Special Features

In milling processes, parts may hit the pane at very high speeds, which can be compared with the speed of bullet from a rifle. Suddenly, there will be parts moving at speeds of 100 m/s to 200 m/s. Since the parts have a mass of 100 g, the impact energy will be much higher thatn the energy of a bullet from a handgun with only 5 g. For this reason, glass is inappropriate for use as a protective material.
Additionally, the forces affecting the fasteners are much lower than in turning machines, since the mass is lower. As a result, design versions with a smaller frame support area (15 mm will be sufficient) are used. A larger support area will most certainly be favourable, since the work piece may break during the milling process under certain circumstances, e.g. due to process errors. A large part of the work piece may then be accelerated to higher speed and may hit the pane. Such part may have a mass of not only 100 g, but the same mass as a work piece. Such work piece may have a mass of 1 – 5 kg. Only the support of the polycarbonate pane will be able to avoid penetration, since a bent pane in expanded condition may be pulled through the window recess.

With milling machines, the aging factor does not have such high importance than with turning machines, since the parts that may hit the pane have a mass of only 100 g, and not of 2.5 kg.
Additionally, the polycarbonate pane, as it is also used in milling machines, does not have time to respond, as it has in a turning machine with speeds of only 60 – 80 m/s. Due to speeds of 180 m/s, the material does not have time to expand. The impact will compress and remove the material. If the occurring force exceeds the actual resistance of the pane, it will cause a hole. It is a basic difference that the deformation of the previously highly expandable material does not have such high importance in milling. This means that failure of a pane is not as predictable as in turning machines. For this reason, the limits specified by EN 12417 must not be exceeded.
The indicator foil makes it evident that the aging process of the polycarbonate starts when the foil is damaged. A 2-year replacement period of polycarbonate is specified. Tests have shown that its resistance will start to decrease already after one year. Experts have agreed on imposing a mandatory replacement period of 2 years. The indicator foil will increase this interval to five years, or one year after the beginning of the disintegration process of the foil. If its transparency should have decreased, it should already be replaced before the end of such period. If the foil starts to disintegrate at a very early stage, the machine operator should fall back on the solution consisting of polycarbonate and glass with stainless steel frame (the classical SEGE window).