This is the second in the series and here I thought we should deal with another common and perhaps slightly more technical issue when evaluating Rubber Sheet materials, and answer the question:
Why do I always get variations in the Hardness of my material?
Letís start with the basics - you need the right equipment for the job. Pushing your nail into a material or trying to compare one piece of rubber with another by pulling and twisting just isnít going to work.
There are a variety of manual and electronic Durometer devices available on the market. Manual, hand held devices tend to be most common as they are easy to use and are relatively low cost.
Hand held digital hardness meters were increasingly popular a number of years ago but most of the main manufacturers have since ceased supply due to the increased level of inaccuracies which became apparent in their use.
Electronic devices can sometimes accommodate testing for a range of products including plastics and metals.
MacLellan Rubber doesnít recommend any particular manufacturer but Iíve included images of typical products below.
Equipment should be recalibrated frequently. Most manual units are spring loaded so performance will deteriorate over time. Most equipment is now supplied with reference blocks to enable regular recalibration.
So what about the mechanics of measuring Ė itís simple. Isnít it?
The first thing to bear in mind when measuring hardness is that there is a general tolerance for many standards which typically allows -4 and +5 Degrees on the stated figure. Therefore it is perfectly acceptable for a 60 Shore A material to measure 56 or 65 Shore A and still be within tolerance.
There are different Shore Hardness scales used with rubber, specifically Shore A and IRHD, which many people assume are interchangeable but are not. MacLellan works to Shore A as detailed on our data sheets. If you require any information on the various hardness scales used in polymers you can Ask George on our web site.
Any measurements need to be taken in the right conditions. If it is too cold the material may measure harder, to hot and it may measure softer. Our experience is that this variation can be as much as 4 degrees, which is enough for someone to think the material is out of specification.
We recommend an ambient temperature of between 15 and 20oC. Additionally the material should be flat, any curvature will affect the reading either but reducing the surface area upon which the Durometer is working or by increasing / decreasing the pressure on the indentation needle.
It is important to realise that most hardness meters will only work accurately with a minimum material thickness of 6mm. If your material is thinner then you should ply this up to gain a true reading.
Also a true hardness reading for a material is determined at a point no less than 13mm in from the edge of the material- you would not believe the number of hardness queries we get against rolls that have never been unwrapped.
You should refer to the manufacturerís data sheet for their hardness figure, which should always show the mean figure. Different manufacturers offer different hardnessís for standard products and as such canít always be compared. Typically you may have one manufacture quoting 70 Shore A as their mean hardness +5 / -4, and another manufacture offering a product with a mean hardness of 65 +5 / -4. In theory both could be considered as a 70 Shore A material but in fact you have a potential range of 61 Ė 75 Shore A which may not be acceptable to your customer.
Lastly the method of testing, typically ASTM D2240 or BS903 A26, requires a short pause between the initial indentation reading and achieving the actual reading. This pause period allows up to thirty seconds for the material to relax against the indentation needle. You may see that your Hardness Meter drops as much as 5 degrees in this period to give a true and accurate reading.
We also need to remember that vulcanised polymers are mixed and cured in large quantities and only batch tested. There will unquestionably be variations in physical characteristics within a single production batch simply down to this fact. It is therefore entirely consistent for the hardness of a roll of rubber to vary within the top and bottom limits across its surface and even from one side to another and consequently is not faulty. Some standards allow three reading to be taken within a defined area and the average considered as the hardness figure.
Below I included a summary of the various hardness scales and what materials they can be applied to and below that a number of the more widely recognised hardness test standards which you may come across.