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Scribe and break method for glass cutting is widely used to separate each Liquid Crystal Displays (LCD) panel from a large mother substrate cell. The scribe-break method does not use water or heat, generates little dust and is an excellent method for preventing product contamination. Scribing is key to the glass cutting process, a median crack created by the scribing without lateral cracks is needed for clear cutting of the mother cell. Several items affect scribing, such as scribe wheel diameter, scribing wheel angle, scribing load, scribing speed. Also scribing conditions depend of course on the used glass type and its scribability. Manual and fully automatic versions are available.
This picture shows an automatic in-line scribe and break machine from Joyo Engineering. For smaller glass plates, manual machines for scribing and for braeking are available.
The upper side of the panel is scribed first. Every display on the glass plate is scribed along both sides, close to each main seal. On the other side of the scribe line is a dummy seal located in order to keep the two waste glass pieces together. The glass plate is now rotated 90° and the remaining scribes are done. The second step is to turn the glass plates and hit the lower glass plate with a rubber breaker. This will break the scribed upper glass plate. Then the scribe procedure for the lower glass plate is performed. The fourth step is the breaking of the lower glass plate after the glass package has been turned over a second time.
On a fully automatic system, all stations of the machine are equipped with pattern recognition systems in order to find the right position for scribe and brake.
The last step is the edge grinding of the displays. From there the displays are then loaded to cassettes and/or forwarded to the LC fill machine.
Optimum scribing condition is due to characteristics of material,
thermal history, construction, scribing machine etc. The condition must be carefully
decided by the test of actual glasses used in production.
When the scribing wheel is run over the glass plates, several kinds of damages happen to the glass surface, which influence the behaviour of the glass during the breaking process. You get a damaged zone, a kind of grove, cut by the wheel. Below you will find medial cracks. They are important for a clear break line of the glass and you can find in the glass so called Wallner lines after the break, which should exhibit uniform curvature when you watch fracture surface after the break.
It should be avoided by the scribe conditions to get chipping and lateral cracks. Lateral cracks continue growing by themselves after scribing and therefore the break step should follow immediately the scribe step.
In order to get a clear break line, the scribe is done as close as possible to the main seal. A dummy sealing is always placed on the other side of the scribe line in order to hold the two remaining glass plates together. Spotted seals are sufficient as a dummy seal.
The cutting angle of the scribe affects strongly medial and lateral cracking. The recommended scribe wheel angels depend also on the glass plate thickness:
The higher the cutting pressure is, the deeper are the vertical cracks created. The following graphics shows the optimum conditions in scribe wheel angle and cutting pressure. The scribe condition must be decided according to test runs.
Cutter depression force is one of the keys to get good scribe.
(Joyo scriber adjust it by air balance system at the pressure of 0.02 MPa ~ 0.2 MPa).
The optimum pressure is decided to achieve both minimum necessary vertical crack depth and minimum chipping.
The maximum vertical crack depth attainable is up to 100~150µm, but the optimum vertical crack depth is considered to be 10 ~ 20% of the glass thickness in general.
The cutter speed also influences the quality of the scribe line. The normal speed range is 200 ~ 300mm/sec. The cutting speed at the edge must be lower and in the range of 10 ~ 20 mm/sec.
The actual cutter penetration is about 10µm and does not vary much. The mechanism to follow an undulation of the glass surface is necessary. If it is too shallow, the scribe line will not be made continuously. If it is too deep, the edge will break. The Joyo machine gives adjustment 0.05mm ~ 0.2mm to enable optimum condition.
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