Bending and shaping MDF

This invention relates to bending and shaping MDF (medium density fibreboard).

Wood is an expensive and rather intractable medium for the construction of articles, and for many years various man-made composites have been used instead, sometimes because of cost and sometimes because of the achievable properties. One such material is that known as MDF, or medium density fibreboard, which has been available since the early 1970s. MDF is basically wood fibres compounded together with amounts of one or more synthetic resin, often one based upon formaldehyde (usually a urea-formaldehyde resin, and sometimes advantageously a melamine-urea-formaldehyde [this type of MDF is often known as MUF, or HMR, the latter standing for high moisture resistance] , and typical such materials available under the generic name MDF are those sold by Medite, Caberboard and Silverpan, the exact details of which are not generally known. The main present uses for MDF are in the manufacture of furniture and kitchen and bathroom fittings (cupboards and shelves, and so on).

MDF is commonly manufactured as flat sheets or boards of various thicknesses (typically from 3 mm to 25 mm) and densities, and may be supplied with a visually-attractive paper- or wood-veneer or plastics surface coating. It is a stiff, very rigid, practically inflexible material, with almost no plasticity, and unless it is provided as very thin sheets, with some limited degree of flexibility by virtue of that very

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thinness, it is difficult if not impossible to bend or otherwise form into the sort of complex shapes often required.

It has now been found that by impregnating MDF with ammonia (used in anhydrous - that is to say, dry gaseous - form) the MDF can be made relatively plastic, and in its plasticised state can quite easily be pressed or bent into almost any shape required.

In one aspect, therefore, the invention provides a method of shaping MDF in sheet form, in which the MDF is treated with ammonia so as to impregnate it therewith, causing it to become relatively plastic, the thus- plasticised MDF is then shaped as desired, and the ammonia is then allowed to dissipate, whereupon the MDF reverts to its normal, relatively unplastic, form, which it thereafter maintains.

The MDF may be any of those materials commonly known as MDF, including especially that known as MUF.

The MDF sheet shaped by the process of the invention may intrinsically be of any appropriate shape, size and thickness. As noted above, typical thicknesses for MDF range from 3 to 25 mm, and the method of the invention can successfully be applied to any of these. Moreover, perhaps slightly surprisingly the MDF sheet may already have a previously indented surface, giving it in section a shaped profile, and this will be substantially undistorted even when the sheet has thereafter been bent and shaped by the method of the invention.

In the process of the invention the MDF is treated - conveniently at room temperature (around 15°C) - with

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gaseous ammonia so as to impregnate it therewith, causing it to become relatively plastic. The ammonia reacts with the MDF, and with the moisture in the MDF, in an exothermic reaction the heat from which raises the MDF's temperature 5 or 10 ^β C. It is believed that the reaction of the ammonia enables the cellulose molecules making the wood, as well as the wood fibres made therefrom, to release their grip one on another and on the bonding agent (typically a urea-formaldehyde resin), and so separate to allow them to be re-positioned as required while any applied bending or impressing occurs. In other words, the altered chemical (and physical) nature of the MDF, coupled with the rise in temperature, causes the MDF to become rather more plastic - that is, able to have its shape changed by an applied force without immediately recovering its original shape when that force is removed - than in its original state. Of course, once the ammonia has been removed, or dissipates, the molecules/fibres re-bond in their new positions, without any distortion of the MDF.

The ammonia, which is preferably in dry (anhydrous) gaseous form, may be applied to the MDF in a variety of ambient physical circumstances. For example, it may be applied in a sealed pressure chamber (which may take the form of a standard autoclave, but can also be nothing more complicated than a sealed plastic bag [albeit one capable of withstanding the pressures and temperatures involved] under a slight pressure of up to 4 Bar (about 4 atmospheres). However, the higher the pressure the more likely the ammonia will be absorbed into the MDF, reacting with it and the moisture therein, so quickly and in such amounts that localised overheating occurs to cause swelling and bubbles in the MDF as it expands, distorting the MDF and even leading to surface unevenness and possibly explosive disintegration of the

MDF. Most preferably, therefore, the applied pressure is kept low, and is about 1 Bar (1 atmosphere).

It may be advantageous to evacuate the pressure chamber containing the MDF prior to feeding in the ammonia, for by doing so the applied pressure of the ammonia can be kept low and yet the treatment period can be reduced (but without fear of distorting the MDF as mentioned above) .

The rate at which the treatment ammonia is absorbed into the MDF depends to some extent upon the moisture in the MDF, the rate being higher the wetter it is. Accordingly, ensuring that the MDF is moist will result in a shortening of the time of treatment - but as observed elsewhere too rapid an impregnation can cause surface blemishes and deformation, and care should be exercised when speeding treatment up in this way.

The period during which the MDF is so treated will also depend to some extent upon the density and thickness of the MDF, and upon the temperature. The higher the density and thickness the longer a treatment is required, while the higher the temperature the shorter the treatment needed. Thus, for example, a 6 mm sheet can successfully be treated for a period of 1 hour and a temperature of 15°C, while a 16 mm sheet might need a longer and/or hooter treatment - maybe for 3 hours and/or at 20 ^β C

Once impregnated with ammonia the MDF becomes relatively plastic, and the thus-plasticised MDF is then shaped as desired. The shaping may be done in any convenient manner, and may cause the MDF to have any suitable form. Typically, a sheet (or part thereof) of MDF can be stamped or impressed with surface patterns so

that it looks embossed, while a strip of MDF can be clamped to and bent round a former into a curve.

Although the plasticised MDF sheet may be bent and shaped entirely by hand, one suitable bending and shaping method involves the use of a jig - that is, a pre-shaped surface or set of points around which the MDF sheet can be bent and clamped into place while it "sets". The art of jig construction and use is in general well-known, and no more need be said about it here.

If the bending or shaping is to involve a stamping or pressing operation, then this can be accomplished in any convenient way - in a conventional press, or using rotating rollers, for instance. This type of shaping method is in general well-known, and needs no further comment here.

The degree of bending attainable without damaging the MDF depends to some considerable extent of the thickness of the MDF, thin sheets being understandably more bendable than thick ones. In tests using the method of the invention it has been possible reliably to bend sheets as follows to make commercially-useful products : -

Sheet Normal Bend Extreme Bend

Thickness (mm) outside diameter (mm)

18.7 300 155

16 230 120

12 190 90

6 50 40

Once the plasticised MDF has been shaped as desired, the ammonia is then allowed to dissipate, whereupon the MDF reverts to its normal, relatively

unplastic, form, which it thereafter maintains. For example, a strip of MDF clamped to and bent around a former in a jig may simply be left there for several hours, during which the ammonia diffuses out of the strip and away, and when removed from the former the strip will then hold its new shape. However, it may be desirable to accelerate this "drying", setting, stage, and various techniques can be employed, including the application of heat (as by radiant heat, or even radio frequency induction, as in a microwave oven) and/or vacuum, and the passing of air currents across the surface (hot air heats, while moist air extracts chemically) . It may be desirable, in order to improve the drying effect, if possible to release those parts of any jig (or other former device) that would otherwise occlude one or more of the sheet s surfaces.

The surface of an MDF sheet treated by the method of the invention may be roughened slightly as a result of moisture absorption. This roughening can usually be removed by a simple buffing operation once the bent and shaped sheet has been dried and thus "fixed" into the required shape.

Although the ammonia treatment of the method of the invention may cause a small amount of swelling of the MDF sheet, this is not significant, and should not cause any noticeable distortion of the finished product.

The invention extends, of course, to shaped MDF articles whenever made by a process according to the invention.

The following Examples are now given, though by way of illustration only, to show details of an embodiment of the method of the invention.

Example 1; Bending and shaping of MDF

Stage 1; Plasticisation of the MDF

Using a purpose-built circular-section autoclave of dimensions 4.2 m by 250 mm internal diameter, there were plasticised a dozen strip-like sheets of MDF each of dimensions 150 mm by 18.7 mm and various lengths from 1.5 m to 3.6 mm. This MDF was obtained from Nelson Pine Industries, of Auckland, under the name THINLINE (this is an MUF material) .

First, the sheets were placed in the autoclave, and the latter was evacuated down to a pressure of 0.1 Bar and stabilised at ambient temperature. The sheets were held at this low pressure for 2 mins, whereupon gaseous ammonia (from a cylinder of anhydrous liquid ammonia) was allowed in over a period of 4 mins, all the while maintaining the temperature more or less constant, until the internal pressure had risen to 4 Bar.

Once all the ammonia had been introduced, the sheet was kept in the ammonia atmosphere for 4| hours at the same temperature and pressure. During this time the chemical action of the ammonia caused the binding forces between the MDF constituents to become less, so that the MDF itself became plastic.

Stage 2: Bending the MDF

At the end of the specified time the ammonia remaining free in the autoclave was drawn off for disposal, the pressures were equalised, and the

autoclave was opened to allow the plasticised MDF sheet to be removed.

The sheets were taken out and placed in a variety of bending jigs of standard form as appropriate. In this way one, for example, was bent into a semicircle outside diameter 800 mm. Thus bent, the sheets were then secured in place in the jigs and allowed to "dry" - that is, the absorbed ammonia was allowed to evaporate off (this evaporation was encouraged by blowing warm air over the sheet). For the first half-an-hour or so each sheet was retained clamped in its jig, but at the end of this time it sheet was undamped and removed from the jig, and allowed to dry "free-standing" for another four or five hours.

-At the end of this drying stage each sheet had recovered its normal physical properties, but retained its new shape and resisted any ordinary attempts to unshape it.

Example 2: More bending and shaping MDF

Two further batches of MDF (this time the non-MUF material MEDITE, available from Medite Ltd) were bent and shaped in much the same overall manner, but with the following significant differences:- a) The container used was a large heavy-duty plastic bag. b) No initial vacuum was applied to one batch, while a 0.1 Bar vacuum was applied to the other. c) The pressure of ammonia in the container was no more than 1 Bar. d) For the pre-evacuated batch the treatment time was hour, while for the other it was 1+ hour.

e) Drying was effected under ambient conditions for 24 hours.

A third batch was treated in much the same way except that it was subjected to a 0.1 bar vacuum after the ammonia withdrawal to hasten the "drying" stage - and in fact the product needed no further drying at all

The shaped MDF products made in these ways were all quite satisfactory.