The extrusion of materials such as plastics or aluminium is a well-known process and this invention finds application within the technical field of extrusion and also in technical, fields concerned with the end use of extrusions formed by the method. It is known to extrude solid circular or hollow circular items such as pipes or tubes in, e.g. plastics or aluminium. It is also known to extrude extrudable materials in lengths which

_» have short widths. There can be practical difficulties in attempting to extrude comparatively wide objects and articles from certain materials. In general, it is relatively conven¬ ient to extrude materials in the form of a solid or hollow pipe and this applies _, particularly with regard to cement-containing extrudable compositions _•

Cementitious compositions which have attractive flexural strength properties and which can be extruded are disclosed in European Patent Application Nos. 21681, 21682, 38126, 30408 and 55035, all in the name ICI pic. In the method of the present invention it is preferred to use cementitious extrudable composi¬ tions as disclosed in the aforementioned Specifications, as exemplified therein and/or as claimed therein.

According to this invention there is provided a method of forming an extrudable material into a secondary shape of part or predominantly linear cross-section from a primary shape of predominantly non-linear cross-section which comprises passing extrudable material through a contoured die wherein a slit is made in the material to form two initially adjacent ends which are separated on passage through the die as the material is formed about a forming block towards an exit of part or predominantly linear cross-section. The invention further provides a die, which may be a two- part die, suitable for use in the method and comprising an inlet of predominantly non-linear cross-section, a forming block extending through the whole or part of the die, an exit of part or predominantly linear cross-section and slitting means capable of forming a slit in extrudable material passed through the inlet. Preferably the inlet profile is substantially circular and hollow and/or the exit profile substantially linear. The forming block.may be integral with the slitting means which latter may

* comprise a wedge having a tapered leading edge. In preferred embodiments the forming block may include one or more elongate slits terminating at the exit. A two-part die is conveniently provided, the parts when assembled defining a passage for extrudable material which itself may be initially solid or hollow, circular preferably or otherwise having a continuous circumference which is separated by the slitting means.

The extrudable material may comprise a hydraulic cement composition comprising a mixture of at least one particulate hydraulic cement (a material which sets and hardens in the present of water) and water in which the weight average mean particle size of the particles of the cement is less than 20jxm, wherein the hydfauliσ cement is substantially free of particles having a size above 30 -m, and in which the proportion of water is not more than 2Q}O by weight of the hydraulic cement in the composition. The hydraulic cement may be a silicate cement, for example Portland cement, or an aluminate cement such as calcium aluminate cement or a calcium sulphate hemihydrate cement, or a mixture of one or more thereof. The composition may include polymer and/or dispersant.

The extrudable material may alternatively comprise a cementitious composition comprising:

(i) at least one hydraulic cement, (ii) water in a proportion of no more than 257° by weight of the hydraulic cement in the composition, (iii) at least one organic polymeric water soluble or water- dispersable additive which is capable of aiding in the processing of the composition in a proportion of 0.5 to 10;- by weight of the hydraulic cement in the composition, and (iv) at least one particulate material insoluble in the composition and having an ultimate particle size of

less than 0.1 um in a proportion of 0.5 to iff/' by weight of the hydraulic cement in the composition. The extrudable material may alternatively comprise a hydraulic cement composition in which the particle size distri- bution of the hydraulic cement iB bimoάαl characterized in that the cement comprises:

(a) at least 50? by weight of particles of size in the range 60 to 110 u ,

(b) at least fo by weight of particles of size in the range 1 to 10 um, and

(d) not greater than 20jδ by weight of particles of size outside the ranges 1 to 10 u and 60- to 110 um. The extrudable material may alternatively comprise a cementitious composition comprising: (a) at least one hydraulic cement,

(b) water in a proportion of not more than 23?» ^" by weight of the composition, and

(c) at least one water-soluble or water/dispersable organic polymer of co-polymer in a proportion of 1 to 1 by weight of the hydraulic cement in the composition, the polymer preferably comprising hydrolised poly (vinyl)acetate. The aforementioned cementitious compositions can be formulated into a mouldable, readily processed dough which is suitable for extrusion in conventional extrusion apparatus. It is, however,

preferred to use a twin screw compounder coupled to a die according to this invention for effecting the extrusion processes. An example of a suitable extruding apparatus comprises a multi-purpose compounder having a variable orifice such as that produced by Baker Perkins and identified by the reference MPC V. Such an extruding apparatus can ^" . thoroughly mix the preferred cementitious compositions for use in the invention and subsequently form the extrudable material before entering the die according to the present invention. In order that the invention may be further illustrated and readily carried into effect, embodiments thereof will now be described by way of example only with reference to the "accompanying drawings and in which:

Figure 1 is an isometric front view of a die, Figure 2 is an isometric rear view of a die,

Figure 3 is a view of the die of Figures 1 and 2 with upper and lower parts separated, Figure 4 is a side elevation, Figure 5 is a rear view, Figure 6 is a front view of the die,

Figure 7 is a profile of extrudable material on entry into the die, Figure 8 shows two superposed extrusions, ifrure 9 shows an alternative profile of extrudable material, and

Figure 10 shows the extruded material of Figure 9 after exit from the die. Referring firstly to Figures 1-6 of the drawings, the die 1 comprises a two-part housing, a contoured aperture housing 2 and a forming block housing 3» The forming block has a substantially flat inlet surface 4 and a contoured surface 9 as clearly shown in Figure 3» Integrally secured to the housing 3 and forming block 4»9 is slitting means.in the form of a sharp edge 5 having tapered edges 6 and 7 and so defining a substantially wedge-shaped component.

Referring to Figures1 and 6, the inlet passageway/3 for receiving extrudable material from an extruding apparatus is defined by the space between the contoured surface of the forming block and the contoured surface 10 of the aperture housing 2. The inlet aperture in this embodiment is thus substantially circular and hollow and the extrudable material on entering the die assumes a shape illustrated in Figure 7 which will be described in greater detail hereafter.

The forming block 4 is provided with first and second sld-ts 11 and 12 respectively which extend the length of the forming block from inlet to exit and maintain a substantially constant cross-section. The inlet aperture 8 in the aperture housing 2 defines the maximum external diameter of extrudable material that can enter the die. The thickness of extrudable material entering is maintained substantially constant throughout

its travel from the inlet 13 to the exit 14, although the particular internal dimensions may not be regarded as critical in this respect. The dimensions of the exit 1 are selected for the particular end use of the extruded material. The number, dimension and positioning of slots 11 and 12 are also selected according to the end use of the extruded product. In one embodiment of this invention which is not illustrated the slots 11 and 12 are absent whereby an extrudable material may enter in hollow cylindrical form and exit in perfectly linear form.

The die 1 may be made of any material capable of with¬ standing pressures and temperatures usually associated with extrusion processes, and it is preferred to extrude cementitious compositions which are initially in the form of a pliable dough through the die apparatus.

Referring next to Figure 1, this shows the profile of extrudable material as it enters the inlet and viewed as in the direction of Figure 5» The extrudable material 15 comprises extrudable cementitious composition which has been extruded from an extrusion apparatus in the form of a substantially circular and hollow cylinder. The material forms ribs 11a and 12a on entry into the respective slots 11 and 12 in the forming block. A slit 1 is formed on entry into the die as the extrudable material contacts the sharp edge 5 of the slitting means. This forms two initially adjacent ends 20,21 which are

caused to separate as the material passes through the die.

Figure 8 shows a pair of extrusions superposed in face- to-face relationship and forming a rectangular channel section divided into three internal channels. The profile of each of the extrusions 1 corresponds with the cross-section of the exit 14 of the die. The arrangement as illustrated may be useful as a double-sided skirting board, wherein electrical wiring and/or piping can be concealed within the internal channels. Such an extrusion of cementitious material, when set hard, can be extremely durable and have high flexural strength. Additionally, such composition may be more economical in use than other extrudable materials such as plastics materials.

Referring finally to Figures 9 and 10, the initial 'form of an alternative shape of extrudable material 1 b is shown having internally directed flanges 16,17 and 18. Such an arrangement requires a die having a modified inlet arrange¬ ment which is nob illustrated. On entry of this material 15b into the respective die a slit 9 is formed between the respective ends 20,21 which are initially adjacent. An isometric view of the extrusion obtained on exit from the die is shown in Figure 10. The ends 20,21 have been separated as far as possible and in both embodiments the cross-sections of the resulting extrusions is predominantly linear. The arrangement of Figure 10 may find application as, for example, a skirting board to house electrical socketry and conceal wiring.

Although fine-particle sized cementitious-based material (having an average particle size of less than 1 mm, preferably less than 0.1 mm) is preferred as an extrudable material because of economics, capability of extrusion and the strength of the extruded products when set this is not the only material useful in this invention.

Other extrudable materials include, for example, wood chip/ glue composition e.g. as currently used in paper or cardboard manufacture, plastics materials, aluminium, aluminium alloy, iron or steel alloys, fibre glass, carbon-fibre or otherwise reinforced glass fibre, Ke lar (trade mark) or glass. when employing the metals/alloys the forming block should be modified to include heating means to keep the material semi-rigid and means to assist travel of the material through the die apparatus such as powered rollers pulling, carrying or otherwise supporting the fed material. For the alloys it would be desirable to use a die of material having a high temperature melting point. ^' When using glass careful temperature control will be required to impart shatter resistance to the extruded products. In embodiments of the invention which are not illustrated (1) the final cross-section of the extrusion is wholly linear and (2) the initial cross-section may comprise a convoluted form to allow for extra length in the extruded material leaving the exit. The extrudable material compounder may in the former arrangement be adapted to present a hollow or solid cylinder for entry into the

-9a- die apparatus. Whatever type or shape of extrusion leaves the exit of the die may be further processed as required or moulded to form a double-skinned shape.

The end uses of extrusions obtained by means of the present invention are widely varied. For example only, such extrusions may find applications in the construction of skirting boards, cabinets, doors or door 'frames, interior or exterior walls, floors, pipes, even buildings and.road surfaces, amongst many others.