ISO 20457:2018

INTERNATIONAL ISO
STANDARD 20457
First edition
2018-09
Plastics moulded parts — Tolerances
and acceptance conditions
Moulages plastiques — Tolérances et conditions de réception
Reference number
©
ISO 2018
© ISO 2018
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ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 2
4.1 General . 2
4.2 Symbols . 2
4.3 Abbreviated terms . 3
5 Tolerancing of plastic moulded parts . 3
5.1 General . 3
5.2 Intention . 4
5.3 General tolerances . 5
5.4 Direct tolerancing (individual tolerancing) . 5
5.5 Tolerancing of draft angles . 5
5.6 Dimensioning, tolerancing and measuring of radii . 6
5.7 Specification of freeform surfaces . 6
6 Moulding compound properties . 6
6.1 General . 6
6.2 Moulding shrinkage and shrinkage anisotropies . 6
6.3 Moulded material stiffness or hardness. 8
7 Dimensional and geometrical tolerancing . 8
7.1 Dimensional tolerancing. 8
7.1.1 Tolerance grades for features of sizes . 8
7.1.2 Determination of the tolerance grades .11
7.2 Geometrical tolerancing .14
7.3 Parting line/Tool offset .16
7.4 Tolerancing of angular dimensions .17
8 Acceptance conditions for moulded part production (ABF) .17
Annex A (informative) Dimensional reference levels for application and production of the
moulded parts .18
Annex B (informative) Causes and influential factors on the moulding shrinkage of non-
porous plastics .20
Annex C (informative) Evaluation of the production expense .21
Annex D (informative) Validation of machine or process capability .24
Annex E (informative) Main causes for dimension, form and location deviations in moulded
part production .25
Annex F (informative) Example for determining the D dimension for application of Table 9 .26
P
Annex G (informative) Feasible acceptance parameters .27
Bibliography .28
Foreword
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different types of ISO documents should be noted. This document was drafted in accordance with the
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This document was prepared by Technical Committee ISO/TC 61, Plastics.
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iv © ISO 2018 – All rights reserved

Introduction
In comparison to metal materials, significantly larger deviations with respect to dimension, form and
location are expected when manufacturing moulded parts. Based on particular properties, such as
high deformability and low stiffness, the functional accuracy requirements in order to economically
manufacture moulded parts are much lower for plastics than for metals.
The physical and chemical properties as well as the material modification options of plastics are vastly
different from those of metals. Properties of plastics relevant to dimensional accuracy in the moulding
application and during processing by the original mould method (injection moulding, compression
moulding, rotational moulding) require a different evaluation and quantification of geometrical
tolerances in comparison to metal materials. The tolerance standards applicable for metal parts,
therefore, cannot be adopted for plastic structures or can only be applied to a very limited extent which
led to the development of this document.
The unique properties of plastics mean that three different dimensional reference levels defined in
Annex A and characterized in respect to the main influential factors are taken into consideration.
The following is the preferred sequence of steps to ensure effective cooperation in the effective design
and development of moulded parts.
a) The part designer specifies the functionally required tolerances based on the application
requirements including, part functionality, use environment, and any assembly requirements.
b) The moulded part manufacturer confirms that the functionally required tolerance is greater
than or equal to the tolerance capability of the manufacturing technology to be used. This is to
avoid impractical tolerances which cannot be achieved without incurring adverse economic or
productivity effects. The functionally required tolerances should always be defined in the design
documentation.
c) The functionally required tolerances should always be defined in the design documentation in
order to establish the basis for determining the moulding shrinkage. This is to prevent situations
in which the functionally required tolerances cannot be achieved, if at all, without excessive
scrap generation and excessive cost. After order placement, calculated values with respect to
the moulding shrinkage should be agreed between the part manufacturer and toolmaker or tool
designer, with consultation with the material supplier as necessary.
Dimensional control of the moulded part is primarily affected by the material specified, the part design
and tool layout, and the processing conditions.
In addition to the factors affecting dimensional control, there are other factors which influence
dimensions, part integrity and mechanical properties. These factors include anisotropic behaviour,
warpage and distortion due to non-uniform thicknesses and resulting non-uniform cooling rates, and
fill profiles. These factors and the basic complexity of polymer systems make standardization much
more difficult in comparison to conventional materials such as metals.
Because of the unavoidable process-induced factors, deviations are therefore expected in the moulded
part. The procedure in case of deviations depends on the function of the moulded part and is subject to
mandatory contractual agreement.
— eliminate deviation by design measures (strengthening ribs, optimized material thickness,
optimized fill profiles, etc.);
— correct deviation by specified retention in the tool, i.e. extended cooling cycles;
— acceptance of non-conformance.
The acceptance of non-conformance requires appropriate documentations including drawing
corrections, production deviation documentations or updated reference parts.
NOTE 1 Process-induced deviations can be reduced both by effective design of the moulded part and by
optimization of the production process.
NOTE 2 The conventional tolerance chain calculation presupposes rigid bodies and is therefore primarily
unsuitable for plastic parts.
vi © ISO 2018 – All rights reserved

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