What tolerances really mean in manufacturing

A tolerance is not a free safety margin. It is a functional requirement that has direct consequences for production costs, the mould, and material behaviour. The tighter the tolerance, the greater the demands placed on the process, the tooling, and the raw material.

In injection moulding, the material largely determines what is achievable. Amorphous plastics such as ABS or polycarbonate shrink predictably and linearly. Semicrystalline materials such as PA66 or POM shrink more and less uniformly. This makes tight tolerances technically more difficult to achieve with semicrystalline materials and more expensive to guarantee.

A common mistake is blindly adopting tolerances from an existing drawing without checking if the chosen material can handle those tolerances. Eurotechniek regularly sees this with reconstructions of existing parts where the material is changed but the tolerances remain the same.

The relationship between shrinkage and dimensional accuracy

Shrinkage is the biggest variable in the injection moulding process when it comes to tolerances. Each material has its own shrinkage percentage, but that percentage is not absolute. Wall thickness, flow direction, process temperature, and cooling rate influence the final shrinkage.

Take glass-fibre reinforced polyamide as an example. Shrinkage in the flow direction is significantly lower than shrinkage perpendicular to the flow direction. This results in anisotropic shrinkage: the part shrinks unevenly. For a flat, symmetrical product, this is manageable. For a complex part with varying wall thicknesses, it becomes a technical challenge that must be addressed during the design phase.

At Eurotechniek, we always process shrinkage data from material suppliers together with our own process data. This provides a more accurate prediction than the average shrinkage value on a technical datasheet alone.

Material selection as a driving factor for tolerance feasibility

Choosing a material based on mechanical properties alone is not enough. The processability, chemical resistance, and shrinkage behaviour must also match the requirements on the drawing. A material that scores perfectly mechanically but has high shrinkage variation makes tight tolerances almost impossible to achieve without intensive process monitoring.

POM is a good example. The material has excellent sliding properties and high rigidity. But the shrinkage of POM is between 1.8 and 2.5 percent, depending on the processing conditions. That is high compared to ABS, which is around 0.4 to 0.7 percent. For a gear that meshes closely with a metal shaft, POM is functionally the best choice. But then you have to explicitly adjust the tolerances and mould design accordingly.

The rule of thumb that we use is: the tighter the tolerance requirement, the earlier in the design process you must fix the material choice. If you leave this until the drawing is finished, you may no longer be able to meet the requirements without adjusting the geometry.

Minimum tolerances per production method

Not every production method delivers the same accuracy. Injection moulding, stamping, CNC milling and 3D printing each have their own tolerance range. These ranges are not a marketing claim but a consequence of the physics of the process.

For injection moulding, IT8 to IT10 is realistic for most plastic parts. For precision parts in amorphous material with controlled process conditions, IT7 is occasionally achievable. Stamping easily achieves IT9 to IT11 in metal, depending on sheet thickness and die quality. CNC milling goes further: IT6 and tighter are achievable, but require more machining time and tool wear.

3D printing, while useful for prototyping, does not typically achieve better than IT11 to IT13 in most technologies. Therefore, additive manufacturing is rarely the final solution for functional assembly parts with tight fits. Eurotechniek uses 3D printing in the design phase to validate geometry, not to prove tolerances.

Challenge and solution: tolerances in practice

Challenge: a customer has a housing for a medical device. The material is polysulfone due to its sterilisation resistance. The mounting holes must have a snug fit with a stainless steel pin. The drawing specifies a tolerance of plus zero, minus 0.05 millimetres.

Solution: Polysulphone has low, easily controllable shrinkage of approximately 0.5 to 0.7 percent. This makes the material suitable for precision injection moulding. However, with the given hole diameter and wall thicknesses, the critical factor was cooling within the mould around the cores. By using beryllium copper inserts in the mould cores, we significantly improved local cooling. The first series production consistently achieved the tolerance.

It is this sort of combination of material insight and mould technology that makes the difference between a part that is correct on paper and a part that is also consistently correct in production.

Documentation and release process as an underestimated component

Recording tolerances on a drawing is step one. Ensuring they are also safeguarded in the production process is step two. This step is regularly skipped, especially with smaller runs or with customers who are working with a particular material for the first time.

A good release process begins with an initial part inspection, where all dimensions are measured and compared to the drawing. Deviations are fed back into the mould design or process settings, not accepted as “close enough”. This is followed by statistical process control for dimensionally critical features in series production.

At Eurotechniek, tolerance control is recorded in a control plan that is drawn up for each project. This plan describes which dimensions are measured, how often, and with which measuring instrument. This way, the quality of the component is traceable, even if the mould undergoes refurbishment after two years.

Frequently asked questions about minimum tolerances and material choice

Which plastic is best suited for tight tolerances?

Amorphous plastics like ABS, polycarbonate, and polysulphone are best suited for tight tolerances. They shrink predictably and minimally, making dimensional accuracy easier to achieve. Semi-crystalline materials such as PA, POM, and PP are functionally strong but require more process control to achieve tight tolerances. The correct choice always depends on the functional requirements of the part.

What is a realistic tolerance for an injection moulded part?

For standard injection moulding, a general tolerance of plus or minus 0.1 to 0.2 millimetres is realistic for most geometries and materials. Tighter than plus or minus 0.05 millimetres is achievable, but requires amorphous material, a well-cooled mould design, and intensive process monitoring. Always discuss which dimensions are critical beforehand. This prevents unnecessary costs for tolerances that make no functional difference.

Wall thickness affects tolerance feasibility.

Uneven wall thicknesses cause uneven shrinkage. This leads to dimensional deviations that cannot be easily compensated for in the process. A constant wall thickness of three to four millimetres is the most stable in most cases. Thick sections next to thin sections create internal stresses that can still cause deformation after injection moulding. Preferably design with uniform wall thicknesses if tolerances are dimensionally critical.

Tolerances and materials: deciding early saves later

The combination of tolerance requirements and material selection determines at an early stage whether a design is producible at acceptable costs. If you defer this consideration until the drawing is finalised, you run the risk of costly mould modifications or process compromises.

Eurotechniek contributes to the design process from the very first phase, considering materials, tolerances, and manufacturability. View our offerings or get in touch for a technical discussion about your project.