GB2241453A

GB2241453A - Sand mould breaking

Info

Publication number: GB2241453A
Application number: GB9101282A
Authority: GB
Inventors: Eric Victor Evans
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1990-03-01
Filing date: 1991-01-21
Publication date: 1991-09-04
Also published as: GB9005395D0; GB9101282D0

Abstract

A sand mould comprising sand held together by a binder into which molten metal is cast may be broken up using a tool (10) comprising a metal probe (20) coupled to a high power ultrasonic transducer (12) so as to resonate at about 20 kHz. Hand pressure applied to the tool (10) is sufficient to drill rapidly into the mould. The tool may be supplied with differently shaped probes for example for breaking up large pieces of mould, or for removing sand from a narrow channel in the casting. <IMAGE>

Description

Sand Mould Breaking This invention relates to a method for breaking up a sand mould.

Casting is a well-known technique for making metal objects. Molten metal (e.g. steel or aluminium alloy) is cast in a mould typically formed of sand held together by organic binders or resins. The sand is usually silica sand, but where a more refractory sand is needed for example for those parts of the mould which come into direct contact with molten metal then zircon or chromite sand may be used; these are denser and more expensive than silica sand. After casting, the mould must be broken away from the metal, and is then usually further broken down into sand grains and recycled. Breaking open the mould can be difficult as it is important to ensure the metal is not damaged, and particular problems arise where the cast object has hollows, crevices, or narrow passageways through it.

According to the present invention there is provided a method of breaking up a sand mould using a sand mould breaking tool comprising an ultrasonic transducer assembly, a waveguide probe coupled to the transducer assembly and of such a length as to resonate at the frequency of ultrasonic waves generated in operation by the transducer assembly, and support means attached to the transducer assembly at a position which in operation is a node, the method comprising holding the tool by means of the support means and urging the end of the waveguide probe into contact with the sand mould while energising the transducer assembly.

If an operator holds the tool at the support means when it is energised, and presses the end of the waveguide probe against the sand mould, the mould is broken down into grains, this process requiring no more than light hand pressure. The preferred frequency is about 20 kHz. The waveguide probe might be of titanium or of steel, and is desirably at least two wavelengths long, so the tool can be used on a hot mould without overheating the transducer.

The probe is desirably tapered or stepped in diameter so the end of the waveguide probe is between 3 mm and 20 mm in diameter, most preferably between 8 mm and 15 mm, for example 10 mm in diameter.

The tool might be provided with a number of replaceable probes of different end diameters, for different purposes.

The invention will now be further described by way of example only and with reference to the accompanying drawing which shows a longitudinal sectional view of a sand mould breaking tool 10. The tool 10 comprises a transducer assembly 12 with a nodal flange 14 by means of which the assembly 12 is supported inside a housing 16. One end of the assembly 12 protrudes from the housing 16, and to it is firmly fixed a titanium probe 20 by means of a short threaded stud 22 which engages in correspondingly threaded holes in the assembly 12 and in the probe 20. The abutting faces of the assembly 12 and of the probe 20 are smooth and flat to maximize the coupling of ultrasonic waves into the probe 20.The assembly 12 and the adjacent part of the probe 20 are both of diameter 33 mm, but the probe 20 is stepped in diameter so its tip 26 and its end part is of diameter 10 mm; the step 24 is provided with a curved fillet. The length of the probe 20 is equal to an integral number of half-wavelengths at 20 kHz, in this embodiment three half-wavelengths, and the step 24 is at a position which in operation is a node.

The transducer assembly 12 comprises a cylindrical titanium coupling block 30 (which also defines the nodal flange 14), and a cylindrical titanium backing block 32, between which are sandwiched two annular discs 34 of pzt (lead zirconate titanate) piezo-electric material polarized in opposite directions. The assembly 12 is held together by an 8 mm diameter steel bolt 36 which is tight enough to ensure that the discs 34 remain in compression in operation. The dimensions and masses are such that the assembly 12 can resonate at about 20 kHz. Such an assembly is available from Sonic Systems, Isle Brewers, Taunton, Somerset. The discs 34 are connected to a suitable 20 kHz signal generator 38 (shown diagrammatically), their outer faces being earthed.

In operation of the tool 10 the generator 38 is energised at an electrical power of 300 W so that the transducer assembly 12 resonates at 20 kHz, the nodal flange 14 being at a node and the ends of the assembly 12 being antinodes. The probe 20 also resonates, the tip 26 being an antinode. Because the tip 26 is narrower than the other end of the probe 20, the oscillation of the tip 26 is of larger amplitude. An operator holds the tool 10 by the housing 16 and pushes the tip 26 of the probe 20 against a sand mould. With no more than gentle hand pressure the sand mould is broken down into grains, effectively drilling a 10 mm diameter hole into the mould. Repeated use of the tool 10 enables the mould to be broken up into lumps.

It will be appreciated that where it is necessary to remove sand from inside a groove or aperture in the casting it may be advantageous to use a probe (not shown) with a narrower tip; for example a probe might taper exponentially down to say 3 mm. Equally, where it is necessary to break up larger portions of sand mould it may be advantageous to use a probe (not shown) with a non-circular tip, for example the tip might be approximately rectangular, 25 mm wide and 2 mm thick. As mentioned above, the probe length must be an integral number of half-wavelengths, and is desirably at least two wavelengths long if it is to be used on a hot mould straight after casting. The probe might be of titanium, stainless steel, or a titanium alloy: it is of course essential that the amplitude of the oscillation at the tip of the probe is not so large that the tensile strength of the metal is exceeded. By way of example at 20 kHz a wavelength in titanium is about 170 mm.

Claims

1. A method of breaking up a sand mould using a sand mould breaking tool comprising an ultrasonic transducer assembly, a waveguide probe coupled to the transducer assembly and of such a length as to resonate at the frequency of ultrasonic waves generated in operation by the transducer assembly, and support means attached to the transducer assembly at a position which in operation is a node, the method comprising holding the tool by means of the support means and urging the end of the waveguide probe into contact with the sand mould while energising the transducer assembly.

2. A method as claimed in Claim 1 wherein the said frequency is about 20 kHz.

3. A method as claimed in Claim 1 or Claim 2 wherein the waveguide probe is at least two wavelengths long.

4. A method as claimed in any one of the preceding Claims wherein the waveguide probe is tapered or stepped in diameter so the end of the waveguide probe is between 3mm and 20mm in diameter.

5. A method as claimed in Claim 4 wherein the end of the probe is between 8mm and 15mm in diameter.

6. A method of breaking up a sand mould substantially as hereinbefore described with reference to, and aa shown in, the accompanying drawings.