Innovations in UV Cured Powder Coatings for Additive Manufacturing Applications
Posted on Thursday, September 21, 2023
By Michael Knoblauch & Rebecca Lonczak
Additive manufacturing (AM), also known as 3D printing, is a technology that creates three-dimensional objects by adding material layer by layer utilizing a computer-aided design (CAD) model. While AM has been around since the 1980s, recent technological, material, and productivity advancements have increased its popularity and commercial use. AM is now used in a variety of industries and product applications.
According to some estimates, the global AM market size was valued as much as $13.84 billion in 2021 and could expand at a compound annual growth rate (CAGR) of 20.8% from 2022 to 2030. The industrial 3D printer segment dominated the market in 2021 and may have accounted for a revenue share of more than 63%. Industries utilizing 3D printing for prototyping, design, and tooling include automotive, aerospace, healthcare, and electronics, among others.
Post processing is defined as any task or process that needs to be completed after a part is printed, including removing supports, cleaning, curing, sanding, polishing, coloring, or finishing. Many AM parts are finished for either decorative and/or functional reasons. There are several ways to add color or apply a coating to AM parts, including dyeing (either pre- or post printing), liquid painting, dip coating, electroplating, electrocoating, or powder coating.
Post processing finishing or coloring can be very time consuming and labor intensive. Often AM parts are painted by hand using a primer, multiple layers, and a topcoat. Coloring AM parts using a dye process can be inconsistent and the color palette is limited. Many companies using solvent or waterborne liquid coating or spray painting are turning to powder coating for their environmental and productivity benefits. Powder coating helps solve the environmental challenges of liquids and offers an unlimited color palette in a single coat. For this article, we will focus on the finishing aspect of post processing, specifically using UV cured power coatings.
Considerations for Finishing AM Parts
Before considering finishing AM materials, it is important to note there are many types of materials used to print AM parts, from polymers, metals, and ceramics to foams, gels, and even biomaterials. Each material has unique properties and a full understanding of the material’s chemistry, performance properties, and limitations are crucial for successful pretreatment and post processing coating of an AM part.
Thermal deformation temperature influences the stability of heat sensitive AM materials during the melt and cure phases of powder coating. Too low a deformation temperature will result in loss of shape and form, rendering the part unusable. UV cured powder coating is ideal when finishing heat sensitive materials, including AM materials, with deformation temperatures above 150 degrees Celsius (302 degrees Fahrenheit).
UV Cured Powder Coating for AM Parts
UV cured powder coatings are increasingly being used to finish AM parts due to lower temperatures and reduced thermal exposure during the UV powder coating process.
The first step, pretreatment, is one of the most important steps for AM parts prior to powder application. Most AM components are non-conductive and require the addition of a conductive pretreatment prior to powder application. Not all AM parts will require the same pretreatment processes.
Several forms of pretreatment can be used including liquid primers, plasma treating, and media blasting. These treatments are adhesion promoters, increasing the wetting and flow of a coating on the substrate or increasing bonding sites, which promote coating adhesion. When using media blasting, operator control of the blast media is important to ensure the part is not damaged during the pretreatment process. The type of blast media must also be evaluated, as some can be too abrasive for an AM part.
Benefits of UV Powder for AM
UV powder is an appropriate finishing solution for AM components, as the fast and energy efficient UV powder system aligns with the small footprint and operational efficiencies of AM printing technology. A UV cured powder coating finishing system has a small plant footprint, reducing energy costs and finishing times while increasing throughput and profits per hour. Users can quickly move from idea, to rapid prototyping, to full production, shortening the time necessary to bring new products to life.
UV cured powder coating allows the user to add color, opacity, and special effects as well as varying texture and gloss levels. These decorative aspects help to differentiate products and improve the perceived quality of the part. Coatings are most often used as a decorative aspect on AM parts, but they can also help improve the part’s performance properties. UV cured powder coating can improve chemical and UV resistance.
Applications and Considerations
Powder coating AM materials can be a challenge. These materials are different from wood and other heat sensitive substrates, and additional research and development is still necessary to develop successful UV cured powder coatings for AM. Typically, AM parts are more complex with many nooks and crannies.
The Faraday effect can be an issue with powder coating AM parts. UV curing is line of sight, meaning the UV light must “see” the powder coated surface to achieve full cure. The light contact time is virtually instantaneous. Curing areas not in direct line of sight can be addressed by rotating the lamp or the part. There are still some instances where an AM part is just too complex and not suitable for powder coating.
The many AM material types or chemistries as well as part geometries make things even less straight forward compared to powder coating metal, wood, or other heat sensitive substrates. It is crucial to understand the AM substrate chemistry makeup and the coating performance requirements before coating AM parts. Successful finishing requires balancing substrate chemistry with the pretreatment process and powder coating application parameters.
AM Performance Test Results
There are several standard performance tests that are used on AM parts finished with UV powder coating to ensure finish performance and adhesion. These are summarized in Table 1.
The specular gloss represents the level of shine of a surface and is evaluated using ASTM D523. This provides a numerical representation, in gloss units (GU), of specular reflectance of a coating surface compared to a black glass standard.
Adhesion is tested using ASTM D 3359, method B and evaluated using a percentage coating loss. A 5B evaluation corresponds to 0% loss, 4B to > 0%-5%, 3B to > 5%-15%, and so on. For most applications, a 4B to 5B is required and achieved on AM parts with UV cured powder coating.
Pencil hardness is a representation of the hardness of a coating’s surface. Generally, the harder the surface, the more resistance the coating will exhibit to scratches and marring. Hardness is evaluated using ASTM D3363.
Solvent resistance provides a quick indication of a coating’s ability to withstand harsh chemicals and can be representative of cure. A commonly used solvent, methyl ethyl ketone (MEK), is used for this test. MEK is applied to the tip of a cotton swab and applied to the surface of the coating using a double rub motion. The coating is evaluated for a decrease in film build and softening.
UV resistance is tested using xenon arc exposure to indicate the ability of the coating to withstand the intense UV energy produced by the sun. Samples are placed in a xenon arc chamber and exposed to high intensity lamps along with sprayed water in controlled conditions using the ASTM D2565 standard.
The test results in Table 1 illustrate the durability and robustness of the UV cured powder, improving the functionality and performance of the tested AM component.
Additive manufacturing has become an essential and established sector in the fields of product development and manufacturing. Process and material improvements are expanding the types of products produced, and AM is growing at a rapid pace. The added performance, process efficiencies, and aesthetic appeal of UV cured powder coating enables new and exciting opportunities for the AM industry. Though the industry is rapidly changing, and advancements are being made every day, one thing is for certain; powder coating for AM parts, which once seemed impossible, has now become a reality.
Mike Knoblauch is president and Rebecca Lonczak is marketing manager for Keyland Polymer UV Powder.