Ever Had a 3D-Printed Part That Just Didn’t Fit?
If you’ve worked with 3D printing, you know the frustration of a part that’s slightly too big or too small. It looks great fresh off the printer, but when you try to fit it into an assembly—bam!—it doesn’t align, screws don’t seat, or mating parts refuse to connect.
That’s where dimensional tolerancing comes in. It’s not just a buzzword; it’s the difference between a precision-engineered product and a pile of near-misses. And in HP Multi Jet Fusion (MJF) 3D printing, understanding and controlling tolerances can take your prints from “almost right” to “spot on every time.”
Why Tolerances in 3D Printing Aren’t as Simple as They Seem
Unlike CNC machining, where cutting tools carve out dimensions with micrometer precision, additive manufacturing (AM) builds parts layer by layer. This introduces factors like material shrinkage, powder spreading, and heat distortion—all of which can throw off your dimensions if not accounted for.
HP’s MJF technology is one of the most precise 3D printing methods out there, with repeatability that rivals injection molding. But even with its accuracy, knowing your tolerances is crucial to ensure a perfect fit every time.
How Precise Is HP MJF? (Hint: It’s Pretty Good)
Dimensional accuracy is often measured using International Tolerance (IT) Grades, a global standard that defines how much variation is acceptable in manufacturing.
HP MJF typically achieves IT10 – IT12, meaning its tolerances are tight enough for most engineering applications. To put that in perspective:
| IT Grade | Typical Application |
| 01 – 4 | Ultra-precise measuring tools (microscopes, calibration instruments) |
| 5 – 11 | Engineering fits, gears, machined metal parts |
| 12 – 16 | Large-scale manufacturing (die casting, stamping, sand casting) |
With IT10 – IT12, HP MJF is ideal for functional assemblies, custom enclosures, and mechanical components—just don’t expect aerospace-grade tolerances without post-processing.
What Causes 3D Printing Tolerance Issues?
Even with HP’s precise system, dimensional shifts happen. Why? Here are some common culprits:
- Material Shrinkage – Most MJF parts shrink by 1-2%, meaning if you don’t compensate in your CAD model, your part won’t match spec.
- Powder Spreading Variability – MJF lays down ultra-thin layers of powder, but microscopic shifts affect fine details.
- Thermal Effects – Parts expand and contract during printing and cooling, which can cause slight deformations.
- Post-Processing Changes – If you machine, polish, or dye your part, tolerances can shift even after printing.
How Do We Measure Process Accuracy?
Manufacturers use a Process Capability Index called CpkC_{pk} to measure how well a process holds tolerances. The higher the CpkC_{pk}, the better the consistency.
Cpk=Cp×(1−2×bias)C_{pk} = C_p \times (1 – 2 \times \text{bias})
In simple terms:
- CpC_p = How repeatable your process is.
- Bias = How far off the average dimension is from the target.
- CpkC_{pk} = The final measure of how accurate AND repeatable your parts are.
How Good Is “Good Enough”?
For real-world manufacturing, a CpkC_{pk} of 1.33 (4 sigma) is the gold standard—meaning 99.94% of parts are within tolerance.
| CpkC_{pk} | Sigma Level | Parts Within Spec (%) |
| 0.67 | 2 | 95.45 |
| 1.00 | 3 | 99.73 |
| 1.33 | 4 | 99.9937 |
| 1.50 | 5 | 99.99966 |
If CpkC_{pk} drops below 1.00, you’re rolling the dice—you’ll need 100% inspection or expect a lot of rework.
How to Get Perfect Fits in HP MJF
Want better results? Follow these best practices:
✅ Compensate for Shrinkage in CAD – Scale up your part 1-2% to offset shrinkage.
✅ Keep Wall Thicknesses Consistent – Thin walls cool unevenly, causing dimensional warping.
✅ Design for Post-Processing – If you’re sanding or machining, leave a little extra material.
✅ Use In-Process Monitoring – HP’s real-time thermal imaging helps ensure layer-by-layer accuracy.
HP’s Secret Weapon: Built-in Process Control
HP has an ace up its sleeve: Jet Fusion 5200 printers include automated dimensional profiling, which self-adjusts printing parameters for better repeatability.
✔ Fleet-Wide Uniformity – Keeps tolerances consistent across multiple machines.
✔ Adaptive Process Tuning – Adjusts parameters in real-time to maintain accuracy.
✔ Repeatability Optimization – Reduces the need for manual tuning and guesswork.
Final Thoughts: The Key to Successful 3D Printing Is Precision
Dimensional tolerancing is often overlooked in 3D printing, but it’s the key to getting parts that actually work. HP MJF is one of the most precise additive manufacturing technologies available, but you still need to design smart, compensate for shrinkage, and monitor your process to get the best results.
Looking to fine-tune your MJF prints and get perfect tolerances?
Reach out to RapidMade at rapidmade.com to optimize your 3D printing workflow!
