Apparatus for producing glass tube and process for producing glass tube via drawing process

Abstract

To provide an apparatus and a method for improved prevention of visible streak formation during the manufacture of glass tubing.SOLUTION: This disclosure relates to an apparatus for producing a glass tube and a process for producing glass tubes via a drawing process. The apparatus and the process avoid the formation of streaks in the produced glass tubes and improve the quality of the glass tubes. The apparatus includes a drawing head having a geometrical shape adapted and specially designed to increase the yield of processable glass melt.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present disclosure relates to an apparatus for producing a glass tube and a process for producing a glass tube via a drawing process. The apparatus comprises a drawing head and a drawing needle, and the geometric shape and dimensions of the drawing head are designed such that streaks that would impair the quality of the glass tube are not formed in the process according to the present disclosure.

[0002] Background Art For the production of glass tubes, the Vello process is known. This is a vertical drawing process in which a viscous glass melt is drawn downward through a ring die. The glass melt is supplied from a melting tank via a feeder channel or duct. At the end of the feeder channel, there is a cylindrical opening at the bottom having a nozzle through which the melt can flow out via a vertical cone, also called a drawing needle, which is adjustable in height and laterally and flares downward in the shape of a funnel. The drawing needle can also assume the shape of a double cone at its lower end, with a first upper cone flaring downward and a second lower cone tapering further downward. The cone is hollow and connected to blowing air via an extension tube. As a result, the glass mass flowing around the valve body remains open and is freely suspended or deflected horizontally with the aid of guides and drawn downward into a temperature-controlled shaft as a glass tube before being drawn by a drawing machine.

[0003] The similarly commonly used down-draw process is similar, except that the formed glass tube is not deflected horizontally but is drawn directly downward vertically.

[0004] The end section of a feeder channel having an opening through which a glass melt is drawn out as a glass tube is often referred to as a feeder head or a draw head. The above-described vertical cone, i.e., the draw needle, can be used to close the glass outlet of the draw head intended to draw out the glass tube. The draw needle is held and moved by a cylindrical extension, i.e., a needle shaft. The needle shaft protrudes from the draw head at its upper open end. Before the glass melt exits the draw head through the glass outlet, it flows around the shank of the draw needle, forming a so-called looping streak in the glass melt. The streaks that can be formed in the glass are chemical inhomogeneities having a slightly different refractive index from the base glass. Especially when the streaks are located on the glass surface, strongly prominent streaks can be seen with the naked eye. It is also possible to visually recognize the streaks in a micrograph taken from a cut cross-section of the glass tube.

[0005] German Patent Application Publication No. 102008009811 discloses a method for controlling and monitoring the temperature in the production of a glass tube using a vero or down-draw process. In this method, a glass melt is introduced into a glass tube needle cell, metered and supplied onto a metering draw needle through a nozzle ring, and the metering draw needle is heated together with its conical glass tube former. The wall of the metering draw needle having a connected glass tube-shaped former is made of a conductive material, and the heating is performed by the flow of an electric current in the conductive wall material. German Patent Application Publication No. 102008009811 also discloses a related device for controlling and monitoring the temperature in the production of a glass tube using a vero or down-draw process. Both the method and the device make it possible to prevent devitrification appearing in the form of streaks on the glass tube.

[0006] U.S. Patent Application Publication No. 2017 / 0320766 discloses a bell assembly device with improved thermal dimensional stability for reducing siding loss during the production of glass tubes, and a glass tube forming device incorporating a bell assembly device with improved thermal dimensional stability so as to reduce undesirable movement of the bell within the delivery orifice of the glass delivery tank due to thermal fluctuations across the bell assembly device during glass tube production.

[0007] U.S. Patent Application Publication No. 2017 / 0341966 discloses a heating device and method for manufacturing glass tubes. The heating device includes a bowl configured to receive molten glass and a plurality of heating elements thermally coupled to the bowl. The bowl includes a tab portion configured to hold the molten glass, a bowl well extending below the tab portion, and an orifice at the distal end of the bowl well. The plurality of heating elements includes a first heating element disposed at a first vertical position along the bowl height and a second heating element disposed at a second vertical position along the bowl height, and the first vertical position is vertically spaced from the second vertical position. The heating device is used to heat the molten glass as it flows out of the orifice through the bowl well from the tab portion. The heating device is configured to maintain the thermal homogeneity of the molten glass exiting the orifice, thereby reducing fluctuations and instabilities in tube dimensions in the glass tube manufacturing process.

[0008] The tendency of glass streaks is that the glass melt undergoes a specific orientation as a result of flowing around the needle shaft and thus appears as loop-shaped streaks. In the prior art, loop-shaped streaks are formed in the drawing head by rejoining the molten glass part after passing through the needle shaft in a stationary region and are assumed to appear as streaks in a glass tube manufactured at a position relatively stationary with respect to their positions on the molded part. If the glass tube is marked so that its position can be tracked when flowing out of the drawing head, the streaks are seen on the side of the glass tube surface opposite to the side of the glass inlet flow when exiting the head. It was previously speculated that the streaks formed on the surface of the drawn glass tube could be prevented by using a head overflow positioned laterally to draw out the part affected by the glass melt. However, this method was insufficient to sufficiently avoid the formation of streaks on the glass tube surface.

[0009] A drawn glass tube having a known drawing head uses a so-called head overflow to suppress the formation of loop-shaped streaks in the glass melt and thus prevent or at least mitigate the formation of streaks on the surface of the manufactured glass tube. As a result, the glass melt drawn through the head overflow cannot be used for product manufacturing, the overall yield decreases, and the production cost increases.

[0010] The object of the present disclosure is to provide an apparatus and method for improving the prevention of visible streak formation during the manufacture of glass tubes.

[0011] Summary of the Invention The stated object is solved by the subject matter of the present disclosure.

[0012] In one aspect, the present disclosure provides an apparatus for producing a glass tube comprising a drawing head and a drawing needle, wherein the drawing head comprises a glass inlet and a glass outlet, the drawing head has an overall height h and an inner diameter d, the ratio h / d is from 2 / 1 to 7 / 1, and the distance a between the drawing needle and the inner wall of the drawing head is from 40 to 300 mm.

[0013] In a related aspect, the present disclosure provides a process for producing a glass tube via a drawing process, the process comprising passing a glass melt through an apparatus according to the present disclosure, the process optionally including a subsequent hot forming step.

[0014] Surprisingly, it has been found that the adaptation of the geometric design of the drawing head can influence the convection in the glass melt, such that the loop-like streaks are no longer visible as streaks on the product. This result was also seen when significantly reducing the glass discharge at the head overflow or without operating the head overflow. In the drawing head according to the present disclosure, the flow of the glass melt within the drawing head is altered such that the streaks move into the manufactured product and thus are no longer present on the surface and are no longer visible on the product.

[0015] The drawing head according to the present disclosure can have a basic cylindrical geometry. The drawing head according to the present disclosure can have an overall height h and an inner diameter d, the ratio h / d can be from 2 / 1 to 7 / 1, preferably the ratio h / d can be from 3 / 1 to 4.5 / 1. Alternatively, the ratio h / d can be from 4 / 1 to 6 / 1.

[0016] In one embodiment of the drawing head, the inner diameter d can be greater than 130 mm, or greater than 150 mm, or greater than 200 mm, or greater than 250 mm, preferably the inner diameter d can be 300 mm.

[0017] In one embodiment of the extraction head with an inner diameter d of 200 mm or more, the ratio h / d can be 3 / 1 to 4.5 / 1, or 3.2 / 1 to 4.3 / 1. In one embodiment of the extraction head with an inner diameter d of less than 200 mm, the ratio h / d can be 4 / 1 to 6 / 1, or 4.2 / 1 to 5.3 / 1.

[0018] In one embodiment of the extraction head, the distance a between the extraction needle and the inner wall of the extraction head can be in the range of 40 to 300 mm. Preferably, the distance a can be in the range of 50 to 150 mm. In the present disclosure, the distance a between the extraction needle and the inner wall can be understood as a position-dependent distance between any point on the surface of the needle shaft and the inner wall, and this distance is measured in the horizontal plane in the radial direction from the horizontal axis of rotation of the needle shaft. The needle shaft may be oriented such that the axis of its rotation is shifted with respect to the axis of rotation of the extraction head, whereby the position-dependent distance a between the extraction needle and the inner wall can result in a sinusoidal functional behavior along the outer circumference of the extraction needle. To account for and / or mitigate the viscosity difference in the glass melt, it may be advantageous to position the needle shaft such that the axis of its rotation is shifted with respect to the axis of rotation of the extraction head. In one embodiment, the needle shaft can be oriented such that the axis of its rotation coincides with the axis of rotation of the extraction head.

[0019] In one embodiment of the extraction head having a distance c between the central axes of the glass outlet and the glass inlet, the ratio c / h can be 0.2 / 1 to 0.8 / 1. In one embodiment, the ratio c / h can be 0.4 / 1 to 0.6 / 1. In one embodiment of the extraction head with an inner diameter d of 200 mm or more, the ratio c / h can be 0.4 / 1 to 0.6 / 1. In one embodiment of the extraction head with an inner diameter d of less than 200 mm, the ratio c / h can be 0.2 / 1 to 0.4 / 1.

[0020] In one embodiment, the extraction needle can have a shaft diameter s of 40 to 70 mm.

[0021] In one embodiment, the inner wall of the withdrawal head that contacts the glass melt during use can be at least partially provided with a noble metal lining. In one embodiment, the noble metal lining can include platinum or a platinum alloy. In one embodiment, the inner wall of the withdrawal head can be provided entirely or at least partially with a noble metal lining in the surface area that contacts the glass melt. A sheet made of a platinum sheet or a platinum alloy is suitable as the noble metal lining for this purpose. On the one hand, this means prevents contact between the glass melt and the material of the inner wall of the withdrawal head, which is generally made of a heat-resistant material. On the other hand, contamination of the glass melt is thereby avoided. In addition, the life of the withdrawal head provided with the noble metal lining is extended.

[0022] In one embodiment, the noble metal lining can be provided with a connection for electric heating and can be directly heated using the current flowing through the noble metal lining. In one embodiment, the noble metal lining can be divided into segments that can be heated and controlled separately, bringing the following advantages.

[0023] 1. Heating can be divided into individual zones and thus can be controlled very precisely for each zone.

[0024] 2. Implementation of rapid temperature changes is possible.

[0025] 3. The temperature can change rapidly over a wide temperature range reaching several hundred degrees Celsius.

[0026] 4. Replacement of the withdrawal head or the withdrawal head tool is possible within a few hours.

[0027] In one embodiment, the withdrawal head can include only a single additional outlet opening in the form of a glass outlet for discharging the glass melt in addition to the glass inlet for receiving the glass melt. In other words, the said embodiment may lack any head overflow part (5).

[0028] The drawing head according to the present disclosure may be particularly preferred for processing aluminosilicate glass and borosilicate glass that melt at very high temperatures and thus require high labor and high costs. The drawing head according to the present disclosure can achieve an increase of about 1 / 5 to 1 / 4 of the amount of directly usable glass melt, which makes the process of producing glass tubes via the drawing process using the apparatus according to the present disclosure very economical.

[0029] In one embodiment of the process, the drawing head may further comprise a head overflow portion, and the amount of glass melt drawn through the head overflow portion is less than 10%, preferably less than 5%, of the amount of glass melt supplied to the drawing head through the glass inlet. In an alternative embodiment of the process, the amount of glass melt discharged through the glass outlet to draw the glass tube may be substantially equal to the amount of glass melt received through the glass inlet.

[0030] Advantageously, the process and apparatus enable the production of glass tubes without visible obvious streaks even when there is no head overflow portion or when the head overflow portion is not operating, so the process minimizes and / or avoids the loss of glass melt drawn through the head overflow portion.

[0031] In one embodiment of the process, the process temperature of the glass melt in the drawing head can be substantially set, preferably controlled, by the electrical heating of the noble metal lining of the drawing head.

[0032] Further advantages and embodiments of the present disclosure are described with reference to the drawings.

Brief Description of the Drawings

[0033]

Figure 1

Figure 2a

Figure 2b

Figure 3

Figure 4

Figure 5

[0034] Mode for Carrying Out the Invention Figure 1 is a scheme not drawn to scale of a conventional drawing head (1) known in the art, into which glass melt enters through a glass inlet (2) in the direction indicated by the arrow. The glass melt flows around a centrally disposed drawing needle (3), which flares conically at the lower end. In the region indicated by the dashed line, loop-shaped streaks (8) are formed. The glass exits the drawing head downward through an annular gap formed between an exit ring (6) and the drawing needle (3). The drawing needle (3) has a channel into which gas is injected, thus ensuring that the drawn glass tube remains hollow and that the emitted glass tube walls do not recombine. To prevent streaks from forming on the surface of the drawn glass tube, a head overflow portion (5) operates. As a result, an amount of about 15% - 25% of the supplied glass melt is withdrawn from the drawing head. Thus, this glass melt can no longer be directly used for glass tube production and, in the best scenario, can be remelted for subsequent use. In conventional Vello and down-draw processes, the amount of glass melt withdrawn through the head overflow portion is about 50 kg / hour.

[0035] Figures 2a and 2b respectively show a side view and a top view of a further drawing head known from the prior art. The glass inlet (entering from the left side in this figure) opens into a substantially bowl-shaped volume. The glass tube is drawn downward through the drawing needle (3), while the glass melt drawn through the head overflow part (5) to suppress streak formation is discharged to the right side (in this figure). The sketched drawing head known from the prior art has a substantially flat or bowl-shaped basic geometry and does not enable the production of a streak-free glass tube unless the head overflow part operates.

[0036] The formation and position of streaks (13) in the glass tube (10) formed in the absence of the operation of the head overflow part using the conventional process are shown in FIG. 3. The glass tube is surrounded by an outer surface (11) and an inner surface (12). The streaks extend from the inside of the glass tube to the outer surface (11), where they become visible to the naked eye as streaks on the glass tube surface. In some cases, the streaks (13) on the outer surface may also result in distinct irregularities. Both the visible streaks and the distinct streaks impair the quality of the manufactured product and lead to an increase in scrap.

[0037] FIG. 4 shows a drawing head (1) according to the present disclosure, which enables the production of a glass tube that is substantially or completely free of visually distinguishable or distinct streaks even in the absence of the operation of the head overflow part (5). Depending on the type of glass being processed and the production parameters, the head overflow part (5) may be completely omitted. The glass melt enters the drawing head (1) through the glass inlet (2) and surrounds the shaft of the drawing needle (3). Due to the specially adapted geometry of the drawing head (1), the flow conditions during the drawing of the glass tube are controlled such that only loop-shaped streaks in the glass melt result in streaks inside the drawn glass tube, and these streaks do not appear on the outer surface (11) or the inner surface (12) of the glass tube and are thus not visually distinguishable or distinct.

[0038] In Figure 4, the position of the surface of the glass melt in the drawing head is indicated by a dashed line.

[0039] The formation of the streak pattern in the cross-section of the glass tube (10) manufactured according to the teachings of the present disclosure is shown in Figure 5. The streaks are only visible on the inside of the tube and do not appear on the outer surface (11) or the inner surface (12) of the glass tube.

Explanation of symbols

[0040] 1 Drawing head 2 Glass inlet 3 Drawing needle 4 Glass outlet 5 Head overflow part 6 Outlet ring 7 Inner wall 8 Loop-shaped streak 10 Glass tube 11 Outer surface of the glass tube 12 Inner surface of the glass tube 13 Streak in the glass tube h Total height d Inner diameter s Shaft diameter c Distance between the central axis of the glass outlet and the glass inlet a Distance between the shaft and the inner wall

Claims

1. An apparatus for producing a glass tube comprising a drawing head (1) and a drawing needle (3), wherein the drawing head (1) comprises a glass inlet (2) and a glass outlet (4), the drawing head (1) has an overall height h and an inner diameter d, and the ratio h / d is from 2 / 1 to 7 / 1, and the distance a between the drawing needle (3) and the inner wall (7) of the drawing head (1) is in the range of 40 to 300 mm, Apparatus.

2. The apparatus according to claim 1, characterized in that the distance a is in the range of 50 to 150 mm.

3. The apparatus according to claim 1 or 2, characterized in that the ratio h / d is from 3 / 1 to 4.5 / 1.

4. The apparatus according to any one of claims 1 to 3, characterized in that it has a distance c between the central axis of the glass outlet (4) and the glass inlet (2), and the ratio c / h is from 0.2 / 1 to 0.8 / 1.

5. The apparatus according to claim 4, characterized in that the ratio c / h is from 0.4 / 1 to 0.6 / 1, or from 0.2 / 1 to 0.4 / 1.

6. The apparatus according to any one of claims 1 to 5, characterized in that the inner diameter d is greater than 130 mm, preferably the inner diameter d is 300 mm.

7. The apparatus according to any one of claims 1 to 6, characterized in that the inner wall (7) of the drawing head (1) that contacts the glass melt during use is at least partially provided with a noble metal lining.

8. The apparatus according to claim 7, characterized in that the noble metal lining contains platinum or a platinum alloy.

9. The apparatus according to claim 7 or 8, characterized in that the noble metal lining is provided with a connection for electrical heating.

10. The apparatus according to any one of claims 1 to 9, characterized in that the drawing head (1) comprises only a single further outlet opening in the form of the glass outlet (4) for discharging the glass melt in addition to the glass inlet (2) for receiving the glass melt.

11. A process for producing a glass tube via a drawing process, comprising the step of passing a glass melt through the apparatus according to any one of claims 1 to 10, wherein the process optionally includes a subsequent hot forming step.

12. The drawing head (1) further comprises a head overflow part (5), and the amount of the molten glass drawn through the head overflow part (5) is less than 10%, preferably less than 5%, of the amount of the molten glass supplied to the drawing head (1) through the glass inlet (2). The process according to claim 11, characterized in that.

13. The process according to claim 11, characterized in that the amount of the molten glass discharged through the glass outlet (4) for drawing the glass tube is substantially equal to the amount of the molten glass received through the glass inlet (2).

14. The process temperature of the molten glass in the drawing head (1) is substantially set, preferably controlled, by the electrical heating of the noble metal lining of the drawing head (1). The process according to any one of claims 11 to 13, characterized in that.

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