Understanding Dimensional Tolerances in Metal 3D Printing

When designing parts for Metal Binder Jetting (MBJ), understanding dimensional tolerances is key to achieving the precision you expect. Here's everything you need to know.

Introduction

When designing parts for Metal Binder Jetting (MBJ), understanding dimensional tolerances is key to achieving the precision you expect. MBJ is gaining momentum as a cost-effective and highly scalable additive manufacturing (AM) process, particularly suited for producing stainless steel parts with intricate geometries and no support structures. But while its advantages are significant, MBJ introduces a unique challenge that every designer and engineer must account for: dimensional and geometrical tolerances.

This article dives deep into the origins of these deviations, the scientific principles behind them, and how to anticipate, manage, and minimize them to ensure high-quality, functional metal parts. Drawing from recent peer-reviewed studies, practical production data, and our own in-house expertise at FINEX, we aim to demystify the topic for both experienced engineers and curious professionals.

What Are Dimensional Tolerances in MBJ?

Dimensional tolerances refer to the acceptable range of variation in a part’s dimensions compared to its CAD model. In traditional machining or MIM (Metal Injection Molding), these variations are tightly controlled. But in MBJ, tolerances must account for two stages of transformation:

1. Printing of the "green" part (layer-by-layer binding of metal powder)

2. Post-processing (debinding + sintering), which densifies the part but also causes shrinkage and deformation

Typical linear shrinkage during sintering can reach 15-20%, but more critically, this shrinkage is anisotropic, it does not happen equally in all directions.

Why Shrinkage Happens

The green part printed by MBJ is a loosely bonded object, fragile and porous. Sintering transforms this object by heating it to near-melting temperatures (~1300-1400°C), causing the metal particles to fuse and densify. This process reduces volume but introduces:

• Linear shrinkage (X, Y, Z)

• Volumetric shrinkage

• Geometric distortion (holes becoming elliptical, flat faces bowing, etc.)

Studies show that Z-axis (vertical) shrinkage is always higher due to gravity, powder settling, and binder distribution. In some cases:

• Z shrinkage: 18.5-19.5%

• XY shrinkage: 16.5-17.5%

Tolerance Tables: What to Expect

At FINEX, we apply the UNI EN 22768-1 (Class "m") standard as a base for expected tolerances:

These apply to dimensions after sintering and assume no post-processing.

Green vs. Sintered State: Double Challenge

If a part is slightly deformed in the green state due to uneven powder compaction or binder saturation, that error will be amplified or modified during sintering. Studies using Coordinate Measuring Machines (CMM) confirm that:

• Cylindricity errors increase with hole diameter

• Inclination of holes with respect to build direction affects shape

• Distortions are higher the farther you get from the tray base (due to less tray contact and more unsupported mass)

At green state, error can be ≤ 0.1 mm. After sintering, this can rise to 0.15-0.25 mm in some geometries.

Minimum Feature Guidelines (Post-Sintered)

To ensure functional and printable results, we recommend the following minimum dimensions:

• Wall thickness: ≥ 0.8 mm

• Pillars / Pins: ≥ 1.0 mm

• Through holes: ≥ 0.8 mm

• Blind holes: ≥ 1.2 mm

• Gap between walls: ≥ 0.5 mm

• Engraved/Embossed text: ≥ 0.3 mm

If your design includes features smaller than these, they may be distorted or lost during printing.

Tolerance Grades: M vs P

At FINEX, you can choose your desired tolerance grade when configuring your file:

• M Grade (Standard) - Standard compensation and tolerances per UNI EN 22768-1

• P Grade (Precision) - Enhanced dimensional control with post-processing (e.g. CNC or EDM), ideal for tight-fitting or mechanical parts

Note: P Grade may involve additional cost and lead time.

Factors Influencing Tolerance in MBJ

1. Material Type: 316L vs. 17-4PH behave differently during sintering due to phase changes and microstructure

2. Hole Orientation: Holes aligned to the printhead direction (Y) are more accurate

3. Position in Build Box: Parts near the edge can shrink differently than those at center

4. Layer Shifting: Slight misalignments during powder spreading accumulate over layers

5. Friction with Tray: Can cause trapezoidal distortion or hourglass shape

6. Binder Saturation & Drying: Poor binder distribution creates weak zones and densification inconsistencies

Cylindricity and Shape Distortion: What the Data Shows

A multi-material study using AISI 316L and 17-4PH stainless steel showed that:

• Holes closer to the tray base (L06) showed best prediction accuracy (<2% error)

• Mid-positioned holes (L09) showed 4-5% diameter error due to shape distortion

• Topmost holes (L12) saw up to 6% error and highest cylindricity deviation

In summary:

• More mass above = gravity load effect

• More distance from base = less tray friction

• More shrinkage = less predictability

MBJ vs. MIM: Which One Is More Precise?

MBJ wins for low-to-medium batches, especially when geometry might evolve.

Common Mistakes to Avoid

• Designing walls <0.8 mm thickness

• Using unsupported overhangs >2 mm

• Assuming uniform shrinkage

• Ignoring hole orientation

• Overusing sharp corners (causes stress concentration)

• Forgetting to annotate critical dimensions in CAD

How FINEX Handles Tolerances

1. Automated compensation based on material, size, and geometry, and calibrated by our engineers

2. Two Tolerance Grades:

   • M Grade: Standard - up to ±0.2 mm variation

   • P Grade: Precision - CNC-enhanced, up to ±0.05 mm variation

3. Real-world calibration using in-house CMM scanning

4. User dashboard alerts when part design is out of tolerance

The users is able to preview dimension ranges directly in the configurator before launching production.

Design Checklist for Uploading

Before submitting your design:

• Use .STEP files (avoid mesh-only .STL if possible)

• Ensure all holes are ≥ 0.8 mm diameter

• Annotate critical features

• Prefer uniform wall thickness

• Avoid cantilevers >2 mm unsupported

• Include escape holes for enclosed voids (minimum 1.5 mm)

Final Thoughts

Dimensional tolerance in MBJ is not a limitation, it’s a characteristic to design for.

With the right process parameters, good orientation, informed design decisions, and compensation models, it is entirely possible to achieve results rivaling traditional machining or MIM.

At FINEX, we leverage both analytical modeling and hands-on experience to deliver dimensional precision. Whether you're prototyping, scaling up, or developing end-use parts, we're here to help you hit your targets with confidence.

Upload your design today, and let our platform guide you through selecting the best tolerance grade for your part.

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