FR2470753A1 - Thixotropic frit material dispensing - by applying shear forces for viscosity correction during transfer to output cylinder - Google Patents

Abstract

In a device for dispensing thixotropic frit materials which are used to seal glass articles at a constant rate, the frit materials are first extruded from a storage cylinder into an output cylinder, thereby exposing them to shear forces which produce a suitable viscosity range. The piston of the output cylinder is then actuated to dispense the frit material with this viscosity at a constant rate. Arrangement ensures a high-quality sealing because the frit materials are dispensed in a suitable viscosity range and at a constant rate.

Description

 The present invention relates to the field of sealing ceramics, and it relates more particularly to a method and an improved device for the delivery of sintered welding glass usable for sealing between them pieces of glass or ceramic.

 Sealing glass compositions such as suspensions of sintered welding glasses are generally called "frits" in industry and are used in manufacturing. many kinds of assemblies including ceramic parts, glass enclosures for electrical devices, glass ceramic components for telescope mirror blanks and; ceramic supports for electrical circuits. A major application frit in the production of seals is the manufacture of envelopes for cathode-ray tubes of color television and the like. Such envelopes are produced by mutual sealing of a part forming a cone and a part forming a bottom or front panel by using '' a frit which is normally applied by extrusion in the form of a continuous ribbon or bead on at least one of the mating surfaces or sealing surfaces foreseen on the front panel and on the canez These parts are then assembled and subjected to a baking intended to give a unit in one piece in which the frit performs a hermetic seal mechanically robust.

 In the past, it was common practice to apply a bead of such a frit to the sealing edge of an article to be sealed by means of a reservoir of the vertical cylinder type having an orifice located near its bottom and container, under constant pneumatic pressure, a charge of the frit to delay vrer. However, since the viscosity of the frit not held in this kind of cylinders varies according to time and temperature and that the level level of pneumatic pressure frit varies as it depletes, the amount of frit deposited by such distributors tends to vary within wide limits, thereby giving mechanically unsatisfactory seals due to a lack or an excess of frit.

Furthermore, in order to obtain a profiled cross section having more frit on the inside of the sealed structure after firing, to thereby obtain a more resistant seal, as well as in order to prevent the frit from overflowing from the edge of sealing of the glass article, it is desirable to deliver a fairly viscous frit with a profiled section.

However, conventional distributors at constant pressure do not allow convenient delivery of frits having the desired high viscosity since the high pressures required simply have the effect of blowing a hole in the center of the sintered paste in the region of the delivery orifices.

An improved method and device for delivering frit which overcomes most of the above-mentioned drawbacks of conventional frit dispensers is described in the patent application of
EUA no 936 343 in the names of BARBER et al. entitled "Method and Apparatus for Dispensing Frit having a nydraulically Actuated Piston", of which this Applicant is assignee. The device described comprises a volumetric distribution which is incorporated in the hydraulic assembly, located at a certain distance from the frit distribution head. The size of the frit tank cylinder described in this patent application and the fact that the hydraulic cylinder is connected directly to this cylinder practically impose on the entry cylinder placed at a certain distance from the dispensing head, the distribution taking place directly from the frit reservoir cylinder.

However, another problem arises when using a thixotropic frit such as that described in the patent application of E, UA, No. 934 348 in the names of CORBETT et al. titled "Gelled Solder Glass
Suspensions for Sealing Glass or Ceramic Parts ", namely that of the notable increase in the viscosity of this thixotropic frit when it is at rest, as is the case between periodic frit deliveries. The frit remaining in the connecting pipe the reservoir cylinder at the dispensing head, as well as the one located inside the dispensing head itself in such an apparatus, becomes very firm when it is at rest between deliveries, and it requires the application of additional pressures to be able to be returned to flow.

However, once the flow is restored and shearing of the frit occurs during this flow, the frit returns to a viscosity within a normal range. The higher initial pressures necessary to establish flow in such a device, however, give rise to an excessive and uncontrolled frit flow rate at the start of each delivery cycle. Thus, the present invention provides improvements to the device which is the subject of the US patent application. no. 936 343 cited above, improvements which remedy the particular behaviors which manifest themselves in the delivery of a thixotropic frit.

 The fundamental aim of the present invention is therefore to provide a method and a device making it possible to deliver a uniform quantity of a thixotropic frit for sealing glass and glass-ceramic articles, regardless of its temperature or its viscosity at rest, by using volumetric means with adjustable displacement involving a preliminary discharge or extrusion of the thixotropic frit passing it from a reservoir cylinder to a distributor cylinder so as to apply shear forces to it and to reduce its viscosity in a range of normal values to have it delivered by the distributor cylinder.

 In its most basic provisions, the present invention provides a method and a device for delivering thixotropic frit for mutual sealing of glass and glass-ceramic articles in a uniform and metered manner in order to provide improved sealing characteristics. The deposition of a bead of non-uniform frit on a sealing edge may make it impossible to obtain a hermetic seal in regions where the quantity of frit deposited is insufficient, and an unsightly bulge and a discontinuity of stresses of the glass are obtained in areas where the amount of frit deposited on the sealing surface is excessive.

A fundamental method of implementing the present invention consists in filling a cylinder-reservoir with thixotropic material to be delivered by retracting a piston provided inside this cylinder. The piston is then advanced in order to push this material into a distributor cylinder of relatively small dimensions compared to the much larger diameter of the reservoir cylinder, this in order to act on the viscosity of the thixotropic material by application to it of forces of shear during this delivery to bring its viscosity into a normal range of delivery viscosites. Then, while the viscosity of the thixotropic material is still within this normal range, a piston located inside the distributor cylinder is advanced. at a constant rate so as to deliver the thixotropic material at a constant rate through a desired orifice in order to obtain a uniform flow of the frit from the start to the end of the delivery by the distributor cylinder.

The characteristics and advantages of the invention will emerge more fully from the detailed description which is given below by way of example with reference to the appended figures in which
Figure 1 is a schematic sectional representation of a preferred embodiment of a device according to the invention intended to deliver frit at a metered flow rate and with a desired configuration by implementing a displacement at constant volume and of a preliminary discharge or extrusion of this frit into a distributor cylinder intended to modify the viscosity thereof; and
Figure 2 is a partial schematic view taken along line 2-2 of Figure i which shows the dispensing head and its perforated plate.

 Reference will now be made to the figures which represent a frit distributor device 10 comprising a hydraulic cylinder 12, a reservoir cylinder 14 and a distributor cylinder 16. The hydraulic cylinder or cylinder 12 and the reservoir cylinder 14 have respective adjacent end portions 18 and 20 which are integral with each other in a sealed manner and comprise a common piston rod 22 which passes through them and is connected to a piston 24 in the reservoir cylinder 14 and to a piston 26 in the hydraulic cylinder 12. The hydraulic cylinder 12 comprises a posterior end cap 28 which faces the adjacent front end portion 18 and is provided with a hydraulic line 30 which communicates with the interior of the cylinder 12 behind the piston 26 through an orifice 32 formed in the end cap 28 Dgune similarly, the front end portion 18 of the hydraulic cylinder 12 is provided with a hydraulic line 34 which is connected to it deed and communicates with the interior of this cylinder in front of the piston 26 by an orifice 36 formed in the terminal portion 18 The adjacent terminal portion 20 of the reservoir cylinder 14 has a vent pipe 38 which is connected to it and communicates with the interior of the cylinder 14 behind the piston 24 through an orifice 40 formed in the terminal portion 20. The front end of the reservoir cylinder 14 includes a terminal cap 42 which is provided with a removable plug 44 to allow loading of the frit in the reservoir cylinder 14.

 The reservoir cylinder 14 is connected to the distributor cylinder 16 by means of a conduit 46 such as a flexible high pressure vapor tight hose in series with which is mounted a suitable valve such as a ball valve 48. At its anterior end , the distributor cylinder 16 comprises a distributor head 50 through which an outlet orifice 52 passes. The duct 46 starts from an outlet passage 45 formed in the end cap 42 of the reservoir cylinder 14 and leads to an inlet passage 47 formed in the distributor head 50 of the distributor cylinder 16. The rear end of the distributor cylinder 16 is provided with an end cap 54 provided with a rod-shaped piston 56 which passes through the latter slidingly. At the outer end of the piston in the form of a rod 56 is fixed a split nut device 58 which is actuated by a pneumatic cylinder not shown so as to cooperate periodically with a threaded shaft 60 which is rotatably mounted in bearings 62 and 63 and is driven by a variable speed electric motor 64. The outlet orifice 52 is provided with a perforated plate 66 of the guillotine or jet cutter type comprising an orifice 68 of desired profile. The profiled orifice 68 of the guillotine plate 66 can slide so as to coincide and out of coincidence with the outlet orifice 52 of the dispensing head 50 by means of a pivot rod 70 and a coupling rod 72 which is itself kinematically coupled to a pneumatic cylinder not shown.

Around the device 10 are arranged several limit switches intended to coordinate its operation. A limit switch LS-1 cooperates with the valve 48 so that the latter acts by its position on the switch LS-1, which controls luimeAme a pneumatic cylinder by making it open and close the split nut 58. A Limit switch LS-2 has the function of cutting the excitation of the variable speed electric motor 64, while a limit switch DS-3 stops the application of hydraulic pressure through line 30 to the hydraulic cylinder 12.Finally, an end-of-travel switch LS-4 which is controlled by the position of the pivot rod 70 causes the electric motor 64 to be excited and consequently the threaded shaft 60 to be driven. various pistons and piston rods are fitted with standard sliding seals as shown in diagram 74.

 In operation, the rod-shaped piston 56 of the distributor cylinder 16 being in its advanced position while the power supply to the electric motor 64 is cut by the limit switch LS-2 with which the split nut 58 is in contact, the ball valve 48 is brought into its open position in the pipe 46. When the valve 48 is brought into its open position, the limit switch LS-1 is actuated, and it acts on the pneumatic cylinder so as to make it open the split nut 58, and the threading of the latter therefore ceases to mesh with the threaded shaft 60. Hydraulic fluid is then applied under pressure to the rear end of the hydraulic cylinder 12 through the pipe 30, the end cap 28 and the passage 32 so as to move the piston 26 forwards. As the piston 26 moves forward, the hydraulic fluid contained in the front part of the cylinder 12 is discharged through the passage 36 and the hydraulic line 34. In addition, because the piston 26 of the cylinder hydraulic 12 and the piston 24 of the reservoir cylinder 14 are mutually coupled by the common piston rod 22 moving the piston 26 forward gives rise to a simultaneous forward movement of the piston 24, thereby subjecting the frit contained in the reservoir cylinder 14 to an extrusion which makes it penetrate into the distributor cylinder 16 through the conduit 46 and the open tap 48 .. It will be noted that during the loading of the distributor cylinder 16, the guillotine plate 66 occupies a closed position of I1 outlet port 58 of the dispensing head 50.

When the thixotropic frit is removed by extrusion from the reservoir cylinder 14 into the distributor cylinder 16, it is subjected to shear forces which reduce its viscosity to a normal value of delivery. In addition, when the frit is extruded into the distributor cylinder 16, the force exerted by the admitted frit on the rod-shaped piston 56 pushes it in the direction of its retracted position, so that when the distributor cylinder 16 is located filled with frit, the split nut 58 connected to the outer end of the rod-shaped piston 56 comes into contact with the limit switch LS-3, which puts an end to the admission of hydraulic fluid in the hydraulic cylinder 12 through the pipe 30.

 The valve 48 is then returned to its closed position shown in FIG. 1, and the limit switch LS-1- is actuated so as to act on the pneumatic cylinder in order to make it close the split nut 58 around the threaded shaft 60. In addition, the pneumatic cylinder coupled to the rod 72 is actuated so as to bring the profiled orifice 68 of the guillotine plate 66 into alignment with the outlet orifice 52. The end switch LS4 stroke is then actuated by the rod 70 so as to excite the variable speed electric motor 64, which rotates 11 threaded shaft 60 thereby causing the slotted nut 58 to be driven along the latter and the displacement of the rod-shaped piston 56 forwards at a well-determined speed inside the distributor cylinder 16 so as to deliver the modified viscosity frit contained therein by exiting it through the profiled orifice 68. At the end from the extrusion, the split nut 58 comes in contact with the limit switch LS-2 to de-energize the electric motor 64 and end the extrusion.

When you want to refill the reservoir cylinder 14, remove the removable cover 44, place a new load of frit over the opening and admit hydraulic fluid into the line 34 to retract the pistons 26 and 24 by means of the common piston rod 22 and then bring in by suction the new frit charge disposed over the opening of the reservoir cylinder.

We have just described the currently preferred embodiment of the invention, but it will be obvious to the specialist that various modifications and variants can be made to the provisions described and shown without departing from the scope of the invention. as defined by the attached claims

Claims (10)

RiVENDICA2IONS  1. Method for delivering thixotropic frit at a constant flow rate for sealing glass articles, characterized in that it comprises the operations consisting in charging thixotropic frit to be delivered into one of the ends of a reservoir cylinder, extruding said thixotropic frit from said reservoir cylinder into a distributor cylinder and simultaneously subjecting said frit to shear forces in order to modify the viscosity to bring it into a normal range of delivery viscosities and, during that the viscosity of the frit is still in said normal range, having said thixotropic frit delivered at a constant flow rate by the distributor cylinder through an orifice of desired configuration.  2. Method according to claim 1, characterized in that it comprises the operations consisting in moving a piston forwards at a constant speed inside said distributor cylinder to deliver the thixotropic frit, and in moving said piston towards the rear in reaction to the force exerted by the thixotropic frit driven by extrusion of said reservoir cylinder into said distributor cylinder. 3. Method according to claim 1, characterized in that it comprises the operations consisting in moving a piston forwards inside said reservoir cylinder to drive out the thixotropic frit by extrusion, and in causing the displacement of said piston from the outside of said reservoir cylinder. 4. Method according to claim 3, charac merise in that it comprises the operations of moving said piston backwards inside said reservoir cylinder to the effect of sucking through a loading opening of the frit thixotropic to deliver, and to cause the displacement of said piston from the outside of said reservoir cylinder.  5. Device for delivering under a constant flow rate of thixotropic frit for sealing glass articles, characterized in that it comprises: a hydraulic cylinder (12), a reservoir cylinder (14) and a distributor cylinder ( 16); a first piston (24) arranged inside said reservoir cylinder (14) and actuated by a second piston (26) disposed inside said hydraulic cylinder for extruding thixotropic frit from said reservoir cylinder into said cylinder distributor in order to simultaneously modify the viscosity of said frit by bringing it into a normal range of delivery viscosities and to move a third piston (56) located inside said distributor cylinder (16) towards the rear of said distributor cylinder ); and motor means (64) capable of moving said third piston forwards inside said distributor cylinder at a constant speed in order to ensure the delivery of the thixotropic frit with modified viscosity by said distributor cylinder. 6. Device according to claim 5, characterized in that it comprises a conduit (10) stéten- dant between said reservoir cylinder (14) and said distributor cylinder (16), a valve member (48) inserted on said conduit and adapted to be opened to allow the frit to pass by extrusion from said reservoir cylinder (14) to said distributor cylinder (16) and to be closed to prevent said frit from returning from said distributor cylinder to said reservoir cylinder when said third piston is moved toward the before to deliver frit 7. Device according to claim 5, characterized in that said motor means comprise a threaded shaft (60), an electric motor with variable speed (64) intended to rotate said threaded shaft and a split nut mechanism (58) connected to said third piston (56) so as to cooperate kinematically periodically with said threaded shaft.  8. Device according to claim 5, characterized in that said distributor cylinder (16) comprises a distributor head, an outlet orifice formed in said distributor head (50) and a guillotine plate (66) through which an orifice (68) profiled and mounted on said dispensing head so as to communicate under command with said outlet orifice.  9. Device according to claim 5, characterized in that said hydraulic cylinder and said reservoir cylinder are adjacent and in mutual axial alignment, and in that said first piston (24) and said second piston (26) are connected by a rod piston (22) common extending between said hydraulic cylinder and said reservoir cylinder. 10. Device according to claim 5, characterized in that said reservoir cylinder is provided with a removable cover (44) intended to enable said reservoir cylinder to be supplied by introduction of a charge of thixotropic frit, and in that said first piston is adapted to ensure suction of said load by retracting.