GB/T 3672.1-2025Tolerances for rubber products - Part 1: Dimensional tolerances (English PDF)

4.1 General rules

For solid products, dimensional measurements shall be taken 16 hours after vulcanization. In cases of dispute, this minimum time may be extended to 72 hours. The buyer's measurements shall be completed, within three months of the date of shipment or before the product is put into use (whichever is earlier); it shall be performed at standard temperature after adjustment according to ISO 23529.Care shall be taken to ensure that the product is not subject to the adverse storage conditions specified in ISO

2230 and that the product does not deform during measurement.

4.2 Measuring instruments

4.2.1 Measurements shall be performed using one or more of the following types of instruments, as appropriate.

4.2.1.1 Solid products can be measured using a graduated micrometer. According to

ISO 23529 and ISO 48-2, the measuring foot of the micrometer shall apply a pressure of 22 kPa ± 5 kPa for rubber with a hardness greater than or equal to 35 IRHD, or 10 kPa ± 2 kPa for rubber with a hardness less than 35 IRHD.

4.2.1.2 Suitable optical measuring instruments.

4.2.1.3 Fixed gauges with upper and lower limits adapted to the dimension to be measured.

4.2.1.4 Other measuring instruments, including tape measures (with or without vernier), vernier calipers, micrometers.

4.2.2 The measurement error of all instruments shall be within the specified tolerance range.

4.2.3 The same measuring instrument shall be used in all measurements intended for comparison.

6.1 General principles

The dimensional tolerances described in this document are wider than those used in some other engineering practices because of the following considerations.

a) All rubbers shrink to some extent after molding and cooling; therefore, this shall be taken into account in mold design. The amount of shrinkage depends on the type of rubber and the compound used; even for the same compound, different batches will have different shrinkage rates. Silicone rubber, fluoro-rubber, other specialty rubber products have higher shrinkage rates, making it difficult to meet

M1 and M2 grade (see 6.2) tolerance requirements using these rubbers.

b) Non-rubber components bonded to the rubber can affect shrinkage, thus impacting the applicable tolerances.

c) Molds are manufactured using different methods depending on the type of product and the required precision. Generally, the precision of the product is not higher than that of the mold; the higher the required precision, the more expensive the mold and other maintenance costs.

d) Caution shall be exercised when using standard tolerances for products with large cross-sectional variations.

e) Rubber products inevitably deform during demolding, which may affect the product's dimensions. Special allowances must be considered in such cases.

6.2 Classification

The following four tolerance grades are specified for the fixed and closed dimensions (see 6.3) of solid molded products.

a) M1 grade. Applicable to precision molded products. These products require precision molds, fewer cavities, tight control of the rubber compound, resulting in high costs. Optical measuring instruments or other similar measuring instruments are needed to minimize deformation of the rubber caused by measuring instruments. These components require precise control and inspection procedures.

b) M2 grade. Applicable to high-quality molded products, incorporating many of the tight controls required by M1.

c) M3 grade. Suitable for molded products of good quality.

d) M4 grade. Suitable for molded products with less stringent dimensional control.

See 6.5 for flash classification.

6.3 Fixed dimensions and mold closure dimensions

When molding rubber products, more rubber is used than is needed to fill the mold cavity; this excess rubber overflows and becomes flash. This flash often hinders the complete closure of mold components, thus affecting the dimensions of the product.

Note. For products molded using the transfer or injection molding method, all dimensions may be considered fixed dimensions.

The following two sets of definitions for fixed dimensions (F) and mold closure dimensions (C) are given.

6.3.1 Fixed dimensions (F). Dimensions unaffected by flash thickness or deformation

caused by lateral displacement of different mold components (upper mold, lower mold, or mold core). See dimensions l1, l2, l3 in Figure 1.

6.3.2 Mold closure dimensions (C). Dimensions that change due to variations in flash

thickness or lateral displacement of different mold components. See dimensions d1, d2, d3, h in Figure 1.

Note. Tolerances for dimensions F and C can only be determined if they are independent of each other.

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