WO1996003262A1

WO1996003262A1 - Shaping wood substitutes

Info

Publication number: WO1996003262A1
Application number: PCT/GB1995/001767
Authority: WO
Inventors: Graham Robert Banks; Graham William Hill
Original assignee: Crosby Limited
Priority date: 1994-07-26
Filing date: 1995-07-26
Publication date: 1996-02-08
Also published as: GB9414988D0; AU3118295A

Abstract

A method of shaping wood based composite materials by bending, impressing or otherwise deforming the material is described in which the material in the form of a sheet (20) or some other appropriate initial shape is first subjected to a plasticising heat treatment, usually using moist steam, and is then deformed to the required shape, preferably in a press, with further heat treatment and under mechanical pressure to prevent any significant surface de-bonding.

Description

SHAPING WOOD SUBSTITUTES

This invention relates to the shaping of wood based composite

materials, generally known as wood substitutes, by bending and

/or otherwise deforming the material.

Even though it is widely used, wood is an expensive and rather

intractable medium for the construction of articles, and for many

years various man-made wood substitute materials manufactured in

board form, like wood, have been used instead. One of these is

medium density fibreboard (MDF) , which basically consists of wood

fibres compounded together with one or more synthetic resins,

often a urea/formaldehyde resin, and sometimes advantageously a

melamine urea formaldehyde - this type of MDF sometimes being

referred to as MUF or HMR, the latter standing for high moisture

resistance. Typical examples of materials available under the

generic name MDF are those sold by Medite, Caberboard, Kronospan,

Finsa and Merbok. MDF has been gaining much favour by

manufacturers of products such as furniture and kitchen and

bathroom fittings.

A second common wood substitute is chipboard, which basically

consists of small wood chips compounded together with one or more synthetic resins, often a melamine/formaldehyde resin mixture.

There are a variety of types and grades of chipboard, ranging

from low density boards to high density boards. Chipboard has

historically been used in the furniture industry and the kitchen

industry.

The third main wood substitute is Oriented Strand Board, known

in the trade as OSB. It basically consists of wood shaving which

are roughly positioned in line and then compounded together with

one or more synthetic resins. Various grades of OSB are

available from numerous producers, and this type of material is

mainly used in the joinery and construction industry.

All these materials are available in sheets of various sizes,

thicknesses (typically from 2.5mm to 25mm or more) and densities,

and can be supplied either plain or with a visually decorative

surface formed usually from paper, a wood veneer, or a plastic

coating. The materials are almost without exception stiff, very

rigid and practically inflexible. They have little if any

plasticity, and unless provided as very thin sheets, so that they

have some limited degree of flexibility by virtue of that very

thinness, they are difficult if not impossible to bend or

otherwise deform into the complex shapes often desired. The invention aims to provide a method by which sheets or boards

of these materials can be deformed to shape in practically any

way desired.

It is well known that solid wood can be bent to shape if it is

first "plasticised" by the application of wet heat, typically in

the form of steam. Indeed, some work has been carried out into

steam bending of MDF and some other composite boards, but without

significant success (and certainly at the moment no such products

are available in the market) . The problem seems to be that while

steam heat treatment of wood substitutes followed by bending does

enable a shaped article to be formed, the surface finish of the

article is poor, the treatment seeming to involve substantial de-

bonding of the wood chips, fibres or shavings from the resin

holding them together, so that the result is a shaped product

with a rough and unsightly surface. We have found that this

problem can be solved by using heat not only in a preliminary

stage, to render the material relatively plastic, but also in a

subsequent stage, in association with pressure, to set the

material in its new shape while at the same time physically

preventing surface de-bonding.

according to the invention, therefore, there is provided a method of shaping a wood substitute material in which the material, in

some appropriate initial physical shape and form, is first

subjected to a plasticising heat treatment, and is then deformed

to the required shape with further heat treatment and under

mechanical pressure to prevent any significant surface de-

bonding.

The invention is particularly applicable to the shaping of wood

substitute materials such as those mentioned earlier, namely MDF,

chipboard and OSB.

The initial physical form may of course be any appropriate to the

final form required, but for the most part will be that of a

sheet or strip which is bent or otherwise deformed into the

desired final, three-dimensional shape. As mentioned before,

typical thicknesses for sheets or strips of wood substitute

materials range from 2.5mm to 25mm and may even be 30mm or more,

and the method in accordance with the invention can be

successfully applied to any of these. Such materials, having

been rendered relatively pliant by the plasticising heat

treatment, may be deformed to the required shape in any suitable

manner, such as be being bent, impressed or simply compressed.

Impression may be a full impression in which the material is impressed from one side to produce a corresponding projection of

the material from the other side (i.e. effectively displacing a

section of the material bodily to one side and involving

localised bending and extrusion of the material) , or it may be

only a surface impression in which one side of the material is

indented without producing any corresponding outward displacement

on the other side.

It may be desirable to pre-cut the material, and possibly even

to pre-mould it, before finally shaping it by the method in

accordance with the invention. In this way there may be avoided

any distortion during the deforming process. Also, pre-cutting

can save much labour, in that no additional after-shaping is

required except for shapes which are of a particularly intricate

design, and less raw material is used.

The plasticising heat treatment in the method of the invention

can take any appropriate form, but a wet treatment is preferred

and is most conveniently a steam treatment - that is to say, a

treatment in which the material is enveloped in wet steam for

sufficient time for it to attain an acceptable degree of

plasticity. The heat treatment will normally be such as to raise

the material's temperature to around 32-38°C, at which point it becomes noticeably more pliable. It is believed that the moi.

heat affects the wood fibres within the board, allowing them to

stretch and to be deformed. Initially, the heat allows the

fibres to be released from the bonding resins (typically a

urea/formaldehyde resin, although some products employ an

isocyanate/based resin for waterproof boards, which reacts in

much the same way) , allowing them to be deformed and relocated

in the next stage before the dry heat and pressure applied in

this stage re-glues the fibres in their new location and sets the

material in the new shape .

The equipment used for the steam treatment may be a steam retort

such as is well known in the wood bending industry, and a typical

retort is that called the ITALPRESSE humidification autoclave

(model SU-4/120") which is available from the Steam Retort Co and

which may be used in the method in accordance with the invention

without modification.

Once the material has been rendered pliable it is then deformed

to the required shape . This shape can be whatever is required

(which naturally depends on the article being made) , and may

range from a simple curved "plank" for use in an archway, to a

small sheet bent in more than one direction to form a seat shell, to a large sheet impressed over its whole surface to form a

complex three-dimensional panel for use as a door panel (or even

a complete door) .

The deformation of the pliable material to the required shape

with the further heat treatment and under mechanical pressure can

be accomplished in any convenient way so long as they are

effected together, for it is this that appears to prevent any

significant surface de-bonding. Most conveniently, however, they

will be achieved using a press which is closed to deform the

material and then held closed to maintain the mechanical pressure

on the material while applying the further heat treatment. The

press may be a heated press in which the platens are heated to

apply the further heat treatment, or the press may be used with

a radio-frequency heating unit to apply the further heat

treatment .

These presses may be similar to those commonly found in a

laminating factory where shaped articles are made by bending and

simultaneously gluing and laminating several sheets of wood

together, as in making plywood. The pressure applied should be

quite high in order to hold the material ' s surface together under

the shaping strains involved. Typically pressures of the order of 0.35xl0⁶ to 3.5xl kg/rrf (500 to 5000psi) will be used, the

higher pressures being required when the shaping needed involves

some element of extrusion. In general the higher the applied

pressure the higher the density and therefore the bond strength

of the product .

It may be desirable to control the speed of the press. Thus, for

deformation involving extrusion, where the wood substitute

material is being both bent and squeezed, it is preferable to

effect the pressing relatively slowly, giving the material time

to "flow" into its new shape without serious disruption of the

material itself, whereas for simpler shapes (such as seat backs)

that require only a little bending pressure, the press can be set

to act faster.

It is believed that the heat and pressure serve to compress and

reset any fibres or chips which have become displaced during the

first stage (steam) heating and the second stage deformation, the

dry heat radiating from the press platens or produced within the

material itself by RF-heating acting firstly to evaporate any

excess moisture left from the initial steam treatment and

secondly to bond the fibres or chips in their new position with

the resin (many wood based composite materials seem to be produced with much of their resin content semi-cured) . So, in

the shaping stage the heat supplied should be sufficient to

substantially dry out the press-shaped material (it is quite wet

after the preferred preliminary steam treatment stage) , and to

ensure that the material will set and so hold its given shape

once the applied pressure and other shaping forces are removed.

If a heated press is used, the platens (and hence the material)

will usually be heated to a temperature of from 40 to 90°C,

although temperatures up to as high as about 240°C may be used

if desired, particularly if the material is thin and high

production rates are required. If a RF heating unit is used, one

providing around 15 KW output power will generally prove

sufficient for most purposes. Obviously, the time for which the

heat must be applied will be dependent on the temperature used

and the volume of the material .

In any board-forming operation the characteristics of the

finished item are paramount when assessing the viability of the

method. It is important that the surface finish be such that the

product can be painted, sprayed or veneered easily without any

costly second-phase finishing. In the method in accordance with

the invention, where pressure and heat are applied simultaneously

to the newly-formed shape, some of the resins rise to the surface of the material and seal any potential blemishes that the initial

heating and the deformation may have caused.

Using the invention, MDF and chipboard can be bent and pressed

into varying shapes leaving the surface with a very smooth

finish, while OSB, which by its very nature normally has a rough

surface, can not only be deformed easily, it can also be given

a smoother surface, enabling the material to be used for products

previously not capable of being made therefrom.

It is also interesting to note that, after treatment in

accordance with the invention, samples of all these three main

types of wood substitutes were tested and shown to exhibit

improved (i.e. decreased) absorption rates.

The invention enables quite considerable degrees of bend to be

applied to these wood substitute materials, although in any

actual case the possible degree of bend and impression achievable

depends upon the composition of the material (fibre lengths and

resin type etc.) , its thickness, the treatment time, and the

temperature of the steam heating. Some examples of bends and

impressions which have been achieved are given in the following

Table. Bendinσ MDF

Sheet Normal Bend, Extreme Bend,

Thickness (mm) Internal Diameter (mm) Internal Diameter (mm)

18 80 45

15 60 34

12 50 30

9 45 22

Impressing thick MDF

Sheet Depth of

Thickness (mm) Impression (mm)

25 12

18 12

9 9

Impressing thin MDF. Chipboard and OSB

Sheet Depth of

Thickness (mm) Impression (mm)

2.5 12 3.0 16 3.2 17 4.0 21 6.0 24

Various embodiments of the invention will now be described, by

way of example only, first with reference to the accompanying

diagrammatic Drawings and then by reference to a number of

specific Examples. In the Drawings Figure 1 shows a perspective view of a mobile s t e a

treatment unit;

Figure 2A show a side view of an open press for deforming

to a desired shape a flat sheet of a wood-based composite

material after it has been steam treated;

Figure 2B shows a side view of the press shown in Figure

2A with the press closed and deforming the sheet;

Figure 3 shows a perspective view of one example of a

shaped article produced by a method in accordance with the

invention; and,

Figure 4 shows a perspective view of another example of a

shaped article produced by a method in accordance with the

invention.

The mobile steam treatment unit of Figure 1 comprises a container

(11) on wheels (12) and with a sealable lid (13) through which

steam can be fed into the container via a pipe (14) connected to

a source (not shown) . The container is of course large enough to hold a planar sheet of a wood based composite material - such

as the sheet indicated at (20) in Figure 2A - while it is heated

up and softened by the steam supplied to the container.

When the wood substitute sheet (20) is suitably hot it is

transferred to a press, as shown in Figure 2A. The press has

upper and lower platens (21u., 211) each of which carries one of

a matching pair of steel tools (22u./ 221) which can be heated

electrically (by means not shown) . When the platens are driven

together (by means not shown) with the sheet (20) located between

them, the tools force the sheet (20) to take up their shape as

shown in Figure 2B, and at the same time supply heat and

mechanical pressure to the surface of the sheet to prevent or

heal any surface break-up.

After a suitable time the platens of the press are moved apart,

and the finished article - i.e. the shaped sheet (and possibly

also cut if the tools have cutting members formed as a part

thereof to trim the sheet as it is bent) - is removed. After

shaping it would normally then be covered with a veneer layer

glued thereon, but if preferred this layer may be applied to the

sheet after the steam heating and prior to its shaping in the

press of Figure 2 so that the veneer layer is deformed and bonded to the wood substitute sheet as the latter is deformed to the

required shape by the press .

Example 1: Bending and impressing MDF

Using a purpose-built steam box of dimensions 2200x1200x200mm

there was heated a single sheet of MDF of dimensions

2100x1100x12m purchased from Medite of Europe, based in Ireland.

The sheet was placed in the steam box, whereupon moist steam was

introduced into the container at an approximate temperature of

98°C. The MDF sheet was held in the steam box for 2 minutes

before being removed and immediately placed in a mould of

dimensions 2200x600mm set inside a press measuring 2600x1550mm.

The mould, which was constructed of wood and lined with 4mm thick

aluminium, consisted of 2 parts in a male and female

configuration defining a transverse curvature with an internal

diameter of 486mm running the whole length of the mould. A small

secondary die measuring 300x200x4mm and depicting a company logo

was placed on the bottom of the female part of the mould.

The press was activated to bring the male and female parts of the

mould together onto the MDF sheet causing the sheet to bend and

take the shape of the mould. The time from activation to

complete closure of the press was approximately 32 seconds, and press pressure was set at about 1.05xl0⁶kg/m² (1500 psi) , which

was found for this application to be the ideal pressure. When

the press was fully closed and the sheet had been bent to shape,

heat was introduced into the sheet by energising a 15 KW Radio

Frequency Heating Unit attached to the press. The drying and

setting period took 4 minutes 20 seconds, during which all the

moisture was removed from the sheet, and the fibres of the sheet

were fixed into their new position and re-bonded with the resin.

As the MDF sheet was bent in the large mould an impression was

made by the small die on the underside of the sheet. The

impression registered clearly to a depth of 4mm without breaking

the surface texture of the sheet.

After the drying and setting was complete, the press was opened

and the newly-shaped sheet was removed. The whole operation -

steam heating, positioning in the press, shaping and drying and

setting - took just over 6 minutes 50 seconds.

Exa le ; Bending MPF

A sheet of MDF measuring 1200x550x18mm was placed into a steam

container and treated with hot moist steam at about 98°C for 60

seconds. The steam heated sheet was then placed into a mono-shell chair

seat mould having 3-dimensional bends and contours with the

minimum internal diameter being 70mm, the mould being located

between the platens of a press. Once the sheet was positioned

in the mould the press was activated so as to close the mould

around the MDF sheet and thus bend the sheet to the shape of the

mould. The press closing time was less than 30 seconds and the

pressure exerted was about 1.05 kg/m² (1500 psi) . Once the mould

had closed and the sheet was bent and under pressure, the sheet

was then heated by means of a 15 KW RF Unit. The heating and

setting process took 3 minutes, and when completed, the press was

opened and the newly shaped MDF monoshell was removed. An

example of such a monoshell chair seat produced in this way is

shown at 30 in Figure 3.

The whole operation took about 4 minutes 40 seconds.

Example 3 : Bending MDF

Four strips of Kronospan MDF of dimensions 1200x200x9mm were

steam heated for 50 seconds in a purpose-built steam container.

The treated strips were then removed from the steam container and

placed side by side between the male and female parts of an

aluminium lined wooden mould with dimensions of 900x800mm and a single 90° degree bend at one end, the bend radius being 45mm.

The male and female parts of the mould were carried by the

platens of a press which, when activated, closed the mould around

the treated MDF strips causing them to be bent to the contour of

the mould. The mould was closed within 10 seconds and the

pressure exerted was about 0.84 kg/m² (1200 psi) . Heat was then

applied to the strips by means of a 15 KW RF unit, and the drying

and setting period took 2 minutes. The press was then opened,

and the bent strips of MDF were removed.

The whole process took about 3 minutes 30 seconds.

Example 4: Bending and impressing thin MDF

A thin sheet of MDF of dimensions 400x400x3.2mm was placed in a

steam container and heated for 40 seconds with moist hot steam

at a temperature of 98°C.

The heated sheet was then placed in a hot conventional oil-heated

steel mould of a press, the mould having male and female parts

for producing a car component - for example a dashboard as shown

at 40 in Figure 4 - having sections 41 impressed to a depth of

19.7mm. The press was activated and the MDF sheet deformed to

the mould's contours. The steel mould was pre-heated to 46°C, and once the press had closed and deformed the sheet at _t

pressure of about 2.1 kg/m² (3000 psi) , the drying and settin

process took 40 seconds. The whole process took just over

minute 20 seconds.

The tool also had incorporated within it the ability to cut out

sections e.g. as shown at 42, from the shaped MDF sheet, and als

to pinch trim the edges of the cut-out sections as well as the

actual edges 43 of the whole sheet.

Example 5 : Bending and impressing chipboard

A sheet of chipboard of dimensions 720x720x6mm was placed in a

steam container and treated with hot moist steam at a temperature

of approximately 98°C for 90 seconds.

The treated sheet was then placed between the male and female

parts of a heated steel mould for forming a 4 panel door skin

impressed to a depth of 12.7mm. The mould formed part of a

heated press and was heated to a temperature of 43°C. The press

was activated to close the mould around the chipboard sheet ,

bending and impressing the sheet to the shape of the mould at a

pressure of about 1.54 kg/m² (2200 psi) . The press took 15

seconds to close, and the setting and drying process took 36 seconds, after which the press was opened and the newly shaped

chipboard panel was removed.

The whole process took 2 minutes 21 seconds.

Example 6 : Bending OSB

A strip of Oriented Strand Board of dimensions 1000x200x9mm was

placed in a steam cylinder and treated for 1 minute.

The treated strip was then placed between the male and female

parts of a double bend mould in a press and having a 45° bend

with a 45mm radius at each end and a distance of 500mm between

the two bends. the press was activated to close the mould and

cause the treated OSB to take the shape of the mould at a

pressure of about 1.05 kg/m² (1500 psi) . With the press closed,

the drying and setting process was effected by introducing heat

into the strip using a 15 KW RF Unit.

The drying/setting period took 1 minute before the press was

opened and the newly shaped OSB strip was removed.

The whole process took just over 2 minutes.

Claims

1. A method of shaping a wood substitute material in which the

material, in some appropriate initial shape and form, is first

subjected to a plasticising heat treatment, and is then deformed

to the required shape with further heat treatment and under

mechanical pressure to prevent any significant surface de-

bonding.

2. A method according to claim 1, in which the plasticising

heat treatment is a stream treatment.

3. A method according to claim 2, in which the steam treatment

is effected by enveloping the material in moist steam in an

enclosure .

4. A method according to any one of claims 1 to 3, in which the

temperature of the material is raised to between 32 and 38°C by

the plasticising heat treatment.

5. A method according to any one of the preceding claims, in

which the material is deformed to the required shape using a press, and the press is held closed to maintain the mechanical

pressure on the material while applying the further heat

treatment .

6. A method according to claim 5, in which the platens of the

press are heated to apply the further heat treatment to the

material .

7. A method according to claim 5, in which the further heat

treatment applied to the material is effected by a radio

frequency heating unit.

8. A method according to any one of claims 5 to 7, in which the

material is heated to a temperature between 40 and 90°C in the

further heat treatment .

9. A method according to any one of claims 5 to 8, in which the

pressure applied to the material by the press is between 0.35xl0⁶

and 3.5xl0⁶kg/m².

10. A method according to any one of claims 5 to 9, in which the

press includes cutting means which trims the material as it is

deformed to shape by ♦"he press.

11. A method according to any one of claims 5 to 10, in whic

a surface veneer layer is placed in the press with the

plasticised wood substitute material and is deformed with and

bonded to the wood substitute material as the latter is deformed

to the required shape by the press.

12. A method according to any one of the preceding claims, in

which the wood substitute material is medium density fibreboard,

chipboard or oriented strand board.

13. A method according to any one of the preceding claims, in

which the initial form of the material is a sheet or strip.

14. A method according to claim 13 , in which the thickness of

the material is between 2.5 and 30mm.

15. A method according to any one of the preceding claims, in

which the material is deformed to the required shape by being

bent and/or impressed.