DIN 16742:2013 Plastic Moulded parts–Tolerances and acceptance conditions-2
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Tolerancing of plastic moulded parts
According to DIN16742-2013A version.
General
The independency principle according to DIN EN ISO 8015 applies when using this standard.
Deviations from this principle (e. g. envelope requirement > size ISO 14405 Ⓔ < or similar data) shall be agreed separately between the contractual partners.
Moulded part drawings or CAD data records correspond to the nominal geometry. The tolerances are symmetrical to the nominal geometry. Asymmetrical tolerances for sizes (e. g. fit dimensions) shall be converted to a symmetrical tolerance field location by the formal nominal dimension modification to tolerance mean dimension C: 100-0,6 99,7 ± 0,3.
The procedure for the verification is to be defined uniquely. In particular in the case of non-dimensionallystable parts, the measuring concept is of special importance (functional orientation, reference system and overdetermination, gravitational influence, pretension etc.), see also DIN ISO 10579.
Unless otherwise defined, plastic moulded parts where the general tolerances are not complied with do not have to be automatically rejected if the function is not impaired.
In the case of multiple component parts, the tolerance group shall be determined for each material and indicated as a separate general tolerance (e. g. hard component according to TG 4, soft component according to TG 7).
The less accurate material forms the basis of the tolerance determination in the case of multiple material sizes. 23 °C ± 2 K and 50 % ± 10 % relative air humidity is defined as a standard atmosphere in the plastic range in DIN EN ISO 291.
It shall be indicated in the labelling field the following note: "Tolerancing ISO 8015 – DIN EN ISO 291:2008-08".
Indirect tolerancing by general tolerances
Only series 1 (standard production) according to Table 8 applies for general tolerances. General tolerances shall be indicated in or on the labelling field, for example:
General tolerances DIN 16742 – TG6. The profile form tolerances apply as general tolerances, a reference system shall be determined for this.
Should general toleranced dimensions be submitted to an orientating dimension control, they have to be indicated with respect to the metrological feasibility in the drawing.
Direct tolerancing by dimension indication at nominal dimension
Acceptance dimensions are all directly toleranced characteristics. All dimensions with general tolerances are not considered in the test record. Position tolerances are not general tolerances.
If required by the function, they shall be entered directly in the drawing.
The dimensional tolerance shall be indicated directly by dimensions for moulded parts dimensions with justifiably high dimensional stability requirements. When doing so, it shall be noted that the dimensional boundary lines or points represent inspection dimensions (reference dimensions, acceptance dimensions). The number of directly toleranced dimensions per moulded part shall be kept as low as possible for economic reasons.
Tolerancing of draft angles
Drafts (also draft angles) are production-induced inclinations on the moulded part in the demoulding orientation of moving tool parts (e. g. punches, gate valves, jaws), which are specified as an integral component of the moulded part drawings or the CAD data records of the moulded part manufacturer for tool design and tool making as well as parts production. Inclination dimension differences specified in terms of design are not a component of dimensional tolerances or form and location deviations.
Measuring points shall be defined at suitable areas for functional dimensions in the specification in order to define two-point dimensions.
5.5 Dimensioning, tolerancing and measuring of radii
Minimum 90° of the circle segment shall be provided as a measurable contour for the specification of radii. NOTE Radii can alternatively be toleranced by profile forms.
Specification of freeform surfaces
Free form surfaces shall be specified with a profile form tolerance. The verification shall be coordinated.
Moulding compound properties
General
This standard does not contain any type lists for moulding compounds or their assignment to attainable production accuracies. Accuracy-relevant properties shall be considered in order to indicate a general assignment scheme for the large number and variety of moulding compounds.
Moulding shrinkage and shrinkage anisotropies
The moulding shrinkage (VS) is the relative difference between the tool contour dimension LW at 23 °C ± 2 K and the corresponding moulded part dimensions LF 16 h to 24 h after production, stored until measurement and measured at 23 °C ± 2 K and 50 % ± 10 % air humidity.
It is calculated according to equation
(1). 1 ×100 [%] = − LLWF VS
(1) Where LF is the moulded part dimension; LW is the tool contour dimension.
The moulding shrinkage for thermoplastics and thermoplastic elastomers is determined (e. g. test panels) according to DIN EN ISO 294-4 and for thermosets according to ISO 2577 on standard test specimens.
Physical causes of the moulding shrinkage and the effect of influencing factors are indicated in Annex B and Annex F. Shrinkage anisotropy is quantified by the absolute difference ∆VS from moulding shrinkage transverse to the melt flow direction VS┴ and the moulding shrinkage parallel to the melt flow direction VSII. See equation
(2). ∆VS = I VS┴ - VSII I (2) Physical main causes are the: moulding impediments as a result of different thermal contraction by solidified boundary layers, material concentrations and locally different tool contour temperatures as well as by the effect of the moulded part design; moulding differences due to anisotropic strengthening materials
(e. g. fabrics, knitted fabrics, rovings); orientation of filling and strengthening materials, molecules and morphological structures due to flowing processes as a result of shear and elongation flows. In particular, particle shape and aspect ratio (lengththickness ratio or side-thickness ratio) of the filling and strengthening materials affect the anisotropy characteristics.
It can be derived from the diverse influences on the moulding shrinkage and shrinkage anisotropy that numerical values are only realistic as range data. The resultant distribution of the moulding shrinkage ∆S is derived from the extreme values VSmax and VSmin. It is calculated according to equation
(3). ∆S = VSmax – VSmin
(3) The size range of the shrinkage distribution can be affected by production conditions (process optimisation), batch-relevant moulding compound differences, moulded part shape and spur technology. Average calculated values of the moulding shrinkage VSR are specifications for tool design, construction and sampling of the tools. It is calculated according to equation (4). VSR = 0,5 (VSmax + VSmin )
(4) This calculated value, which is a basis for the tool design, is primarily expected from the moulded part manufacturer, as the latter can actively influence the shrinkage in limits and usually has corresponding data.
They can be generated as a by-product from dimensional check measurements. In special cases, the shrinkage values are to be made more precise by sampling with similar tools. In addition, the moulded part manufacturer can use corresponding data and experience of the moulding compound manufacturer.
In the case of distinct shrinkage anisotropy, the shrinkage differences can be considered to a limited extent by dimensional provisions in the tool. Computer-assisted shrinkage and deformation statements might be able to provide information in respect to this. The shrinkage distribution is also of major significance for the attainable production accuracy. This value range is to be estimated according to experience of the moulded part manufacturer.
NOTE If the shrinkage anisotropy cannot be considered adequately in the contour calculation, a larger shrinkage distribution and hence deformation is to be expected. A timely coordination between the customer and moulded part manufacturer is necessary in respect to this.
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