Question.
I am looking for a portable hardness tester for film rolls and all I find on Internet is for paper and it does not seem to be efficient for plastic film. Durometers are not very useful either. Does someone know a device that could give us a good hardness reading ?
Answer.
The hardness of rolls can be quite important. If a roll arrives too loosely wound it can easily telescope on being handled or during the initial vacuum pump down process. If the rolls are wound hard they can highlight any profile variations and so a soft wound roll may immediately raise suspicions of either poor handling or poor quality film. If there is any profile variation such as an area of over thickness then this will show up as a gauge band. If the thickness variation is large and the hardness too high this can stretch the film along the gauge band and put some permanent deformation into the film.
No matter what state of hardness the film arrives in by the time it has been re-wound in the vacuum system it will be a hard roll. This is because there is no air interleaving between the film layers as the film is re-wound. The hardness is in effect a measure of the quantity of air that is wound into the roll.
It is common to want to assess the roll hardness and often operators can be seen to knock the rolls with a knuckle to get a feel and listen to the sound as a method of testing the roll. In fact this method has been improved upon by use of a stick (Billy Club) to do the knocking. These methods are subjective and so can be variable from operator to operator and depend on their experience.
It is possible to purchase a simple test meter that standardises the same process. There are several products on the market that can be calibrated and give a reproducible measure of roll hardness. The method of testing can vary from having a spring loaded plunger that for a given force will press into the roll and a measure of the depth will give a reading of the hardness. Another test is to use a projectile, such as a ball bearing, that is dropped from a standard height and the height of the bounce is measured. The harder the roll the higher will be the bounce. This same principle can have the projectile mechanically fired so that the test can be carried out at any angle and not just vertically downward.
In the book by Good & Roisun (1) the following methods or instruments are described, the Billy Club and its Variants, RhoMeter and RhoHammer (used to be the Beloit RhoMeter), Backtender's Friend, Schmidt (Concrete) Hammer, Parotester and TAPIO RQP (2 – 5)
It is best to measure the roll several times across the whole width so that any variations are picked up. Some of the above meters now have the facility to store many data readings and to be able to transfer the data to computers to make recording the data easy.
One word of caution. Measuring the hardness whether by the knuckle or Billy stick or by automatic machines that bounce a projectile off the surface or press a probe into the surface, all can cause damage to the film. The indentation can penetrate more than 25 layers down into the roll. The harder the roll the greater the depth the damage can penetrate. This damage may only be of consequence to some of the more demanding products such as transparent conducting oxides or similar ceramic coatings as used in optical or electronic applications. Decorative metal coatings may be completely unaffected by this type of testing.
1. Good J.K.G & Roisum D.R. ‘Winding: Machines, mechanics & measurements’ Pub. DEStech Publications Inc. 2007
ISBN: 978-1-932078-69-5
2. www.testingmachines.com Schmidt Hammer
3. www.millassist.com/rhometer.htm Rhometer
4. www.tapiotechnologies.fi/paper_roll_hardness_more.html TAPIO RPQ
5. www.proceq.com PAROtester
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Posted by: lalit | April 15, 2007 at 03:08 PM