This blog is translated from German with DeepL.
From October 2009, when the previous standard (DIN 16901) was withdrawn, until October 2013, there was no official standard for the tolerancing of molded plastic parts. Like many others, we continued to rely on the withdrawn DIN 16901 during this time.
In this Knowledge Tip, we would like to take you on a short journey into the realm of standards and show you what this means for us as an engineering company.
1. Scope of application
The DIN 16742 standard applies to the following plastic molded parts:
I. Injection molding
– Liquefied plastic which is poured into a mold (tool) under pressure.
– The mold is closed and the individual parts of it are secured to each other.
II. injection stamping
– Liquefied plastic, which is poured into a mold (tool) with pressure.
– The mold is slightly open during pouring and is closed only during the pouring process in order to obtain the same pressure everywhere.
III. injection molding
– A predefined amount of liquid plastic is poured into the mold (more than is actually needed to minimize air pockets).
– Mold parts are pressed together only at the end and held until the plastic has cured.
IV. Pressing of non-porous molded parts made of thermoplastics
V. Thermoplastic elastomers and thermosets
How to proceed and what to consider:
Scope, tolerancing, reduction of tolerance zones and GEA normal case TG5 – TG7.
Among other things, symmetrical tolerances are new. Whereas in the previous version, e.g. 20+0.4/-0.3 were still approved, the new standard even prescribes symmetrical tolerancing (e.g. 20 ± 0.4). This takes into account the fact that nowadays the toolmaker usually continues to work with the 3D CAD model, and his work is made considerably easier if the model has already been designed with tolerances in mind.
Furthermore, a distinction is made between shape-related and non-shape-related dimensions.
The size of the tolerance zone is determined by the space diagonal.
The figure below illustrates what this means exactly. To determine the plus-minus tolerance for dimension 12.45, the tolerance values for nominal dimension 76.766 (space diagonal) must be found from the tolerance table and entered.
This also takes into account the warpage of plastic parts, among other things.
3. Reduction in the size of the tolerance zones.
As a result of increasingly better and more precise manufacturing processes, more precise tolerances can be specified and achieved than in DIN 16901.
What used to be listed in DIN 16901 under the most precise tolerance group is found in the new standard approximately in the tolerance series TG4 and TG5. The tolerance zones of the TG1 to TG3 series have been significantly reduced again compared to the withdrawn standard, at least for the lower nominal dimension ranges.
4. GEA normal case TG5 – TG7
DIN 16742 comprises 9 tolerance groups (TG1 – TG9). These can be approximately assigned to the ISO basic tolerances (IT01…IT18 according to SN EN ISO 286-1). As a result, it is now also possible to communicate via the ISO tolerance series if required. This assignment did not exist with DIN 16901.
A material- and production-dependent evaluation system is used to determine the tolerance group. Despite the wide range of tolerance series, a certain standard is emerging in our company.
Normally, we work with the tolerance series TG5 – TG7 (of course, everything is done under the principle: “As exact as necessary, as rough as possible”).