Rubber tolerances need to be considered when designing products; it is essential to consider the characteristics of elastomeric materials.
It is a fact that all elastomers exhibit some level of shrinkage, which will vary not only depending on the specific material, but can also be affected by the tool design, moulding method, geometry of the part, and hardness of the compound, to mention just some of the major influences. Notwithstanding this, it is entirely possible to manufacture high precision, repeatable components if this is what is required, however this may come at a considerable cost, so it is very important to establish whether any critical features are identified and acceptably controlled. For a more detailed explanation of Shrinkage, see our Blog on the topic.
Rubber tolerances are quite different to those of many other materials, and our experienced technical staff are always ready to provide guidance, and interpret the implications of BS 3734 (ISO 3302:1995) for Tolerances of Rubber Mouldings or Extrusions. Should a tolerance band better than M2 or E2 be required, please contact us early in the design phase. As a general rule, clearly the most cost effective solution is to open out the tolerances to the widest acceptable degree whilst maintaining fitness for purpose, but this important step is often overlooked, typically when designers do not have previous experience of working with elastomeric components, and make assumptions based on general engineering best practice.
The method of manufacture of any rubber product will have an influence on the tolerances that can be achieved, and to assist in good design practice we have a general guide here. This indicates the typical range of tolerances that can be expected when moulding, cutting or machining rubber and allied products, however, we always recommend that designers should discuss tolerances at an early stage with our Technical Staff so that the specific requirements of any particular application can be well understood, and the most appropriate solution applied.
Dimensional measurement of rubber products is always a challenge when fine tolerances are required, and as such Martin’s often use a non-contact Shadowgraph for assessing the detail required, and First Article Inspection Reports are routinely completed using this technique when required.
It is also important to consider the Storage Conditions for rubber products as this will affect both shelf life and dimensional stability, for guidance please review the general conditions laid out in BS ISO 2230:2002 in conjunction with our technical staff.
Hardness tolerance of rubber products is assessed using a ball indent type Dead Load hardness meter, measuring International Rubber Hardness Degrees (which broadly correlates to Shore A°) on standard sample plaques, which are required to be 6mm thick in order to conform to the requirements of the test. Therefore it is always a challenge to try and assess the hardness of a product itself, which may not have a suitable “standard” area to measure, and here our hand held hardness meters will provide an indication of hardness. However the best way to meet this need is to mould a plaque of material specifically for testing, from the same batch of material as is used for the products in question, and if required, this can be accommodated in our processes. For a wider discussion on rubber hardness, see our Blog on this topic.