3D Printing Post-Processing: A Complete Guide to MJF and SLS Parts

If you work with MJF (Multi Jet Fusion) or SLS (Selective Laser Sintering) 3D printing, you know that the printed part isn’t the final product. Post-processing is key to unlocking their full potential—whether you need a smooth surface, better water resistance, or electrical conductivity. This guide breaks down every critical post-processing step for MJF and SLS parts, with real-world examples, data, and tips to solve common challenges.

1. Understanding MJF and SLS: Why Post-Processing Matters

First, let’s recap: both MJF and SLS are powder-based 3D printing technologies that use thermoplastics (like nylon PA12) or elastomers. Unlike FDM (Fused Deposition Modeling), they don’t need support structures—but their raw printed parts still have limitations:

• Rough surfaces from leftover powder
• Porosity that affects water tightness
• Limited color options (MJF prints gray; SLS prints white/gray)
• Basic mechanical properties (no conductivity or extra hardness)

Post-processing fixes these issues. Below is a detailed breakdown of the most effective methods, with pros, cons, and real use cases.

2. Essential Post-Processing Steps for MJF & SLS Parts

We’ve organized the most common processes by goal—from cleaning to enhancing performance. Each section includes data and examples to help you choose.

2.1 Powder Removal & Shot Peening: The First Step

Every MJF/SLS part starts with leftover powder (trapped in crevices or on the surface). This step is non-negotiable.

How it works:

1. Powder removal: Use a soft brush or compressed air to blow loose powder off the part. For tight spaces (like hollow tubes), use a vacuum nozzle.
2. Shot peening: Blast the part with high-pressure compressed air mixed with tiny glass beads (100–150 μm in size). This removes remaining powder and smooths minor surface roughness.

Key Data:

Real Example:

A manufacturer of SLS nylon PA12 gears used shot peening to remove powder from tooth gaps. The process cut assembly time by 40% (no more manual picking of powder) and reduced friction between gears by 15%.

Pros:

• Fast and low-cost (no special chemicals)
• No effect on part size or shape
• Works for all MJF/SLS geometries

Cons:

• Only removes powder (doesn’t fix deep scratches or add color)

2.2 Media Tumbling (Vibration Polishing): For a Satin Finish

If you need a smooth, matte surface (like for consumer products or medical devices), media tumbling is ideal.

How it works:

Place 10–50 parts in a vibrating drum filled with small ceramic chips (3–5 mm) and a mild detergent. Let it run for 4–8 hours—the chips gently rub against the part, reducing roughness.

Real Example:

A startup making SLS nylon watch cases used media tumbling. The process turned rough, powdery surfaces into a satin finish (Ra from 8 μm to 2 μm) and allowed them to process 30 cases at once—saving 60% of labor time vs. hand polishing.

Pros & Cons (At a Glance):

2.3 Dyeing: Add Color Without Altering Size

Dyeing is the cheapest way to add color to MJF and SLS parts—perfect for branding or aesthetic needs.

How it works:

Immerse parts in a heated water bath (60–80°C) with acid dyes. Let them soak for 10–15 minutes (longer for darker colors). The dye penetrates the surface but not the core.

Critical Differences: MJF vs. SLS

Real Example:

A toy company dyed 500 SLS nylon PA12 figurines green. They processed 100 at a time, costing \(0.20 per part—far cheaper than painting (\)1.50 per part). The only downside: a scratch on the figurine revealed the white base (since dye only goes 0.5 mm deep).

Pros:

• Low cost (\(0.10–\)0.30 per part)
• No effect on part size or weight
• Fast (10–15 minutes per batch)

Cons:

• Dye wears off with heavy use (only 0.5 mm deep)
• Doesn’t smooth surfaces (rough parts stay rough)

2.4 Painting: For Gloss, Metallic Sheen, or Extra Protection

Painting goes beyond color—it adds physical benefits like water resistance and scratch protection.

How it works:

1. Pre-treatment: Clean the part with alcohol to remove oil/dust (critical for adhesion).
2. Painting: Apply 3–5 thin coats of paint (instead of 1 thick coat) because MJF/SLS parts are porous. Use acrylic or enamel paints for best results.
3. Curing: Let each coat dry for 1–2 hours before applying the next.

Real Example:

An automotive supplier painted MJF nylon PA12 sensor housings with a metallic silver paint. The paint added a glossy finish (popular with customers) and improved water resistance—testing showed the housings could withstand 500 hours of rain without leaking (vs. 200 hours for uncoated parts).

Pros:

• Adds gloss, metallic, or matte finishes
• Improves water tightness and scratch resistance
• Works for both MJF and SLS parts

Cons:

• Labor-intensive (parts must be painted one by one)
• Pre-treatment is critical (bad cleaning = paint peeling)
• Adds minor weight (0.5–1 g per part)

2.5 Watertight Coatings: For Sealing Porous Parts

While raw MJF/SLS parts have basic water resistance, coatings like silicone or acrylate make them fully watertight—ideal for underwater or medical devices.

Key Coating Options:

Real Example:

A medical device company used silicone coating on SLS nylon PA12 insulin pump reservoirs. The coating made the reservoirs 100% watertight—no leakage even when dropped in water for 30 days. It also didn’t affect the part’s flexibility (critical for pump use).

Pros:

• Excellent water resistance
• Silicone coatings preserve flexibility

Cons:

• Thicker coatings (0.1–0.3 mm) may affect small part dimensions
• Requires curing time (2–4 hours for silicone)

2.6 Metal Plating: For Strength and Conductivity

Metal plating is for high-performance parts—think aerospace components or electrical connectors that need extra strength or conductivity.

How it works:

1. Prep: Clean the part and apply a thin conductive layer (like graphite) because plastic isn’t conductive.
2. Electroplating: Submerge the part in a solution with dissolved metal (copper, nickel, gold, etc.). Pass an electric current—metal particles stick to the part’s surface.

Common Metal Options:

Real Example:

An electronics firm nickel-plated SLS nylon PA12 connectors. The plating increased the part’s tensile strength by 30% and made it conductive—perfect for linking circuit boards. The downside: each connector cost \(5 to plate (vs. \)1 to paint).

Pros:

• Boosts strength and conductivity
• Creates a high-quality, durable finish

Cons:

• Expensive (3–10x cost of painting)
• Adds weight (5–10% of part weight)

3. Yigu Technology’s Perspective on MJF/SLS Post-Processing

At Yigu Technology, we’ve supported hundreds of clients in optimizing MJF and SLS post-processing. Our experience shows that choosing the right process depends on three factors: end-use, budget, and volume. For example, small-batch consumer products (like phone cases) benefit from media tumbling + dyeing (cost-effective and fast), while medical devices need watertight coatings + quality checks. We also recommend combining processes—e.g., shot peening followed by painting—to get both smoothness and protection. By aligning post-processing with your part’s purpose, you can reduce waste and improve performance.

4. FAQ: Common Questions About MJF/SLS Post-Processing

Q1: Can I skip powder removal if my MJF part looks clean?

No. Even if the surface looks clean, powder trapped in crevices (like holes or slots) can cause cracking or warping over time. Always do shot peening after brush/vacuum removal.

Q2: Why can’t MJF parts be dyed bright colors (e.g., red or green)?

MJF parts have a jet-black core. Bright dyes can’t cover this—they’ll look dull or uneven. Stick to black dye for MJF; SLS parts (white/gray core) work with any bright color.

Q3: Is metal plating worth the cost for non-aerospace parts?

It depends on your needs. If you need conductivity (e.g., electronics) or extra strength (e.g., industrial tools), yes. For basic parts (e.g., brackets), painting or media tumbling is a cheaper alternative.