Overview
Bending Skills
General
Unlike other glass fabricating processes such as glass cutting, glass edging, glass bevelling, glass drilling, glass washing and glass insulating, glass bending requires more operating technique during bending process. A good glass cutting machine can score glass properly with high precision. A good edging and a good beveling machine can make good edge and bevel finishes with proper tools. A good drilling machine can make a good hole and chamfer with proper drilling bits. A good glass washing machine can produce clean and dry glass. A good insulating glass can make good insulated glass with proper sealants, spacer bar and desiccant. These machines require very few or even nil processing technology. These machines do everything. Operators require no skill but only glass loading and unloading. Good quality of laminated glass requires a good laminating machine, good autoclave and qualified interlayer films as well, for example PVB and EVA. Glass tempering requires only little skill to make good temper glass. A good tempering furnace plays more important role in making good temper glass.
However, glass bending oven is another story. Other than a well design and fabricated bending oven, processing technique and proper design of bending mould plays important roles in making good quality of bend glass especially glass with deep bending depth and small radius of curvature such as L and U shapes glass.
Ovens
A well design glass bending oven should include, but not least, advantages,
- Selected Heating Resistors Materials
- Correct Heating Power Of Each Heating Resistor
- Proper Position & Distribution Of Heating Resistors Across Oven
- Individual Control of Each Heating Resistor
- Individual Control of Each Heating Zones
- Proper Heating & Annealing Control
- Well Thermal Insulation & Heat Preservation
- Steady & Easy Wagon or Cart Movement
Moulds
Well mould designs would be helpful in making good quality of bend glass. For example, a counter weight three sections mould may be required for deep bend glass with small radius of curvature. How the mould is divided? Where is the hinge? How heavy is the counter weight? They are experiences learnt by trail and error. A well design mould produces good quality of bend glass with certain thickness. It may not produce same good quality of glass with different thickness of glass. Another example is should the mold be designed as upward bend or downward bend? Well design moulds and some little tricks result in
- Fast Bending & Forming
- Glass Free Of Mould Marks
- Compatible For Different Glass Thicknesses
- Easy Fabricating
- Mould Material Available Everywhere
- Low Distortion During Frequent High Temperature Heating
- Simple Fabricating Tools
- Low Mould Materials & Fabricating Investment
- Less Mould Maintenance
- Long Lasting Moulds
Process
Heating Profile
Bend glass heating profile is a curve of temperature versus time. Which heating resistors should be turned on at a certain time? Which heating resistors should be turned off at a certain time? Technology know how is required to define these data for a certain type of bend glass. When all data are found out and connected together, it becomes a curve which is known as heating, temperature or straightforward, bending profile.
When glass and bending mould are put inside heating furnace, all heating resistors are usually turned on. Glass is heated and when it becomes soft, it slowly falls down and approaches to bending mold by means of gravity. Operator begins to turn off proper heating resistors with respect to glass curvature until glass edges get in contact around the rim of bending mould. Middle heating resistors are usually turned off first if it is an ordinary bent glass. Side heating resistors are usually turned off next one after one until all heating resistors are shut down. If curvature of bent glass is symmetric, two corresponding symmetric side heating resistors are usually turned off simultaneously. Heating resistors are usually shut down before corresponding glass section falls down and gets in contact with bending mold. This is because heat inertia keeps softening the glass even though heating resistors turned off. Too much heat inertia will create sag in the middle of bend glass. However, automobile glass designers may design windshields with certain sag depth. In this case, sag is necessary and a technique is required to make sure all windshields have the same sag depth.
Automatic Bending
The heating profile of a new type of bend glass is always set up manually even though the oven is design for automatic bending. When the heating profile is set, the profile parameter is stored as a file in the memory of oven computer. When same type of glass is bent, the parameter file is re-loaded from the computer and the heating profile is repeated without any interference manually. This is so called automatic bending. Automatic glass bending is very helpful for bending of a batch of glass with same curve, for example, windshield. If heating profile of every piece of bend glass is different or glass batches with same curve is very rare, careful consideration is required whether it is worthy to invest in automatic glass bending system.
Single Curvature Bending
Do not under estimate glass bending skill for a piece of bend glass with slight bend single curvature. Glass curve rim approximately 150 mm away from straight edge is sometimes found not in perfect contact with bending mould especially for thick glass. This is because heating resistors are not properly distributed across the oven. Heat in concerned area is not enough to "soft" the glass. Glass should be loaded such that curving curvature should cross heating resistors.
Deep Bending
When bending of glass with large bending depth and small radius of curvature, heating resistors may be required to lower down close to bending wings providing more heat. Sometimes, bottom heaters are also required for auxiliary heating.
Angle Bending
It is very easy to have mould mark at bending angle. Extra heating resistors putting close to bend angle are required. Advance skill is a must to produce angle bending glass with all pieces of batch identical.
Mixed Bending
In order to save energy and increase productivity, it may attempt to heat and bend several small pieces of bend glass inside a big oven within the same batch. Glass may be with different thickness, curvature, bending depth. Some glass may be over cooked while some may not be bent enough.
Mould Marks
Glass becomes soft at softening point during bending. Soft glass easily gets scratched when it is sliding on mould during bending. Fiber glass or chalk is usually applied on mould to prevent scratching during bending.
Glass Surface Defects
Sometimes unusual higher temperature is required for certain bending. However, too high temperature may result in glass surface defects. Proper technique is required to maintain the glass at high temperature and meanwhile, free of glass defect.
Breaking during Bending
During bending, two side of glass push towards middle part. When inward force is larger than middle resist force, glass breaks. Adequate skill could overcome.
Breaking during Annealing
If annealing is too fast, glass surface becomes cool while glass core remains hot. Glass will break. Annealing should be done gradually especially for thick glass. Proper oven annealing design and annealing skill would shorten annealing time while remaining glass unbroken.
