ISO 3302-1 Tolerance

General

The dimensional tolerances stated in this part of ISO 3302 may be wider than those used in some other engineering practice. The following considerations apply:

a) 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 rubber type and the mix used, but also varies from batch to batch of the same mix.

Products made from some silicone rubbers, fluorocarbon elastomers and other special purpose elastomers are subject to larger shrinkages; therefore tolerance classes M1 and M2 (see 5.2) are very difficult to obtain with these rubbers.

b) Non-rubber parts bonded to the rubber will affect the shrinkage and, therefore, the practicable tolerances.

c) 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 become the moulds and their maintenance.

d) Care shall be taken in applying the standard tolerances to products having wide sectional variations.

e) 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.

Classification

This subclause establishes four classes of tolerance for fixed and closure dimensions (see 5.3) for products moulded in solid rubber, namely:

a) Class M1 for precision mouldings. Such mouldings require precision moulds, fewer cavities per mould, close mix controls, etc., which results in high cost. Optical comparators or other, similar, measuring devices may be required to minimize distortion of the rubber by the measuring instrument. This type of part requires expensive control and inspection procedures.

b) Class M2 for high-quality mouldings involving much of the close control required for class M1.

c) Class M3 for good-quality mouldings.

d) Class M4 for mouldings where dimensional control is non-critical.

Fixed dimensions and closure dimensions

In moulding a rubber product, more rubber is used than is required to fill the cavity, and the excess is flashed. This flash tends to prevent the mould sections from fully closing and thus affects the finished part dimensions.

NOTE 3 For products moulded by transfer or injection, it is possible to regard all dimensions as fixed.

Two sets of tolerances, F and C, are given and are defined below.

fixed dimensions (F): Dimensions which are not affected by deforming influences like flash thickness or lateral displacement of different mould parts

(upper and lower parts or cores). See figure 1, dimensions l 1 , l 2 and l 3 .

closure dimensions (C): Dimensions which can be altered by variation ·in the flash thickness or lateral displacement of different mould parts. See figure 1, dimensions d1, d2, d3 and h.

NOTE 4 The dimensions for F and C can only be tolerance insofar as they are independent of each other.

Tolerances

The tolerances to be applied shall be chosen, by agreement between the interested parties, from the classes of tolerance described in 5.2. Standard tolerances are given in table 1. Fixed tolerances (F) are related by size to each dimension. but all closure tolerances (C) are determined by the largest closure dimension (h, see figure 1).