FIELD OF THE INVENTION

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

BACKGROUND

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

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.

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

The use of a Venturi apparatus allows for improved feeding of material and reduces congestion observed in the prior art.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawing, in which:

FIG. 1 is a schematic diagram of an apparatus for putting the method of the present invention into practice.

DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in the accompanying drawing, the present invention provides a method and apparatus comprising a first hopper (10) for a component material which feeds onto a first feeder (14) under the influence of gravity, and a second hopper (20) for a component material which feeds onto a second feeder (24) under the influence of gravity, wherein the component material fed into the first hopper (10) is powdered casting plaster and the component material fed into the second hopper (20) is a pre-mixed mixture of powdered limestone and hemp fibre.

The hemp fibre is preferably of Manila variety and less than 4 mm in length. Further, the use of hemp fibre is found to increase the strength of resultant products whilst keeping the weight down. For example, the resultant products may have a specific gravity of less than 0.4.

The first hopper (10) and second hopper (20) are provided with vibrating means to ensure a steady flow of component material to respective feeders (14/24) in use. The first feeder (14) and second feeder (24) are arranged in use to feed component material towards a third hopper (30).

The first feeder (14) and second feeder (24) are vibrating feeders of known type which are designed to provide a steady flow rate of component material to the third hopper (30) under the influence of gravity. The flow rate may be adjusted by means of a rheostat control.

The resulting mixture of the component materials, hereafter referred to as “the primary mixture (first stage)”, may consist of about 85 per cent by weight of casting plaster, about 4 per cent by weight of powdered limestone and about 11 per cent by weight of hemp fibre.

The third hopper (30) feeds the primary mixture (first stage) into a Venturi apparatus (40) under the influence of gravity. A gap maybe left between the outlet of the third hopper (30) and the inlet of the Venturi apparatus (40). The Venturi apparatus (40) may have multiple small holes in the casting. Compressed air is supplied to the Venturi apparatus (40) through an inlet pipe (41), causing air and solid particles to be sucked into the Venturi apparatus (40).

A gap maybe left between the outlet of the Venturi apparatus (40) and an elongated chemical tube (50). A primary mixture (first stage) becomes entrained in the air flow and flows through the Venturi apparatus (40) into the elongated chemical tube (50). The tube (50) may be formed of a suitable plastics material.

The elongated chemical tube (50) is provided with an inlet nozzle (52) arranged to receive dilute phosphoric acid. The phosphoric acid is sprayed into the elongated chemical tube (50) to combine with the primary mixture (first stage) to create a primary mixture (second stage). The dilute phosphoric acid is preferably 5% phosphoric acid supplied at a rate of about 7 litres per minute.

The elongated chemical tube (50) is provided with an inlet nozzle (54) arranged to receive a mixture of water mist with detergent agent. This mixture is sprayed into the elongated chemical tube (50) to combine with the primary mixture (second stage) to create a primary mixture (third stage). The water mist with detergent agent may be supplied at a rate of 1 litre per minute.

The elongated chemical tube (50) is provided with an inlet nozzle (56) arranged to receive compressed air. The compressed air is used to boost pressure through the elongated chemical tube (50). The elongated chemical tube (50) delivers the primary mixture (third stage) to a mixing and spray head (60). Typically, the operating pressure is 8 bar and provides free air capacity of about 50 cubic feet per minute.

Modifications and variations in which would be apparent to a skilled addressee are deemed to be within in the scope of the present invention. For example, a change in the reactivity of the plaster supplied may enable greater production and less time in the mould.