Injection Molding Tolerances: Standards, Charts & Design Guidelines

Your design file says ±0.1mm. Your molder quotes ±0.2mm. Your customer requires flatness within 0.05mm across the whole sealing surface. Three different numbers — none of them speak the same language. That’s the core problem with tolerancing in 사출 성형: linear dimensions and geometric tolerances are not the same thing, and confusing them can cost you an entire production run.

This guide explains what geometric tolerances actually mean in injection molding, how GD&T symbols translate to mold and part requirements, and what you can realistically hold in production — with specific numbers, not vague ranges.

What Are Geometric Tolerances in Injection Molding?

사출 성형에서 기하 공차는 이 섹션에서 설명하는 주요 범주 또는 옵션입니다. 벤더를 비교하거나 조달을 계획 중이라면, injection molding supplier sourcing guide covers RFQ prep, qualification, and commercial risk checks.

Geometric tolerances define the permissible variation in the shape, orientation, location, and runout of a feature — not just its size. In injection molding, a part may measure within ±0.1mm in diameter but still fail assembly because its mating surface is 0.3mm out of flat. That failure is a geometric tolerance problem, not a dimensional one.

The formal system for specifying geometric tolerances is GD&T — Geometric Dimensioning and Tolerancing — standardized under ASME Y14.5 and ISO 1101. GD&T divides tolerances into five categories: form (flatness, straightness, circularity, cylindricity), orientation (parallelism, perpendicularity, angularity), location (true position, concentricity, symmetry), runout (circular runout, total runout), and profile (profile of a line, profile of a surface).

For injection-molded parts, the most commonly applied GD&T controls are flatness (sealing surfaces, mounting faces), true position (boss locations, snap-fit hooks), and perpendicularity (walls, ribs, pins). Each of these tolerances must account for how plastic behaves during cooling — something a purely dimensional callout cannot capture.

What Tolerance Levels Can Injection Molding Actually Hold?

Standard commercial-grade injection molding holds ±0.2mm on non-critical features. Fine-tolerance production reaches ±0.05–0.1mm on critical dimensions with controlled materials and validated tooling. Anything tighter than ±0.05mm typically requires secondary machining or precision tooling with temperature-controlled presses.

The SPI (Society of the Plastics Industry) tolerance guidelines categorize parts into three classes. Commercial class allows ±0.25mm on most features and suits consumer products. Fine class targets ±0.13mm for functional components. Precision class aims for ±0.05mm on critical features and applies to medical, aerospace, and automotive sealing interfaces.

Geometric tolerances add another layer. Even when a dimension is within spec, the form may not be. A flat boss face specified at 0.1mm flatness is far more demanding than a ±0.1mm dimension callout — it requires the entire surface to lie within a 0.1mm tolerance zone, regardless of where the part falls dimensionally.

Material selection drives tolerance capability as much as tooling does. Amorphous resins like PC and ABS shrink uniformly and typically hold tighter tolerances. Semi-crystalline materials like nylon and POM have higher and more variable 수축¹ rates, making geometric controls harder to achieve without compensating the mold.

How Does Plastic Shrinkage Affect Geometric Tolerances?

Shrinkage is the primary variable that separates geometric tolerance theory from production reality. Every plastic material shrinks as it transitions from melt to solid — typically 0.1% to 3% — and this shrinkage is never perfectly uniform across a complex part. Non-uniform shrinkage creates warp, which directly violates flatness and perpendicularity callouts.

The mold is intentionally oversized to compensate for shrinkage. A part nominally 100mm long with a 0.5% shrinkage rate requires a mold cavity of 100.5mm. But if wall thickness varies — say, 2mm in one zone and 4mm in another — the thicker section shrinks more and later, pulling the part out of flat even when each zone individually measures within the linear tolerance band.

This is why geometric tolerances require 금형 흐름 분석². Without simulating flow and cooling, you cannot predict where differential shrinkage will concentrate, which zones will warp, or whether a GD&T flatness callout of 0.1mm is achievable before any steel is cut. Mold flow analysis converts geometric tolerance requirements into design constraints — wall thickness limits, gate positions, cooling channel layouts — before tooling begins.

Warpage vs. Shrinkage: Two Different Problems

Shrinkage is predictable and compensated in the mold. Warpage is the residual deformation that remains after compensation — caused by differential shrinkage, residual stress, or uneven cooling. A part can have correct average dimensions but still fail a flatness callout by 0.3mm due to warpage. The distinction matters because you solve them differently: shrinkage is a mold dimension problem; warpage is a cooling and packing pressure problem.

Warpage is measured against a datum plane defined in the GD&T drawing. If the part rocks on its primary datum, every downstream geometric callout becomes unreliable — positional tolerances reference datums that don’t sit flat. Establishing stable datum surfaces is therefore the first step in a geometric tolerance analysis for injection-molded assemblies.

Material Shrinkage Comparison Across Common Resins

Different materials shrink at vastly different rates, which directly impacts how tight a geometric tolerance can realistically be held. Below is a comparison of common injection molding resins and their typical shrinkage ranges, along with the practical flatness tolerance achievable in production.

ABS and PC shrink 0.4–0.7% and consistently achieve ±0.1mm linear tolerances with 0.15–0.2mm flatness in production. Nylon 6/6 (PA66) shrinks 1.0–2.0% with significant anisotropy when glass-filled, requiring mold compensation and careful cooling design to hit ±0.15mm linear and 0.25mm flatness. POM (acetal) shrinks 1.5–3.5% but is predictable, allowing ±0.1–0.15mm on precision-tooled parts. PEEK and engineering grades shrink 0.1–0.5% but require specialized tooling and process control to achieve their inherently low shrinkage consistently.

Glass-filled grades complicate geometric tolerances further. Glass fibers orient along the flow direction during injection, creating anisotropic shrinkage — the part shrinks differently in the flow direction versus cross-flow. This differential contraction bows flat parts and shifts boss positions out of true position tolerance. When specifying geometric tolerances on glass-filled parts, build in 20–30% additional tolerance or validate with mold flow analysis first.

How Does GD&T Apply to Mold Design?

GD&T callouts on a part drawing directly translate into mold steel requirements. A flatness callout of 0.05mm on a sealing surface means the mold cavity must be machined and polished to better than 0.02mm flatness — accounting for the fact that the mold face must be significantly more accurate than the part it produces, to allow for tool wear and process variation.

True position callouts on boss and pin locations drive EDM and CNC machining tolerances in the mold. A true position of ±0.1mm on a connector pin pattern requires the mold to hold core pin positions to ±0.04mm or better, because the molding process introduces its own variation through packing pressure and thermal cycling.

분할선은 금형 설계 기하 공차와 가장 직접적으로 상호작용합니다. 분할선 표면은 평평해야 하며 두 금형 반쪽에 걸쳐 정확히 일치해야 합니다. 분할선의 모든 스텝 또는 갭은 플래시를 생성하고 분할면 근처 표면을 참조하는 모든 기하 치수 기준에 전파되는 기준 오차를 유발합니다. 고정밀 부품의 경우, 분할선 평면도는 일반적으로 금형에서 0.02–0.03mm, 이로 인해 성형 부품에서는 0.04–0.07mm로 유지됩니다.

Datum Selection in Injection-Molded Part Drawings

The datum scheme chosen in a GD&T drawing must align with how the part is actually fixtured — in the mold, in the assembly, and in the CMM inspection fixture. If you select a datum surface that is adjacent to the parting line, you will almost certainly have datum instability from parting line mismatch and flash burrs. Best practice: place primary datums on surfaces formed by a single mold half, not at parting surfaces.

For injection-molded parts, the three-datum rule applies rigorously. Datum A (primary) should be the largest, most stable surface — typically a flat base formed in the cavity half. Datum B (secondary) constrains rotation. Datum C (tertiary) constrains translation. When this hierarchy is violated in the drawing, inspection results become ambiguous and incoming quality disputes are nearly impossible to resolve.

What Are the Most Common Geometric Tolerance Failures in Injection Molding?

사출 성형에서 가장 흔한 기하 공차 부적합은 이 섹션에서 설명하는 주요 범주 또는 옵션입니다. 밀폐 표면의 평면도 부적합은 사출 성형에서 기하 공차 불량의 대부분을 차지합니다. 근본 원인은 거의 항상 차등 냉각입니다 — 부품의 한 영역이 더 빨리 고화되어 표면을 그릇 또는 안장 모양으로 당깁니다. 부품은 각 지점에서 치수 사양 내에서 측정되지만 전체 표면에 걸친 평면도 공차대에는 부합하지 않습니다.

True position failures on boss and hole patterns are the second most common rejection. Differential shrinkage between the boss zone and surrounding wall displaces the boss centerline from its nominal position. On a 200mm long part with four mounting bosses, ±0.5mm shrinkage variation shifts outer bosses by 0.3–0.5mm — easily exceeding a ±0.2mm true position callout without any mold machining error.

Perpendicularity failures on snap-fit hooks and latch arms occur when uneven wall thickness causes the vertical feature to lean during ejection. The base of the snap is stiffer and shrinks less; the tip cools last and contracts, pulling the hook out of perpendicular. The fix is usually a small rib behind the snap arm — a 10-minute DFM change that prevents a tolerance failure that cannot be corrected in the mold after tooling.

Tolerance Stack-Up in Assembled Plastic Subassemblies

Geometric tolerance failures rarely appear in isolation. In an assembly of three or four injection-molded parts, each with its own flatness, position, and perpendicularity variation, the worst-case stack-up can prevent proper fit even when all individual parts pass incoming inspection. This is the tolerance stack-up problem, and it is especially severe with plastic because part-to-part variation is higher than with machined metal components.

The solution is statistical tolerance analysis — RSS (root sum square) or Monte Carlo simulation — during the design phase, not after first articles fail. For assemblies with more than three molded components, statistical stack-up should be a mandatory design gate before tooling authorization. The alternative is discovering in production that a 100% yield on individual parts produces 20% assembly rejects.

How Do You Specify Geometric Tolerances on a Plastic Part Drawing?

Start with function, not with tradition. Ask: what does this surface need to do? A sealing face needs flatness. A bearing bore needs cylindricity. A connector pin pattern needs true position. Assign only the geometric controls that the function actually requires — each additional callout adds inspection cost and creates rejection risk.

Always specify material and process conditions on the drawing. GD&T callouts for injection-molded parts should reference the measurement state: as-molded, 24-hours post-ejection, or conditioned at 23°C/50% RH per ASTM D5947. A flatness callout measured 5 minutes after ejection will read differently than one measured 24 hours later after stress relaxation — sometimes by 0.1–0.2mm on large parts.

Coordinate with your molder before finalizing the drawing. A tolerance that is technically achievable in one material may be impossible in the material your supply chain specifies. Get your molder’s DFM input on geometric callouts before the drawing reaches revision lock — changes after tooling authorization cost 10–50× more than changes in the design phase.

Use a DFM review to validate geometric callouts against production capability before cutting steel. A DFM review takes 4–8 hours and surfaces tolerance conflicts that would otherwise appear as first-article failures — at a fraction of the cost of a mold modification.

자주 묻는 질문

사출 성형이 유지할 수 있는 가장 엄격한 기하학적 공차는 무엇인가요?

정밀 사출 성형은 온도 제어 금형, 검증된 재료, 과학적 성형 공정 제어를 통해 중요한 선형 치수에서 ±0.025–0.05mm, 평면도에서 0.04–0.08mm를 유지할 수 있습니다. ±0.025mm보다 엄격한 공차는 일반적으로 사출 성형만으로는 달성할 수 없으며, 성형 후 2차 CNC 가공 작업이 필요합니다. 달성 가능한 기하 공차는 재료 수축률, 부품 형상 복잡성, 벽 두께 균일성, 냉각 시스템 설계 및 제어되는 특정 GD&T 특성에 크게 의존합니다. 많은 사출 성형 부품 형상에서 평면도 요구사항은 위치도보다 일반적으로 달성하기 어렵습니다.

How does material choice affect geometric tolerances in plastic parts?

재료 수축률과 이방성은 기하 공차 능력에 지배적인 요소입니다. ABS, PC, PMMA와 같은 비정질 수지는 모든 방향으로 균일하게 0.3–0.7% 수축하며, 반결정성 재료보다 더 타이트한 기하 공차를 일관되게 달성합니다. PA66, POM, PP와 같은 반결정성 수지는 1–3% 수축하며 상당한 방향 변화를 보여, 보상 금형 형상 없이는 평면도와 위치 치수 기준을 유지하기 어렵게 만듭니다. 유리섬유 첨가 등급은 유동 방향 이방성을 도입하여, 수정된 금형 설계와 검증된 충전 시뮬레이션이 없으면 200mm 부품에서 0.3–0.8mm 뒤틀림을 유발할 수 있습니다.

What is the difference between a linear tolerance and a GD&T geometric tolerance?

선형 공차는 부품 상의 두 점 사이의 거리를 제어하며, 굽힘, 비틀림, 테이퍼 또는 측정 지점 간의 정렬 불량을 감지할 수 없습니다. GD&T 기하 공차는 정의된 공차 구역 내에서 표면 또는 형상의 전체 모양, 방향 또는 위치를 제어합니다. 이는 점 간 거리가 아닌 전체 표면을 제약합니다. 부품은 모든 측정 지점에서 ±0.1mm 선형 공차 내에 있을 수 있지만, 동시에 0.1mm 평면도 요구사항을 충족하지 못할 수 있습니다. 이는 표면이 치수 검사로 포착할 수 없는 방식으로 측정 지점 사이에서 휘어지기 때문입니다.

Can I use GD&T true position instead of ±XY coordinates for boss locations?

네, 그리고 위치도 공차는 일반적으로 사출 성형된 보스 패턴에 더 나은 선택입니다. 위치도 공차는 명목 위치를 중심으로 하는 원형 공차대를 정의하며, 여전히 조립 기능을 보장하면서도 어떤 단일 축에서도 약간 더 많은 변동을 허용합니다. ±0.1mm XY 치수 기준은 정사각형 영역을 제공합니다; 지름 0.14mm 위치도 공차는 동등한 최악의 경우 면적을 가진 원형 영역을 제공합니다. 위치도 공차는 CMM 소프트웨어로 검사하기 더 쉬우며 기능적 조립 요구 사항을 더 잘 나타내어, 생산 보스 및 핀 위치 제어를 위한 선호되는 방법이 됩니다.

Why do injection-molded parts often fail geometric tolerances even when dimensions are in spec?

차등 수축률은 점 대 점 선형 치수가 완전히 놓치는 형태 오류를 생성합니다. 부품은 양 끝점에서 정확히 100.0mm를 측정하는 동시에 중앙에서 0.3mm 만큼 휠 수 있습니다 — 길이 공차 내에 있지만 명백히 0.1mm 평면도 치수 기준을 벗어납니다. 게이트 압력 구배, 두꺼운 벽과 얇은 벽 영역에 걸친 불균일한 냉각, 그리고 급격한 벽 두께 변화는 모두 이젝션 후 기하 변형으로 해소되는 내부 잔류 응력을 생성하며, 측정 지점에서의 치수 오프셋으로 나타나지 않습니다. 이것이 기능성 플라스틱 조립체에 기하 관리가 필수적인 이유입니다.

몰드 부품의 기하 공차를 관리하는 데 도움이 되는 소프트웨어 도구는 무엇인가요?

SolidWorks, Creo, CATIA와 같은 CAD 패키지에는 3D 모델의 형상에 직접 공차 기호를 부착하는 내장 GD&T 모듈이 포함되어 있습니다. 시뮬레이션을 위해 Moldflow와 Moldex3D는 강철이 절단되기 전에 GD&T 요구사항에 대한 수축 및 뒤틀림을 예측합니다. 검사에서는 PolyWorks 및 Calypso와 같은 도구가 CMM 프로브 데이터를 기하 공차 사양에 대한 편차 맵으로 변환하여 부품 출하 전에 공차 이탈 상태를 쉽게 발견할 수 있게 합니다. 시뮬레이션과 GD&T 인식 검사를 결합하면 생산 환경에서 초도품 불량률을 크게 줄일 수 있습니다.

Ready to Tolerance Your Injection-Molded Parts Correctly?

Quick rule: assign flatness to sealing surfaces, true position to boss patterns, perpendicularity to snap fits, and cylindricity to precision bores. Specify measurement state on the drawing. Run mold flow analysis before finalizing callouts on glass-filled or semi-crystalline materials. And validate your datum scheme against your CMM fixture before first articles arrive.

At ZetarMold, our engineering team reviews geometric tolerance callouts as part of every DFM process — flagging unrealistic specs before tooling, not after. If you have a drawing with GD&T callouts you’re not sure a molder can hit, send it our way. We’ll tell you exactly what’s achievable and what needs adjustment.

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1. shrinkage: 수축률: 수축률은 성형 부품이 냉각 및 경화되면서 겪는 치수 감소를 의미하며, 원래 금형 캐비티 치수 대비 백분율로 측정됩니다. 재료 및 벽 두께에 따라 일반적으로 0.1%에서 3%입니다. ↩

2. mold flow analysis: 몰드 흐름 분석: 몰드 흐름 분석은 용융 플라스틱이 금형 캐비티를 어떻게 채우는지 예측하는 데 사용되는 CAE 시뮬레이션 방법으로, 엔지니어들이 강철 절삭 전에 게이트 위치, 벽 두께, 냉각을 최적화할 수 있게 합니다. ↩

3. parting line: 분할선: 분할선은 금형의 두 반쪽이 만나는 사출 성형품의 경계를 말하며, 완성된 부품을 이젝션하기 위해 사용되는 분리 평면을 정의합니다. ↩