POLYCARBONATE SCREENS AND INSPECTION WINDOWS

POLYCARBONATE SCREENS AND INSPECTION WINDOWS

Poorly designed screens and inspection windows represent one of the main critical points in machinery guard systems.

In cases in which there is a risk of ejecting metal particles, as typically in the case of fast turning machines, machinery should be fitted with inspection screens in polycarbonate coupled with glass sheets which are resistant to the impact with the metal particle ejected from the machine during the production cycle.


Best Practice references for the design of on-board inspection windows in polycarbonate coupled with glass sheets. This technical in-depth analysis aims at providing useful guidelines for the designing of screens and inspection windows for fast turning machines and more in general machines where the risk of particle ejection is present.

Recent research carried out by HSE (Health & Safety Executive UK) have shown that all polycarbonate materials generally employed for the manufacturing of inspection windows on board of machinery (the most popular brands are LEXAN by GE and MAKROLON by Bayer) may lose the minimum mechanical qualities needed to resist to the impact with metal particles ejected during the production cycle particularly in presence of liquids generally employed for the mechanical working process.

Recent research carried out by HSE (Health & Safety Executive UK) have shown that all polycarbonate materials generally employed for the manufacturing of inspection windows on board of machinery (the most popular brands are LEXAN by GE and MAKROLON by Bayer) may lose the minimum mechanical qualities needed to resist to the impact with metal particles ejected during the production cycle particularly in presence of liquids generally employed for the mechanical working process.

The chart below shows the typical decay curves of the mechanical qualities along the years as a consequence of exposition to aggressive liquids in comparison to situations in which the working area is clean or the panels are protected. Variations in decay speed are connected to the exposition factor which can be total or partial in cases in which the polycarbonate panels are protected on one side only or if the protection is damaged in some points or if it is not sealed.


Considering the findings and the consequent severity risk factor, we believe issues regarding design should be reconsidered along with a revision of the main concepts involved in estimating the screen's maximum surface, the polycarbonates' thickness, the guard system and its maintenance in the case of turning machines.

First of all an assessment of the ejection risk should be carried out, therefore the following characteristics should be taken into account:

a) mandrel turning speed during a specific production cycle
b) mandrel types and weights
c) mandrels range of action
d) force employed to block the worked part
e) if the part is blocked from the outside or the inside
f) maintenance status of the mandrel: lubrication, cleaning etc.
g) mandrel balancing
h) magnitude of the forces applied in the working process

Once assessed all the above, an evaluation of the worst possible scenario should be carried out by estimating the ejected particle weight, the action range calculated from the centre of the turning device in relation to its speed, bearing in mind that an increase in weight implies a proportional increase in energy, the increase in action range or speed implies more than proportional increases, which means that a smaller object at a greater speed is more dangerous than a greater object at a lower speed.


Ejection energy in Kj


When using this table to calculate the ejection energy, you must consider that the effective weight varies the proportional result, that the action range at the ejection point is normally considered as 25% bigger than the effective mandrel action range and that the maximum rotation speed must consider the possibility of a damage to the rotation limiting device, that the final figure should be rounded off in excess.

Once the ejection energy has been calculated, you need to define the materials to be employed in the production of the window. A general option is to go for polycarbonates, glass or coupled materials.

Glass thickness generally between 4 and 8 mm, even if strengthened, is not considered suitable to stop a particle ejected at a high speed

Polycarbonate thickness generally between 4 and 20 mm, has a high resistance factor but undergoes envsteelmental decay if used over a long period of time

Coupled material made of two glass sheets and an inner polycarbonate sheet with a thickness between 9 and 30 mm,if appropriately sealed, represents the state of the art because the external glass and the sealing preserve the polycarbonate mechanical qualities in time, in calculating the window thickness you need to consider only the polycarbonate thickness without the glass sheets.

To conclude, in order to determine the appropriate polycarbonate thickness which better responds to the machinery protection, we have devised a table which considers the mechanic properties supplied by the manufacturers, the ejection energy calculated thanks to table 1 above and the impact surface (print mark), in those cases in which the polycarbonate screen is not protected i.e. no sealed coupling has been applied, we have estimated the decay period.


tabella spessori policarbonato

Machinery operator manuals should report non-protected polycarbonate screen maintenance periods (and substitutions) and evaluate a safety coefficient by increasing the thickness of the screen originally employed.

This study was carried out by Access after a great number of reports by some customers regarding the decay status of guards fitted with polycarbonate screens. Access then modified its guards engineering principle in order to make it possible to substitute the polycarbonate screens at the expiry date reported in the technical data sheets.



Access now fits screens and inspection windows on a steel sheet panel which has been laser cut and the thickness of which has been conventionally fixed as a third of that established for the polycarbonate screen (such an estimate has been derived from the typical properties of steel and carbonium S235JR). The screen is then fixed by mechanic blocks placed inside the guard fencing which can be removed for substitution.



Access' LEXAN panelsrespect the modular features of all our range of products and can be fixed with our patented CE certified Meta® fixing systems.
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