Thermally molded elastomers are subject to the following factors:
- Cure time
- Mold tolerance
- Mold registration
- Compound variation
Shrinkage occurs on all elastomers depending on the following:
- Tool design
In order to account for the variables that are part of the manufacturing process, it is necessary to have tolerances defined to achieve the level of precision required. Numerous organizations around the world have developed tolerance designations for use in development and processing of elastomer products.
The International Organization of Standardization is a worldwide federation of national standards bodies. ISO 3302-1 specifies classes of dimensional tolerances and their values for moulded, extruded, and calendared solid rubber products. The relevant test methods necessary for the establishment of compliance with ISO 3302-1 are also specified.
The tolerances are primarily intended for use with vulcanized rubber but can also be suitable for products made of thermoplastic rubbers.
There are four tolerance classes ranging from M1 = fine to M4 = coarse. Depending on the press direction of the mould, differences are also made between dimensions bound to the mould and the two component adhesion system measurements bound to the mould closure.
The dimensional tolerances stated in this part of ISO 3302 may be wider than those used in some other engineering practices. The following considerations apply:
- All rubber shows some shrinkage when cooled after moulding, and allowance for this is made in the mould design. The amount of shrinkage is dependent on the elastomer type and the mix used, but also varies from batch to batch of the same mix. Products made from some silicone compounds, fluorocarbon elastomers, and other special purpose elastomers are subject to larger shrinkages; therefore tolerance classes M1 and M2 are very difficult to obtain with these rubbers.
- Non-rubber parts bonded to the elastomer will affect the shrinkage and, therefore, the practicable tolerances.
- Moulds are made in various ways depending on the type of product and accuracy demanded. In general, the product can be no more accurate than the mould, and the greater the degree of accuracy demanded, the more expensive the moulds and their maintenance.
- Care shall be taken in applying the standard tolerances to products having wide sectional variations.
- In cases where the rubber product is unavoidably distorted during removal from the mould, the dimensions of the products may be affected, and special allowance may be needed.
The tolerance classes are defined as:
- M1 for precision mouldings. Such mouldings require precision moulds, fewer cavities per mould, close mix controls, which results in high cost. Optical comparators or other measuring devices may be required to minimize distortion of the compound by the measuring instrument. This type of part requires expensive control and inspection procedures.
- M2 for high-quality mouldings involving much of the close control required for class M1.
- M3 for good-quality mouldings.
- M4 for mouldings where dimensional control is non-critical.
ISO 3302 specifies the following geometrical tolerances for moulded and extruded solid rubber products, including those with metal inserts:
- Flatness Tolerance
- Parallelism Tolerance
- Perpendicularity Tolerance
- Coaxiality Tolerance
- Positional Tolerance
The tolerances are primarily intended for use with vulcanized rubber, but may also be suitable for products made of thermoplastic rubbers.