You may have seen this before. The product looks superb in the CAD file, but real-life parts look like scrap. That’s because plastics have a mind of their own. They breathe, shrink, and fight back. Engineers who understand injection molding tolerances can anticipate and design with these changes in mind.
Tolerance is the language of precision manufacturing. It’s not a suggestion. Every decimal point on a drawing is a promise that must be kept over a million cycles, and throughout the part’s lifespan.
What are Injection Molding Tolerances?
An injection molding tolerance is the acceptable deviation from a part’s dimension. It is a plus or minus value often measured in mm or inches.
For instance, when a CAD file calls for 100 mm diameter, a tolerance of 0.05mm tells us whether at 100.05 or 99.95 mm the part will still fit and function well.
Tolerances can be tight or high precision. Speaking in general, a tight tolerance is approximately plus or minus 0.05 mm. This is acceptable for mating fixtures.
A high precision tolerance is approximately plus or minus 0.025mm. This is a requirement for mission- critical aerospace or medical parts.
Tolerances must account for:
- Variations in material batches
- Changes in the cooling water temperatures
- Tool wear on parting lines
No manufacturing is perfect. A part’s dimensions will always naturally vary from the original design.
The tighter the tolerance, the higher the cost of manufacturing. The looser the tolerance, the more unreliable the part performance.
The goal for injection molding tolerancing is to set the widest possible dimensional limits within which a part can function 100% efficiently and be manufactured quickly.
Why is Tolerancing so Critical?
Tolerancing ensures that parts work as expected. In real-life applications, tolerances directly affect safety, operational efficiency, and total cost of ownership.
If a surgical component is off by even a few tenths of a millimeter, the medical procedure could fail, risking the patient’s life. If an automotive clip is too loose, it could lead to a damaging electrical short.
Frequent mechanical failure due to improperly fitting parts causes expensive downtimes and increases maintenance costs.
When a part is produced with appropriate tolerances, it performs better, lasts longer, and costs less to produce.
Typical Tolerances for Injection Molded Parts
| Industry Sector | Example Parts | Typical Tolerance (mm) |
| Medical | Surgical valves and dosing tools | ± 0.01 – 0.05 |
| Electronics | Micro-connectors and SIM trays | ± 0.02 – 0.05 |
| Aerospace | Avionics housings and seals | ± 0.020 – 0.050 |
| Cosmetics | Lipstick twist mechanisms | ± 0.030 – 0.080 |
| Automotive | Fuel injectors and sensors | ± 0.050 – 0.100 |
| Industrial | High-load gears and bearings | ± 0.080 – 0.125 |
| Packaging | Leak-proof bottle caps | ± 0.100 – 0.150 |
| Consumer Tech | Phone cases and laptop shells | ± 0.125 – 0.200 |
| Toys | Building blocks and action figures | ± 0.10 – 0.5 |
| Housewares | Storage bins and laundry baskets | ± 0.5 mm or greater |
Why Tight is not always better
There is a common misconception in tolerancing. It goes like “by requesting the tightest injection molding tolerances, we can guarantee excellent product outcomes.”
That’s the tail wagging the dog. Do not specify a tolerance that is tighter than the resin’s thermal expansion or contraction. If you do, the part goes out of spec whenever the weather changes.
The ultimate best tolerance is built for mechanical freedom. This is intuitive geometry that allows for a wide expansion and contraction range, while still guaranteeing top performance, for reliability in different usage conditions.
Types of Injection Molding Tolerances
Dimensional tolerances
A dimensional tolerance is the acceptable variation on the part’s length, width, and diameter that still allows for seamless performance.
Geometric dimension and tolerancing enable manufacturers to control warpage, alignment, and roundness of parts meant to fit and operate in 3D spaces.
Plastic doesn’t expand or shrink uniformly, and that’s why designers and engineers assign different tolerances to the parts X, Y, and Z axes using mold flow simulations.
Dimensional tolerances include commercial and fine tolerances as governed by the ISO 20457 and DIN 12020-2 standards [1].
In both cases, the permissible tolerance increases as the part’s nominal dimension increases.
| Dimensional Tolerance Group | Tolerance ranges (mm) | Example applications |
| TG3 | 0.03- 0.15 | Micro-electronics, aerospace |
| TG4 | 0.04 – 0.20 | Medical, aerospace, high-end electronics |
| TG6 | 0.07- 0.57 | Household, consumer goods, packaging, automotive trims |
| TG9 | 0.50 – 1.50 | Heavy industrial, construction |
Commercial dimensional tolerances common in consumer goods applications have a standard or range of plus or minus 0.07–0.57mm. This range falls within TG6.
Under fine dimensional tolerances required in complex mechanical assemblies where parts must snap in place, the range falls between 0.03 – 0.20 mm (TG4).
Straightness and Flatness Tolerances
Straightness and flatness tolerances govern the outer shape of parts.
When you’ve been in this business long enough, you understand that differential shrinkage is an inevitable part of plastic injection molding. The outside of the part often cools faster than the core, and as the core cools, it pulls on the surface, causing uneven flatness.
But consistent flatness is a prerequisite for seal integrity to prevent leaks and ensure reliable assembly. Therefore, you need accurate straightness and flatness tolerancing.
The allowable straightness and flatness ranges are governed by the length of the part as detailed in the ISO 1101 and ISO 20457 [2] standards.
The permissible straightness and flatness variance increase as the length of the surface increases.
| Flatness/Straightness Tolerance Group | Tolerance Range (mm) | Example applications |
|---|---|---|
| TG3 | ± 0.05 – 0.10 | Aerospace, medical |
| TG4 | ± 0.20 – 0.40 | Automotive, electronics |
| TG5 | ± 0.50 – 1.00 | Toys, consumer goods packaging |
Hole Diameter and Depth Tolerances
These tolerances control the size and depth of holes in a molded part and govern how the part interacts with fasteners, pins, and fluid channels.
Diameter and depth tolerances range between ± 0.03 – 0.1 mm, depending on the part’s nominal size.
These dimensions are also governed by ISO 20457, but they are harder to maintain because of the way plastic flows around a core pin.
If the temperature fluctuates, weld lines form a flat spot. The plastic must knit together well for the hole to stay within tolerance limits.
Tolerances for Concentricity and Ovality
When manufacturing a pipe with an inner and outer wall, you would want the wall thickness to be consistently equal all the way round. That’s concentricity, and these tolerances help to minimize deviations.
Ovality looks at how perfectly round the part is. For example if a plastic bottle cap is too oval, it will not fit properly. Ovality tolerances aim to improve radius uniformity by preventing unevenness during cooling.
| Tolerance type | Tolerance Group (ISO 20457) | Typical Tolerance | Example applications |
| Circularity (Ovality) | TG4 | ±0.03 mm | High-speed impellers or turbine blades. |
| TG6 | ±0.15 mm | A plastic bottle cap or a standard tube fitting | |
| Concentricity | TG4 | ±0.05 mm | Dual-shot medical syringes or gear assemblies. |
| TG6 | ±0.20 mm | Battery compartments or simple pen barrels. |
Tolerances and Tolerance Stackups
It might be easier to achieve and maintain individual injection molding tolerances. But the real challenge starts when several parts are fit together in assembly.
Small acceptable deviations can compound into large unacceptable errors. This is called tolerance stackup. It’s one of the leading causes of product failure.
The fix? Perform rigorous worst-case or statistical tolerance stackup analysis before tooling. These scenario analyses reveal the best individual part tolerances needed to prevent stackup-related failure.
ASME Y14.5 and ISO 1101 [3] standards mandate that stackup tolerancing must account for both dimensional and geometric (form/positional) variations.
Factors Affecting Injection Molding Tolerances
| Factor | How it Affects Tolerance | |
| Part Design | Wall thickness | Uniform walls lead to stable tolerances. |
| Draft angles | Large draft angles eliminate the risk of part surface deformation during ejection. | |
| Part complexity | Complex parts are harder to tolerance because of dead spots. | |
| Material Selection | Crystalline vs. amorphous | It’s harder to maintain tolerance with crystalline plastics (like Nylon) because they shrink way more than amorphous plastics (like ABS). |
| Crystalline plastics shrinkage rates (%) | PEEK 1.2 – 1.5 POM (Acetal) 1.8 – 3.5 PA 66 (Nylon 6/6) 0.7 – 3.0 PP 1.0 – 3.0 HDPE 1.5 – 4.0 | |
| Amorphous plastics shrinkage rates (%) | PC 0.5 – 0.8 ABS 0.4 – 1.6 PMMA 0.1- 1.0 PS 0.1 – 0.8PVC 0.1 – 0.6 | |
| Warpage | Different areas of a part shrink at different speeds. | |
| Thermal expansion | Plastics expand and contract when processing or operational temperature fluctuates which effectively changes tolerances. | |
| Tooling Considerations | Tooling quality | A Class 101 tool offers more thermal stability. |
| Tooling precision | Molds must be machined to tighter tolerances than the actual parts. | |
| Parting line mismatches | Slight misalignment may happen where two halves of the mold meet. | |
| Process Control | Maintaining injection parameters | Separate packing pressure from filling pressure to minimize stretch Maintain stable melt temperature and cooling temperature for dimensional consistency. |
| Multiple Components and Tolerance Interplay | If you have two or more parts fitting together, individual tolerances multiply. |
Important notes
1. Use uniform wall thickness. If one side of your part is 3mm and another is 5mm, the tolerance outcome is unfavorable.
2. Use 1–3 degrees of draft (depending on surface finish) to achieve seamless part surfaces.
3. After ejection, you must wait for 24 hours before measuring part tolerance. Plastic releases the stress you put into it during molding by shrinking or warping.
4. Use a “steel safe” design for accurate tolerancing. Leave a bit of extra metal in the mold cavities. If the part comes out too small, you can grind away the extra steel.
5. In any given assembly, you want all parts to be made from the same material groups so that they expand and contract at the same rate.
6. For insert molding or overmolding, use extremely tight tolerances for the first part so that the second part doesn’t flash.
International Tolerance Standards and Rules
ISO 20457:2018
ISO 20457:2018 is the most current tolerance standard, which replaced the DIN 16901 and DIN 16742.
ISO 20457:2018 groups production accuracy into tolerance groups.
The ISO 20457:2018 tolerance groups that specifically apply to injection molding include:
- TG1 to TG3 for extreme precision parts
- TG4 for high-precision parts
- TG5 is the baseline (standard) precision applications
- TG6 (for coarse parts) where loose tolerances are acceptable
- TG7 to TG9 (for very coarse parts) where the shrinkage rate is high or unpredictable
For details, refer to our initial tables in “types of injection molding tolerances”.
DIN 16901
DIN 16901 has since been withdrawn. But it’s still a widely referenced German standard that focuses on general tolerances. This standard provides a very simple way of assigning materials to specific tolerance categories known as series.
Series 3 is for ultra-precision tolerance applications, series 2 is for precision, while series 1 is for standard commercial tolerances. This standard then uses a second breakdown that provides the actual permissible numerical deviations in mm and nominal ranges [4].
| Nominal Range (mm) (of part sizes) | Type A (± mm) | Type B (± mm) |
| 0 – 1 | 0.18 | 0.08 |
| 1 – 3 | 0.19 | 0.09 |
| 3 – 6 | 0.20 | 0.10 |
| 30 – 40 | 0.30 | 0.20 |
| 90 – 120 | 0.51 | 0.41 |
| 315 – 400 | 1.30 | 1.20 |
Type B variances apply to parts in the single half of the steel block, while type A variances apply to moving parts in the mold.
ISO 8062
ISO 8062-1:2007 defines the terms to use for tolerancing, specifically the Geometric Product Specifications for flatness and roundness of parts.
ISO 8062-3:2007 lists the needed dimensional (DCTG) and geometrical (GCT) tolerances for molded parts.
Under this standard, injection molding tolerances range from DCTG 4 to DCTG 8 for standard precision.
High precision tolerances range from DCTG 1 to DCTG 3 for critical components. These ranges are influenced by individual material shrinkage rates.
Methods and Tools for Measuring Tolerances
Injection molding tolerances are measured using CMM (Coordinate Measuring Machines) and Optical Comparators.
A CMM uses a probe to map the part’s surface in 3D with sub-micron accuracy.
These methods can also be combined with automated vision systems. These photograph the parts as they fall from the mold and relay the images to ML and AI solutions that check and verify dimensions instantly.
Best Practices to Achieve Tight Injection Molding Tolerances
Design for Manufacturability (DFM)
Start tolerancing early in the design stage. Having a 20-minute tolerance conversation during DFM can save you tens of thousands in part modifications later.
The main purpose of DFM in tolerancing is to find the best variance where the part is as loose as possible for speedy and cost-effective production and yet as tight as possible for reliable fit and performance.
During DFM, this is done by:
- Optimizing wall thickness uniformity
- Choosing resins that offer more dimensional stability
- Using gate and parting line placements that minimize weld lines and ensure uniform cavity filling
- Relaxing non-critical dimensions to standard tolerances
EzraMade
Design for Injection Molding: A Practical DFM Guide
Understand the Environment
Consider the part’s usage environment when tolerancing. Plastic has a high coefficient of thermal expansion, so your part might assemble perfectly in a 60°F lab but fail in a 110°F warehouse.
- Always apply the material’s coefficient of thermal expansion to the tolerance stackup
- Oversize the mold if you are dealing with resins that absorb moisture, e.g., nylon
- Simulate and anticipate how UV, chemicals, and cleaning agents can affect the dimensional straightness, flatness or roundness of the part
Mold Flow Analysis
This is the part that comes before you cut the steel. Mold flow analysis uses software that helps you see how the plastic will fill the mold and where weld lines, warping, or other issues can occur.
Use mold flow analysis to adjust the tolerances in the virtual world before you spend a single penny on real steel:
- Change gate location
- Adjust cooling channel placement
- Switch mold cavity dimensions
- Add vents where air seems to be compressed
Automation
The use of robotics eliminates human error so that every cycle is identical. This creates extreme consistency for tight tolerance bands in critical industries like medical and aerospace.
Robotics can be deployed in closed-loop process control to auto-adjust pressure and temperature based on set tolerance targets. They can help in precision part removal and integrated vision inspection, too.
Scheduled Maintenance and Quality Checks
Maintaining the mold and machine in good condition prevents tolerance drift. Manually scrub the mold cavity and remove resin residue from vents to prevent backup pressure buildup.
Regularly swap out seals and O-rings to prevent pressure drops and thermal spikes, which are not good for dimensional consistency.
Regularly check for washout at the gates and parting lines to ensure the steel hasn’t eroded. Similarly, verify that the machine’s thermocouples accurately reflect temperature to prevent viscosity shifts.
Conclusion
Tolerancing involves much more than preventing dimensional deviation. The best tolerance isn’t necessarily the tightest. And this number isn’t just a figure on a CAD file. It is a value that promises two things that are almost always conflicting – easy manufacturing and top part performance. It is hard-earned by ensuring that design, metallurgy, material science, and processing physics never drift apart in any given cycle.
Frequently Asked Questions (FAQ)
The tightest tolerance achievable in injection molding is typically around ±0.01 mm (0.0004 inches) under highly controlled manufacturing conditions.
The steel safe design maximizes metal and reduces plastic. It is easy to remove metal from the mold cavity but it’s hard to add metal to the mold. Therefore, the mold cavity is intentionally made slightly smaller, so the molded part comes out slightly larger. Engineers can then remove steel from the mold to gradually achieve the final target dimensions.
Because plastic fights back. The part will eventually release the stress or pressure added to it during molding and this can be noticed as post-mold shrinkage or warping. Therefore, engineers must always wait 24 -48 hours after molding before measuring tolerances so as to avoid false positives.
Sources:
- SCRIBD: DIN ISO 20457 TG6 Tolerance Overview
- ISO/DIS 20457(en): Plastics moulded parts — Tolerances and acceptance conditions
- CMM Quarterly: Key Differences between ASME and ISO GD&T
- Mould Hauss: Plastic Tolerances DIN.16901-130
