PNEUMATIC MICRO-SANDBLASTING APPARATUS HAVING A DRIER FOR ABRASIVE POWDERS

Abstract: A pneumatic micro-sandblasting apparatus comprises a reservoir containing an abrasive powder. The reservoir comprises an upper container (12) into which the abrasive is supplied and which is provided with a resistor (40) and a pressurizable lower container (14) connected to a source of pressurized air and a discharge nozzle.

Description: The present invention relates to a pneumatic micro-sandblasting apparatus comprising a source of pressurized air, an upper container into which the abrasive powder is supplied, a pressurizable lower container, connected to said source, discharge nozzle means connected to said lower container and valve means interposed between the containers and adapted to enable the abrasive powder to fall by gravity from the upper container to the lower container. A sand-blast apparatus having the above features is disclosed in THE OFFICIAL GAZETTE OF THE UNITED STATES PATENT OFFICE, vol. 74, no. 6, 11th February 1896 page 678, abstract no. 554299, Washington, US; J.E. PARKER et al.: "Sand-blast apparatus for cleaning molds". Such apparatus is useful only for cleaning moulds and similar items and it is not suitable for the abrasive used in the micro-sandblasting process. The abrasive used in the micro-sandblasting process, comprising pure aluminium oxides for mechanical pickling, are notably hygroscopic and thus present the problem of lump formation and possibly that of packing if the moisture content exceeds a particular value, normally less than the equilibrium value with the atmospheric humidity. Furthermore, in the use of the micro-sandblasting technique for the cleaning of precious metal castings used in dental technology for the production of dental prostheses or in gold-working for the production of necklaces and the like, it has been found that the lower the moisture content of the abrasive, the better the quality of the surface treated. In the particular case of dental prostheses, the use of alumina (A1₂O₃) with a low moisture content leads to better adhesion of the resin or ceramic to the surface treated. Moreover, with an abrasive which has a low moisture content, there are considerable advantages in the more regular delivery of the blasting sand with a reduced composition of air and abrasive powder. Some attempts have been made to dry the abrasive powder by placing it in an oven (a device which is always present in a laboratory of gold smiths or dental mechanics). This kind of drying is not effective since the dried powder out of the oven reabsorbs quickly the moisture from the atmosphere. The object of the present invention is to provide a pneumatic micro-sandblasting apparatus which enables the the problems caused by the use of an abrasive which has not been dried to be overcome, and which is simple and cheap to produce. According to the invention, this object is achieved by virtue of the fact that the upper container is cup-shaped and is provided with electric heating means and with an upper closure cover and that the pressurizable lower container has an inverted cup shape, is connected to the upper container in correspondence with its bottom and has a lower closure cover. By virtue of these characteristics, it is possible to dry the abrasive to the required degree and admit it directly into the circuit for sending the compressed air to the discharge nozzle means avoiding any contact between the atmosphere and the abrasive powder which would lead to an increase of the moisture content of the powder. Moreover, only the required quantity of abrasive is dried, so that there is no waste of energy. Further advantages and characteristics of the apparatus according to the invention will become clear from the following detailed description, provided purely by way of non-limiting example, with reference to the appended drawings in which: Figure 1 is a schematic view of a pneumatic micro-sandblasting apparatus according to the invention, and Figure 2 is a longitudinal section of a drier according to the invention. With reference to the drawings, a pneumatic micro-sandblasting apparatus for the cleaning of precious metal castings used in dental technology is generally indicated 8. The apparatus comprises a drier 10 for powdered abrasives (for instance aluminium oxides) which is installed between a compressor C and a micro-sandblasting machine M having a discharge nozzle (not illustrated). The drier 10 comprises an upper cup-shaped container 12 and a lower container 14 with a shape substantially identical to the upper container 12 but in an inverted configuration. The containers 12 and 14 have respective end walls 12a and 14a provided with respective central holes 12b and 14b. The connection between the containers 12 and 14 is achieved by means of a tubular element 16 inserted in correspondence with the holes 12b and 14b and made fast to the end walls 12a and 14a of the containers by means of a ring nut 13. The upper container 12 has an upper cover 18 having a central hole 20 coaxial with the tubular element 16 on a central axis X-X of the drier 10 and a lateral aperture 22 for the loading of abrasive powder. A tubular guide 24 is fixed in the central hole 20 of the upper cover 18 and, substantially in correspondence with the cover 18, has an internal annular shoulder 14a. A rod 26 is slidably mounted within the tubular guide 24 and has a first end 26a to which a knob 28 is fixed and a second end 26b to which is fixed an obturator adapted to cooperate with a valve seat V defined by an end of the tubular element 16 connecting the two containers. A helical spring 32 is interposed between the internal annular shoulder 24a of the tubular guide 24 and the knob 28, and is adapted to urge the obturator 30 into the closed position. A resilient split ring 36 is mounted externally of the tubular guide 24 and supports a plate 38 to which are fixed the terminals 40a of an electrical resistance 40 wound in a spiral around the guide 24. A spacer bush 34 coaxial with the tubular guide 24 is interposed between the support plate 38 and the upper cover 18. A lower cover 44 is fixed to the bottom of the cup-shaped container 14 with the interposition of an elastomeric sealing ring 42, and has a first central aperture 46 for connection to a compressed-air inlet line A and a second aperture 48 for connection to a delivery line B to the micro-sandblasting machine M. A shaped outlet tube 50 whose end 50a essentially faces the end wall 14a of the lower container 14 is fixed to the second aperture 48 of the cover 44. During installation of the drier 10, as well as its connection to the delivery and inlet lines A and B respectively, the terminals 40a of the resistor 40 are connected to an electrical control circuit of the micro-sandblasting machine including a switch and a thermostat generally indicated E. A predetermined quantity of the powdered abrasive to be dried is then introduced into the upper container 12 through the loading aperture 22 in the upper cover 18. The control of the quantity introduced is facilitated by the fact that both the upper container 12 and the lower container 14 are made of transparent plastics material, for example, polycarbonate. In order to dry the abrasive introduced, the operator operates the switch E with the associated thermostat, so that the abrasive contained in the upper container 12 soon reaches a predeterminated temperature, normally 60-70°C. The water vapour which is liberated by the abrasive during drying is evacuated through the same loading aperture 22 of the upper cover 18. After a period of time (normally 1 hour) during which the abrasive is kept at the above predetermined temperature, by pushing the knob 28 downwardly the operator causes the obturator 30 to move axially away from the valve seat V to enable the dried abrasive to fall by gravity into the lower container 14 which, at this stage, is kept at ambient pressure. Compressed air is then sent to the first aperture 46 of the cover 44 and the abrasive is transported with the air flow into the outlet tube 50 and subsequently into the delivery line B to the micro-sandblasting machine M. Whilst the machine M is operating, with the lower container 14 under pressure, further abrasive can be dried simultaneously in the upper container 12 so that, when the dried abrasive contained in the lower container 14 is used up, the operator can supply the abrasive dried in the upper container 12 without loss of time.

Claims: A pneumatic micro-sandblasting apparatus comprising: a source of pressurized air (C), an upper container (12) into which the abrasive powder is supplied, a pressurizable lower container (14) connected to said source, discharge nozzle means connected to said lower container, valve means (V, 30) interposed between the containers and adapted to enable the abrasive powder to fall by gravity from the upper container to the lower container, characterised in that the upper container (12) is cup-shaped and is provided with electric heating means (40) and with an upper closure cover (18) and in that the pressurizable lower container (14) has an inverted cup shape, is connected to the upper container (12) in correspondence with its bottom (14a) and has a lower closure cover (44). An apparatus according to claim 1, characterised in that the upper container (12) is provided with an aperture (22) in the upper closure cover (18) for the loading of the abrasive powder and for the discharge of the water vapour, and in that the lower pressurizable container (14) is provided with a central aperture (46) in the lower closure cover (44) for the inlet of compressed air and an offset aperture (48) for the outlet of the air and abrasive. An apparatus according to claim 1, characterised in that the upper and lower containers (12, 14) have a shape identical to each other. An apparatus according to claim 1, characterised in that the valve means comprise: a valve seat (V) formed in correspondence with a tubular element (16) connecting the two containers (12, 14), and an obturator (30) adapted to cooperate with the set (V) and operated by a rod (26) coaxial with the first cup-shaped container (12) and axially slidable relative to the upper closure cover (18), resilient means (32) being provided for keeping the obturator (30) in the closed configuration. An apparatus according to claim 4, characterised in that the electric heating means comprise an electrical resistance (40) wound on a spiral coaxial with the control rod (26) of the obturator (30).

Inventors: MINI, GIORGIO
Applicants: DENTALFARM S.N.C. DI CATELLA GIUSEPPE E FIGLI
Application year: 1988
Main classification: B24C7
IPC codes: B24C7, F26B9