embodiment shown, the third and fourth core members are made from the same material as the first core member. The materials may be chosen according to the particular demands of the frame structure. Preferably, the second core member is made of heat treated wood to improve the overall insulating properties of the frame structure, and the first, third and fourth core members of a wooden material having aesthetically pleasing properties.
In all of the above embodiments, the first core member of each core piece faces inwards with respect to the frame opening. However, other configurations are of course conceivable.
In Fig. 13 a photo of a corner joint of the window frame can be seen. A part of the plastic coating 3 has been cut away for illustration of the finger joint. One core 2 comprises three fingers 2a, 2b and 2c, engaged with four fingers 7a, 7b, 7c and 7d of another core. The joint between fingers 7b, 2b and 7c is an interference fit, so this joint will be able to fix the mutual position of the two cores. This means that at manufacture, the cores may be assembled and placed in a mould. The joint between other fingers is a loose fit, so there is a gap into which the plastic coating material may flow and set to fix the joint, as can be seen at reference numeral 8.
As will be clear to the skilled person the finger joint should not be considered restricted to the embodiment with core member of heat-treated wood, as the advantages of the joint is evidently also present with core members of other materials, such as untreated wood, plywood etc.
In Figs 14 and 15, an embodiment of an assembled frame core 12 can be seen prior to moulding of the plastic coating around the members of the frame core 12. The frame core 12 is assembled of straight pieces of heat-treated wood having uniform cross-section along the length thereof. At a corner joint auxiliary wedge parts 13 may be provided as can be seen to strengthen the joint and provide a smooth transition between the jointed members, if the core members differ in size. The wedge parts 13 are not necessarily made of heat-treated wood, but may be made of for example plywood.
An embodiment of an assembled sash core 20 can be seen in Figs 16 and 17. The construction of the sash core 20 is a little more complicated and comprises additional plywood parts 21, 22 and 23 attached to the core parts of heat-treated wood.
A prototype of the frame structure was made using a polyurethane foam of the integral type as the plastic material. The polyurethane was foamed in the mould, and the density of the resulting layer of PUR depending on the free space available in the mould, as e.g. a void will result in a lower density. It will be evident to the skilled person that other types of plastic material could be used with good results.

Claims

A method for manufacturing a frame structure comprising the steps of:
providing a core member of heat-treated wood, which has been treated at a temperature of 150-240°C for 0.5-4 hours,

pre-treating the surface of the core member to facilitate adherence,

assembling a core piece of a plurality of core members,

placing a set of assembled core pieces in a mould,

injecting a setting plastic material in the mould,

removing the finished frame structure from the mould.
The method according to claim 1, wherein the core member is machined in the appropriate shape by moulding, chamfer, or the like.
The method according to claim 1 or 2, wherein ends of the core members are machined to provide engaging finger joints with a plurality of fingers of which one set of fingers are adapted to provide an interference fit, whereas the rest of the finger joints are loose fits.
The method according to any of the claims 1-3, wherein the step of pre-treating the surface of the core member is a step of roughening the surface.
The method according to claim 4, wherein the setting plastic material is polyurethane.
A frame structure, such as a window sash or a frame for a window or door, including side, top and bottom pieces, said frame structure comprising a core including a wooden core member and a plastic covering encasing the wooden core member, manufactured by the method according to any one of claims 1-5.
A frame structure according to claim 6, wherein the core includes a plurality of core pieces, each said core piece including at least one core member of heat-treated wood.
A frame structure according to claim 7, wherein said plurality of core pieces totals four core pieces corresponding to the side, top and bottom pieces of the frame structure.
A frame structure according to claim 8, wherein at least one of said core pieces includes a plurality of core members.
A frame structure according to claim 9, wherein a first core member and a second core member of said core piece are connected with each other at a first set of mutually facing contact surfaces, said first or second core member being formed from heat treated wood having a density lower than the density of the second or first core member and better thermal insulating properties than the second or first core member.
A frame structure according to claim 10, wherein said core piece comprises a third core member connected with the first and/or the second core member at a second set of mutually facing contact surfaces.
A frame structure according to claim 11, wherein said third core member is made from the same material as the first core member.
A frame structure according to claim 11 or 12, wherein said third core member is connected with the second core member, said second set of contact surfaces extending substantially in parallel with said first set of contact surfaces.
A frame structure according to claim 11 or 12, wherein said third core member is connected with at least the second core member, said second set of contact surfaces extending substantially perpendicularly to said first set of contact surfaces.
A frame structure according to any one of claims 10 to 14, wherein the core piece comprises a fourth core member connected with the first, second and/or third core member at a third set of mutually facing contact surfaces.
A frame structure according to claim 15, wherein said fourth core member is made from the same material as the first core member.
A frame structure according to claim 15 or 16, wherein said third core member is connected with the second core member, said second set of contact surfaces extending substantially in parallel with said first set of contact surfaces, and wherein said fourth core member is connected with at least the second core member, said third set of contact surfaces extending substantially perpendicularly to said first and second sets of contact surfaces.
A frame structure according to any one of claims 10 to 17, wherein the first core member of each core piece faces inwards with respect to the frame opening.
A frame structure according to any one of claims 6 to 18, wherein the plastic covering in cross section covers only part of the surfaces of the wooden core member.
A frame structure according to claim 19, wherein the plastic covering in cross section comprises two groups of each at least two parts provided on substantially opposite surfaces of said core member.
A frame structure according to any one of claims 6 to 20, wherein the plastic covering covers parts of the exterior surfaces of the wooden core member visible to a user in a mounted state of the frame structure.
A frame structure according to any one of claims 6 to 21, wherein the core member of heat treated wood has a density between 350 and 500 kg/m³,
A frame structure according to any one of claims 6 to 22, wherein the wooden core member is chamfered to the desired shape.
A frame structure according to any one of claims 6 to 23, wherein the wooden core members are assembled with finger joints with a plurality of fingers, of which one finger joint is an interference fit, whereas the rest of the finger joints are loose fits.
A frame structure according to any of claims 6 to 24, wherein the plastic material encasing has a thickness in the interval of 1 to 8 mm, preferably at least 2 mm and maximum 5 mm.