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.