If you have ever received a plastic part that looked right on paper but failed in assembly, the root cause often traces back to tolerances — specifically, how they were defined, communicated, and controlled. For decades, engineers relied on DIN 16901, a German standard from the early 1980s, to specify dimensional tolerances for injection-molded plastic parts. It was useful, but incomplete. It addressed size, not shape. It accounted for material type, but not process variation, shrinkage anisotropy, or the reality that error grows with distance from a datum.

ISO 20457:2018 was developed to fix exactly those gaps. It is now the internationally recognized standard for plastic part tolerances — covering not just dimensional limits, but geometric tolerances, acceptance conditions, and a structured method for determining what tolerance grade is actually achievable given your material, process, and production setup. Whether you are designing a consumer electronics enclosure, an automotive bracket, or a medical device housing, understanding ISO 20457 helps you specify tolerances that are realistic, communicable, and manufacturable.

This guide breaks down every key element of ISO 20457 in plain engineering language — the tolerance grade system, the W/NW dimension classification, the five-factor scoring method, geometric tolerancing, and what the standard's acceptance rules mean for your production program. We also connect each concept to real manufacturing decisions so you can apply this knowledge from the first prototype through to volume production.

Why Plastic Part Tolerances Are Not Like Metal Tolerances

A common and costly mistake in product development is applying metal machining tolerances directly to injection-molded plastic parts. The two materials behave fundamentally differently during and after processing. When a CNC machine cuts aluminum, dimensional variation comes primarily from tool deflection and thermal growth — both of which are small and predictable. Plastic parts, by contrast, are shaped by heat, pressure, cooling rate, material flow, and mold geometry all interacting simultaneously. The result is a part whose final dimensions are influenced by a far larger set of variables.

Plastic shrinks as it cools, and that shrinkage is rarely perfectly uniform. A glass-fiber-filled nylon part shrinks differently in the flow direction than across it. A large flat panel cools unevenly from edge to center, introducing warpage that no tolerance band can fully anticipate. Add to this the mechanical realities of the mold itself — parting lines, side actions, ejector pins — and you have a system where dimensional variation is not just inevitable but structurally embedded in the process. The job of a tolerance standard is to provide a rational, consistent framework for specifying what is acceptable given these realities. ISO 20457 does this more completely than any predecessor standard.

From DIN 16901 to ISO 20457: Why the Standard Evolved

DIN 16901, published in 1982, organized tolerances into six groups (110 through 160) based on material type and nominal size. Tighter group numbers meant tighter tolerances. It was a straightforward system that gave engineers a starting point, and it remained widely used across European manufacturing for over three decades. But it had real limitations that became increasingly apparent as plastic parts grew more complex and global supply chains demanded clearer documentation.

The most significant limitation was scope. DIN 16901 only defined size tolerances — it said nothing about flatness, position, or surface profile. For a simple bracket, that might be acceptable. For a snap-fit enclosure or a sealing surface that must mate with a gasket, size alone is not enough. The standard also assigned tolerance groups based only on material and size, ignoring process factors like shrinkage rate variability, production effort, and molding method. This led to tolerance specifications that were sometimes too loose to be functional and sometimes too tight to be manufacturable — with no systematic way to tell the difference before production started.

ISO 20457:2018 replaced this approach with a more rigorous and internationally harmonized system. It expanded from six tolerance groups to nine tolerance grades, added geometric and positional tolerancing, introduced a structured scoring method to determine the correct grade, and established formal acceptance conditions for molded parts. It also aligned with the broader ISO GPS (Geometrical Product Specifications) framework, making plastic part tolerancing consistent with the language used across the rest of mechanical engineering.

ISO 20457 Tolerance Grades: TG1 Through TG9 Explained

ISO 20457 defines nine tolerance grades, labeled TG1 through TG9. TG1 is the tightest — demanding the highest dimensional precision and the most controlled production conditions. TG9 is the loosest, applicable to parts where dimensional variation has minimal functional consequence. In practical production, most injection-molded parts fall within TG3 to TG6, with TG1 and TG2 reserved for precision components that require exceptional process control, specialized tooling, and often a qualification program before production begins.

The tolerance value for any given grade increases with nominal dimension size. A 10 mm feature at TG4 carries a smaller tolerance band than a 200 mm feature at the same grade, because larger dimensions accumulate more absolute error from shrinkage and process variation. This scaling is built into the standard's tolerance tables and reflects the physical reality of how plastic parts behave. Understanding which TG grade applies to your part is therefore not just a matter of selecting a number — it depends on your material, your process, and the production conditions your supplier operates under. That determination is made through the five-factor scoring system, which we cover below.

W and NW Dimensions: The Two Categories Every Engineer Should Know

One of the most practically useful concepts in ISO 20457 is its classification of all part dimensions into two categories: W (werkzeuggebunden, or tool-specific) and NW (nicht werkzeuggebunden, or non-tool-specific). These categories determine how much tolerance variation is expected and, consequently, how tight a tolerance is achievable.

W dimensions are those whose size is determined entirely by a single, fixed region of the mold. Think of a pocket depth machined into the core, or the diameter of a hole formed by a fixed pin. Because these features are cut into one solid block of steel and do not cross any moving boundary within the mold, their dimensional consistency is limited mainly by mold manufacturing precision and material shrinkage. They are inherently more repeatable, and ISO 20457 assigns them tighter tolerance allowances.

NW dimensions, by contrast, span across a mold boundary — the parting line, a side action, a lifter, or the interface between core and cavity. Every time the mold closes, there is some small variation in how these surfaces align. Over thousands of cycles, that variation accumulates into a wider dimensional scatter. Wall thickness across a parting line is a classic NW dimension. So is any feature that involves a slider or a collapsible core. ISO 20457 acknowledges this by assigning NW dimensions a larger tolerance allowance than W dimensions at the same tolerance grade. Recognizing which of your critical dimensions falls into which category is one of the first steps in writing a tolerance specification that your molder can actually meet. This is exactly the kind of design review that the team at NICE Rapid's plastic injection molding service provides as part of its engineering-driven manufacturing support.

The Five-Factor Scoring System (P1–P5): How Your TG Grade Is Determined

Perhaps the most important structural improvement ISO 20457 makes over DIN 16901 is the introduction of a quantifiable method for determining which tolerance grade is realistically achievable for a given part. The standard defines five factors, each scored independently, and the sum of those scores maps to a TG grade.

• P1 – Molding process: Different plastic processing methods produce different levels of dimensional consistency. Injection molding generally achieves finer tolerances than blow molding or rotational molding, so the process itself carries a score that reflects this.
• P2 – Material stiffness: Stiffer materials resist warpage and spring-back more effectively, supporting tighter dimensional control. A rigid glass-filled engineering plastic scores better here than a soft, flexible elastomer.
• P3 – Shrinkage rate: Materials with lower, more predictable shrinkage rates are easier to dimension accurately. High-shrinkage materials like unfilled polypropylene require wider tolerances than low-shrinkage materials like filled POM or PMMA.
• P4 – Shrinkage anisotropy: This factor captures how differently a material shrinks in different directions — flow direction versus cross-flow direction. Highly anisotropic materials, particularly long-fiber-reinforced grades, introduce directional dimensional error that must be accounted for in the tolerance band.
• P5 – Production effort: This factor reflects the investment in tooling quality, process control, and inspection. ISO 20457 defines four production series (Series 1 through 4), where Series 1 represents standard production and Series 4 represents the highest level of precision manufacturing with validated processes and tight statistical control.

Once all five factors are scored and summed, the total maps to a TG grade. This makes tolerance negotiation between designer and manufacturer much more structured. Rather than debating whether ±0.1 mm is achievable on a given feature, both parties work from the same scoring framework. It also provides a rational basis for discussing trade-offs: if a material with high shrinkage anisotropy forces a higher TG grade, the designer can evaluate whether switching to a more dimensionally stable material justifies the change.

For teams using CNC machining or 3D printing during prototyping, it is worth noting that neither process is governed by ISO 20457 — that standard is specific to molded plastic parts. But understanding the TG grade your production part will need to achieve helps you design prototypes with appropriate functional margins, so that what works in prototype also works in production.

Geometric Tolerances: What ISO 20457 Adds That DIN 16901 Missed

DIN 16901 defined only size tolerances — the permissible deviation in a linear or diameter measurement. ISO 20457 goes significantly further by incorporating geometric tolerancing for plastic parts, aligning with ISO 1101 (form and position tolerances), ISO 5458 (pattern tolerancing), and ISO 5459 (datums and datum systems).

In practical terms, this means ISO 20457 now provides tolerance tables for positional tolerances (how far a feature can deviate from its theoretically exact location) and surface profile tolerances (how far a freeform surface can deviate from its nominal geometry). Both of these are critical for parts with assembly interfaces, sealing surfaces, or complex curvature — categories that cover the vast majority of precision plastic components in automotive, medical, and consumer electronics applications.

Importantly, ISO 20457 introduces the concept of Dp — the distance from a feature to its datum origin — as a variable that directly affects position and profile tolerances. This acknowledges a physical reality that DIN 16901 ignored entirely: error in plastic parts accumulates with distance from the datum. A feature 150 mm from the datum origin will exhibit more positional scatter than a feature 15 mm from it, due to the compounding effects of shrinkage gradients, warpage, and thermal history. By scaling tolerances with Dp, ISO 20457 produces specifications that are both more accurate and more achievable. This kind of geometric rigor becomes especially important in low volume manufacturing programs, where part-to-part consistency must be validated before committing to full production tooling.

ABF Acceptance Conditions: When a Part Is Considered Good

One of ISO 20457's most practically impactful additions is Chapter 8, which defines the ABF (Acceptance Conditions for Moulded Part Production). Prior standards, including DIN 16901, provided tolerance tables but left acceptance criteria undefined — meaning that how a part was measured, what sampling plan was used, and how deviations were judged remained open to interpretation and disagreement.

ISO 20457's ABF framework addresses this directly. It specifies what must be inspected, how to handle dimensions that fall outside the general tolerance but still meet functional requirements, how to distinguish between functional features (where deviation has a direct consequence on performance) and non-functional features (where deviation affects appearance or perception), and how shrinkage variation should be managed in long production runs. It also establishes the principle that a part is considered acceptable if it meets its functional and usage requirements, even if some dimensions fall outside the general tolerance — unless a specific deviation is contractually excluded. This is a significant and often misunderstood point: ISO 20457 is a functional standard, not a purely geometric one.

For product teams managing supplier relationships, the ABF section provides a concrete foundation for quality agreements. Rather than writing custom acceptance criteria for every program, engineers can reference ISO 20457 Chapter 8 as the governing framework and focus their agreements on the specific critical features that need tighter definition. Services like mid volume manufacturing and high volume manufacturing benefit most from this clarity, since acceptance disputes become more costly as volumes increase.

Applying ISO 20457 in Practice: What Product Teams Need to Know

You do not need to read every table in ISO 20457 to use it effectively. In practice, applying the standard comes down to a few consistent habits. First, classify every toleranced dimension as W or NW before specifying a value. This single step prevents a common error: specifying tight tolerances on NW features that span the parting line, where the mold structure makes those tolerances structurally unachievable. Second, use the P1–P5 scoring system — or ask your manufacturing partner to apply it — to determine the realistic TG grade for your material and process combination before freezing your design. This is most valuable in the early design phase, when geometry can still be modified to support tighter tolerances if needed.

Third, apply tight tolerances only where they are functionally necessary. ISO 20457 supports a tiered approach: critical features like snap-fit engagement zones, sealing surfaces, and alignment datums receive formal tolerance callouts at the appropriate TG grade, while non-functional surfaces are governed by the general tolerance for the part's overall TG grade. This approach reduces tooling cost, simplifies inspection, and produces a drawing that communicates intent clearly. For teams using vacuum casting to validate assemblies before injection mold tooling is cut, understanding your target TG grade also helps you assess how much functional margin to build into cast prototype testing — since vacuum cast parts will not replicate the exact shrinkage behavior of injection-molded production parts.

Finally, when your program involves materials other than standard thermoplastics — silicone, liquid silicone rubber, or thermoplastic elastomers — be aware that ISO 20457 applies specifically to rigid and semi-rigid molded plastics. Soft and highly elastic materials require different tolerance conventions, and suppliers experienced in LSR molding or silicone molding will typically specify dimensional acceptance based on functional fit and agreed measurement conditions rather than standard ISO 20457 tables.

Frequently Asked Questions

Does ISO 20457 replace DIN 16901 completely?

In practical terms, yes — ISO 20457:2018 is the modern international successor to DIN 16901 and provides a more complete system for specifying, determining, and accepting tolerances on molded plastic parts. DIN 16901 is still referenced in some legacy programs, particularly in German-speaking markets, but it has not been updated and is no longer considered the leading standard for new product development.

Which tolerance grade should I specify for a typical injection-molded part?

Most commercial injection-molded parts fall within TG3 to TG6, depending on material, part complexity, and production conditions. TG4 or TG5 is a reasonable starting point for general-purpose structural components. Functional interfaces, snap fits, and sealing surfaces often require TG3. TG1 and TG2 are reserved for high-precision applications and require significant investment in tooling and process control.

Can ISO 20457 be applied to processes other than injection molding?

ISO 20457 is written broadly enough to apply to multiple plastic molding processes, including compression molding, blow molding, and transfer molding. However, the achievable TG grades will differ by process — injection molding typically supports the tightest grades, while blow molding and rotational molding operate at wider tolerances due to the nature of those processes.

What happens if a dimension has no tolerance specified on the drawing?

Under ISO 20457, untoleranced dimensions are governed by the general tolerance for the part's designated TG grade. A part is considered acceptable as long as it meets its functional and usage requirements, even if some untoleranced dimensions fall slightly outside the general tolerance — unless the drawing or quality agreement explicitly states otherwise.

How does ISO 20457 apply during prototyping?

ISO 20457 applies to molded parts, so it becomes directly relevant once you move to injection mold tooling. During prototyping phases using CNC machining, 3D printing, or vacuum casting, different tolerance conventions apply because those processes have different dimensional capabilities and error sources. Understanding your production TG target early, however, helps you design prototype validation tests with the right functional margins.

ISO 20457 represents a genuine step forward in how the industry thinks about plastic part tolerances. By moving beyond simple size tables to a structured system that accounts for process factors, material behavior, geometric form, and acceptance conditions, it gives designers and manufacturers a common language for specifying what is achievable and what is acceptable. The result is fewer disputes, fewer failed assemblies, and faster programs from prototype to production.

The standard rewards early engagement. Classifying dimensions as W or NW, scoring the five P-factors against your material and process, and limiting tight tolerances to functionally critical features are decisions that pay dividends throughout the product lifecycle — from first sample approval through to high-volume production. The teams that understand ISO 20457 are the ones writing drawings that their manufacturers can actually build to, on schedule and to cost.