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Whats Wrong with My Material? - Part 3 Why Does The Thickness Vary?

07 Aug 2015 | Posted by Andrew Onions

Or perhaps I should say that we are more commonly asked why isnt 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 regularly 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 havent the following images will give you some idea of the scale of these beasts

Two typical designs
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.
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.

Wrong Tool - Vernier

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.
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.

Right Tool - Caliper Gauge
Having established our preferred equipment the method of measuring should be consistent.
Material must be flat and relaxed before measuring any flex in the sheet will affect the reading
Measurements should be taken at least 50mm into the roll to gain a true reflection of the thickness - Measuring the edge will give an inaccurate reflection of the true thickness because rolls will always taper at the edge where pressure during curing is at its lowest. Trimming rolls should eliminate this but cannot be guaranteed in a world where maximising production volumes and yields is so important.
Use the same equipment consistently and service regularly
Measure material in an ambient room temperature. Extremes in temperature will affect the material and equipment and therefore the final reading
MacLellan Rubber manufactures in a number of locations across the globe and we work with those sites to ensure consistency and repeatability of quality, through investment and training.
We aim to ensure thickness tolerances are within the industry expectations through multiple checks during the production processes, and pre-despatch, however with in excess of 3,000,000kg of material moving through our premises in any twelve month period there will be some instances that even we miss. However where genuine issues are identified you can rely on MacLellan Rubber to replace product quickly and with minimum fuss and inconvenience to you.