GB2201495A - Furnace - Google Patents

Abstract

A furnace comprises a plurality of furnace chambers 3 each defining an opening through which access can be gained to the chamber interior. A single furnace energising circuit is switchable to energise any one of the furnace chambers. One or more furnace doors 4 are positionable to close only one of the furnace chamber openings at a time, and the closed furnace chamber is selectively energised. The chambers can be used sequentially so that as one is being energised the or each other furnace chamber is cooling down from a preceding energisation. <IMAGE>

Description

FURNACE The present invention relates to a furnace and in pa-rticular to the type of furnace which can be used to fire porcelain of the type used in for example dental workpieces.

Vacuum furnaces are used widely for the production of porcelain workpieces. A disadvantage of all the known vacuum furnaces is that they require a considerable "down time" after the porcelain work pierce has been fire, the down tire allowing the workpieces to cool at a controlled rate from the elevated temperatures achieved during firing. The length of the down time depends upon live nature ot the workpieces being fired but in the case of for example firing some porcelain-to-metal bonded bridge work restorations a protracted cooling period during which the work piece remains in the furnace is required and this can mean that the furnace is occupied by cooling workpieces for a period of ten minutes or more.During this cooling period the furnace cannot of course be used to fire further workpieces.

British Patent Specification No. 1 587 020 describes a furnace having a furnace chamber defining an upwardly facing opening which during use of the furnace is closed by a furnace door. The door can be opened automatically by releasing the vacuum within the furnace chamber on completion of the firing process. Even with this furnace however it is necessary to leave workpieces for a considerable period of time within the furnace chamber to obtain the required rate of cooling. The single furnace chamber is associated with complex electronic and pneumatic circuits to control the heating up of the furnace chamber during firing and to control the evacuation of the furnace chamber. All of these components are left idle during down time.

It is an object of the present invention to provide an improved furnace which obviates or mitigates the problems outlined above.

According to the present invention there is provided a furnace comprising a plurality of furnace chambers each defining an opening through which access can be gained to the chamber interior, a single furnace energising means switchable to energise any one of the furnace chambers, one or more furnace doors positionable so as to close any one of the furnace chamber openings one at a time, and means for switching the energising means to selectively energise the said one closed furnace chamber.

In the case where the furnace is a vacuum furnace means may be be provided for selectively evacuating a furnace chamber closed by the furnace door.

Preferably, there is a single furnace door which is selectively positionable above each of the furnace openings.

The furnace door may be in the form of a lid mounted on a vertically extending shaft, the lid being swingable on the shaft over the top of each of the furnace chamber openings. Means may be provided to automatically sense the position of the furnace door so that when it is pushed down onto any one furnace chamber opening only that chamber is energised and evacuated.

Where two furnace chambers are provided the chambers can be used alternately so that as one is being energised the other is cooling down from a preceding energisation. It is still necessary to leave fired workpieces in the- furnace chambers to cool down and each furnace chamber therefore still has a down time but all the components of the furnace other than that single furnace chamber can still be used to initiate a firing cycle in the- other furnace chamber. A substantially continuous firing process can thus be achieved effectively doubling the productivity of the furnace for very little expenditure as 'compared with purchasing two independent furnaces.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig 1 is an illustration of the outer appearance of an embodiment of the present invention; Fig. 2 is a partial sectional view taken on the line 2-2 of Fig. 1; Fig. 3 is a schematic perspective illustration of the inner mechanism of a door support structure of the embodiment of Fig. 1; and Fig. 4 is a front elevation view of the door.

support structure of Fig. 3.

Referring to Fig. 1, the illustrated furnace comprises an outer casing 1 supporting an instrument panel 2 and two furnace chambers 3. A furnace door 4 is suppotted on a vertical shaft 5 extending through the outer casing 1. A door height adjusting screw 6 is positioned adjacent the shaft 5.

The instrument panel comprises meters and manually operable buttons and rotary knobs which enable a heating programme to be established to control the heating of each of the furnace chambers 3, 'only one of the chambers being heated at any one time. The instrument panel also controls a vacuum pump which when turned on can evacuate one of the chambers 3 at a time. The interior of the furnace chambers 3 is lined with an insulating muffle supporting an electrical heating element. Any appropriate heating muffle may be used.

Referring now to Fig. 2, a cross section of one furnace chamber 3 and the structure supporting the shaft 5 and door 4 are illustrated schematically.

The shaft 5 is mounted in vertically aligned bearings defined in brackets 7 which are secured to a back-plate 8 that in turn is secured to the casing 1. The screw 6 extends through the casing 1 and a tubular support member 9 to engage a nut 10 connected to one end of a tension spring 11. The other end of the spring is connected to a pin 12 extending substantially horizontally and radially through the shaft 5.

Rotation of the screw 6 enables the position of the upper end of the spring 11 to be adjusted, and hence the vertical position of the shaft 5 in which the spring 11 balances the weight of the shaft 5 and door 4.

The furnace door 4 is supported on an arm 13 and comprises a casing in the form of an inverted dish supporting an O-ring 14. A door insulation plug of ceramic fibrous material is engaged in the door casing, the insulation plug having a tapering lower portion 15 which is a snug fit within the top of each of the furnace chambers.

Each furnace chamber comprises a cup-shaped body 16 to the top of which an annular plate 17 is secured, an O-ring 18 sealing the joint between the body 16 and the plate 17. The plate projects radially inwards beneath a circular opening defined in the casing 1 to define a horizontal ledge 19 against which the sealing ring 14 supported by the door bears when the door is closed.

The chamber encloses an insulating base liner 20, a tubular insulating wall liner 21, a tubular muffle 22 of the type described in British Patent Specification No. 1 587 020, and an annular collar 23. The collar 23 defines a tapering opening into which the lower portion 15 of the door is received when the door is closed. The size of the opening can be selected to give a desired rate of cooling to the furnace. The smaller the opening, the slower is the rate of cooling. The collar 23 thus provides an inbuilt restriction limiting the maximum rate of cooling.

As can be seer. from Figs. 3 and 4, the pin 12 cooperates with two end stops 24 and a gate member 25 which are fixedly mounted on the base of the outer casing 1. The gate 25 supports microswitches 26 which extend across gaps 27 defined between the gate 25 and the end stops 24.

The door 4 may be swung from the position shown in Fig. 1 in which it is located above one furnace chamber to a position in which it is located above the other furnace chamber. In each of these two alternative positions the pin 12 bears against a respective one of the end stops 24. When the pin 12 is in contact with one of the end stops 24 the shaft can drop vertically downwards so that the pin 12 enters the gap 27 and depresses the associated microswitch 26. When the door is in an intermediate position the shaft cannot drop vertically downwards as the pin 12 bears against the upper edge of the gate 25. When the door is released it assumes a position dependent upon the tension on the spring 11. Thus the spacing between the lower portion 15 of the door and the collar 23 can be adjusted by turning the screw 6.As this spacing determines the rate at which heat can escape from the chamber, this arrangement provides a simple means for controlling the rate of cooling of workpieces fired within the chamber.

In use, the door 4 is positioned above one of the furnace chambers 3 and a workpiece to be fired is inserted into the other furnace chamber. The shaft 5 is then pulled upwards manually and swung round so that the door is positioned over the chamber into which workpieces have been inserted. The door is then pushed down so that the pin 12 mounted on the shaft 5 actuates the appropriate microswitch 26. The heating programme predetermined by the setting of the controls on the control panel 2 is then performed, the chamber identified by actuation of the appropriate microswitch 26, that is the chamber with which the door 4 has been engaged, being energised and evacuated. The heating programme will generally comprise a pre-dry portion during which full vacuum is not required. A calibrated bleed of air is allowed into the chamber under the control of a solenoid.The partial vacuum in the chamber is sufficient to hold the door closed. During pre-drying the chamber is scavenged of gassed off impurities whilst controlled ventilation is provided for full carbon burn out of expendibles in the workpiece. After the pre-drying portion of the programme, the solenoid valve is closed to enable a full vacuum to be established.

curing the period that one chamber is being evacuated and energised, the other chamber is not energised as its associated microswitch 26 has not been actuated. As the heating programme in the chamber into which the workpieces have been loaded is carried out further workpieces can be inserted into the vacant furnace chamber. When the heating programme is terminated the vacuum within the heated chamber is released and the dpor 4 moves upwards under the influence of the tension spring 11. The heated chamber then begins to cool down. The door 4 can then be swung round over the unheated chamber, immediately, or after pre-cooling of the chamber has been completed.Once over the unheated chamber, the door 4 is pressed down to seal the opening in the unheated chamber and the same programme can then be carried out on the workpieces located within the previously unheated chamber. The previously heated chamber then cools down gradually and in due course the fired workpieces therein can be removed and replaced by fresh workpieces. The alternate energisation and cooling of the two chambers can thus be carried out in a way which does not prevent use of the furnace during the 'down time' normally associated with such furnaces where workpieces have to be left for some time in a previously heated chamber. The productivity of the unit as compared with a single chamber furnace is thus effectively doubled without it being necessary to duplicate the control circuitry required to evacuate and energise the chambers.

In the illustrated embodiment of the invention the two furnace chambers are alternately closed by a single door which is positionable above either chamber. It would however be possible to provide a plurality of doors, one for each chamber. It would be necessary however to provide interlocks which were operative to prevent more than one chamber from being closed and energised simultaneously.

Claims (7)

CLAIMS:

1. A furnace comprising a plurality of furnace chambers each defining an opening through which access can be gained to the chamber interior, a single furnace energising means switchable to energise any one of the furnace chambers, one or more furnace doors positionable so as to close any one of the furnace chamber openings one at a time, and means for switching. the energising means to selectively energise the said one closed furnace chamber.

2. A furnace according to claim 1, wherein the furnace is a vacuum furnace, and means are provided for selectively evacuating the said one closed furnace chamber.

3. A furnace according to claim 1 or 2 comprising a single furnace door which is positionable above each of the furnace chamber openings.

4. A furnace according to claim 3, wherein the furnace door is in the form of a lid mounted on a vertically extending shaft, the lid being swingable on the shaft over the top of each of the furnace chamber openings.

5. A furnace according to claim 3 or 4, wherein means are provided to automatically sense the position of the furnace door so that when it is pushed down onto any one furnace chamber opening that chamber is energised.

6. A furnace according to claim 4 or 5, wherein means are provided to control the spacing between the door and the chamber opening when the door is positioned above that chamber opening.

7. A furnace substantially as hereinbefore described with reference to the accompanying drawings.