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FLARE MAKING
© Philips Lighting B.V. (department I.S.G.T.) 2002 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of Philips Lighting B.V., dept. I.S.G.T. - the Netherlands.
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INFORMATION FOR THE MECHANICAL GLASS HANDLING COURSE
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-------------------------------------------------------------------------------------------------------------subject: Flare making.
contents: 1. 2. 3. 4. 5. 6. 7. 8. 9. General product requirements. Cracking off by thermal shock. Flanging. Cooling. Glazing. Perfect process adjustment. Hot cutting with two knives. Sulphur dioxide. Flare machine lay-out.
10. Process lay-out for thermal shock. 11. Process lay-out for hot cutting.
International Support group Glass Technology. Jan 2003.
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© Philips Lighting B.V. (department I.S.G.T.) 2002 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of Philips Lighting B.V., dept. I.S.G.T. - the Netherlands.
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1. GENERAL PRODUCT REQUIREMENTS.
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Dimensions: According to specifications: - length. - diameter. - depth (gauge with reference circle) or angle. Strength: - wall thickness T1 = T2. - correct strainpattern in the flare. - properly glazed. - strength to be checked with drop test and polariscope. Shape: - rim not overglazed: D in same as original inner diameter tube. - eccentricity and ovality: max. 3% of flange diameter. - flange: flat, round and not damaged. - rim round and straight.
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2. CRACKING OFF.
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Relative positioning of burner and scratching wheel. The scratch has to be made exactly in the zone of the maximum thermal strain. This means in the centre of the heated zone. This zone must be as small as possible. The crack will start from the scratch and will follow the highest strain.
If the scratch is not situated in the zone with the maximum strain, the crack will move from the initiating point e.g. the scratch, towards the zone with the highest strain. This causes stepping or waving of the crack. Specially ‘stepping’ may cause axial cracks in the stemtube, causing rejects during stemmaking. (cracked stemtube).
Although these types of crack may seem very small and sometimes can be closed again during glazing of the cutting rim, they must be avoided.
As a starting point for the adjustment of the correct burner height, the height level of the scratch can be compared to the height level of the burner. For this adjustment the machine has to driven by hand, from the scratching wheel position back to the burner position. In most cases this will not be the final adjustment, but it does give a good starting point for proper adjustment.
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Positioning of the scratching wheel: The scratching wheel must be positioned horizontally in all directions. The axis of the wheel holder must be directed towards the centre of the machine. The wheel must be well fixed and its bearing must be stable. Since we meet a number of tolerances, such as positioning of the heads, outer diameter of the glass tubes etc. the adjustment of the scratching wheel in radial direction of the machine is somewhat questionable with respect to the scratching depth. A compromise has to be found between on one side the combination of the smallest glass tube diameters and the heads positioned somewhat inwards to the machine and on the other side the biggest tube diameters and heads positioned somewhat outwards. Also the speed of touching has some influence. As a guide line, a scratch with a length of 2 to 4 mm is an indication of a correct adjustment. The spring force must be set so that all tubes will be scratched. It should not be so strong that mechanical damage (chips, cracks) are observed. The scratching wheel has to be sharp and grinded correctly. (see GLV 109-15-25 pages 310-312). Positioning of the burner and flame setting: The burner has to be positioned horizontally in all directions. The axis of the burner must be directed towards the centre of the machine. The flame has to be parallel with the axis of the burner (check for correct mounting and stacking of the plates, contamination and connection of gas and oxygen). Since the zone with strain has to be as small as possible, the flame has to be sharp and stable causing a concentrated heat in a limited zone. Flame spread must not be observed.
Scratching is only possible in solid glass. Consequently the temperature of the glass has to be below the setting point. (maximum temporary strain possible). The strain that is generated is compressive on the outside and tensile on the inside of the tube and will change into tensile outside after scratching and cooling.
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During heating however, there is also an effect of radial strain build up, which is tensile outside. The tube will expand in radial direction, but only very locally, which causes a ‘bending’ in the tube.
This effect can cause early cracking of the tube before it reaches the cooling/cracking position. Positioning of the burner in the radial direction must be so that the distance of the burner to the glass is sufficient for the formation of a well shaped flame. This is the reason for a minimum distance. In practice a distance of about 2 to 3 mm gives an optimal adjustment. The quality of the product decides the final adjustment. Air blowing in the cracking position: The air has to cool down the surface of the glass, thus reversing the compressive and the tensile strain in the glass. The strain which will cause the crack has to be developed as fast as possible and must be as high as possible. The air nozzle has to be directed over the centre of the position and pointing towards the centre of the machine. In horizontal sense the pipe must be directed towards the centre of the heated zone (and also the height of the scratch). Amount of air: the cooling action must be sufficient for the glass to crack before it is touched by the pin. The glass must not crack before it reaches the full position. If the crack develops too slowly, it will not be smooth. Cooling before the burner position: This cooling determines the temperature of the glass before the thermal shock process. Too little cooling will leave the inside of the tube too hot and will hinder the build up of temperature difference in and outside (temporary strain) needed for the crack off. Proper cooling enables a large temperature difference in and outside during heating. Excessive cooling might create unwanted strains over a big area of the glass tube, which can cause an irregular crack.
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Adjustments:
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3. FLANGING. Heat distribution: The lower part of the glass tube has to be deformed the most, while the upper part hardly needs any deformation. Consequently, the lower part has to be heated more than the upper part. For the correct burner positions see drawings.
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The upper reference line indicates the top level of the deformation. The upper part of the flame has to be positioned somewhat below this line. If the glass is heated too high, the cylindrical part of the tube will be deformed. Too much heat in the lower part will cause too much contraction (surface tension) of the glass. The opening will become too small for the reamer plate to enter the tube. In the third heating position the inner and outer wall temperature must be equal. Most of the heat must be brought into the glass in the first and second heating position. In the third heating position the outer wall temperature may be finally corrected and the inner wall temperature must be brought to the same value. This is so called stabilising of the temperature.
Sulphur dioxide is applied extra in the third and fourth position. In the third position the SO2 is dosed by means of the bottom burner, for lubrication and reinforcement (strengthening) of the glass surface. In the fourth, the flanging position, the SO2 is applied for lubrication of the reamer plate.
Flanging position: The reamer plate must spread the glass evenly, it should not push the glass. Pushing or spreading will influence the shape of the flange. Therefore the flare plate must be positioned as indicated in the drawings below. The plate can be adjusted in the horizontal and the vertical position. The vertical position determines the diameter and the depth of the flare both, the horizontal position determines the shape of the flare. Vertical adjustment can be done while the machine is running. The horizontal positioning must be done when the machine is stopped. For big flare diameters a reamer mechanism instead of a reamer plate is used. The reamer mechanism is based on a double movement: first it enters into the hot tube, then it moves outwards for the flanging movement.
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© Philips Lighting B.V. (department I.S.G.T.) 2002 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of Philips Lighting B.V., dept. I.S.G.T. - the Netherlands.
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4. COOLING. The influence of the cooling with respect to the strain: The transition from the soft state to the solid state may cause strain in the glass. Strains will develop when the glass is not cooled evenly in- and outside. (for more details see the chapter ‘glass properties’). Excessive cooling will cause a compressive strain in the outer layer of glass, while the inner layer will be under tensile strain. Normally glass is very strong under compressive strain and very weak under tensile strain. When the tensile strain is completely surrounded by a compressive strain, the tensile is not dangerous. A glass that is cooled from one side will have compressive strain on that side. When it is cooled from both sides, there will be a compressive strain in both cooled sides, enclosing the tensile strain. Since the flare is the most handled part of the lamp during production, the flare is exposed to mechanical damage. Therefore a flare should be resistant to mechanical damage. This is realised by cooling of the glass directly after flaring, thus creating a strain pattern as described above. This cooling, also known as tempering, is done with a flare cooler in position 6. see drawing. There are several different shapes of cooling blocks for different flare machines.
From the above it may be clear that the use of a ‘kisser’ in position 5 is dangerous. It may produce the desired flat shape of the flare, but it can also cool down the glass locally, thus
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creating an unwanted strain in the flare. Consequently the kisser should not touch the flare. The flange must be made in such a way that the kisser is obsolete.
The strength of a flare can be tested with a drop test. Regular checks will give an indication of the strength in time. Imperfections of the cooling will show lower values with the drop test. The dimensions of the weight determine the minimum and maximum drop height for the test.
5. GLAZING. The aim of the glazing is to flame polish the cutting rim to a smooth and rounded surface. This can be checked with the fingernail: a smooth and rounded surface without roughness or inequalities must be felt. The cylindrical part must remain straight. The rounding must be symmetrical. Check mechanical adjustments as: concentricity of burner and flare axis, distance burner to glass etc. For the strength as described above, it is important that the strainpattern in the flare is not changed during glazing. This can be checked with the polariscope.
6. PERFECT ADJUSTMENT FOR THERMAL SHOCK PROCESS. To bring a ‘reasonably running’ machine to a state of perfect adjustment it is recommended to do the following: Check the crack for waving or stepping, if present, check and readjust the burner height. The quality of the crack must be checked before glazing! Check the scratching wheel: Check if the wheel is fully horizontal and stable. Check also the sharpness and the spring tension. When the crack is still not perfect: check the flame setting. Heating a too wide area or heating to a too high temperature will cause bad cracks. Check the amount of air in the pre cooling positions and in the cooling in the cracking position.
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7. HOT CUTTING WITH TWO KNIVES.
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Some older flare machines are equipped with hot cutting with two knives in stead of thermal shock cutting. The principle can be compared with cutting with scissors. The positions 8, 9, 10 & 11 are used for heating, while in position 12 the actual cutting with two knives is done. 1. Heating. Heating is done with relative stiff, small flames to create a small heated ring in the tube. In position 11 the temperature is stabilised so that there is no temperature difference between inner and outer glasswall. The required temperature of the glass when it reaches position 12 is somewhat dependent on the sharpness of the knives but should be just under the American Softening Temperature. Cutting above A.S.P. will cause sticking immediately after cutting so the tube and flare will not separate. Cutting at Philips Softening Temperature (or below) will cause breakage of the glasstube. When small particles of glass are visible on top of the big knife, the temperature is slightly too low. 2. Cutting. Position 12 is equipped with two knives : a big knife on the outside and a small one inside the tube. The big knife rotates by friction when it touches the glasstube, the small knife is machine driven. The distance between big knife and glasstube should be approximately 1 mm. During the cutting motion both knives should touch the glass simultaneously. The horizontal overlap of the knives must be at least equal to the wallthickness of the glass. usually an overlap of 1-1.5 mm is maintained. The distance (vertical) between the knives should be as small as possible. (Think of the scissors: not touching, but also no gap in between). To realise a properly glazed finished product it is essential that the small knife pushes the hot glass slightly outwards, so that during glazing, due to surface tension, the tube does not contract too far.
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The heating in positions 8 to 11 must be done with stiff flames, to heat up only a narrow ring. Heating of a broad ring can cause too much deformation of the glass tube above and below the cutting area. Check the motion of the cutting wheels with a glass tube and by moving the flare machine by hand. Both the movement for the small as the big knife can be adjusted in the horizontal plane. The distance between the knives in vertical sense can be adjusted by raising or lowering the small knife.
8. SULPHUR DIOXIDE. SO2 is added to lubricate the contact between the flare plate and the glass tube. The SO2 is fed into the injector block in the gas line. A restriction fitted into the injector block prevents from overdosing the mixture which otherwise could extinguish the flame.
lubrication: In position 3 the lower burner adds some SO2 to the glass. At high temperatures sulphurdioxide can oxidate further to sulphur trioxide: SO3 (reaction: 2SO2 + O2 ⇐ 2SO3) SO3 and metal oxidise and make the salts, named sulphides. This happens by means of the positive metal ion and the negative SO4 ion. example: Na2O ⇐ 2NA+ + O-- and SO3 + O-- ⇐ SO4-example: 2NA+ + SO4-- ⇐ Na2SO4 Or the short-cut: Na2O + SO3 ⇐ Na2SO4 Similar reactions with Pb: PbO + SO3 ⇐ PbSO4 or with Kalium: K2O + SO3 ⇐ K2SO4 These are ledsulphide and kaliumsulphide, liquids that have a lubricating effect in the case of softglass. In hardglas it is Sodiumsulphate that is the lubricant. In position 4 the burners for the flare plate also add some SO2 on the metal, forming a metal sulphate on the plate, again for lubrication.
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Re-inforcement:
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The SO2 reacts a.o. with the Na of the glass. The Na is a softener, and when the reaction with SO2 takes place some of the Na of the glass surface is removed. This makes the outer layer of the glass slightly harder, thus creating a compressive layer on the outside of the glass. This increases the strength of the flare. See also the chapter ‘glass properties’.
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9. THE FLARE MACHINE LAY-OUT.
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Circular process lay-out: position Hot cutting Thermal shock heating heating heating flanging kisser or empty tempering/cooling length adjustment cooling cooling cooling heating scratching cooling/cracking
1 heating 2 heating 3 heating 4 flanging 5 kisser or empty 6 tempering/cooling 7 length adjustment 8 heating 9 heating 10 heating 11 heating index 12 cutting with 2 knives Mechanical adjustments: A: Height level small knife. B: Horizontal movement small knife. C: Horizontal movement big knife. D: Height level reamer plate. E: Length adjustment.
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1. schliff, showing compression-tension-compression.
2. Left: good strain pattern, right: tensile strain on the inner surface (not acceptable).
3. Top view, showing the four quadrants of the polariscope.
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