Your printed part is only the starting point. The right post-processing step turns a raw MJF part into a finished, production-ready component, shaping it precisely the way you need it.
Table of Contents
Overview of HP Jet Fusion Secondary Post-Processes
HP Multi Jet Fusion (MJF) produces functional, accurate parts straight out of the printer. But depending on your application, whether it needs a specific color, a smoother tactile surface, tighter tolerances, or a production-grade appearance, MJF secondary post processing can make a significant difference to the final result.
Introduction
Secondary post-processing is an optional but often highly valuable step in the end-to-end workflow for parts printed with Multi Jet Fusion technology.
While the raw part, once it has been suitably cleaned by MJF bead blasting or water jet blasting, can be used as is, without further treatment, the specific application of the final part may require processing to fulfill technical requirements, such as a colored finish or a smooth tactile surface.
This HP MJF post processing guide focuses on practical, production-ready solutions that improve part performance, appearance, and usability.
The techniques in this guide are organized into two main categories:
- Cosmetics – covering color and appearance
- Surface Roughness – covering smoothness and finish quality.
Some techniques deliver both outcomes simultaneously. Electroplating, for example, adds a metallic appearance while also acting as a functional coating that reduces surface roughness. Vapor smoothing improves surface finish while also sealing the part and enhancing its resistance to moisture and bacteria.
In addition to many available cosmetic finishes, post-machining can improve part quality by improving geometric tolerances and localized surface finish. Typical machining tolerances are tighter than 3D printing, and milled surfaces are glossy and jet black.
Quick-Pick: Which Post-Processing Method Is Right for Your Part?
Not sure which finish your part needs? Use this table as a starting point.
Technique | Best For | Result | Category |
Dyeing | Visible parts, wear-facing parts | Uniform color penetrates the interior | Cosmetic |
Painting | Custom colors, UV/scratch resistance | Smooth film, multiple colors | Cosmetic |
Graphite Blasting | Visual prototypes, metallic look | Metallic appearance, lower friction | Cosmetic |
Smoothing Blasting | Pre-treatment before dyeing | Matt to glossy, scratch-resistant | Cosmetic + Surface |
Electroplating | Conductive, EMI shielded, decorative metal finish | Metal surface layer | Cosmetic + Functional |
Vibratory Finishing | General smoothing, pre-electroplate | Reduced roughness, slight polish | Surface |
Chemical Polishing | Internal cavities, complex geometries | <1 µm Ra, gloss or matt | Surface |
CNC Post-Machining | Threads, bearing housings, tight fits | ±0.05 mm tolerances | Dimensional |
For a specific recommendation, talk to our team.
Cosmetics
Cosmetic finishing covers any post-process that improves the color, uniformity, or visual appeal of your MJF part.
These MJF finishing methods are typically selected based on how the part will be used, whether it will be handled, and the level of visual quality required.
There are five primary techniques: dyeing, painting, graphite blasting, smoothing blasting, and electroplating.
Each serves a different purpose, and the right choice depends on your application, the color of your raw part, and whether the finish needs to be durable or purely aesthetic.
Dyeing
The MJF dyeing process is the most widely used cosmetic finishing technique for Multi Jet Fusion parts. It involves immersing the part in a hot dye for a prescribed period of time so that the dye penetrates the part completely, until a specific color or color homogeneity is achieved across the entire part surface.
This makes it one of the most reliable MJF post processing techniques for parts that will be handled, worn, or exposed to regular use.
It is suitable for applications where the part is visible or subject to wear since the color penetrates the interior. The results obtained depend on the color starting point of the raw part, either gray or white printed parts, depending on the HP Multi 3D Jet Printing Solution. A white raw part offers a much wider range of achievable colors and tones than a gray part, so if color accuracy matters, specify white as your base.
The cosmetic category includes five key techniques: Dyeing, painting, graphite blasting, smooth blasting and electroplating.
Manual Dyeing
Manual dyeing is an inexpensive, efficient technique that uses a dye bath or vat and a hotplate. The dye mix is made up in the dye bath and then conditioned. It is then heated to 80-100 °C, and the parts are immersed in the mix for approximately 8 minutes. The dyeing time will depend on the color of the raw part and the desired color intensity. The newly dyed parts are then transferred to the rinse bath containing water heated to 60 °C, in order to remove any excess dye from the parts and avoid staining. The parts are then left to dry, either naturally or by placing them in a drying oven at 50 °C.
Automatic Dyeing
Automatic dyeing solutions are a convenient and efficient alternative that requires less operator intervention.
These systems include programmed cycles for dye bath mixing, conditioning, dyeing, part rinsing, dye disposal, and dye bath cleaning. In some automatic dyeing machines, the dyeing mix can be reused up to 5 to 10 times, and the built-in stirring ensures the dye solution is always homogeneous, achieving better results.
Dyes are available in powder and liquid formats. The concentration of the dye mix depends on the dye specifications and the desired color intensity. Powder dyes are used at approximately 7 g/L at 100 °C. Liquid dyes use a ratio of 0.1 L dye to 10 L water at 80 °C.
Best practice: For the best results when dyeing, parts must have been thoroughly cleaned to ensure they are completely free of any unfused material. Parts must be able to move freely within the dye mix, so the dye bath should be adequate for the volume of parts to be dyed. Small, fragile parts should be placed in a mesh bag to protect them from the effects of stirring in the dye mix.
If stains appear on the parts due to scratch marks or where the dye has become too concentrated, even after rinsing, such marks can be removed by MJF bead-blasting. Avoid dyeing large, thin, flat parts under 2 mm thickness; thermal shock during the process can cause warping.
Need your MJF parts dyed to a specific color?
RapidMade handles dyeing in-house with fast turnaround at our Tualatin, OR facility.
Painting
In the process of painting, a pigmented liquid is applied to the part surface in thin layers, each drying to a solid film. It delivers color uniformity and improved surface texture, and, depending on the paint formulation, can also provide UV resistance, wear and scratch resistance, and watertightness.
Both solvent and water-based paints can be applied to Multi Jet Fusion parts. Depending on the final application where the part will be used, the raw part can be painted directly or pre-processed first. For the best result, prime and sand the surface first; any imperfections in the raw part will show through once painted.
Several layers may be required according to the final application. The surface roughness of HP Jet Fusion 3D printed parts can be improved by adding layers of paint; the greater the number of layers, the smoother the surface.
Paint can be applied manually or automatically using a spray gun.
Graphite Blasting
The principle of graphite blasting is similar to bead blasting, although its intent is different. Rather than cleaning the part, graphite blasting provides a uniform, metallic-looking surface finish that is visually distinctive and suitable for functional parts where a lower friction surface is an advantage.
Parts are placed in a blasting chamber where a controlled mix of glass beads (70–110 µm) and graphite, with graphite at 0.7% of the total mix, is blasted across the part surface.
Excess graphite is then removed using air blasting. A 5 kg glass bead-graphite mixture is suitable for 1–2 full print jobs.
The settings for air pressure and distance to part are similar to manual bead blasting, being 3 bars, depending on the part‘s fragility, and 10–15 cm distance to part.
Since graphite blasting is necessarily a two-step process, as parts are blasted first in the bead chamber and then air-blasted to remove excess graphite, the total time required is around 2–5 minutes per part, allowing 1–5 minutes per part for bead blasting in an automatic chamber and a subsequent 5 seconds per part for air blasting.
As with bead blasting, small and fragile parts can be processed in batches using a sinter box.
Graphite-blasted parts are prone to staining due to excess graphite on the surface, the degree of which varies depending on the blasting time; however, air blasting can reduce the level of staining.
Graphite blasting is well-suited to visual prototypes and mechanical parts where lubrication at contact surfaces is beneficial. It is not recommended for end-use parts that receive frequent handling, as the graphite coating fades over time and is not wear-resistant.
Smoothing Blasting
Smoothing blasting propels an abrasive media, typically plastic or ceramic rather than glass, onto the part surface at high pressure. The goal is a better surface finish, not just cleaning.
The surface finish ranges from matte to glossy, depending on the media type and processing time. The resulting surface is more resistant to scratches, dirt, and liquid absorption. Key variables are air pressure, blast media type, and time.
At a minimum distance of 200 mm and a pressure of 5 bar, a batch of parts can typically be processed in 10–20 minutes. Smoothing blasting is most effective when combined with other processes, particularly dyeing, and is commonly used as a preparatory step before electroplating or for correcting dye stains.
Electroplating
Electroplating and metal coating are techniques used to add a layer of metal to the surface of a printed part.
This is done to improve the look and feel of the part for cosmetic applications or to improve physical properties such as conductivity, EMI/RFI shielding, mechanical strengthening, and heat conductivity, as well as other desirable attributes such as a functional coating like antibacterial properties.
For a better, mirror-like finish, raw parts must be pre-processed to reduce surface roughness to 1 or 2 μm, using a vibratory tumbler, chemical polishing, or smoothing blasting, although small holes due to porosity may still be observed.
Electroplating involves adding a thin layer of metal to a metallic object. At a basic level, it consists of dissolving one metal in a solution and subsequently attaching it to another metallic surface using an electric current. The target surface must be able to conduct electricity.
Since Multi Jet Fusion 3D parts are printed in polyamide, a semi-crystalline thermoplastic, the part must be treated to make the surface conductive before the electroplating process.
There are three methods to do this:
- Electroless plating – the most common method. Deposits palladium particles into micro-cracks in the surface, creating an initial metal layer. Requires mechanical or chemical etching first.
- Gas activation technology – uses ionized gas to make the surface conductive without etching. Fast and allows selective metallization, though it leaves a small unplated area where the electrode connects.
Conductive coatings – the simplest and most economical option, but the weakest in adhesion and long-term durability.
The method chosen to make the surface of the raw part conductive depends greatly on the application and desired attributes for the final part. The most commonly used method, however, is electroless plating.
Electroless plating adds small particles of palladium inside micro-cracks in the part’s surface, which will create an initial layer of metal. It is a complex process, which is dependent on the geometry of the part for its effectiveness.
Electroless plating is a multi-step process in which, first, the part needs to be etched mechanically, i.e., sandblasted with an abrasive medium, to enhance the surface-adhesive capabilities of the part. For raw or tumbled parts, processing the part with mechanical etching is sufficient; however, for parts treated with chemical polishing, chemical etching may be required.
In chemical etching, the surface is attacked chemically using a chromic acid-based solution. Any excess chromic acid is then neutralized. Next, a solution consisting of palladium and tin salts is applied to the part. The part is then submerged in an electroless plating solution, which coats the surface of the material with either nickel or copper, depending on the solution, creating a pre-plate layer.
Gas activation technology uses an ionized gas to make the surface conductive. It does not require mechanical or chemical etching to prepare the surface. It is fast and gives selective metallization; however, it results in a small area where the electrode is connected without electroplating.
Coating the part is the easiest and the most economical way to make the surface conductive. However, it is not a very robust solution, as adhesion of the coating to the surface is weak, meaning it does not provide durability over time.
Once the surface has been made conductive by electroless plating, gas activation, or with a conductive coating, the standard procedure for electroplating can be done, depositing a metal coating on the surface of the part. Various metals can be used to plate the surface: notably, nickel, copper, chrome, and precious metals such as gold or silver.
To overcome possible porosity or irregularities on the surface, a priming spray paint can be applied after mechanical polishing
and before electroplating for the best results.
Reducing Surface Roughness
Because MJF is a powder bed fusion process, the distribution of fused particles is not perfectly uniform across every surface orientation.
This means raw MJF parts typically have an inconsistent surface roughness, ranging from 8 µm to 12 µm Ra, depending on how each face was oriented during printing.
MJF surface finishing techniques help reduce this roughness and create a more consistent and production-ready finish.
There are two main techniques that can smooth the printed part’s surface:
• Vibratory finishing
• Chemical polishing
Vibratory Finishing
Vibratory finishing can be either a wet or a dry process, with the media being adapted accordingly.
In wet vibratory finishing, coarse ceramic and plastic media are usually used, resulting in a more polished finish. The wear on the media tends to be less, and the overall process is usually quieter. However, the waste resulting from the liquid-abrasive media mix needs to be filtered and treated.
Dry vibratory finishing, on the other hand, is a cleaner process, resulting in easier treatment of waste output (only worn dry media). However, it tends to be more aggressive, depending on the media.
There are several key factors that influence the results achieved in vibratory tumbling:
- Abrasive media: Ceramic media is denser and more aggressive, delivering lower Ra values faster. Plastic media removes less material and better preserves fine details, but requires longer cycle times. Steel, synthetic, and organic media are also available for specific roughness targets.
- Media size progression: Start with larger media and move to finer sizes as the target roughness is approached, similar in principle to sanding. Note that larger media cannot reach narrow internal features, while very small media may become lodged in crevices and require a cleaning step afterward.
- Time and RPM: The process is most effective when the media is fresh, losing efficiency over time. Optimal cycle duration varies by application but typically runs for several hours.
Important: Vibratory tumbling will round out sharp edges and can affect overall part dimensions. It is not recommended for parts with fine details, intricate features, or fragile walls below 2 mm.
Chemical Polishing
Chemical polishing, also known as chemical smoothing, is a physicochemical process that smooths the surface of thermoplastic polymer parts, including internal cavities.
It is a non-line-of-sight process; in other words, it acts on the surface of the part without degradation of the part’s mechanical properties and can achieve a surface finish with a roughness value of less than 1 μm and different levels of glossiness.
It is compatible with polyamide (PA 11 and PA 12) as well as TPE and TPU.
Since the process can be controlled, it can result in a matt, gloss, or shiny surface without losing any fine detail on the part or affecting the part’s dimensional accuracy to any great extent.
One practical advantage: when applied to gray MJF parts, chemical polishing cuts through the partially fused outer layer to produce a clean piano-black surface, avoiding the need for a separate dyeing step.
Limitations to note: Chemical polishing is not recommended for sections thinner than 1 mm due to the risk of structural deformation.
It is also less effective on sharp edges. Since the process can result in minimal dimensional change, with a maximum of approximately 0.4% variation, it is important to factor this in for precision-fit parts.
Need a smooth, production-ready finish on your MJF parts?
Recommendations for Post-Machining MJF Parts
Introduction
HP Multi Jet Fusion (MJF) technology allows for the design and production of accurate parts with small features, complex geometries, and functional assemblies. These advantages can be enhanced by adding complementary post-processes such as machining, especially for applications where very tight tolerances are required, such as small threads, bearing housings, or engineering fits.
A machining post-process can add value to the following aspects:
• Dimensional accuracy: A standard machining post-process can provide very tight tolerances, up to ±0.05 mm, in a particular area or for a critical feature where high function and tolerance are required.
• Geometry references: In addition to dimensional accuracy, a machining post-process can also improve geometric requirements like flatness, concentricity, perpendicularity, or parallelism, reaching very tight tolerances up to ±0.08 mm.
• Small features: A machining post-process also allows for the implementation of small features such as small threads, ensuring accurate results.
• Surface roughness: By default, the mean surface roughness or roughness average (Ra) of a part that is 3D printed using HP MJF technology is between 8 µm and 12 µm, depending on the face orientation. This surface roughness can be significantly improved with machining post-processes such as milling or turning, which have roughness values of up to 0.30 µm and 0.60 µm, respectively.
• Repeatability: A machining post-process can also decrease the variation in applications that require repeatable specs.
Design Recommendations for Machinable MJF Parts
When designing a part for HP MJF, it is important to bear in mind the requirements for the final part. Some applications may
require adjustments in the design in order to machine the printed part accordingly.
Surface Grinding
This abrasive post-process removes material to create very flat surfaces with fine finishes and very accurate tolerances. For this reason, the machined surface needs to be designed with additional material to achieve a suitable result. The minimum recommended thickness is 0.5 mm, bearing in mind that an excess of material will lead to increased costs as well as manufacturing and post-processing time.
Process Recommendations
When machining a part for HP MJF, selecting the correct parameters for each operation is critical to achieving the intended result without damaging the underlying printed geometry.
Milling
Raw MJF parts have a surface roughness of 8–12 µm Ra. Milling can bring critical faces down to 0.30 µm Ra, making it the right choice when precision surfaces, flat references, or tight fits are required.
The main risk when milling MJF parts is heat buildup. The material does not conduct heat away from the cut zone like metal does; if heat accumulates, the surface softens instead of shearing cleanly. Sharp carbide tooling, shallow passes, and climb milling over conventional milling all reduce heat at the cut zone and produce a consistently better surface finish.
For through-holes that will house bearings or press fits, helical interpolation is more reliable than straight plunging. It distributes cutting force evenly and produces a cleaner hole wall.
Design note: add a minimum of 0.5 mm machining stock to any surface intended for milling before printing. Skim passes on surfaces printed to the final dimension are too shallow to be stable.
Turning
Turning is the most reliable post-process for achieving tight cylindrical tolerances on MJF parts; bearing journals, press-fit diameters, and threaded shafts all respond well.
Use sharp carbide tools and keep the cutting process moving. If the tool stays in one place too long, heat builds up, and the part can lose accuracy. During turning, MJF parts can also create long chips, so tools that break chips help prevent surface damage.
For production runs, using custom carbide or cobalt tools that match your part size helps keep results consistent. A properly fitted chuck also reduces vibration, which leads to a smoother surface finish.
Need tight tolerances on your MJF parts?
RapidMade combines in-house MJF printing and CNC machining for a seamless, single-source workflow.
Choosing the Right MJF Post-Processing Services for Your Application
The best post-processing approach depends on three things: the purpose of the part, the material it’s printed in, and the finish requirements of the end application. Here is a practical summary:
- For visible, handled parts: Dyeing is the most cost-effective choice. It penetrates the interior and holds up to wear.
- For custom colors or UV resistance: Painting gives the most control, especially for multi-color or branded finishes.
- For smooth, production-grade surfaces: Vapor smoothing delivers the best results, near injection-molding quality, without compromising mechanical properties.
- For watertight or medical-grade parts: Vapor smoothing is the preferred choice, with documented biocompatibility and water resistance for PA12 parts.
- For tight tolerances and precision features: CNC post-machining after printing is the most reliable path to ±0.05 mm accuracy.
- For metallic appearance or EMI shielding: Electroplating adds both the aesthetic and functional metal layer in a single process.
At RapidMade, we handle all of these MJF post processing services in-house.
Our team can help you select the right combination of finishing steps for your part, and execute them with the same quality controls that serve our aerospace, medical, and industrial customers every day.
Ready to Finish Your MJF Parts the Right Way?
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©Copyright 2019 HP Development Company, L.P. The information contained herein is subject to change without notice.
The information contained herein is provided for information purposes only. The only terms and conditions governing the sale of HP 3D printer solutions are those set forth in a written sales agreement. The only warranties for HP products and services are set forth in the express warranty statements accompanying such products and services. Nothing herein should be construed as constituting an additional warranty or additional binding terms and conditions. HP shall not be liable for technical or editorial errors or omissions contained herein and the information herein is subject to change without notice.
4AA7-5666ENW, August 2019