Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – With measuring – controlling – sensing – programming – timing,...
Reexamination Certificate
2000-08-02
2003-02-11
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
With measuring, controlling, sensing, programming, timing,...
C065S382000, C065S393000, C065S404000, C065S489000, C065S491000, C065S435000, C065S381000
Reexamination Certificate
active
06516636
ABSTRACT:
This invention concerns a method for the manufacture of a tube made of a vitreous material, especially of quartz glass, in which a hollow cylindrical semifinished product made of a vitreous material is carried essentially vertically to a heating zone, wherein it is heated and drawn off downwards—without the use of tools—to the tube by forming a transitional area from semifinished product to tube, while diameter and wall thickness of the tube are continuously measured, and the tube's measured geometrical data being used to generate a control signal with the aid of which a pressure difference is regulated between pressure P
1
in the interior space of the semifinished product, the transitional area and the tube, as well as pressure P
2
in the heating chamber which is regulated in the heating zone at least in the transitional area from semifinished product to tube and its adjacent tube area.
A method, as specified above, is known from EP-A1 394 640. For the manufacture of tubes, quartz glass or highly siliceous glasses in the form of a hollow cylinder as semifinished products are drawn off to a tube in a vertical method, where the pressure in the tube's inside space is higher than the outside pressure, i.e. that pressure which is acting upon the outside of the semifinished product and of the tube in the heating zone. With this method, only such tubes can be manufactured in which the ratio of the tube's outside diameter to its inside diameter is at most equivalent to the ratio of outside diameter to inside diameter of the highly cylindrical semifinished product; however, tubes are generally obtained with the known method in which the ratio of the tube's outside diameter to inside diameter is smaller than the ratio of outside diameter to inside diameter of the semifinished product.
Furthermore, the manufacture of capillary tubes is known from EP-A1 259 877, i.e. of such tubes having a very small inside diameter. Moreover, the outside diameter of these capillary tubes is typically up to 5 times greater than the inside diameter. The manufacture of such capillary tubes is done by arranging several glass tubes of different diameters into one another, with the individual inside and outside diameters of these glass tubes being adjusted to each other. After these glass tubes are fit into one another, the arrangement is heated and the individual tubes are molten with each other by drawing into longitudinal direction. However, this procedure means that elaborate stockkeeping of glass tubes of varying inside and outside diameters is required. Moreover, the coaxial arrangement requires a support of the tubes on both ends during the drawing and melting with high precision. Furthermore, the individual, prefabricated glass tubes which are then fit into one another must be manufactured very precisely in their geometrical dimensions to ensure especially a uniform combination of the individual tubes.
Starting from the above specified state of the art, this invention is based on the task of developing the initially specified method further so as to enable the processing of a broad spectrum of hollow cylindrical semifinished products to tubes with the desired inside and outside dimensions.
The above task is solved, starting from the initially specified method, such that—for the manufacture of tubes with the ratio of their outside diameter (D
Ra
) to their inside diameter (D
Ri
) being greater than the ratio of the outside diameter (D
Ha
) to the inside diameter (D
Hi
) of the semifinished product—pressure P
1
in the inside space of the semifinished product, the transitional area and the tube is being kept at a value which is smaller than the value of pressure P
2
in the heating chamber, with pressure P
1
being maintained by means of a suction pump.
Due to the defined settings of pressure P
1
on the inside of the semifinished product or tube, respectively, and that of pressure P
2
on their outside and regulation by means of a nominal/actual adjustment, it is possible to adjust—over a broad range—the ratio of the outside diameter to the inside diameter of the tube to be manufactured. In particular, the method according to the invention also allows the manufacture of tubes in which the ratio of wall thickness to outside diameter is greater than in the semifinished product. The possible application of available semifinished products for tube manufacturing is considerably expanded thereby, and the flexibility and efficiency of existing tube drawing facilities is thus increased.
For an adjustment of pressure P
1
, it is preferable—during manufacture—to keep that end of the tube closed which is facing away from the semifinished product. Pressure P
1
on the tube's inside will then be adjusted and kept at a precalculated value by means of a suction pump and by supplying a flow of gas which is passed into the inside of the semifinished product, the transitional area and the tube. Thus, a defined negative pressure can be maintained in the tube and defined in adjustment.
It has been shown that a tube quality can be greatly influenced by the pressure variations on the inside of the tube, even if they are only minor. To counter such pressure variations, another embodiment of the method has the flow of gas lead—prior to its introduction into the semifinished product—first to a vessel serving as buffer, being inserted between suction pump and the tube's inside space.
Furthermore, it proved advantageous to introduce an inert gas into the inside space of the semifinished product and the tube. Inert gas offers the advantage that oxidations are avoided in case of contact with hot parts, possibly even with hot furnace parts.
Especially when pressure P
1
in the inside space of the tube and the semifinished product is kept at a value which is 5 mbar to 200 mbar smaller than the value of pressure P
2
in the heating chamber, semifinished products can then be produced in which the ratio of semifinished product outside diameter (D
Ha
) to semifinished product inside diameter (D
Hi
) is within a range of between 1.5 and 4.5, thus in a very wide range.
The method is designed very simply when pressure P
2
in the heating chamber has approximately atmospheric pressure.
During the manufacture of the tube from the semifinished product, it is possible to exercise an influence on the inside and outside diameter of the tube to be manufactured—such that, if there is a deviation of the ratio D
Ra
to D
Ri
to a specified nominal value—a control signal is produced and, with it, a valve upstream of the suction pump is being controlled and closed so long until the nominal value is reached again. The valve will again be opened after the nominal value is reached. This control action is permanently repeated to keep the tube to be manufactured within specified nominal value limits with regard to its inside and outside diameter. Here, the diameter (outside and inside diameter) of the tube is advantageously measured directly, seen in the tube drawing direction, after the transitional area, thus still in the heating chamber; and these geometric data are used for producing the control signal for regulating the pressure difference.
It has been shown that the specified method is especially suitable for the manufacture of tubes in which the ratio (D
Ra
/D
Ri
) of tube outside diameter to tube inside diameter is greater than 1.5; moreover, this ratio (D
Ra
/D
Ri
) should preferably be within the range of between 1.8 and 5.0. Moreover, the method can be optimized for tubes in which the ratio D
Ra
to D
Ri
is greater than 1.5 when a semifinished product is used whose ratio of semifinished product outside diameter D
Ha
to semifinished product inside diameter D
Hi
is within the range of between 1.5 and 4.5.
The method according to the invention is particularly suitable for the manufacture of capillary tubes having a small inside diameter and high breaking strength since tubes with relatively thick walls can be produced in one process step.
In particular, the method is also used for the manufacture of a tube for th
Bogdahn Thomas
Gänsicke Frank
Ganz Oliver
Hain Harald
Humbach Oliver
Heraeus Quarzglas GmbH & Co. KG
Hoffmann John
Tiajoloff Andrew L.
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