Sheet thickness is often stated by the manufacturer or it can be measured using a micrometer, as using a ruler would be impractical for such small sizes. The thickness of thin sheet material such as plastic sheets needs to be known as it will influence properties such as the sheet stiffness – i.e. how easily the sheet can flex.

This is relevant to designers who need to specify plastic sheeting – for example in the sign industry. The correct thickness of a plastic cover sheet for a snap frame is different to that required for a hinged-door poster case or free-standing print holder. The former needs to be flexible, the latter needs to be rigid. Communicating the correct specification requires an understanding of potentially ambiguous terminology – for instance for US companies wishing to trade outside North America.

The Mils Muddle

In the metric system of measurement, such as used in the UK, the millimetre (mm) is a convenient small unit of measurement of sheet thickness. However for thicknesses below 1 mm it is often convenient to divide 1 mm into 1000 parts and these units are called microns. Thus 0.5 mm is 500 microns.

In USA ‘mil’ is a measurement equal to 1/1000th of an inch. Mil is the same as the old imperial unit Thou (one thousandth of an inch). Americans frequently state the plural version as mils. This is potentially very confusing to metric users as it sounds identical to the colloquial for millimetres – spoken as mils though always written as mm. The meaning changes as one crosses the Atlantic.

To give some examples applicable, for example, to flexible plastic cover sheets of many poster frames (such as snap frames):

  • 400 micron or 0.4 mm = 15 mils (to be precise 15.75 mils). Americans might also write.015″ thickness. This thickness would only be suitable for small poster frames.
  • 500 micron or 0.5 mm = 20 mils. In North America it might be written.020″ thickness. This is a very common thickness for a poster protector sheet for medium size poster frames such as snap frames. This sheet could be rolled up for convenient shipping if spare covers are required.
  • 1000 micron or 1mm = 39 mils (to be precise 39.37 mils). At this thickness plastic sheets are fairly rigid and could not be rolled up easily, so shipping single sheets becomes problematic.

Stiff plastic sheets

When the application calls for stiffer plastic sheet a typical thickness is 3mm (approx 1/8th inch). This would apply to the plastic ‘glazing’ used in the hinged door of a poster case or covered notice board. In this case the plastic functions like a window rather than a protective film. Plastic sheeting, such as acrylic or polycarbonate, with thickness 2mm or 3mm would also be specified for items such as free-standing plastic table-top print holders which need to be self-supporting.

Defining by weight per unit area

For thin sheets the weight of the sheet may be stated, rather than the thickness – in Europe stated as g/m2, usually abbreviated to gsm. Depending on the density of the particular material the g/m2 weight and micron thickness can coincidentally be quite close in number and may be confused.

Measurement systems which may appear back-to-front

Metals are also supplied in thin sheets and in this case the thickness is often referred to as the ‘gauge’. Gauge can be a little confusing to the uninitiated as it appears to be a ‘backwards’ scale. The higher the number the thinner the sheet. For example, 1.5 mm thick metal sheet is 16 gauge (16 SWG) whereas the much thinner 0.5 mm metal sheet is 24 SWG – SWG is the abbreviation for standard wire gauge, a long-established imperial unit of measure. It is not generally used for plastic sheets.

The ISO (International Standards Organisation) metric paper size nomenclature can also catch people out because it too can seem to be a backwards scale: A8 paper is very small, A4 is closest to US letter size, and A0 is used for large posters. Most European users will be very familiar with A sizes whilst being baffled by the term ‘letter size’. In US and Canada the reverse may be true.

By appreciating these points confusion over measurement units can be reduced or eliminated in transatlantic trade relating to thin plastic sheet materials.

Source by Nigel Spelman

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