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Advances in the manufacturing of toughened laminates
  
The last three years have seen an increase in demand for toughened laminates with several high profile projects requiring both the structural strength and integrity that only a toughened laminate can provide.
 
Mr. John Gilleeney, Technical Director - Manufacturing of the South African company AGI takes an interesting approach to toughened laminates in his Case Study. My case study covers two projects using toughened laminate glass. The first project was for balustrades on the Nelson Mandela Bridge in Johannesburg, and the second project was for the gorilla enclosure at the National Zoological Gardens in Pretoria, he explains.
   
The first step for the successful manufacture of toughened laminates is to cut and process the glass as accurately as possible. This is especially important when the toughened laminate panel has holes, as was the case with the glass panels used on the Nelson Mandela Bridge: Any deviation in the position of the holes, results in the dilemma whether to line up the edges or the holes, and often a compromise has to be made. Good processing also reduces furnace losses, which is important when trying to produce matched sets.
  
Step 2 is to toughen the glass as flat as possible. Some deviation from absolute flatness is inevitable, hence the need to produce matched sets so that the glasses can be nested together.
  
Some of the factors affecting the direction or degree of roller wave are common knowledge, e.g. glass orientation through the furnace, furnace settings, glass type and substance, and the influence of glass coatings. We learned from our recent experiences that there are other factors which, while being obvious, are not always taken into account. For example, the loading position of glass on the furnace bed plays a role. My presentation will discuss these other factors and how we significantly reduced our losses during the production of the Mandela Bridge laminates.
  
The third step is the assembly of the laminate. AGI found that it was preferable to use a single layer of PVB. For this reason, where possible, we have changed from using two layers of 0.76mm to a single 1.14mm Butacite interlayer.
  
For de-airing AGI use either a nip roll or a vacuum bag process. The nip roll has a lower manufacturing cost but a higher risk of producing a reject. The higher manufacturing cost of vacuum bagging is justified for multi-laminates or where the laminate does not have a symmetrical construction. AGI used nip roll de-airing for the Mandela Bridge, while for the gorilla enclosure they provided matched sets of 4 x 10mm toughened glass panels to Rene Turck and Associates, a company that specialises in producing special laminates by a vacuum bag de-airing process. They produced 20 large panels (3000mm x 1100mm) without any losses.
  
The autoclave cycle also plays an important role and we have benefited from putting a step into our cycle, which maintains a lower pressure until a pre-determined temperature has been reached. Then the cycle continues to reach the maximum temperature and pressure. This has reduced the incidence of edge bubbles due to blow in.