Ά METHOD FOR THE MANUFACTURE OF FOAMED PLASTER"

FIELD OF THE INVENTION

[0001) The present invention relates to a method for the manufacture of foamed plaster.

BACKGROUND

[0002 The present invention attempts to overcome at least in part, the disadvantages of previously known methods for the manufacture of foamed plaster

SUMMARY OF THE INVENTION

[0003] It is known to provide foamed plaster for the manufacture of moulded components for use in industry. The present invention provides a new and improved method for manufacture of foamed plaster.

[0004] In accordance with one aspect of the present invention there is provided a method for the manufacture of foamed plaster which comprises feeding powdered casting plaster from a supply thereof to a Venturi apparatus, feeding the powder from the Venturi apparatus to a supply of phosphoric acid admixing the powder and phosphoric acid, and forwarding the resulting mixture to a dispensing and spray head so as to feed the mixture to a moulding means.

BRIEF DESCRIPTION OF DRAWINGS

[0005] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an apparatus for putting the method of the present invention into practice.

Figure 2 is a view of a compressed air supply;

Figure 3 is a schematic side view of air being fed to a Venturi apparatus.

Figure 4 is a view of a spray head which may form part of the present invention; and

Figure 5 is a cross section of the spray head of Figure 4.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0006] As shown in the accompanying drawings, the present invention provides a method and apparatus comprising a feed means (10) for powdered material which powder to a platform (14) under the influence of gravity.

[0007] The feed means (10) is provided with a vibrating means (11) to ensure a steady flow of material to the vibrating platform (14) in use. The vibrating platform (14) is arranged to feed the powder towards a Venturi apparatus (16).

[0008] The supply of powder feed may consist of about 93 per cent by weight of casting plaster with the balance of about 7 per cent of powdered limestone.

[0009] A compressed air supply (18) is arranged to provide a flow of air to draw in 25-50mm such as 40mm between the output of the compressed air supply (18) and the adjacent input end of the Venturi apparatus (16).

[0010] The powdered plaster falls from the platform (14) under the influence of gravity. This powdered plaster becomes entrained in the air flow from the compressor of the compressed air supply (18) and is fed into the Venturi apparatus (16) as shown

[0011] The platform (14) is part of a vibrating feeder of known type which is designed to provide a steady flow rate of powder to the Venturi (16). The flow rate can be adjusted by means of a rheostat control. Variation of the vibration of the platform (14) urges the powder on the platform (14) to move forwardly to the right as seen in Figure 1. To enhance the flow of air through the Venturi (16), a separate compressed air supply (20) is provided adjacent a narrow end on the Venturi (16). As shown in Figure 3, the air supply (20) is directed towards the wider end of the Venturi (16).

[0012] The wider end of the Venturi (16) is connected to an elongated chemical tube (22). The tube (22) may be formed of a suitable plastics material.

[0013] Further, the tube (22) is connected to an elbow (24) remote from the Venturi (16). The elbow (24) may be formed of a high-density polyethylene having an internal diameter of 65mm

[0014] The 135% elbow (24) is provided with an inlet nozzle (26) arranged to receive dilute phosphoric acid. Further, the 135% elbow (24) is connected to a length of chemical tape (28). The phosphoric acid is sprayed into the elbow (24) to produce a mixture of the acid and the powder from the Venturi (16). The hose (28) delivers the phosphoric acid and plaster mixture to a mixing and spray head (40) via a further elbow 30 which has an inlet (31) for further compressed ait to ensure steady flow in the spray head (40).

[0015] In a variation of the method of the present invention, a quantity of manila hemp fibres may be added to the material being processed. The manila hemp fibres are preferably less than 4mm in length. Further, the use of manila hemp fibre is found to increase the strength of the resultant products whilst keeping the weight down. For example, the resultant products may have specific gravity of less than 0.4. It has been found that the Venturi system described above works well with the manila hemp but requires the addition of the air supply (18) to transfer the powder to the Venturi.

[0016] Further, it has been found that a rotary screw compressor with air dryer, overcame problems in the compressed air supply with minimal variation in pressure and moisture. Typically, the operating pressure is 8 bar and provides free air capacity of 50 cubic feet per minute.

[0017] As shown in the drawing, the Venturi apparatus (16) has a relatively narrow end adjacent to the air supply (18) and a relatively wide end remote from the air supply (18). By way of example, the narrow end could be 30mm in diameter whilst the wide end could be 65mm in diameter.

[0018] As show in Figure 2, the air supply (18) may have an outlet plate with a plurality of apparatus (36) which ensure that a high velocity air flow is provided to transfer the powder into the Venturi (16).

[0019] In Figures 4 and 5 of the accompanying drawings there is shown a mixing and spray head (40) comprising a tubular member (42) having an inlet end (44) and an outlet end (46).

[0020] At the inlet end (44) the tubular member (42) is connected to a hose (28). At the outlet end (46) the tubular member (42) has mounted thereto an outlet nozzle (48).

[0021 ] Further, the tubular member (42) contains spaced apart mixing baffle members (41) and (43).

[0022] The baffles (41) and (43) are arranged to induce turbulence in the tubular member (42) in use. As seen in Figure 4, the baffle member (41) extends from one side of the tubular member (42) whilst the baffle member (43) extends from an opposite side of the tubular member (42). The result is that the flow of air is initially interrupted by the baffle (41) and then returns to the full diameter section of the tubular member (42). Subsequently the flow of air encounters the baffle (43) and is again interrupted and then returns to the full diameter of the tubular member (42). The result is that the flow of air is subject to variable back eddy currents.

[0023] In the arrangement described the baffle member (41) blocks flow in part of the elongated member (42) which tends to throw air pressure sideways. The baffle member (43) is of similar shape is therefore desirable to give an opposing throw to balance the elongate member (42) in use. The baffles (41) and (43) have an effect on the elongated member for about 180 degrees. These could be replaced by three balanced baffles set at 120 degrees or four balanced baffles set at 90 degrees. In an operation, a mixture of casting plaster with calcium carbonate is entrained in the flow of air the hose (18) prior to entering the tubular member (42).

[0024] In addition, in the tubular member (42) it is desirable to induce shear in the mix. This could be achieved for example, by altering the shape of the baffles to give differential flow on one side thus tending to mix the for and after flow of the particulate matter. This enables a fraction of the flow to move forward and to be realigned with the Coriolis Effect to give a twist and provide stability to the outgoing jet of slurry. In that regard, results of tests in the Southern Hemisphere would require adj ustment to the mirror image in the Northern Hemisphere.

[0025] It has been found that in practice, the airspeed through the mixing and spray head (40) should preferably be at least 200 km per hour to keep the fine particles in suspension. Further, frictional losses in pipes of less than 50 mm diameter tend to restrict the distance carried. Tests with pipe lengths of 3 to 5 meters have been carried out with pipes of 2½ inch internal diameter which corresponds to about 56mm. A nozzle (50) preferably provides a restricted outlet about 0.6 to 0.8 of the diameter of the tubular member (42) At the nozzle (50) particulate foamed plaster is ejected from the tubular member (42). Further, a preferred option is to provide a small water spray adjustment (49) at the spray head (40) to ensure that the resultant mixture has sufficient moisture content if required.

[0026] The present invention will now be described with reference to the following example. [0027] It has now been found that the method using the apparatus of the present invention can reduce the specific gravity of the foamed plaster from, for example, 0.6 to 0.4.

[0028] Thus, it is possible to convert 18kg of casting plaster together with 7% by weight of calcium carbonate and deliver it as dry powder together with 4% phosphoric acid at 6 litres per minute to produce 48 litres of dry foam plaster at a specific gravity of 0.4 per minute.

[0029] Modifications and variations in fact would be apparent to a skilled addressee are deemed to be within in the scope of the present invention.