Tight tolerances determine whether an injection moulded part can be assembled, seals reliably and stays consistent across the entire production run. Unlike machined metal parts, that accuracy is not created on the part itself but in the mould – and in the process that fills it.
This article shows what dimensional accuracy in plastic injection moulding actually depends on, what role shrinkage and cooling play, and how precision can be secured over a series life.
The key points at a glance
- The mould defines the tolerance. Dimensional accuracy is created in the mould – what is not designed in can hardly be corrected on the finished part.
- Account for shrinkage. Plastic shrinks as it cools. The mould is therefore built larger than the target dimension, depending on material, wall thickness and fibre content.
- Cooling determines warpage. If one side of a part cools faster than the other, the part distorts. Even cooling is core design work, not a detail.
- Keep the process window stable. Melt temperature, holding pressure and cooling time must remain reproducible across the series – even small deviations shift dimensions.
- Tolerance costs money. Tighter tolerances mean more tooling and inspection effort. It is sensible to specify tight tolerances only where function requires them.
Why precision works differently in injection moulding
With a machined part, material is removed until the dimension is right – and measured on the part. In injection moulding it is different: the plastic fills a cavity, solidifies inside it and shrinks as it does so. The part dimension therefore results from the mould dimension minus a shrinkage that depends on material, wall thickness, fibre orientation and process control.
Precision therefore begins not in production but in mould design. Anyone needing tight tolerances must allow for them when the mould is built – a dimension that turns out too small can hardly be corrected afterwards, one that is too large not at all.
What influences dimensional accuracy
- Material and shrinkage behaviour: semi-crystalline plastics such as polyamide or POM shrink more, and more directionally, than amorphous ones such as PC or ABS. Glass-fibre reinforcement reduces shrinkage but makes it direction-dependent.
- Wall thickness distribution: thicker areas cool more slowly and shrink more. Uneven wall thicknesses cause warpage and sink marks.
- Gate position: the injection point determines flow direction and thus the orientation of molecules and fibres – and therefore in which direction the part shrinks more.
- Cooling: channels in the mould remove heat. Wherever they act unevenly, warpage occurs.
- Holding pressure and cooling time: holding pressure pushes in additional material while the gate is still open. It directly influences part dimension and weight.
- Number of cavities: in multi-cavity moulds every cavity must fill evenly, otherwise dimensions scatter between the parts of one shot.
Specifying tolerances sensibly
Not every dimension needs a tight tolerance. In practice it pays to identify function-critical dimensions clearly – fits, sealing faces, bearing seats – and keep the rest more generous. This noticeably reduces tooling and inspection costs without compromising function.
For plastic parts, DIN 16742 defines tolerance groups that take shrinkage behaviour into account. It is the usual reference where a drawing specifies no values of its own. What matters is agreeing tolerances early – ideally before mould design begins.
Securing precision across the series
A dimensionally accurate first sample says little about whether the part will still be within tolerance after 200,000 shots. What counts is controlling the process and keeping an eye on mould condition.
- Sampling and documentation: during initial sampling the parts are measured completely and the results recorded in an initial sample inspection report.
- Frozen process parameters: once all dimensions are stable, the parameters are documented and fixed – so the part can be reproduced even after a production break.
- Ongoing measurement: spot checks and gauges reveal deviations before they lead to scrap.
- Mould maintenance: wear on guides and sealing faces shifts dimensions gradually. Defined maintenance intervals keep the mould in its target condition – this also applies to moulds taken over via tool transfer.
Precision parts at Schröder + Heidler
We manufacture technical plastic components for customers in the automotive, electrical engineering, consumer goods and mechanical engineering sectors. Mould making, injection moulding and subassembly assembly are located at one site – if sampling reveals a need for correction, the mould is adjusted in house, without shipping or cross-company coordination.
Our processes are certified to ISO 9001 and IATF 16949. This includes documented traceability: for every part it can be established when it was produced and inspected.
Frequently asked questions
What tolerances are achievable in injection moulding?
That depends on material, part size and geometry. DIN 16742 assigns tolerance groups to plastic parts that account for shrinkage behaviour. A reliable statement is only possible once the drawing has been reviewed – blanket figures are misleading.
Why do parts vary despite coming from the same mould?
Because the dimension depends not only on the mould but also on the process. Fluctuating melt temperature, altered holding pressure or a shortened cooling time shift dimensions. In multi-cavity moulds, fill balance between cavities adds to this.
What is warpage and how can it be avoided?
Warpage occurs when areas of a part cool at different rates or shrink to different degrees. Even wall thicknesses, well-considered cooling and a suitable gate position are the most effective remedies – all three are determined in mould design.
How is dimensional accuracy monitored in series production?
Through documented process parameters, spot measurements and, depending on requirements, gauges or full measurement. The basis is the inspection plan defined during sampling.
Conclusion: precision is created before the first shot
Dimensionally accurate plastic parts are not the result of particularly precise machines, but of mould design that considers shrinkage, cooling and gating together – and of a process that stays reproducible. Anyone needing tight tolerances should therefore name them early, not at sampling.
Send us your drawing – we will assess which tolerances are realistically achievable in injection moulding and what that means for mould and process. Talk to us about your part.