OVENS FOR TREATING LAMELLAE

Abstract: A method of heating lamellae (11) which comprises conveying the lamellae through a tunnel oven (42) in spaced array with their planes generally parallel and approximately perpendicular to the direction of travel, the lamellae being preceded and succeeded by baffle plates or doors (47) which move with the lamellae, lie in planes approximately parallel to the lamellae and have dimensions which approximately match the cross sectional dimensions of the tunnel. The baffles may be attached to racks (13) in which the lamellae are carried but they may be movable through the oven independently of the lamellae. A preferred procedure is as follows: position two doors, A and B, at the entrance end of the oven, move the doors forward with the first door A being releasably held at the exit end and the second door B being releasably held at the entry end of the oven, then introduce a batch of lamellae (11) to the entry end of the oven while moving the second door B through the oven to the exit end and moving the first door A out of the oven; then introduce third and fourth doors C and D to the entry end of the oven and move the third door C through the oven behind the lamellae to the exit end and releasably hold the third door there; and hold the fourth door D at the entry end.

Description: OVENS FOR TREATING LAMELLAE This invention relates to ovens for treating lamellae. While the invention is particularly suitable for use with ovens for handling printed circuit board blanks during the production process and before the application of components thereto, the invention may be applied to ovens for other lamellae in the form of rigid or semi-rigid sheets, plates, cards or the like; the invention is particularly, but not exclusively, concerned with ovens in which the lamellae to be heated are held at only one edge to avoid damage to the broad surfaces of the lamellae or, for example, to fragile or uncured coatings thereon. In the production of printed circuit boards, rigid or semi-rigid substrates require oven treatment, for example for the thermal curing of coatings thereon. In one system which is in use, the lamellae are conveyed through a tunnel oven in spaced array with their planes generally parallel and approximately perpendicular to the direction of travel. While various heating systems may be used, in a particular arrangement hot air is blown across the oven between the printed circuit boards as they pass through the oven the air being admitted at one side and exhausted from the other and this, creates laminar air flow over the boards. It is desirable to maintain as near as possible uniform temperature across the width, length and height of the tunnel. To achieve this and to reduce energy requirements the apertures at the entrance to and exit from the tunnel may be provided with means to reduce the exchange of air between the interior and exterior and examples of such means are air curtains, doors and shutters. However, the apertures must, for reasons of economy, be sufficiently large to handle the largest format of printed circuit board commonly in use. The largest size in common use today is 650mm by 610mm and it is usual for the apertures at the entrance and exit of the tunnel to be of the order of 760mm by 700mm. There is thus a substantial gap around even the largest printed circuit boards and when the oven is used with smaller printed circuit boards, the gap is very substantial. The present invention provides a means of heating lamellae in a tunnel oven, in a way which minimises energy requirements without the use of air curtains, doors or shutters. According to the present invention, the lamellae are conveyed through a tunnel oven in spaced array with their planes generally parallel and approximately perpendicular to the direction of travel, the lamellae being preceded and succeeded by baffle plates which more with the lamellae lie in planes approximately parallel to the lamellae and have dimensions which approximately match the cross sectional dimensions of the tunnel. According to one aspect, the invention may be seen to consist in a method of heating lamellae in the manner described. In one version, the doors are movable through the oven independently of the lamellae, a leading door being releasably held at the exit end of the oven and a trailing door being releasably held at the entry end of the oven during operation of the oven and except during exit and entry respectively of lamellae. Preferably, in this version, the method uses at least four doors which are independently movable through the oven and the procedure is as follows: 1) position one door at the exit end of the oven and releasably hold it there and a second door at the entry end of the oven and releasably hold it there; 2) introduce a batch of lamellae to the entry end of the oven while moving the second door through the oven to the exit end and releasably hold it there and moving the first door out of the oven; 3) introduce a third door to the entry end of the oven and move the third door through the oven behind the lamellae to the exit end and releasably hold the third door there; and 4) position a fourth door at the entry end of the oven. According to another aspect of the invention a tunnel oven has a conveyor with means of supporting lamellae in spaced array with their planes generally parallel and approximately perpendicular to the direction of travel, and baffle plates carried by the conveyor to be located ahead of and behind lamellae supported on the conveyor, the baffle plates having dimensions which approximately match the cross sectional dimensions of the tunnel. According to a third aspect of the invention, a rack for supporting lamellae during their passage through a tunnel oven has a base having a plurality of transversely extending holders for the receipt of the lower edges of lamellae and at or adjacent each end of the base a baffle plate extending upwards from the base. It will be realised that the invention may be applied to an oven having a conveyor which itself is provided with means for supporting the individual lamellae, such as holders or slots, and to an oven of the kind in which a plain conveyor supports separate racks in which the lamellae are supported. Preferably the oven is preceded and succeeded by plain insulated tunnel sections which co-operate with the baffles, effectively to seal off the tunnel of the oven itself. The invention may be carried into practice in various ways and two particular ovens operating in accordance with the principles of the present invention, together with some modifications thereof, will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is side elevation of the first oven; Figure 2 is an end elevation of the first oven; Figure 3 is a side elevation to larger scale of a rack used in conjunction with the oven shown in Figures l and 2; Figure 4 is an end elevation on a much enlarged scale of a holder forming part of the conveyer of an alternative construction of oven; Figure 5 is a side elevation of another oven; Figure 6 is a side elevation of a single mobile door used with the oven shown in Figure 5; Figure 7 is an end elevation of the door shown in Figure 6; Figure 8 is a diagrammatic longitudinal section to a larger scale of part of the oven shown in Figure 5 illustrating a catch; Figure 9 is a view similar to Figure 5 with two mobile doors in position shortly after commencement of operation of the oven; Figure 10 is a view similar to Figure 9 showing the position at a later stage; and' Figure 11 is a view similar to Figures 9 and 10 showing the position at a yet later stage. The oven shown in Figures 1 and 2 comprises a base 1 on which there is mounted an oven 10 comprising a first oven section 2 and a second oven section 3 contiguous to the first oven section 2. A conveyer 4 extends through the oven 10 extending beyond the oven to provide a loading section 5 and an unloading section 6. The oven 10 and the conveyor 4 make a tunnel 7 the walls of which are insulated and provided with means, not shown, for passing a flow of hot air from one side 8 to the other side 9 of the tunnel, apertures being provided in these walls to permit flow of the hot air. Apart from the apertures, the walls of the tunnel are substantially smooth and the tunnel is of constant cross section. The walls of the tunnel are heavily insulated. The oven is intended for the drying of coated lamellae 11 which, as shown in figure 3, are conveyed through the tunnel 7 in racks 12, one of which is shown in figure 3. Each rack 12 comprises a base 13 which is shaped to fit onto and be conveyed by the conveyor 4 and has an upper surface provided with slots 14 which are shaped to receive the lower edges of the lamellae 11 so that the lamellae are supported in spaced array with their planes generally parallel and close to vertical. At one end the rack 12 rigidly supports a front end baffle 15 and at the opposite end it rigidly supports a rear end baffle 16. These baffles are vertical and are rectangular in shape having dimensions which are very slightly smaller than the cross sectional dimensions of the tunnel 7. The top edges and possibly the side edges of the baffles 15 and 16 may be provided with narrow flexible sealing strips 17. Upstream of the first tunnel section 2 there is an inlet tunnel section 21 while at the downstream end of the second oven section 3 there is an outlet tunnel section 22. These tunnel sections have tunnel openings of the same cross section as the oven sections 2,3 but omit the apertures for the cross flow of air. The lengths of the sections 21 and 22 in the direction of travel of the conveyor 4 are substantially equal to, or slightly more than, the distance between the baffle plates 15 and 16 of the rack 12. The operation of the oven will now be described. Lamellae to be heated, for example to cure coatings on their surfaces, are placed in a rack 12 in the manner shown in Figure 3, this being done at a location away from the oven. The filled rack is then placed on the loading section 5 of the conveyor 4 and the rack with the lamellae thereon is transported through the oven. As the front baffle 15 reaches the inlet section 21 the aperture of the tunnel will be substantially closed at this end thus preventing the transfer of air through this aperture. As the front baffle 15 reaches the oven section 2 (by which time the rear baffle 16 will have entered the section 21) the rack becomes a miniature oven within the main oven giving good control over air flow and heat distribution. This condition holds good until until the rear baffle 16 reaches the downstream end of the oven section 3. At this time the forward baffle 15 will leave the exit section 22 but the rearward baffle 16 will continue to close off the tunnel from the exterior until the whole rack has left the exit section 22. Racks can follow one another with a relatively small gap between them and if this is done the tunnel through the oven sections 2 and 3 will remain closed at all times. If a small quantity of lamellae is to be treated or an interruption to flow occurs so that the rear baffle of a rack goes beyond the entrance to the oven section 2 without a subsequent rack being in position, thus allowing air to pass through the aperture of the inlet tunnel section 21, a sensor (not shown) is activated to provide an audible or visual warning to the operator who may introduce an empty rack fitted with front and rear baffles 15 and 16 to re-seal the oven. Alternatively the oven could be provided with doors or other conventional sealing means for use in such circumstances. In an alternative construction separate racks are not used but the conveyor 4a is provided with a plurality of parallel holders of the section shown in Figure 4. These holders provide slots 31 with open mouths 32 to receive the lower edges of lamellae. With this arrangement, a baffle is introduced into a slot ahead of a batch of lamellae and a further baffle is introduced into a slot to the rear of the batch of lamellae so that these baffles, which are of similar dimensions to the baffles 15 and 16 shown in Figure 3, perform the same function as those baffles to seal the tunnel as the lamellae passes' through the tunnel. It will be appreciated that where the conveyor is kept full of lamellae baffles will be introduced into the conveyor at suitable intervals so that the baffles are spaced apart by distances approximately equal to the lengths of the inlet and outlet sections 21 and 22. Although the dimensions of the various parts of the ovens described may vary, in a typical example intended to take printed circuit boards of sizes from 300mm by 300mm up to 650mm by 610mm, the tunnel has cross sectional dimensions of 760mm wide by 700mm high and the clearance between the edges of the baffles 15 and 16 and the top and side walls of the tunnel 7 is of the order of 5mm. Flow through the 5mm gap is minimised by the sealing strips 17. Each of the sections 2 and 3 of the oven are 600mm long while the lengths of the inlet sections 21 and 22 and the distance between the baffles 15 and 16 of the rack 12 are approximately 300mm. With the rack shown in Figure 3 the lamellae or boards 11 are loosely held in the slots 14 and accordingly tend to lean over very slightly. In an alternative construction the front end baffle and the rear end baffle are interconnected by light bars which run on either side of the boards 11 and are shaped in the manner of combs, the teeth extending horizontally between adjacent boards to hold the boards in a vertical position. Similarly a top bar may extend between the baffles 15 and 16 and have depending teeth extending between the boards at their upper edges. These bars not only serve to maintain the boards upright but also add rigidity to the baffles. With the oven shown in Figure 5 a somewhat different mode of operation is adopted and instead of providing front end baffles and rear end baffles attached to the racks 13 as shown in Figure 3 single separate baffles are used as will be described. These baffles will also be referred to as doors. The oven shown in Figure 5 comprises a base section 41 containing conveyor means for advancing racks carrying boards through an oven 42 in the direction of the arrow 43. The conveyor comprises a stationary ball bearing bed on which the racks travel and a conveyor belt having entraining means to advance the racks and the baffles. The oven 42 has an infeed section 44, an oven area 45 and an outfeed section 46. Each of the single baffles or doors 47 used with the oven shown in Figure 5 comprises a support section 48 of downwardly open channel section and a door section 49 which consists of a rectangular plate which is secured by welding to the channel 48. Depending from the web of the support section 48 is a striker plate 50. The conveyor comprises two parallel rails 51 which guide the racks 13 and the doors 47, the rails passing between the ends of the striker plate 50 and the flanges of the support section 48. The oven includes two overridable pressure catches 52,53 the construction of one of which is shown in detail in Figure 8. The catch comprises a rocker 54 located beneath the bed of the conveyor and pivoted at a centre point on a horizontal pin 55. Extending upwardly from the downstream end of the rocker '54 is a detent plate 56 which passes through an aperture 57 in the bed of the conveyor into the path of the striker plate 50. A compression spring 58 biases the rocker 54 in a counter-clockwise direction as seen in Figure 8 by an amount which is limited by an adjustable stop 59. If sufficient pressure is applied to the detent plate 56 by the striker plate 50 the rocker 54 is moved clockwise against the bias of the spring 58 to allow the striker plate 50 to pass beyond the detent plate 56. Operation of the oven shown in Figures 5, 9, 10 and 11 will now be described. Upon startup two single doors A,B of the construction shown in Figures 6 and 7 are fed into the oven from the left hand end as shown in Figure 5. As the two doors travel together along the conveyor they reach the first catch 52. The first door A is pushed over the catch by the second door B which in turn is held in position by the catch 52. The first door A continues its motion until it is retained by the second catch 53. In this position which is shown in Figure 9 the oven area is effectively sealed to promote rapid warm-up. When the oven is at the desired temperature racks 13 carrying the boards are fed into the infeed section 44 of the oven as shown in Figure 10. As the racks progress through the oven they come into contact with the door B overriding the catch 52 and pushing the door B along the conveyor. This sequence is repeated when the racks reach the door A and the catch 53 is overridden. At the end of each of each batch of boards two further doors C and D are fed onto the conveyor. As the racks travel through the oven door C is pushed over the first catch 52 by the door D which in turn is retained by this catch as shown in Figure 11 thus maintaining a seal at the entrance to the oven. The door C stops when it reaches the second catch 53 thus sealing the exit to the oven and returning the oven to the situation shown in Figure 9. The oven area is now sealed thus retaining the temperature for the next batch of boards when the process will be repeated.

Claims: Claims: 1. A method of heating lamellae which comprises conveying the lamellae through a tunnel oven in spaced array with their planes generally parallel and approximately perpendicular to the direction of travel, the lamellae being preceded and succeeded by baffle plates which move with the lamellae, lie in planes approximately parallel to 'the lamellae and have dimensions which approximately match the cross sectional dimensions of the tunnel. 2. A method according to claim 1 in which the doors are movable through the oven independently of the lamellae, a leading door being releasably held at the exit end of the oven and a trailing door being releasably held at the entry end of the oven during operation of the oven and except during exit and entry respectively of lamellae. 3. A method according to claim 2 using at least four doors which are independently movable through the oven, the procedure being as follows: 1) position one door at the exit end of the oven and releasably hold it there and a second door at the entry end of the oven and releasably hold it there; 2) introduce a batch of lamellae to the entry end of the oven while moving the second door through the oven to the exit end and releasably hold it there and moving the first door out of the oven; 3) introduce a third door to the entry end of the oven and move the third door through the oven behind the lamellae to the exit end and releasably hold the third door there; and 4) position a fourth door at the entry end of the oven. 4. A tunnel oven comprising a conveyor with means of supporting lamellae in spaced array with their planes generally parallel and approximately perpendicular to the direction of travel, and baffle plates carried by the conveyor to be located ahead of and behind lamellae supported on the conveyor, the baffle plates having dimensions which approximately match the cross sectional dimensions of the tunnel. 5: A tunnel oven according to claim 4 in which the conveyor itself is provided with means for supporting individual lamellae and the baffle plates, the baffle plates being constructed to be separately introduced on to the conveyor. 6. A tunnel oven according to claim 4 which includes a rack comprising a base having means for supporting individual lamellae and a baffle plate secured to the base at each end of the rack. 7. A tunnel oven according to claim 5 or claim 6 having an oven section which is preceded and succeeded by insulated tunnel sections. 8. A tunnel oven according to claim 4 which includes individual baffle plates movable independently through the oven by the conveyor and, adjacent each end of the oven, means for releasably holding a baffle plate during operation of the conveyor. 9. A tunnel oven according to claim 8 in which each baffle plate has a striker plate and each holding means comprises a spring-loaded detent plate to releasably engage the striker plate. 10. A rack for supporting lamellae during their passage through a tunnel oven having a base having a plurality of transversely extending holders for the receipt of the lower edges of lamellae and at or adjacent each end of the base a baffle plate extending upwards from the base.

Inventors: LUCAS ALAN WILLIAM, LUCAS, ALAN, WILLIAM
Applicants: NUBAL ELECTRONICS LTD, NUBAL ELECTRONICS LIMITED
Application year: 1991
Main classification: F27B9
IPC codes: F27B9, F27D1, F27D3, F27D23, H05K3