TECHNICAL FIELD The present invention relates to the manufacture of wax sheets, and in particular to the manufacture of wax forming sheets for use in casting metal articles, such as dental crowns by the lost wax process. BACKGROUND ART

In casting metals it is known to produce a model of the shape to be cast in a material such as wax or plastic, inverting the model in a mould of a refractory material and heating the mould to a high temperature at which the wax or plastic is burnt away leaving a void. The metal is then poured into the void in a molten state and cooled in the mould.

In intricate metal casting it is often required that the cast article will fit snugly with a complementary part, and a known way of achieving this is to heat a sheet of wax or plastic so that it is malleable and form it over the complementary part or a model of the complementary part by applying pressure or a vacuum. The formed sheet is cooled and combined with a wax or plastic model of the remainder of the part to be cast, and the part is cast by the lost wax method described above.

However in some intricate metal casting, in areas such as dentistry and jewellery, where the complementary part (e.g. a tooth or a gemstone) has a shape and surface which are smooth and may only be altered to a limited extent, if at all, it can be difficult to achieve a snug fit between the cast metal part and the complementary part. In such cases it is desirable that the surface of the metal part

which is to come into contact with the complementary part has as great an effective surface area as possible, i.e. it is provided with a relief pattern, such as a series of ridges. Since dental crowns and jewellery are small intricate objects, it is desirable that the relief pattern on the surface of the wax should be as fine as possible. It is known to produce a pattern on the surface of wax by mechanical means, such as using a combing device on the wax before it has set. However, it is difficult to produce a combing device which will produce ridges which are sufficiently fine for the wax to be suitable for manufacturing jewellery and dental crowns. Furthermore combing devices tend to become clogged up with wax, which may reduce the quality of the product, and results in inefficiency since production has to be stopped to clean wax deposits from the combing device. DISCLOSURE OF INVENTION According to the present invention there is provided a method of manufacturing wax sheets, the method comprising the step of applying molten wax to a relatively cool surface to create a wax film on the relatively cool surface and the molten wax being subjected to a thermal shock upon contact with the relatively cool surface, whereby ridges are created in the wax film.

In a preferred embodiment, the cool surface is dipped into molten wax, and removed with a wax film thereon, the surface of the wax film adjacent the said cooled surface undergoing the thermal shock which produces ridges thereon.

Preferably the cool surface is the surface of a drum which rotates so that for part of each rotation its surface passes through a bath of molten wax.

Alternatively, the molten wax is sprayed onto the relatively cool surface which may be the surface of a rotatable drum.

The rotatable drum, in either the dipping or spraying process, may be replaced by a convenient form of endless conveyor itself providing the relatively cool surface or carrying plates which provide a plurality of individual relatively cool surface.

The term "spraying" is to be construed as including "controlled pouring".

Preferably the wax film is removed by paring or scraping means.

According to the present invention there is further provided apparatus for manufacturing wax sheets in accordance with the above defined method, the apparatus comprising a drum adapted to rotate so that for part of its rotation the surface of the drum dips into a bath of molten wax, the surface of the drum being cool relative to the wax, and paring means which separates the wax film from the surface of the drum.

Preferably the drum is driven by a motor.

Preferably the drum is cooled by means of a liquid coolant passing therethrough.

Preferably the paring means is a doctor blade, for example a plastics sheet, located adjacent the top of the drum.

Preferably the level of wax in the bath is controlled by means of a float on the surface of the wax, the level of which is sensed by mechanical or electronic means.

According to the present invention there is further provided a wax sheet, a surface of which is provided with fine ridges.

Preferably the wax sheet is formed by the method and/or the apparatus according to the present invention as hereinbefore defined. DESCRIPTION The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Fig. 1 is a side view of part of apparatus according to the present invention; Fig. 2 is a plan view corresponding to Fig. 1;

Fig. 3 is a perspective view from above of part of the apparatus of Fig. 1;

Fig. 4 is an enlarged representation of part of a wax sheet manufactured in accordance with the method and the apparatus of the present invention;

Fig. 5 is a perspective view of the complete apparatus according to the present invention;

Fig. 6 is a sectional view of a wax bath and height adjustment mechanism which may be employed in the apparatus of Fig. 5;

Fig. 7 is a perspective view of the wax bath of the apparatus of Fig. 5;

Fig. 8 is a table and graph of optimum operating conditions; and

Fig. 9 is a diagrammatic perspective view of a second embodiment of the apparatus according to the present invention.

The apparatus comprises a wax heating arrangement 20 from which fluent wax is delivered to a wax bath 21, a rotatable drum 22 within the bath 21, means 23 for driving the drum 22, a drum cooling means 24, a doctor blade 25 cooperating with the drum surface for removing formed wax sheeting, and a microprocessor control means 26 which may be omitted if desired.

The wax heating arrangement 20 comprises a hopper 27 provided with heating means such as electric immersion heating elements 28, or alternatively external gas burner elements. The hopper 27 is provided with an outflow valve 29 near its bottom incorporated in a delivery pipe 30 extending from the side of the hopper 27. This valve 29 may be manually operated as indicated at 31, or may be provided with an electrically operated solenoid control system 32. The delivery pipe 30 leads to the wax bath 21 and provides a drip feed thereto.

The hopper 27 also contains a temperature sensing device 33 for continuously measuring the temperature of the molten wax within the hopper 27.

The wax contained within the hopper 27 is maintained at a predetermined temperature, or within a predetermined temperature range, by the electric immersion heating elements 28 controlled by the microprocessor 26 which receives temperature levels feedback from the sensing device 33 (see lines A and B). The microprocessor 26 also services to control operation of the solenoid control system 32 of the valve 29 (see line C). Wax in solid state is supplied to the hopper 27 by any convenient known means.

The wax bath 21 is provided with heating means such as electric immersion heating elements 34, or alternatively external gas burner elements, which immersion heating elements 34 are controllably linked to the microprocessor 26 as shown (see line D) . The wax bath 21 (see Fig. 6) is also provided with a visual level indicator device 35 whereby the level of wax in the wax bath 21 is readily discernible. Such level indicator device 35 may simply comprise a scale

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36 on the inside of a wall of the wax bath 21. A float 37 can be provided in the wax bath 21 operatively associated in any convenient manner known to those skilled in the art with the valve 29 to operate the latter to permit feeding of molten wax to the wax bath 21 from the hopper 27 when necessary. Alternatively, an electronic level sensor device (not shown) for indicating and controlling wax level in the wax bath 21 may be provided operatively linked to 0 the microprocessor 26. The level indicator and control device of whatever construction enables the user to monitor and maintain constant the depth of wax in the wax bath 21 by manual (see 31) or microprocessor control (see 32, 26) of the valve 29. 5 A temperature sensing device 38 also linked to the microprocessor 26 (see line E) permits temperature of the molten wax in the wax bath 21 to be controlled to a predetermined temperature or within a predetermined temperature range similarly to 0 the wax in tthe hopper 27.

The rotatable drum 22 is a hollow drum whose outer surface may be of chromium plated steel or other suitable material impermeable to water and possessing high thermal conductivity. The drum 22 is 5 provided with a hollow drive shaft 39 supported in bearings 40 and rotated by the driving means 23. The latter comprises a variable-speed electric motor 41, whereof the output shaft 42 is coupled to the hollow drive shaft 39 by a speed reduction, endless toothed 0 belt transmission system 43 to 45. The drum rotation may alternatively be manually effected using a crank handle 46 adapted for coupling to a drive formation 47 on the end of the motor output shaft 42.

The drum cooling means 24 comprise a compression 5 or absorption refrigerator 47 coupled to a water

supply tank 48 which communicates with the hollow drive shaft 39 and consequently the interior of the rotatable drum 22 via a supply pipe 49 incorporating a pump 50 and a rotatable joint 51. A return pipe 52 leads from the interior of the rotatable drum 22 back to the refrigerator 47. The latter serve ^' s to ensure that the surface temperature of the drum 22 is maintained at a temperature of 7° Celsius for example. The cooling of the drum surface is achieved by pumping by the pump 50 cold water from the tank 48 cooled to near freezing point through the pipe 49 and 52 into and out of the drum interior in a continuous circulation process. The temperature of the water is maintained at the desired temperature by thermostat means 53 associated with the refrigerator 47 and linked to the microprocessor 26 (see line F). The coolant may, instead of water, be another liquid, or a gas, or a liquid/gas mixture.

The doctor blade 25 comprises a blade of plastic or other suitable material provided with a scraping edge 54 and merging with a flat supporting surface 55. The doctor blade 25 is disposed tangentially of the drum 22, with the scraping edge 54 optimally touching the roller drum surface.

The drum 22 is located in the wax bath 21, spaced above the bottom of the wax bath preferably at a height of 50 mm to 70 mm from the bottom of the wax bath 21. The depth of immersion of the drum 22 in wax in the bath 21 may be varied either by varying the level of wax in the wax bath 21 or by providing the wax bath 21 with a jacking means 56 (see Fig. 6) to move it up or down relative to the drum 22 within a range of 0.55mm to 0.85 for example. Guide rods 57 are provided between the wax bath 21 and supporting framework 58 of the apparatus when this jacking

facility is provided.

Selection of the type of corrugated surface wax determines the operating conditions of the apparatus. A preferred performance is achieved by the following procedure.

In use, the apparatus being microprocessor controlled, wax is heated in the hopper 27 to the desired temperature, for example a constant 78° +10% Celsius. When this temperature has been attained the solenoid control system 32 triggers the opening of the valve 29 to permit egress of wax from the hopper 27 into the wax bath 21 until the electronic level sensor device in the wax bath 21 senses that the wax has reached a predetermined depth and causes the valve 29 to be closed. The depth of wax in the bath 21 is maintained by the automatic opening and closing of the valve 29 in response to the temperature and depth readings of the thermostats 33 and 38 and the level sensor device. The wax in the wax bath is maintained at a temperature of 78° +_10% Celsius by the immersion heating elements 34 in the wax bath 21 which are also thermostatically controlled (see 38).

The drum 22 is maintained at a temperature substantially below that of the wax, optimally at 7° Celsius as aforesaid. The drum 22 is rotated at a constant speed in direction X, the speed varying with the depth of immersion according to the graph (see Fig. 8) and depending on the gauge of the wax sheet it is desired to produce. As it rotates the drum 22 picks up molten wax from the wax bath 21. As the warm wax comes into contact with the cooled surface of the drum 22 it undergoes a thermal shock, the effect of which, combined with the rotation of the roller drum, is to form fine ridges 60 on the surface of the wax sheet 61 adjacent the drum 22 (Fig. 4),

the other side of the wax sheet 61 remains, as is shown, of flat configuration.

As the surface of the drum 22 coated' with wax reaches the highest point of its rotation it is contacted by the doctor blade 25 which pares the wax sheet 61 from the drum surface. The resulting wax sheet 61 is pushed along the flat supporting surface 55 where it cools in a flat condition with the lower surface of the wax maintaining the ridged configuration 60 occasioned by the contact with the drum 22. On reaching room temperature, optimal storage conditions being in the range 10° to 20° Celsius, the wax sheet 61 may be cut into disks or other shapes for use, for example, in casting dental crowns by the lost wax method.

If variation from the preferred conditions set out above becomes necessary, the apparatus may operate efficiently with variation of temperature of the wax in the wax bath within the range of +_10%. If such variation occurs, the speed of rotation of the drum 22 and the depth of immersion of the drum 22 in the wax bath 21 must be adjusted accordingly; an increase in the temperature of the wax reduces its adhesiveness to the surface of the drum 22 and thus the speed of rotation must be reduced accordingly; cooler wax forms a thicker layer on the drum 22 and in order to produce corrugated wax of the same gauge the speed of rotation of the drum 22 must be increased. Similarly the constant temperature of the surface of the drum 22 may be varied to +10% of the preferred temperature, but speed of rotation and depth of immersion must be correspondingly adjusted to deal with this for production of wax sheeting with the acceptable characteristics. The maintenance of the desired wax temperature.

speed of rotation of drum, drum surface temperature and depth of immersion of the drum in the wax is ensured by and variations effected by the central controlling microprocessor 26. Alternatively, in a very simple embodiment of the apparatus, visual observation of thermometers located in the heating arrangement, the wax bath and the drum, and hand cranked driving means provided with variable preset governor, together with visual indicators of depth of immersion of the drum (for example by means of maintaining the drum at a fixed height above the bottom of the wax bath and varying the depth of the wax, with depth indicator markings on the inside wall of the wax bath) and manual operation of the valve from the wax supply hopper may be employed.

In the embodiment of Fig. 9, parts identical with those of the embodiment of Figs. 1 to 8 are designated by the same reference numerals with a suffix "A".

The hopper 27A is, in this embodiment, provided with a motor-driven agitating or stirring device 65, the motor 66 being controlled by the microprocessor 26A (see line F) . The agitating or stirring device 65 assists in keeping the wax in the hopper 27A fluent and prevents same from congealing.

The fluent wax which leaves the hopper 27A by gravity or under the influence of a pump (not shown but disposed between the hopper 27A and the valve 29A) is delivered to a spray arrangement 67, or a controlled pouring arrangement, which directs the fluent wax onto the cooled surface of the drum 22A where it is subjected to thermal shock and solidifies into sheet form for removal by the doctor blade 25A onto the flat conveying surface 55A.

Excess wax, or wax not removed by the doctor blade 25A, is collected in a waste reservoir 68 below the drum 22A for recycling to the hopper 27A.

This embodiment of apparatus eliminates the necessity of a wax bath or heating elements therein and the need for carefully maintained depths of immersion of the drum in the wax bath. In this embodiment, the rate of flow of the molten wax from the hopper and the temperature of the surface of the drum and its speed of rotation is controlled to produce a desired gauge of wax which is pared off the drum by the doctor blade as described. These variables are preset and controlled by the microprocessor. In a modification, the drum is replaced by an endless belt or drum onto which are releasably mounted a series of plates which may be pre-cooled to the desired operating temperature in a cooling bath or by other means prior to immersion in or application thereto, of molten wax. These plates may be mounted directly onto the drum or the endless belt. here such plates are used, when they reach the top of the drum or upper surface of the endless belt, the plates are removed manually from the drum or belt and set aside to cool; replacement plates being mounted in their place, so that a series of discrete plates of wax material is produced, eliminating the doctor blade arrangement disclosed above and providing pre-shaped wax wafers.

The molten wax may simply be applied to flat cooled plates (not drum or endless belt) by dipping, spraying or pouring.