Below are the most frequently asked questions linked to the answers:
- What is Toughened Glass?
- Why do we need toughened glass?
- How do we do it?
- What type of glass can we toughen?
- What are the different types of toughening machine available?
- Which type of toughening plant performs the best?
- What should you be looking for when buying a tempering system?
What is Toughened Glass?
Toughened or tempered glass as it is sometimes known is normal soda-lime silica glass, which has been subjected to a process involving controlled heating and cooling to introduce stresses into the glass. It has a much higher resistance to both temperature and mechanical force than normal annealed glass; typically it will withstand 4 times the mechanical stress and as much as 6 times the thermal stress.
Why do we need toughened glass?
Toughened glass is considered to be safe because when broken it rapidly disintegrates into very small particles which are unlikely to cause serious injury. Most countries now have laws that make the installation of safety glass mandatory in many areas for both new and replacement glazing in buildings. The same applies to glass used for furniture and interior fixings such as shower screens and partitions. Its comparatively light weight/strength ratio and its suitability for bolted fixings make it and ideal medium for structural roles, doors and screens. Glass is now available with coatings to reduce heat loss, restrict light transmission and even to reduce noise; these can all be successfully toughened and are being frequently specified by architects.
How do we do it?
The glass is heated to a uniform temperature throughout its body of around 640°C to 650°C, it is then passed to a cooling section where its surfaces are cooled rapidly, the centre of the glass is then cooled more slowly, this difference in the rates of cooling causes the surfaces of the glass to contract less than the centre. As a result of these differences stresses are introduced in the glass, compressive in the surface regions and tensile in the centre. In good quality toughened glass these stresses are likely to be in the region of 15000psi to 20000 psi (103N/mm² 138 N/mm²)
What type of glass can we toughen?
Almost all clear, patterned and tinted float glass can be toughened, coated glasses such as Pilkingtons “K” glass and the other insulating glasses can be toughened if like “K” they have a “Hard coat” if the coating is a “Soft coat” then it may be possible to toughen but the glass manufacturer should be consulted. Soft coats sometimes have a sacrificial topcoat applied specifically to allow toughening. If the glass has been painted or silk-screened then it can be toughened provided that the correct paints or ceramics are used. Both sand blasted and acid etched decorative glasses can also be toughened. Prior to the toughening process the edges of the glass will need to be either polished, bevelled, ground or arrised this is to prevent cracks from opening up during the cooling process. If this is not done then the glass can still be toughened but losses during the process will be very high possibly in excess of 25%. The glass can be supplied in a vast range of shapes and with holes and cut-outs however there are fixed rules for the position and sizes of these holes. Once toughened the glass cannot normally be cut, shaped or drilled.
What are the different types of toughening machine available?
Tougheners come in two main types either horizontal or vertical, equipment for producing “bent” can be either horizontal or vertical. Very few companies now manufacture vertical tougheners as the glass is suspended from tongs during the process, which causes marks in the top edge, and vertically toughened glass tends to be of worse optical quality then horizontally toughened. Horizontal tougheners transport the glass on rollers, in the heated section these are normally of a fused silica material. Horizontal tougheners generally use electric heating and large fans for cooling, gas heating is becoming popular in some countries and compressed air is occasionally used for cooling.
Which type of toughening plant performs the best?
In horizontal tougheners the process remains the same regardless of the type of heating and cooling used. The quality of the finished product is dependent on the quality of machine manufacture and operator training, major factors affecting quality are temperature control and the size, pitch and surface finish of the rollers. Provided that the equipment is well manufactured then there is no difference in the types of glass or coatings which can be processed, but thickness may be restricted, manufacturers may ask for a large premium for machines which can toughen 3mm and sometimes increase the price for machines capable of toughening the thicker glass sizes (> 15mm).
What should you be looking for when buying a tempering system?
Specification: Can the machine toughen your production sizes and will satisfy your capacity needs Is the maximum load size big enough for your products and will the roll pitch allow you to run your smallest lites. What is the optical quality of the finished product like; ask to visit existing customers to see this. Design: Is the machine of a low maintenance design? What happens when a lite breaks in the furnace can you carry on running or do you have to shut the machine down to prevent damage to the lower heating elements? How easy is it to clear a broken lite from the quench area? What is the incidence of major failures with this manufacturers equipment? Maintenance: What is the manufacturers average service response time and spares availability? Do they provide 24-hour 7-day cover, this is important if the machine stops at midday on Friday are you going to be out of production until the engineer arrives sometime the following week? How far has the engineer got to travel to reach you? In the event of a major failure such as a fan disintegrating how long will you be out of production, is this likely?
Remember it is important to always ask the opinions of a number of the manufacturers existing customers rather than relying on their salesman.
