As an alternative to more traditional methods that typically use solvents, industrial inks and coatings can be cured (dried) by ultraviolet light (UV) or electron beam radiation (EB). These methods of curing, known collectively as energy-curable (EC) or radiation curing, have been around for many years. Examples include curing wooden flooring/panels and architectural coatings, and, more recently, printing inks for labels and packaging, to deliver higher press speeds.
Energy-curing especially comes into its own for products that require fast processing or for substrates that are sensitive to heat.
But an area which is now attracting attention is combining radiation curing with 3D printing. Using radiation curing (usually UV) as part of the process enhances the properties of the final product.
Used by 3D Printing Pioneers
The potential has been recognised for some time. The early pioneer, 3D Systems, having grown organically and by acquisition, manufactures 3D printing machines which use a variety of technologies, including stereolithography, selective laser sintering (SLS), colour-jet printing (CJP), fused deposition modelling (FDM), multi-jet printing (MJP) and direct metal sintering (DMS). The systems vary in their materials, print capacities and applications. Stereolithography uses UV curing as an integral part of the process.
In recent years, 3D Systems has applied for patents for radiation-curable compositions useful in image projection systems. In particular, the compositions are liquid and radiation-curable and are suitable for the production of three-dimensional solid articles in solid, free-form fabrication systems. They include a cationically polymerizing alicyclic epoxide having at least two epoxy groups; at least one cationic photoinitiator; at least one free radical photoinitiator; and at least one dendritic polymer having either at least six hydroxyl functional groups or at least six (meth)acrylate functional groups.
New 3D Technology Cures Layer by Layer
The newest variant of additive manufacturing, called material jetting or 3D inkjet printing, directs ultra-thin layers of polymer from a multiple-nozzle printhead onto a platform, and each layer is individually UV cured. It creates a complete, crosslinked plastic structure, which is supported by a gel-like material that allows the creation of complex structural geometries and overhangs, and delivers a very hard and strong result that can combine different materials and different colours in the same product construction. A small, rigid, plastic model car, for example, may be furnished in-line with soft rubber tyres.
UV curing is a key enabler in innovation in the technology, because it offers easier tool path creation and smaller files; a wider variety of materials and chemistries than other options; and higher detail. As the cost of consumer electronics is reduced, the versatility of UV-based systems will continue to expand the market for 3D printing.
Not all 3D printing processes require radiation curing. Selective laser sintering (SLS) directly fuses particles together layer by layer through the heat of a high energy pulse laser. Similar to SLA, this process starts with a tank full of bulk material, but this time in powder form. The bulk material is typically heated to just under its transition temperature to allow for faster particle fusion and print moves. This method increases the range of materials that can be used, including both plastics and metals, creating much stronger and more durable prototypes.
Fused filament fabrication (FFF) is a relatively new method which works by laying down consecutive layers of material in filament form at high temperatures, allowing the adjacent layers to cool and bond together before the next layer is deposited.
The German plastics specialist Igus has developed a new tribo-polymer filament for 3D printers, developed specifically for structurally robust prototypes or small batch components. The material is up to 50 times more resistant to wear and abrasion than the filaments originally used, such as ABS (Acrylonitrile Butadiene Styrene) and PLA (Polylactic acid).
Table 1: Advantages & Limitations of Radiation Curing (UV/EB)
|Seconds to cure vs. minutes or hours||EC coatings can have high shrinkage, which adversely affects adhesion to some substrates|
|Low overall cost (per cured part)||Tear resistance|
|Single component formulae eliminate mixing errors found in two-component systems||Lack of solvent coupled with a fast cure reduces the formulator’s ability to meet low gloss, low film build requirements.|
|High gloss||Overall cure of 3D parts: additional lamps are needed to cure 3D parts since EC is a line of sight cure method.|
|High hardness||Additional capital cost|
|Scratch and stain resistance||Retrofitting can sometimes be difficult|
|Fast cure||EB is expensive, but is becoming more cost competitive|
|Works best with flat substrates||Materials can be more expensive (though offset by improvements in productivity, superior appearance)|
|Avoids solvent use in most cases – very low volatiles||Re-training of operators|
|Smaller equipment footprint||UV/EB materials inks perform differently compared to conventional inks & coatings|
|Open pot life|
|Consistent printing and colour|
RadTech International N.A. (the Association for UV and EB Technology)
Radiation curable compositions useful in solid freeform fabrication systems
Patent Application #20100190881 PDF, 3D Systems
Tribo-filament for 3D printers
Igus (UK) : 08 October, 2014
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