Injection molding tolerances are the allowed size variations for part features and overall dimensions. They are usually shown as plus or minus values in millimeters or inches. This acceptable variation depends on part size, resin, wall thickness, tooling quality and process repeatability.
A skilled team can estimate shrinkage for different resins, but the final result still depends on geometry, material choice and cooling behavior. If the resin has a high shrinkage rate, plastic injection molding tolerances become harder to hold.
Injection Molding Tolerance Stackups
A tolerance stackup describes how several parts must still fit together when each one has its own allowed variation.
For example, three molded parts may each have a hole that is technically within specification. That is useful, but the holes still need to align so a fastener can pass through the full assembly. This is where dimensional deviation becomes practical rather than theoretical.
For complex assemblies, the Moldflow TolTool can help engineers compare simulation results with real measurement data and evaluate whether deviations stay within the required limits.
Types of Injection Molding Tolerances
There is more than one injection molding tolerance to consider. Each feature can have its own requirement depending on the function and risk.
- Dimensional: Overall size of the part. Larger parts usually need wider limits because shrinkage accumulates over distance.
- Straightness or flatness: Controls warpage in larger flat areas.
- Hole diameter: Larger holes usually need wider limits because shrinkage is harder to control.
- Blind hole depth: Deeper pins can deflect under pressure, which affects depth accuracy.
- Concentricity or ovality: Thin cylindrical parts can shrink unevenly and lose circularity.
Standard injection molding tolerances are usually divided into commercial and fine classes. Commercial values are less strict and reduce costs. Fine values are used when the function requires tighter control, but the tool and process become more expensive.
Injection Mold Tolerances by Feature Size and Material
Part values are connected to tool values. At the same time, injection mold tolerances must reflect feature size and material behavior.
The tables below show reference values for common materials. ABS means acrylonitrile butadiene styrene. PC is polycarbonate. HDPE and LDPE are polyethylene grades. PA is nylon. PA GF 30% is nylon with 30% glass fiber. PMMA is acrylic. POM is polyoxymethylene. PP is polypropylene. SAN is styrene acrylonitrile.
Dimensional Tolerances
| Type | Commercial | Fine | ||||
| Feature size: | 1-20 mm | 21-100 mm | 101-160 mm | For every 20 mm over 160 | 1-20 mm | 21-100 mm |
| ABS | 0.100 | 0.150 | 0.325 | 0.080 | 0.050 | 0.100 |
| ABS/PC | ||||||
| HDPE | 0.125 | 0.170 | 0.375 | 0.100 | 0.075 | 0.110 |
| LDPE | ||||||
| PA | 0.075 | 0.160 | 0.310 | 0.80 | 0.030 | 0.130 |
| PA GF 30% | 0.060 | 0.120 | 0.240 | 0.080 | 0.030 | 0.100 |
| PC | ||||||
| PMMA | 0.075 | 0.120 | 0.250 | 0.080 | 0.050 | 0.070 |
| POM | 0.075 | 0.0160 | 0.310 | 0.080 | 0.030 | 0.130 |
| PP | 0.125 | 0.170 | 0.375 | 0.100 | 0.075 | 0.110 |
| SAN | 0.100 | 0.150 | 0.325 | 0.80 | 0.50 | 0.100 |
A small feature can be controlled more tightly than a larger section because dimensional variation grows as the molded distance increases.
Straightness/Flatness Tolerances
| Type | Commercial Tolerance | Fine Tolerance | ||
| Feature size | 0-100 mm | 101-160 mm | 0-100 mm | 101-160 mm |
| ABS | 0.380 | 0.800 | 0.250 | 0.500 |
| ABS/PC | ||||
| PA | 0.300 | 0.500 | 0.150 | 0.250 |
| PA GF 30% | 0.150 | 0.200 | 0.080 | 0.100 |
| PC | ||||
| POM | 0.300 | 0.500 | 0.150 | 0.250 |
| PP | 0.850 | 1.500 | 0.500 | 0.850 |
| SAN | 0.380 | 0.800 | 0.250 | 0.500 |
Flatness is strongly affected by cooling balance and wall thickness. A part can meet local feature dimensions but still fail flatness if it warps after ejection.
Hole Diameter Tolerance
| Type | Commercial Tolerance | Fine Tolerance | ||||||
| Feature Size | 0.3 | 3.1-6 | 6.1-14 | 14-40 | 0.3 | 3.1-6 | 6.1-14 | 14-40 |
| ABS | 0.050 | 0.050 | 0.080 | 0.100 | 0.030 | 0.030 | 0.050 | 0.050 |
| ABS/PC | ||||||||
| HDPE | 0.050 | 0.080 | 0.0100 | 0.150 | 0.030 | 0.050 | 0.050 | 0.080 |
| LDPE | ||||||||
| PA | 0.050 | 0.080 | 0.080 | 0.130 | 0.030 | 0.040 | 0.050 | 0.080 |
| PA GF 30% | 0.050 | 0.050 | 0.080 | 0.080 | 0.030 | 0.040 | 0.050 | 0.050 |
| PC | 0.050 | 0.050 | 0.080 | 0.080 | 0.030 | 0.040 | 0.050 | 0.050 |
| PMMA | 0.080 | 0.080 | 0.100 | 0.130 | 0.030 | 0.050 | 0.050 | 0.080 |
| POM | 0.050 | 0.080 | 0.080 | 0.130 | 0.030 | 0.040 | 0.050 | 0.080 |
| PP | 0.050 | 0.080 | 0.100 | 0.150 | 0.030 | 0.050 | 0.050 | 0.080 |
| SAN | 0.050 | 0.050 | 0.080 | 0.100 | 0.030 | 0.030 | 0.050 | 0.050 |
Hole values depend on diameter, pin stability and material shrinkage. Blind holes can be harder to hold because longer core pins may move under pressure.
How Material Affects Molding Tolerances
Plastic molding tolerances depend heavily on resin properties. Shrinkage, flow, cooling rate, additives and thermal expansion all influence part accuracy. ABS, PP, PC, nylon and glass-filled materials do not behave the same way in a mold.
The shrinkage rate is especially important. ABS may shrink far less than polypropylene, so changing resin after the tool is built can move the final part outside its required range. Glass-filled plastics often shrink less in the flow direction, but they may behave differently across the transverse direction.
Resin selection should also consider the end-use environment. A plastic resin may meet the drawing at room temperature, but change dimensions during service. For tight tolerance injection molding, engineers need to review shrinkage, expansion, stiffness and expected operating temperature together.
Part Shrinkage and Tolerances
To compensate for cooling shrinkage, engineers design the part to nominal dimensions and scale the cavity by the expected shrinkage rate. Injection molding simulation helps test that assumption before steel is corrected.
Shrinkage can be calculated using ASTM D955-style testing:
Shrinkage Rate = 100% × (Lc – Lp) / Lp
Lc means cavity length. Lp means part length after cooling. For fiber-filled materials, the result can differ by direction because material behavior is not always symmetrical.
Typical Shrinkage Values
| Material | Shrinkage range, % |
| ABS | 0.7-1.6 |
| PC/ABS | 0.5-0.7 |
| Acetal/POM (Delrin®) | 1.8-2.5 |
| ASA | 0.4-0.7 |
| HDPE | 1.5-4 |
| HIPS | 0.2-0.8 |
| LDPE | 2-4 |
| Nylon 6/6 | 0.7-3 |
| Nylon 6/6 Glass Filled 30% | 0.5-0.5 |
| PBT | 0.5-2.2 |
| PBT Glass Filled 30% | 0.2-1 |
| PEEK | 1.2-1.5 |
| PEEK Glass Filled 30% | 0.4-0.8 |
| PEI (Ultem®) | 0.7-0.8 |
| PMMA (Acrylic) | 0.2-0.8 |
| PC | 0.7-1 |
| PC Glass Filled (20-40%) | 0.1-0.3 |
| Polyethylene Glass Field | 0.2-0.6 |
| Polypropylene Homopolymer | 1-3 |
| Polypropylene Copolymer | 2-3 |
| PPA | 0.5-2.2 |
| PPO | 0.5-0.7 |
| PPS | 0.6-1.4 |
| Rigid PVC | 0.1-0.6 |
| SAN (AS) | 0.3-0.7 |
| TPE | 0.5-2.5 |
| TPU | 0.4-1.4 |
These values are general references. For typical injection molding tolerances, consult the actual material datasheet and consider the molding conditions.
Mold Flow Analysis
For complex geometry, mold flow analysis is one of the best ways to estimate shrinkage. The software shows how resin fills the cavity, where pressure drops occur and where cooling may create local movement.
Simulation can also show shrinkage variation across a part. This helps engineers check whether the finished molded part can meet drawing limits before tooling changes become costly.
Designing for Moldability: Part Dimensions
Moldability should be reviewed early. A design for manufacturing analysis checks whether geometry, draft, walls and ribs support stable production.
Wall thickness is one of the strongest drivers of dimensional accuracy. Thick areas cool slowly and can create sink marks. Uneven walls cool at different rates, which can create warpage and non-uniform shrinkage. Consistent wall thickness improves dimensional consistency.
Draft also affects dimensions. If a part has too little draft, the geometry may need to change before molding. A typical starting point is 1 to 2 degrees, but the right value depends on material, texture and tool construction.
Best Practices for Optimizing Injection Molding Tolerances
The best results come from early review, stable processing and realistic specifications. Process optimization helps connect CAD intent with measurable production capability.
Design For Manufacturability (DFM)
DFM checks part complexity, undercuts, flow paths and fill behavior before the tool is built. Complex geometry may need deeper review and mold fill analysis to prevent shrinkage, warpage and plastic injection molding tolerances issues.
Parting Line Mismatches
Parting line mismatch can create steps, flash or local measurement errors. Engineers should review shut-offs, alignment features and wear areas early, especially when critical features sit close to the split line.
Material Selection
Material should be selected during DFM and tooling planning, not after the tool is designed. Engineers should check datasheets, shrinkage values, expansion behavior, mechanical needs and whether the part needs commercial values or tight tolerances.
Process Control Parameters
Injection pressure, holding pressure, mold and melt temperature, fill time, cooling time and resin viscosity all affect injection molding tolerance standards in production. These variables should be validated against the target feature limits before full release.
Tooling Considerations
Mold tools are often built slightly oversized to compensate for shrinkage, so material choice must be known early. Single-cavity tools are easier to control. Multi-cavity and family tools need balanced cooling, consistent cavities and stable gate locations. Poor cooling can create lower deviation or upper deviation problems across cavities, especially in tight-tolerance injection molding.
Implement repeatable process controls
Sensors, monitoring and standardized setup sheets help keep the process stable. Real-time feedback allows teams to react when pressure or temperature begins to drift. For tight tolerances, parts should also be measured under consistent environmental conditions because plastics can change size as temperature changes.
When to Work with Injection Molding Experts on Tolerances
Not every dimension needs tight control. Moldflow Software experts can help define critical features, relax non-functional dimensions and reduce unnecessary cost.
Expert review helps align injection molding tolerance standards with real function, tooling cost and process stability.
FAQ
What are the tolerances for injection molding?
Injection molding tolerances are the allowed dimensional limits for molded part features. Values depend on size, resin, part quality and process stability.
Which injection molding tolerances do you need to specify?
Specify dimensional, straightness or flatness, hole diameter, blind hole depth and concentricity or ovality where they affect function.
How does tooling affect plastic injection molding tolerances?
Tool quality affects cooling, shrinkage, gate balance and repeatability. Better tooling improves consistency and supports tighter feature control.
How does overall size and wall thickness affect part tolerance?
Larger parts usually need wider limits. Uneven wall thickness increases shrinkage variation, warpage and measurement risk.
What is the ISO standard for injection molding tolerances?
ISO 20457:2018 is the main standard for plastic molded parts tolerances. It defines possible manufacturing injection molding tolerances and supports agreement between suppliers and customers.
What are the 4 types of tolerances?
The main engineering types are unilateral, bilateral, limit and geometric tolerances. In molded parts, they are applied based on function and inspection needs.
