This is the long awaited third 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 does the thickness of my Rubber Sheeting vary?
Or perhaps I should say that we are more commonly asked why isn't the thickness exactly what I want it to be?
This can perhaps be the most challenging area for any manufacturer and customer to agree upon. It certainly seems to be an area for concern and disagreement.
As a starting point in this discourse we should state that there are various standards relating to thickness tolerances in rubber which are quoted to MacLellan Rubber, most of which relate to moulded or extruded products and are therefore not transferable to calendared rubber sheeting.
Some of you may have visited a rubber sheeting manufacturing plant and witnessed the large, room sized, calendaring and rotocuring machines in operation, if you haven't the following images will give you some idea of the scale of these beasts.
Processing rubber sheeting is a dirty, heavy business using big bulky machinery. Adjusting these machines to run different thickness is not a simple task and requires skill and judgement by the trained operators, notwithstanding this all of our manufacturing partners have in-line measuring equipment to ensure production tolerances are maintained across the full width of the roll throughout a production run. These will record and if necessary automatically adjust the pressure being applied by the drum to the material being processed to keep it within tolerance.
Although machinery is regularly maintained there can be localised variances in pressure and profile of the heated drum within the rotacure and as such thickness can vary between top and bottom tolerances across the width and length of a roll. It is therefore essential to measure across the entire width of a roll to ascertain a true value and not simply measure the edge.
Manufacturers offer tolerances on their rubber sheet as a general guide, however these will vary between manufacturers and in many cases can be put down to the quality of the production processes and is generally reflected in the final cost.
Using the right Tools!
Key to the issue of thickness is the measuring equipment used. There is a general assumption that digital veniers are the tool of choice to measure the thickness of rubber sheeting, this is not the case.
The specified tool for thickness measurement is a Major Caliper Gauge. This tool is specified for several reasons, the main one being that the operator cannot influence the reading by applying to much or too little pressure. A Major Caliper Gauge also offers a wider surface reading area and with a deep jawed gauge it can measure further into a roll.
A Digital Vernier can only measure the edge of the roll or piece of material. It also requires the user to apply pressure to achieve a reading and cannot therefore provide a true and accurate measurement of the material thickness.
Having established our preferred equipment the method of measuring should be consistent.