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?
Let’s 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 isn’t 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 doesn’t recommend any particular manufacturer but I’ve 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 – it’s simple. Isn’t 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 manufacturer’s data sheet for their hardness figure, which should always show the mean figure. Different manufacturers offer different hardness’s for standard products and as such can’t 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 – 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.
Everyone has an opinion about the Rubber Sheeting they buy and not simply that it’s too expensive or not as good as ‘in the old days’. Common questions we are asked in relation to Rubber Sheeting materials involve hardness queries, thickness tolerances and surface finish. The problem in trying to respond too many of these issues relate to variances in methodology, equipment available, individual interpretations of standards and quite simply custom and practice.
I thought it would be a good idea to ruminate through a blog on these issues and more importantly publish how MacLellan Rubber assess, measure and approve its own materials, with a view to these ruminations being a guideline for Storage; Hardness Measuring, Thickness Measuring and Handling, many of which relate to British and European Standards and where appropriate common sense.
I may not be right about everything but in publishing this and follow up articles we hope to clarify some of our customers concerns and remove some of the areas of conflict that can so easily arise.
The Customer is of course always right, it’s just that they are not always correct!
I thought we should start with one of the most common issues which impacts on quality concerns which is the Storage of Rubber Sheeting and Rubber Matting
The principal thing to remember is that we are dealing with a vulcanised rubber which is an elastic product and susceptible to changes in the physical properties if storage conditions vary. Areas which are readily affected by incorrect storage and handling are hardness, flexibility, surface cracking and deterioration. Long term implications of incorrect storage and handling may be a reduction in performance characteristics such as elongation, tensile strength and compression set
These are covered by standards such as BS3734, ISO 2230 amongst others but the main points to follow are:
Store in a Cool environment – Optimum temperature will be 10° C, but shouldn’t be below 0° C or exceed +30° C as the polymerisation within the material may start to break down. Where temperatures do drop below 0o C you should allow the material to warm slowly before commencing work on it.
Store away from Direct Sunlight – Exposure to UV will cause materials to harden and crack.
Material should be wrapped in an opaque, non-reactive packing material for best practice.
Store away from Heat Sources – Extreme Heat will cause material to soften and degrade.
Maintain Humidity - A relative humidity between 45 and 75% is optimum. Low humidity may cause material to harden and crack.
Store in a Relaxed Condition – Rubber Sheet may take on a compression set or deformation if stored incorrectly or under load. Ideally store standing on its end rather than lying on its side. Re-roll the material so that it is tightly coiled and retains some rigidity. If the material has to be laid on its side then avoid putting too much load on top which will crush the material and may effect some compression set.
Despite following these recommendations we need to remember that vulcanised Rubber Sheet still has a defined Shelf Life after which there can be no expectation that it will perform to anything like the stated characteristics such as elongation or tensile strength.
ISO 2230:2002 amongst other standards categorise materials into one of three groups to determine their initial shelf life when stored correctly and an extension period that can be agreed if the material is show to be in good condition. These are summarised accordingly:
For a full list of Polymers by Classification please refer to our website.
Note shelf life is dependent upon correct storage conditions and cannot be taken for granted. Incorrect storage conditions may reduce the shelf life by as much as 50%.
As a final point in this blog, I should say that all of the team at MacLellan Rubber are regularly updated on changes to standards that affect our business and are willing to provide advice and recommendations where appropriate.
I hope that this first instalment gives all of those with the time and inclination to read it something to consider, whether or not you agree with the statements made. If you do, we look forward to working with you. If you don’t I guess we will potentially have more discussions in the future.