Tolerances

Tolerances used for moulding in the rubber industry

The following is an extract from the introduction to the British Standard BS3734 - Dimensional Tolerances of Solid Moulded and Extruded Rubber.
The dimensional tolerances stated in this International Standard may be wider than those used in some other engineering practices. The following considerations apply:

• All rubber shows some shrinkage after moulding,and allowance for this is made in the mould design. The amount of shrinkage is dependent on the rubber and mix used, but also varies from batch to batch of the same mix. Products made from silicone rubbers, fluorocarbon rubbers and polyacrylates are subject to large shrinkage; therefore, classes M1 and M2 tolerances [sub--clause 1.1] are very difficult to obtain.
• Non-rubber parts bonded to the rubber will affect the shrinkage and therefore, the practicable tolerances.
• Moulds are made in various ways depending on the type of product and the accuracy demanded. In general, the product can be no more accurate than the mould, and the greater degree of accuracy demanded, the more expensive become the moulds and their maintenance.
• Care should 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.

Section One of this international standard establishes four classes of tolerance for fixed and closure dimension [Sec..3:1 and 3:2] for products moulded in solid rubber, namely:

CLASS M1 for precision mouldings

CLASS M2 for high quality mouldings involving much of the close control required for Class M1

CLASS M3 for good quality mouldings

CLASS M4 for mouldings where dimensional control is non-critical.

• Extruded rubber products require greater tolerances in manufacture than those produced by moulding since, after being forced through a die, swelling of the rubber takes place, and during subsequent vulcanisation shrinkage and deformation usually occur.
• Deformation can be reduced by the use of supports during vulcanisation,the nature of the support depending on the section being produced and the degree of control required. These characteristics control the class of tolerance applicable to given dimensions.
• When particular physical properties are required in the product, it may not always be possible to provide these in a compound which is capable of extrusion to close tolerances and it is advisable, in these circumstances, that consultation should take place between the interested parties. In general, the softer vulcanisation [i.e.of hardness below 50 IRHD] need greater tolerances than the harder ones.
• The tolerances chosen for use in any particular application should take into account the dimensional changes occurring after extrusion and vulcanisation; these will depend on the nature of the rubber and mix used.
• In the case of certain synthetic rubbers, extrusion class El tolerances [table 2] are not directly obtainable.
• The closer tolerance classes should not be demanded unless the application requires it and should be restricted only to those dimensions which are critical. The greater the degree of accuracy demanded, the closer the control which must be exercised during manufacturing, and hence the dearer the product becomes.
• The standard specifies the dimensional tolerances of solid moulded and extruded rubber products.

The standard establishes eleven classes of tolerance for extrusions in solid rubber related to particular ranges of dimensions, namely:

1.2.1.1 Three classes of tolerances on nominal cross-sectional dimensions of unsupported extrusions:
E1 = high quality, E2 = good quality, E3 = non-critical

1.2.1.2 Three classes of tolerance on nominal cross-sectional dimensions of mandrel- supported extrusions:
EM1 = precision, EM2 = high quality, EM3 = good quality

1.2.1.3 Two classes of tolerance on outside dimensions [normally diameters ] of surface--ground extrusions [ tubing ] together with two classes of tolerance on wall thickness of these extrusions:
EG 1 AND EW1 = precision, EG2 AND EW2 = good quality

1.2.1.4 Three classes of tolerance for the cut length of extrusions, and three classes of tolerance on the thickness of cut sections of extrusions:
L1 and EC1 = precision, L2 and EC2 = good quality, L3 and EC3 = non-critical

It also prescribes the relevant conditions for the measurement of dimensions: