Tolerances of moulded parts: what ISO 20457 sets out
On a cap drawing, every dimension carries a tolerance. For a machined steel part, everyone knows how to read it. For an injection-moulded plastic part, which shrinks as it cools and keeps moving after ejection, another grammar is needed: that of ISO 20457.
By Tecmom editorial team · 2 min read
Why plastics have their own standard
A moulded part shrinks: as it cools, it becomes smaller than the cavity that formed it. This shrinkage varies with the material, the wall thickness, the orientation of the chains and the press settings. The part even keeps changing in the hours after ejection: this is post-shrinkage.
Applying metal-part tolerances to such parts would lead to impossible requirements, or to unjustified rejections. Hence dedicated standards. See our article on parting lines and shrinkage.
From a German standard to an international one
For decades, Europe used the German standard DIN 16901, replaced by DIN 16742. ISO 20457, published in 2018, took over at international level, and a second edition appeared in 2026.
The standard does not set a single tolerance. It defines tolerance groups, from tightest to widest, and tables that give, for each group, the tolerance as a function of the size of the dimension. The group depends on what the process and the material can achieve.
Not all dimensions are equal
A dimension formed entirely within one part of the mould varies less than one that crosses the parting line or depends on a moving part, such as a slide. Tolerance standards for plastic parts have long taken this into account.
The standard also sets the measurement conditions: a plastic part is measured after it has stabilised, at a defined temperature. Measuring a warm cap straight from the press says little about the cap that will reach the customer.
Specifying a cap or a pill container
On a well-made drawing, most dimensions refer to a general tolerance, chosen from a group of the standard. Only the functional dimensions receive a specific tolerance: the sealing diameter, the thread, the height under the band. They decide screwing and sealing; see our advice page on the bottle neck finish.
Tightening every dimension without distinction makes the mould and the inspection more expensive without improving the part. It is a discussion for the buyer and the mould maker from the very start of the project.
A tighter tolerance does not make a better cap. It only makes the mould more expensive and the inspection longer. The right tolerance is the one that protects the function.
Glossary
- Tolerance
- The permitted deviation between an actual dimension and its nominal value.
- Shrinkage
- The reduction in a plastic part's dimensions as it cools.
- Post-shrinkage
- The change in a part's dimensions after ejection.
- Functional dimension
- A dimension that directly governs the part's function.