Dimensional tolerances are the silent make-or-break factor in every plastic injection molding project. A deviation of 0.05mm might be invisible to the eye, but it can mean the difference between a snap-fit that clicks perfectly and one that rattles loose. This comprehensive guide covers ISO 2768, DIN 16901, and the practical realities of achieving tight tolerances with engineering plastics.
Why Tolerances Matter in Plastic Parts
Unlike metals, thermoplastics shrink during cooling — and they shrink anisotropically, meaning differently in flow and cross-flow directions. Add glass fiber orientation, mold temperature gradients, and varying wall thickness, and you have a complex dimensional puzzle. Getting tolerances right affects assembly fit, sealing performance, aesthetic quality, and ultimately your rejection rate and total cost.
ISO 2768 General Tolerances for Plastics
ISO 2768 provides general tolerance classes for linear dimensions and angular dimensions where no specific tolerance is indicated on the drawing. For plastic parts, the relevant standard is ISO 2768-1 (linear and angular dimensions) with the mK tolerance class typically applied.
| Tolerance Class | 0.5–3mm | 3–6mm | 6–30mm | 30–120mm | 120–400mm | Типичная область применения |
|---|---|---|---|---|---|---|
| f (fine) | ±0.05 | ±0.05 | ±0.1 | ±0.15 | ±0.2 | Precision gears, optical |
| m (medium) | ±0.1 | ±0.1 | ±0.2 | ±0.3 | ±0.5 | Most injection molded parts |
| c (coarse) | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | Low-precision housings |
| v (very coarse) | ±0.5 | ±1.0 | ±1.5 | ±2.0 | ±3.0 | Large non-critical parts |
For most engineering plastic parts, ISO 2768-m is a reasonable starting point. ISO 2768-f is achievable only with careful mold design, stable processing, and materials with predictable shrinkage.
DIN 16901 — Injection Molding Specific Tolerances
DIN 16901 is the gold standard for injection molding tolerances. Unlike ISO 2768, it accounts for material-specific shrinkage behavior by grouping thermoplastics into shrinkage categories. This makes it far more practical for mold makers and quality engineers.
| Материал | Shrinkage Group | Typical Shrinkage | Grade A (tight) | Grade B (standard) | Grade C (loose) |
|---|---|---|---|---|---|
| PA6/PA66 unfilled | Group 2 | 1.0–2.0% | ±0.1% | ±0.2% | ±0.4% |
| PA66 GF30 | Group 1 | 0.3–0.7% | ±0.05% | ±0.1% | ±0.2% |
| POM (ацеталь) | Group 2 | 1.8–2.5% | ±0.1% | ±0.2% | ±0.4% |
| PC (поликарбонат) | Group 1 | 0.5–0.7% | ±0.05% | ±0.1% | ±0.2% |
| ABS | Group 1 | 0.4–0.7% | ±0.05% | ±0.1% | ±0.2% |
| PP unfilled | Group 3 | 1.5–2.5% | ±0.2% | ±0.4% | ±0.6% |
Основной вывод: Glass fiber reinforcement dramatically improves dimensional stability. PA66 GF30 (Group 1) can achieve tolerances nearly as tight as unfilled PC, while unfilled PA66 (Group 2) needs wider tolerance allowances due to higher and more variable shrinkage.
How Shrinkage Affects Achievable Tolerance
Shrinkage is the single largest variable in plastic part tolerances. Here is how different materials compare:
- Nylon 6/66 unfilled: 1.0–2.0% shrinkage. A 100mm dimension can vary by 1–2mm just from the material alone, before considering mold and process variation.
- Nylon 66 GF30: 0.3–0.7% in flow direction, 0.7–1.0% cross-flow. The glass fiber constrains shrinkage but creates anisotropy — dimensions differ depending on fiber orientation.
- POM: 1.8–2.5% — the highest shrinkage of common engineering plastics, which is why tight-tolerance POM parts need very precise mold compensation.
- ПК: 0.5–0.7% — excellent dimensional stability, making it a preferred choice for optical and precision applications.
The mold maker must calculate cavity dimensions as: Nominal Dimension × (1 + Shrinkage Rate), then fine-tune after first-shot samples. Modern Moldflow simulation predicts shrinkage within ±0.1% accuracy when properly calibrated.
Tolerance Stack-Up Analysis
When multiple toleranced features interact in an assembly, their individual tolerances accumulate. The practical formula for worst-case stack-up is:
Ttotal = T1 + T2 + … + Tn
For statistical (RSS) stack-up, which is more realistic for production volumes:
Ttotal = √(T1² + T2² + … + Tn²)
Common mistakes include forgetting to account for the mold split line tolerance, ignoring thermal expansion differences between assembled materials, and treating shrink rates as constants rather than ranges. Always run a tolerance analysis before finalizing mold steel — it is far cheaper than discovering interference at first-shot inspection.
Design for Tolerance — Best Practices
Uniform wall thickness: The single most effective way to improve dimensional control. Thick-to-thin transitions cause differential cooling and warpage.
Gate location: Position the gate so the melt front fills the cavity uniformly, minimizing anisotropic shrinkage. A poorly placed gate creates asymmetric flow patterns that warp the part.
Mold steel selection: For tight tolerances (±0.02mm or better), use hardened tool steel (H13, S136) rather than P20. Hardened steel holds dimensions longer and provides better surface finish, reducing the need for post-molding compensation.
Draft angles and ejection: Ejector pin placement affects flatness. Uneven ejection force distorts the part while it is still warm, creating permanent dimensional errors.
Measurement Methods for Plastic Parts
| Метод | Точность | Лучшее для | Уровень затрат |
|---|---|---|---|
| Caliper | ±0,02 мм | Quick checks, simple features | $ |
| Micrometer | ±0.001mm | Wall thickness, precision diameters | $ |
| Gauge pins | ±0,005 мм | Hole diameters, go/no-go | $$ |
| Optical comparator | ±0,005 мм | Profiles, radii, 2D geometry | $$$ |
| CMM (Coordinate Measuring Machine) | ±0.001mm | Full 3D dimensional inspection | $$$$ |
| 3D scanning | ±0.02–0.05mm | Complex freeform surfaces, comparison to CAD | $$$ |
For production QC, a combination of gauge pins (fast go/no-go for critical bores) and periodic CMM inspection (full dimensional report for PPAP/ISIR) is the industry standard.
The Cost of Tighter Tolerances
Every decimal place in your tolerance specification increases cost. Here is a practical cost pyramid for injection molded nylon parts:
| Tolerance Band | Mold Cost Premium | Part Cost Premium | Rejection Rate |
|---|---|---|---|
| ±0,5 мм | Исходные данные | Исходные данные | 0.5–1% |
| ±0.2mm | +5–10% | +3–5% | 1–3% |
| ±0.1mm | +15–25% | +10–15% | 3–5% |
| ±0,05 мм | +30–50% | +20–30% | 5–10% |
| ±0,02 мм | +60–100% | +40–60% | 10–20% |
The premium is not just financial: tighter tolerances also increase mold lead time (additional EDM and polishing) and require more frequent QC inspection during production.
Заключение и рекомендации
Specifying tolerances for injection molded plastic parts requires balancing functional requirements with manufacturing reality. For nylon parts, the sweet spot is typically DIN 16901 Grade B (standard) — it provides adequate precision for most mechanical applications without excessive cost premiums. Glass-filled grades can reliably achieve Grade A tolerances thanks to their lower and more predictable shrinkage. Always involve your mold maker early in the tolerance specification process: their experience with your specific material and geometry is worth more than any general standard.
Часто задаваемые вопросы
Каков минимально достижимый допуск для нейлона, полученного методом литья под давлением?
For unfilled nylon (PA6/PA66), practical tight tolerance is ±0.05mm for dimensions under 10mm, and approximately ±0.1% of the nominal dimension for larger features. With PA66 GF30, you can reliably achieve ±0.03mm for small features due to the glass fiber’s shrinkage-constraining effect. Achieving tighter than ±0.02mm requires post-molding CNC machining.
Как указать допуски на чертеже пластиковых деталей?
В качестве общего стандарта допусков следует использовать ISO 2768-mK, а для критических размеров отдельно указывать более узкие допуски. В частности, для деталей, изготовленных методом литья под давлением, следует руководствоваться стандартом DIN 16901 и указывать класс допуска (A/B/C) наряду с группой усадки материала. Всегда указывайте, применяются ли допуски при формовании или после 24-часовой кондиционирования, поскольку размеры изделий из нейлона изменяются при поглощении влаги.
Улучшает или ухудшает стекловолокно допуски на размеры?
Стекловолокно значительно улучшает допуски на размеры за счет уменьшения и стабилизации усадки. Усадка PA66 GF30 составляет 0,3–0,7% по сравнению с 1,0–2,0% у ненаполненного PA66. Однако стекловолокно вызывает анизотропную усадку (различную в направлении потока и поперечно к нему), которую конструктор пресс-формы должен компенсировать за счёт правильного расположения литника и корректировки размеров полости. В целом это оказывает весьма положительное влияние на контроль размеров.
What does ‘free tolerance’ mean in plastic manufacturing?
Свободный допуск означает, что для данного размера на чертеже не указан индивидуальный допуск, и поэтому по умолчанию применяется общий допуск, установленный в соответствующем стандарте (обычно ISO 2768-m для пластиковых деталей). Для размера 50 мм в соответствии с ISO 2768-m свободный допуск составит ±0,3 мм. Свободные допуски позволяют упростить чертеж и снизить производственные затраты, однако их следует использовать только для нефункциональных размеров, не связанных с посадкой деталей.
