Optical Clarity through Post Processing and Clear Coating

Optical Clarity through Post Processing and Clear Coating

Best Practice

p/n 31-D109 Rev A


Some user applications require optically clear parts. Optical clarity or transparency refers to the visual distinctness with which an object can be seen when viewed through the part. There are many rapid prototyping needs for optical clear parts, like automotive lenses, bottle design, and packaging. Traditional, high-volume plastic injection molding is able to produce professional quality optically clear parts making them common in many everyday consumer devices that benefit from rapid prototyping. High optical clarity is also important for many research and development needs for things like flow visualization.


Numerous customer use cases require high optical clarity including automotive lenses, bottles, jars and containers, as well as flow visualization and for purely aesthetic purposes 


In addition, it can also be advantageous to be able to see through a part during the rapid prototyping phase of a project. This is especially true for small parts or with complex internal geometry like snap fits, stiffening ribs and small electronics assemblies and connections, and all types of wire routing.


M2G-DUR (ProFlex),  Semi-transparent electronic assembly

showing internal snap fits and electronics 



M2G-CL (Armor),  Clear coated (left) and as printed (right) transparent electronic assembly showing internal electronics, wire routing, screw bosses, mechanical articulation, and assembly fits


MJP Optical Clarity Overview

The ProJet MultiJet Printer (MJP) 2500 is designed to be used by an individual user up to a large group and offers numerous materials for a wide variety of customer needs. One unique aspect of the ProJet 2500 is that three different clear materials are available for the printer and they span a wide range of material properties from rigid and higher temperature (polycarbonate-like), to stiff and tough (ABS-like), and also flexible and extremely tough (polypropylene-like). This means that engineers do not have to make the tradeoff between strength and clarity for a wide variety of rapid prototyping needs. Also, all materials still benefit from the world-class ease-of-use, part accuracy, and feature fidelity that MJP technology is well known for.  All three of these clear materials achieve very high optical clarity and visual quality and are ideal for all aspects of the design cycle including concept models, functional prototyping, jigs and fixture development, and small volume early manufacturing needs. 


The ProJet 2500 offers three different clear materials with a wide range of material properties

M2R-CL rigid clear material can be used to achieve excellent optical clarity. This material has relatively higher stiffness, strength, and heat deflection temperature compared to the engineering clear materials. 


ProJet M2R-CL rigid clear material part with complex internal passageways

The M2R-CL rigid clear is a strong and stiff material with tensile strength 35-45 MPa, a modulus of up to 2000 MPa and yet still achieves an elongation before break of up to 30% and an as-printed heat deflection temperature of 51°C.   


M2R-CL rigid clear transparent flashlight lens

The engineering materials M2G-CL (Armor) and M2G-DUR (Proflex) maintain the high quality and fidelity of MJP, but were designed especially for the most aggressive engineering applications including high impact applications, complex snap fits, and living hinges. The material M2G-CL (Armor) is an engineering clear material that has a very good balance of stiffness and toughness. 


M2G-CL (Armor) threaded bottle cap lid with entrapped support marking

The Armor material is designed to have ABS-like properties to simulate injection-molded plastic parts. Armor has an impact strength of 40-50 J/m, and yet still maintains a tensile strength of 30-35 MPa and stiffness modulus of 2000 MPa and elongation before break between 55 to 65%.  


M2G-CL (Armor) snap buckle with entrapped support logo marking

The engineering material M2G-DUR (ProFlex) is the most flexible engineering material and has the highest toughness. The ProFlex material is designed to have polypropylene-like properties and is extremely flexible and tough making it nearly impossible to break for even the most aggressive applications.


M2G-DUR (ProFlex) printed assembly with functional hinge and snap fit

ProFlex has an impact strength of 70-80 J/m, and is much more flexible with a stiffness of 250 to 350 MPa and elongation before break up to 75%. All three of these materials are optically clear and can be professionally dyed in an array of colors and then clear coated for a highly professional appearance.


Rigid clear (M2R-CL) color keyring with clear coating dyed with standard colors and clear coated


Producing Optically Clear Parts

For some applications, the ProJet 2500 is able to produce optically clear parts right out of the printer.  However, to achieve the highest optically clarity and for highly complex surfaces, extra steps are required in the post processing. All MJP parts to be clear coated should be processed normally with the EasyClean system or an oven to remove the bulk wax and with mineral oil. EZ Rinse-C is not recommended for fine wax removal as it can inhibit the paint curing process often leaving a painted part tacky. If EZ Rince-C is used one should wipe the parts down with isopropyl alcohol and then take special care to sand or bead blast the surface. If mineral oil is used, dawn soap should be used to remove all the mineral oil residue from the surface. For very complex parts it may be advantageous to apply the dawn soap directly to the part and use a brush to insure all the mineral oil is removed. For both solvents, a good water rinse should be used as the final cleaning step.


For MJP painted parts apply the Dawn soap directly to the part and use a brush if needed.


As Printed Optical Clarity


If some thought goes into the design and for certain applications, the optical clarity of as-printed MJP parts may be sufficient. In particular, both upward and downward facing flat surfaces (+/- z direction) achieve very good optical clarity as-printed and with only the standard MJP post processing. 

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Left, M2R-CL rigid as-printed vs. right M2R-CL processed and clear coated

For more complex surfaces, as-printed part surface uniformity and clarity is improved somewhat by aggressively brushing the surface with a stiff nylon brush or a buffing wheel.

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A Stiff nylon brush can be used to improve surface optical clarity

This method might be advantageous for a part with a nozzle or other critical to function holes that might get clogged or negatively impacted by the application of paint. 


Left as printed, center sanded and polished, right sanded and clear coated

Brushing or sanding will improve uniformity and clarity, but only slightly. To achieve a more optically clear part the most important step is in the surface preparation prior to post processing and before painting. 


Sanding and/or Blasting to Achieve the Best Optical Clarity

A bead blasting or a sanding process on the surfaces of the parts followed by a clear coating is highly recommended in order to achieve the best optical clarity in parts. Hand sanding is best applied to flat surfaces where bead blasting is best for complex surfaces. A wet sandpaper like 3M Wetordry is recommended for sanding MJP parts. Wet sanding is highly recommended using a progression of finer grit sandpaper. Set the sandpaper on a flat surface preferably near a sink. Apply water to the part and the sandpaper. Push the part against the paper and sand the part either in long strokes or in a circulatory motion. Move the part to un-used portions of the paper often. Rotate the part 90°deg periodically attempting to sand the entire surface uniformly. 


3M Wetordry sandpaper


Apply force during sanding in a way to equally distribute the pressure across the surface of the part being sanded. Wash debris off the part and sandpaper often by running cold water over them. Don’t let the debris build as it can cause surface defects and impede the normal sanding process. Start with 240 grit and sand the surface until it is uniformly finished and flat. Don’t proceed if the surface has high or low spots or curvature. If the part has printed natural curvature that you want to maintain, hold the part in your hand and sand it with a folded piece of paper that allows compliance over the surface of curvature. Periodically dry the part and look at it in reflected light to visualize areas that are not being sanded sufficiently. After the 240 grit, move to 600 grit and then to finer 1200, 1500 or 2000 grit using the same procedure with water and washing away debris often. The finer grit sanding should go relatively fast as long as the initial 240 grit achieved a flat and uniform surface. 

For complex shaped surfaces, a faster and high quality surface preparation for clear coating can be achieved by using a dry glass bead blasting after the post process. Use 40 to 90 nominal diameter micron range small glass beads with a 60-80 psi pressure on the entire surface.

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Dry Glass Bead Blasting


For higher volume processing of parts, one might consider the use a water vapor blasting system. Also known as “wet abrasive” or “dustless blasting” or “water honing” it is an abrasive blasting technique that mixes the blasting media with water. 

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Water Vapor Blasting


The addition of water buffers and lubricates the particle providing a cushion that can protect both the media and the surface from excess damage. This method also increases the effectiveness of the media by lowering media breakdown and by preventing impregnation of foreign materials into the surface. 


Dry vs. Wet blasting (water vapor blasting)


Dry and wet blasting are both commonly used to process 3D printed parts for many technologies including Selective Laser Sintering and Stereolithography parts. An industrial wet blasting machine is able to process a larger of volume of parts with less effort compared to a dry bead blasting. After sanding and/or dry or wet blasting, the part should be thoroughly rinsed and dried and is ready to be painted. Again, these sanding and/or blasting steps are the most important steps that must be done to a sufficient degree in order to achieve good painted optical clarity.


Clear Paint Type and Application Process

Clear coating can be done by a professional shop using a two-part polyurethane with a paint and catalyst or with standard clear enamel/acrylic paint that can be found at most hardware or hobby stores or on-line at locations like Amazon.com or Walmart.com. Very good results can be achieved without the need for professional skills or equipment. Most clear paints can be used. For example, Rust-Oleum Ultra Cover 2x, Rust-Oleum Triple Thick Glaze or Krylon ColorMaster Cover Max are all good choices. Triple thick glaze is recommended if available. 

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Clear gloss spray paints good for MJP parts 

Minwax Helmsman Spar Urethane also works well to clear coat parts. A high gloss surface can be achieved in fewer coats and is less sensitive to application technique. However, it can leave a light yellow tint on the parts. 


Yellowing of a clear part from Minwax Spar Urethane

Minwax Helmsman Spar Urethane is a very good choice to paint clear parts that have been dyed because the light yellow tint is not noticeable, the application is simpler, and the result looks very good.


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Automotive lenses dyed with all 12 of the standard Rit Synthetic dyes followed by clear coating


Painting should be done according to the recommendations on the can. Use an overhead light that enables good visualization of the part surfaces. Hold the can 10-12 inches away from the part and apply paint until a complete coat is achieved. Start painting the surfaces that are the hardest to reach. Other more accessible surfaces will get coated naturally as these difficult areas are painted. After the difficult areas are coated with a uniform layer, touch up any of the easier to reach areas.


M2R-CL (Rigid Clear) highly complex and three-dimensional statue


Be careful to never apply so much paint that it runs. The goal is to create a uniform layer on all surfaces without resulting in any dripping or running of the paint. Proper technique will often result in a lot of wasted paint. Three or four coats are typically needed for acrylic paints and one to two coats for Spar Urethane. Apply the coats in rapid succession with 3-5 minutes between coats. If the paint is allowed to dry for a longer time, let it sit overnight and apply the remaining coats the next day. If drips or runs are formed during painting, it is best to let it dry completely and attempt to sand the drips off these surfaces. Subsequent coats will typically hide most over- or under-coverage issues and reprocessing sanding marks in the paint but will not hide large drips of paint. The part will look very clear after the paint is applied, but the clarity will often subdue and lose its solid glossy look after curing for the first few coats. However, after multiple coats are applied, the coating will remain optically clear and with high gloss after drying. 

If you have your parts professionally painted with a two-part polyurethane (like an automotive clear coat or plastic injection molded clear coat), try a 7-1-2 mixture of paint, catalyst and thinner. Use three coats applied in rapid succession. Initial part surface quality is more important for clear coating. Always consider sanding or glass bead blasting for best results. 


M2R-CL and M2R-GRY parts professionally painted with a two-part polyurethane

7-1-2 paint/catalyst/thinner – 3 coats in rapid succession


MJP Clear Coating Examples


M2R-CL Rigid clear with entrapped support colorization and graphical pattern


Rigid clear (M2R-CL) automotive lenses dyed and painted with a two-part polyurethane


M2G-DUR ProFlex box with robust and flexible hinge


M2G-CL Armor educational STEM sugar molecule


M2G-CL Armor educational STEM molecules



M2R-CL medallion with entrapped text, grayscale colorization and patterning



M2R-CL floating entrapped logo and patterning


M2R-CL medallion with entrapped text, grayscale colorization and patterning



M2G-DUR optically clear flexible storage container with entrapped text and logo


M2G-DUR printed assembly with functional hinge and snap fit.  The part was water vapor blasted and clear coated with Rust-Oleum Triple Glaze



M2R-CL flow visualization part for carbon scrubber flow visualization



M2R-CL transparent part with entrapped support colorization and highly detailed aesthetic structures visually showing the high quality and high fidelity capability of MJP technology


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Transparent clear but colored parts can be created with “glass” type translucent acrylic paints  



Transparent polypropylene-like storage containers made with M2G-DUR (ProFlex)



Rigid clear (M2R-CL) color keyring with clear coating dyed with standard colors with 1% concentration and clear coated with Minwax Spar Urethane