GB2139615A - Forming holes in vitreous sheets - Google Patents

Abstract

A hole may be formed in a vitreous sheet 1 by fracturing the vitreous material, e.g. using a laser, along a closed path and removing vitreous material 11 circumscribed by that path from the surround 13 to leave a hole in the sheet. After fracturing the vitreous material, the circumscribed plug 11 is placed in contact with a heat sink 26 at low temperature so that the plug 11 contracts to afford clearance between it and the surround 13 to permit removal. The heat sink may be cooled with carbon dioxide, liquid nitrogen, technical grade neon or other coolants depending on the temperature desired. <IMAGE>

Description

SPECIFICATION Forming holes in vitreous sheets This invention relates to a method of forming a hole in a vitreous sheet by fracturing the vitrequs material along a closed path and removing vitreous material circumscribed by that path to leave a hole in said sheet.

Vitreous sheets with holes in them are required for various purposes. One such purpose is as supports for information recording layers for example as specified in co-pending application No. 8307590 (publication No. GB A).

The forming of holes in vitreous sheets presents problems not encountered in other vitreous sheet severing operations. For example sheet glass can be cut into smaller sheets by scoring and flexing about the scored lines. Such flexing is in general not possible or not effective when forming a hole in a sheet. Again, a disc may be cut from a sheet by scoring and breaking away the surrounding material as waste. This is clearly not a possible course of action when the surrounding material constitutes the required product. It would of course be possible to make a hole in a glass sheet by severing and breaking away the glass from the area of the hole, but only if the hole was sufficiently large for this to be done without breaking the surround. But even this involves high risk of initiating fractures in the surround.Circular holes of smaller diameter have hitherto required to be formed by a grinding operation, for example using a diamond core drill which grinds away vitreous material to leave sufficient clearance for a core of the vitreous material to be removed from the surround: but this is time consuming.

It is an object of the present invention to provide a more efficient method of forming a hole in a vitreous sheet.

According to the present invention, there is provided a method of forming a hole in a vitreous sheet by fracturing the vitreous material along a closed path and removing vitreous material circumscribed by that path to leave a hole in said sheet, characterised in that the hole is formed in the vitreous material by fracturing it along said path whereafter the circumscribed plug of material is placed in contact with a heat sink at low temperature whereby the plug is caused to contract to afford clearance between it and its surround to permit removal.

The present invention provides an efficient and relatively simple way of forming a hole in a vitreous sheet.

It is surprising that cooling a plug in that way will give sufficient clearance for its removal since it is clear that in the absence of heating the surround, there will be a maximum temperature difference of less than 300K between the plug and the surround.

In order to provide maximum cooling of the plug, it is preferred that both sides of said plug of circumscribed material are placed in contact with a said heat sink.

It will be apparent that the clearance afforded by cooling a plug of circumscribed material in a method in accordance with the present invention will depend on the temperature difference between the plug and its surround, the size of the plug and the coefficient of expansion of the vitreous material. In preferred embodiments of the invention, the or each heat sink is maintained at a temperature not exceeding 220 K. This can readily be achieved by cooling the or each heat sink by carbon dioxide as is provided in some preferred embodiments of the invention. The sublimation temperature of carbon dioxide at 1 atmosphere is 194.5K.Using carbon dioxide as coolant in the forming of holes in ordinary soda-lime glass sheets, plugs having a diameter of 50 mm have been separated from their surrounds, the plugs being formed by scoring to effect pressure fracturing of the glass and being cooled from one side only.

In some preferred embodiments of the invention, the or each heat sink is maintained at a temperature not exceeding 100K. Various coolants can be used to achieve such low temperatures. Among them may be cited liquid air, oxygen (90K), argon (87K), carbon monoxide (82K) and nitrogen (77K), the approximate temperatures given being those at which the respective fluid has a vapour pressure of 1 atmosphere. Of these coolants, oxygen is highly active, argon is expensive and carbon monoxide is toxic. It is accordingly preferred that the or each such heat sink is cooled by liquid nitrogen. Using liquid nitrogen as coolant, plugs of 40 mm diameter formed by pressure-fracturing soda-lime glass sheets have been separated from their surrounds by cooling from one side only.

If it is desired to maintain a said heat sink at an even lower temperature, for example below 50K or even below 30K, then neon, hydrogen or helium may be used. Hydrogen however forms an explosive mixture with air so its use is not recommended despite its cost advantages in relation to neon and helium.

The approximate temperatures at which these materials have a vapour pressure of 1 atmosphere are neon 27K, hydrogen 20K, and helium 4K. If it is desired to cool to such low temperatures it is recommended to use technical grade neon which is a 75% neon, 25% helium mixture commercially available as a by-product of the distillation of oxygen and nitrogen from liquid air.

In the most preferred embodiments of the invention, tehe vitreous material is fractured along said closed path by means of laser radiation. Laser fracturing gives considerable advantages. In particular, laser irradiation enables the formation of a very clean fracture which is perpendicular to the plane of the sheet. Thus less clearance between a plug and its surround is required in order to separate them. Also the use of laser radiation enables complicated shaped holes to be cut.It is especially preferred to use a laser severing method as specified in either or both of copending application Nos 8313288 and 8313287 respectively filed under agents ret- erences 263/Bermuda and 262/Variser and published under Nos and In some preferred embodiments of the invention, the surround is heated, preferably to a temperature not exceeding 200C while the plug is cooled. This increases the temperature difference and thus the clearance between the plug and its surround. Such heating can readily be effected by resting the sheet on a heated table.

The invention is especially suitable for forming concentric circular holes in circular sheets, and it extends to a vitreous sheet having a hole made by a method as herein defined.

A preferred embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 illustrates apperatus for forming a closed fracture path in a vitreous sheet, and Figure 2 illustrates heat sink apparatus for cooling a vitreous plug circumscribed by such fracture path.

In Fig. 1 a vitreous sheet 1 for example in the form of a disc is located on a turntable 2 having an axle 3 mounted in bearing 4 for rotation about an axis 5 in order to form a circular fracture in the sheet whose centre lies on that axis 5. A laser light source 6 emits a laser beam 7. which is focused by a system of plane 8 and concave 9, 10 mirrors onto the vitreous sheet 1 a desired distance from the axis 5 of the turntable 2. On rotation of the turntable 2, the laser beam 7 will scan a circle on the sheet 1 whose radius is equal to that desired distance to fracture the sheet and form a cylindrical plug 11 (Fig. 2).

If desired the laser beam 7 may be directed through a screen (not shown) to modify its cross-section so that it has one or two apexes which or each of which points along the desired path of fracture. Such an arrangement forms the subject of British Patent Application No. 8313288 (BERMUDA) referred to above.

Alternatively or in addition, a central portion 1 2 of the top surface of the turntable 2 may be provided with a reflective surface so that any laser radiation penetrating the vitreous sheet is reflected back to the sheet to irradiate registering volume increments on the desired path of fracture from opposite sides. Such an arrangement forms part of the subject of British Patent Application No. 8313287 (VAR ISER) referred to above.

As a preferred optional feature, the reflective surface of the central portion 12 may be shaped so as to refocus emergent laser radiation back within the thickness of the sheet 1.

In an alternative arrangement, the laser beam is caused to scan a stationary sheet to effect fracture.

In order to effect removal of the plug 11 from its surround 1 3 of the vitreous sheet 1, the plug is contacted with a heat sink generally indicated at 14 as shown in Fig. 2.

The heat sink 14 comprises a metal reservoir 1 5 for coolant fluid 1 6 which surrounds a metal core 1 7 in thermal contact with the base 18 of the reservoir. The reservoir 1 5 has a roof 19 and is enclosed in thermal insulation 20. An inlet 21 is provided for the supply of coolant liquid and a vent 22 is provided for escape of coolant vapour which can be recycled in known manner if desired.

A threaded hole 23 is provided in the base 18 of the reservoir 1 5 beneath the core 1 7 for receipt of a stud 24 of a contact member 25 provided with a boss 26. This contact member is suitably of bronze or brass in view of the properties of those materials at low temperatures and in view of their high thermal conductivity. The boss 26 is-thus in good thermal contact with the coolant fluid 1 6. The contact member 25 is detachable so that contact members having different sized bosses 26 can be used for the cooling of different sized plugs 11 of vitreous material.

In use, the boss 26 of the heat sink 14 presses the plug 11 of the vitreous sheet 1 against an anvil 27. The boss 26 is marginally smaller in area than the plug 11 and the anvil 27 is also smaller in area than the plug 11 so that on contraction of the plug 11, the surround 1 3 of the vitreous sheet 1 can fall or be pressed down onto a layer 28 of soft material, such as felt, on the anvil support 29.

If desired, the anvil 27 may be constituted as a boss of a second heat sink. Such a heat sink could be of broadly similar construction to that shown at 14 in Figure 2. For example of the heat sink 14 and its coolant inlet and outlet openings 21, 22 appropriately relocated it could be inverted so that the boss 26 constituted an anvil 27.

It will be noted in the drawing that the anvil 27 is part conical in form to provide clearance for the surround 1 3 as it descends. Such anvil could alternatively be cylindrical.

As an alternative, the glass is rested on a support. having a hole through which the plug is pushed by the boss 26. Such a support may optionally be heated, preferably to a temperature of less than 200"C so that the surround 1 3 is heated while the plug 11 is cooled.

Claims (10)

1. A method of forming a hole in a vitreous sheet by fracturing the vitreous material along a closed path and removing vitreous material circumscribed by that path to leave a hole in said sheet, characterised in that the hole is formed in the vitreous material by fracturing it along said path whereafter the circumscribed plug of material is placed in contact with a heat sink at low temperature whereby the plug is caused to contract to afford clearance between it and its surround to permit removal.

2. A method according to claim 1, wherein both sides of said plug of circumscribed material are placed in contact with a said heat sink.

3. A method according to claim 1 or 2, wherein the or each heat sink is maintained at a temperature not exceeding 220K.

4. A method according to claim 3 wherein the or each heat sink is cooled by carbon dioxide.

5. A method according to claim 3, wherein the or each heat sink is maintained at a temperature not exceeding 100K.

6. A method according to claim 5, wherein the or each heat sink is cooled by liquid nitrogen.

7. A method according to any preceding claim, wherein the vitreous material is fractured along said closed path by means of laser radiation.

8. A method according to any preceding claim, wherein the surround is heated, preferably to a temperature not exceeding 200"C, while the plug is cooled.

9. A method according to any preceding claim, wherein said sheet and hole are circular and concentric.

10. A vitreous sheet having a hole made by a method according to any preceding claim.