|The Production of Bullet Resistant Glass – Part 2|
|The second article in our technical series about bullet resistant glass (BRG) discusses the various complications encountered when entering this sphere of the safety glass industry. The production processes are in some ways very similar to that of manufacturing normal automotive windscreens equally, they share some of the same processing problems. This article will focus on the key aspects of BRG manufacturing, or more specifically the key problematic areas.|
Laminated automotive safety glass is usually in the order of 6mm in thickness and has a degree of flexibility which assists in its ability to conform to a final shape tolerance specification, lift from fixture etc. However, a fully laminated BRG windscreen has “zero” flexibility and therefore has to be absolutely correct. At this point it should be pointed out that incorrectly manufactured BRG “is very expensive scrap”. Attention to detail cannot be over stated and is the primary difference between a good manufacturer and a bad manufacturer.
It is critical to establish the correct glass shape data which may be via a drawing or sample from which the “flattened shapes” can be established; this information is usually digitised and used for both cutting and edge working.
A BRG windscreen can be designed to fit within the existing vehicle body opening, therefore it is essential to establish all the correct sizes at the beginning of the exercise. Arrissed edge working is simply not good enough. Therefore all glass edges should be either flat ground with appropriate chamfering or the application of a pencil edge finish. All glass sections that make up the laminate should be fully inspected for size and finish prior to being further processed.
All further pre-processing operations are in many respects similar to those used for the production of standard auto glass.
Picture 1 passed BRG application after shot test
This represents the most difficult and technically challenging aspect of the manufacturing processes related to curved BRG, and is undoubtedly, the most important part of the process. The object of the process is to bend glass to a final shape that is of the correct size, shape, cross curvature and is optically acceptable. Heating and bending thick glass packs to a required shape and then annealing and cooling the pack without breakages and deformations is an essential part of the craft and it requires know-how, as well as quality mould tooling.
Picture 2 bend BRG windshield in visual inspection
A well trained and experienced furnace operator is undoubtedly a great asset to your company, but they need good tooling to define the correct and final glass shape. It’s true to say that the tooling is very special and that the final Picture 2 bend BRG windshield in visual inspection product is a total reflection of the tools shape, quality, and the actual applied bending processes. The engineering of great bending tooling is obviously not going to be discussed at length within this article however; we can supply this expertise as part of our technical package.
LAMINATION AND AUTOCLAVING
After the completion of the bending process, the glasses are separated and properly cleaned in order to remove all traces of the interleaving powders. At this point the glasses are thoroughly inspected for size, shape, optical quality and any other processing defects – if all is acceptable then the glasses are washed in a suitable liquid and passed on to the lamination room.
The glass and all other requirements/components are assembled in the correct order and relative positions, trimming etc. is all based on tried and proven lamination techniques. Vacuum bagging systems should be of the very best quality and again applied with tried and proven techniques. When the vacuum has been fully established a suitable electronic leak detector should be used to check for any leaks in the bag or connections.
After a suitable period of vacuum de-airing the assemblies can be moved to the autoclave. Autoclave processing programmes are fairly well documented and advised by the lamination film manufacturers however; individual laminators usually establish their own set of parameters that suit their equipment and product requirements.
After the assemblies have been carefully removed from the vacuum bags they should be visually inspected for bubbles, cracks or any other surface or internal damage/defects as per normal inspection routines. The assembly should then be subjected to optical distortion test procedures and all other required windscreen test procedures as per normal windscreens.
Picture 3 BRG windshield zebra board inspection for optical quality
As a final note, the light transmission of BRG can be significantly reduced by the overall thickness of the laminated section, I therefore recommend that this subject be addressed and a minimum light transmission value agreed upon with the client. This value can be adjusted with the use of different glass types etc.
Each of the individual manufacturing processes contain significant degrees of difficulty and any failures within these processes will contribute towards sub standard products which, can be extremely detrimental to the manufacturers reputation as a competent supplier of BRG products. But most importantly, the most serious result of poorly manufactured BRG products is the risk of exposing the end user to an unacceptable degree of "life threatening component failure".
Safety Glass Experts can assist with any BRG product development program, legislative test standard approvals, production start ups in either new or existing plants and any other client requirements including training programs. Safety Glass Experts can significantly reduce BRG project time scales which, enables our clients to rapidly meet their production and marketing ambitions.
For more information visit www.sge.fi or contact:
Safety Glass Experts International Oy Ltd
Talviseisaus 2 D 8
FI-20400 Turku Finland
Phone: +358 400 979 300
Fax: +358 2 6518 2539
PICTURE SOURCE: Besel Glass Co - Armour Protected Glass
All graphics, photographs, and text appearing in this article belong to Safety Glass Experts International Oy Ltd. Redistribution or commercial use is prohibited without express written permission.
Photos: Besel Glass Co - Armour Protected Glass
Last review: May, 2011
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