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Benefits of UV Curing

Manufacturers across a wide range of industries including commercial printing; product decoration; medical device and electronics assembly, furniture, cabinet, and flooring production; fiber optics; IML/IMD hard coats, automotive; and many others gravitate toward UV curing for the inherent advantages the technology offers. A brief list of benefits include: 

  • High Speed:  Compared to thermally cured formulations, ultraviolet inks, coatings, and adhesives cure extremely fast; almost instantly. This makes UV an ideal choice for high speed production lines including printing, fiber optic cable, wide web converting and others. 

  • Low Temperature:  With UV curing, light replaces heat as the force that drives the chemical reaction. UV sources (especially LEDs) are well suited to curing parts made of materials easily distorted or damaged by excessive heat.  

  • Low Contamination:  When an ink, coating, or adhesive cures rapidly, the reduction in open time presents less opportunity for dirt and other airborne contaminants to get into the finish and cause cosmetic or functional defects. Unlike typical convection ovens, UV curing generally has far less air movement stirring up unwanted, foreign materials. For aesthetically demanding or high value parts, a positive result of UV curing is less costly scrap and rework due to dirt and contamination. 

  • Reduced Space & Processing Time Requirements:  UV production lines can be compact in size since the need for long thermal drying tunnels is eliminated. In addition, parts exit the line immediately ready for post cure processing and packing. This combined space and time savings allows for leaner manufacturing and just-in-time production which eliminates work-in-process (WIP) and reduces production scheduling hassles. Plastic parts for example can be taken from the mold, placed directly onto the UV finishing line, cured, and then directly packaged for shipment.

  • High Performance:  UV cured formulations are highly cross-linked polymers with numerous bonds among the resins. The result is harder, scratch-resistant surfaces with excellent durability and mar and chemical resistance. But this doesn’t mean that UV cured materials are brittle and inflexible. UV formulations for flexible, soft touch, and other tactile or haptic effects are used for many commercial applications.

  • Environmentally Friendly:  Many UV coatings are formulated with few if any solvents. 100% solids UV materials eliminate hazardous air pollutants (HAPS) and volatile organic compounds (VOCs) altogether. UV cured materials frequently incorporate low molecular weight monomers and oligomers as reactive diluents in place of traditional solvents. This makes UV materials and UV curing an excellent green technology. 

  • Less Waste & Clean-Up:  Since UV formulations only cure when exposed to ultraviolet light, materials can be left in the machine and application equipment for extended periods with minimal risk of gumming up dispensing nozzles or setting-up in the pan, tray, pail, or pump. UV also makes clean-up easier as spills, spitting, or over-spray can be more easily wiped away.

The fist column in the following table lists a number of benefits associated with all UV curing systems (arc, microwave, LED, and spot cure). The second column provides an additional set of advantages that are characteristics of UV LED systems.

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Designing UV Curing Applications

There is an ever-increasing abundance of UV curing systems, formulations, and vendors from which to choose. All promote very similar features and operational benefits making it difficult to discern which products are actually best suited for an application. For UV emitting lamps, the primary differentiators are the source type (arc, microwave, LED, or spot cure), spectral output, peak irradiance, energy density, cooling method, lamp form factor, and operational life. With respect to formulations, the intended use, formulation consistency across batches, and final cure properties for a given application at the desired press or process speed are equally important.

While knowledge of both lamp and formulation characteristics is required for proper application design, it can still be a challenge to match a UV system to the exact needs of the formulation and process, particularly in cases where something novel is being attempted or the established manufacturing norms are being pushed to new limits. Technical spec sheets do not always provide explanations as to which product(s) are best suited for a given application or how the technology should be scaled from the lab or pilot line to production. For these situations, the following general guidelines can be used to provide additional insight when attempting to match a UV system to the chemistry and application method.

  • Final Cure Properties:  Formulations are not universal and are instead intended for specific uses. The desired physical and aesthetic properties of the final cure as well as the particular product use (such as low migration food packaging, medical device, dermal contact, outdoor use, necessary post cure processing) should be specified and are instrumental in driving the formulation chemistry and ultimately whether a UV curable solution is possible. Be sure to communicate the desired post-cure properties and the intended use to equipment suppliers, formulators, and integrators. Straying outside of specific applications intended for the formulation can pose potential liability issues. ALWAYS CHECK WITH YOUR SUPPLIER.

  • Application Method and Production Process Needs:  Just as formulations and light sources are designed to deliver specific final cure properties, they are also crafted for particular application methods (flexo, screen, digital, coater, spray, etc.). Viscosity and shear forces vary across application methods and must be factored into the formulation design. In addition, other process elements influence the selection of formulation and light source including working distance, line speed, machine width, part profiles including drastic contours and shadow areas, substrate or part material construction, as well as dual stage applications which utilize UV light to partially cure the A Stage formulation followed by a B Stage or final cure process that may involve heat, pressure, moisture, or some combination. All of this is relevant information and should be communicated to suppliers and factored into selection criteria.

  • Wavelength:  With respect to broadspectrum lamps, mercury is the most commonly used across all markets. Gallium doped lamps are optimal for white inks and white coatings, and iron doped lamps are often preferred in industrial applications. For LED, commercial curing wavelengths include 365, 385, 395, and 405 nm. For most ink applications, 395 nm is the preferred wavelength with 365 and 385 nm wavelengths used to a lesser degree. Adhesives typically work best with 365 or 405 nm, depending on the formulation, but also cure similarly with 385 or 395 nm. Graphic over print varnishes tend to match the ink wavelength of 395 nm, and when it comes to industrial coatings (both functional and hard), there is no consensus as UV LED development is still very much in its infancy and may require more evolution of UVC and UVB LEDs. While lamp (bulb) types are interchangeable in mercury systems, UV LED systems are supplied fixed with one wavelength or blended with a few wavelengths. In either case, LED wavelengths cannot be changed after being assembled into a curing device, cassette, or module. 

  • Irradiance Window:  The formulation chemistry must be cured within a minimum and maximum peak irradiance (Watts/cm2). Operating below a minimum irradiance will result in insufficient cure and increasing irradiance beyond the maximum does not necessarily produce better results. There is no universal irradiance that meets the needs of all formulations. Each application is different, and the optimal emitted irradiance window could be anywhere between a few hundred mW/cm2 and 50 Watts/cm2. More important than the emitted irradiance, however, is the actual irradiance that reaches the cure surface as irradiance decreases rapidly with distance traveled. While irradiance operating windows are not narrow, they should be identified for process control purposes. Don't be content to operate at a setting that works without understanding the outer boundaries. When the day comes that your process falls outside the optimal operating window, which it will, it may not be clear how to recover without a previously documented reference.

  • Energy Density Window:  The formulation and the manufacturing line speed determine the necessary energy density (Joules/cm2). A greater energy density generally results in a better overall cure, allows for a faster line speed, and sometimes enables a lower lamp peak irradiance. Not all systems emitting the same peak irradiance deliver the same energy density. Refer to UV Curing Sources and UV Measurement for more information. Energy density can be increased by using a lamp which by design emits greater energy density or by increasing the exposure time. Exposure time can be increased by either using wider lamps, multiple lamps in combination, or slowing the line speed.

 

  • Working Distance:  This is defined as the offset between the face of the lamp head or UV LED emitting window and the cure surface. It must be specified for the application and machine set-up as irradiance decreases quickly with distance. In order to accommodate greater working distances, consider more powerful lamps (either greater irradiance or greater energy density or both) or a solution that incorporates optics or reflectors which direct or contain the light. Conventional arc lamps typically have a peak irradiance at the focus. While it varies by system, the focus is typically 50 mm (2") from the surface of the lamp head. While LEDs can be supplied with a greater irradiance than conventional lamps, LED peak irradiance occurs at or near the emitting window, not 50 mm (2") away. Unlike conventional lamps, UV LED systems do not have a focus. UV LED output without the use of optics or reflectors diverges quickly. Both LED and mercury UV curing systems have limits in effective working distance; although, it varies based on system and application.

 

  • Cooling Mechanism:  UV curing systems are cooled with either negatively or positively forced air or circulated liquid coolant. The plant environment including air quality and cleanliness, temperature, humidity, etc.; preference of the OEM or end user; and curing system design determine the cooling mechanism (air or liquid). When engineered correctly, the cooling method does not negatively impact the UV output. When air-cooling is used, filters should be monitored and either cleaned or replaced as needed. When liquid cooling is used, it is important to follow OEM guidelines regarding which type of solution, concentration, and dilution material to use. Never mix different solutions as it will result in precipitates falling out of solution and gumming up the lines. Coolant levels should be maintained and flushed periodically.

  • Part or Substrate Heat Sensitivity:  Spectral output is energy. Different wavelengths within spectral output as well as the magnitude of the wavelengths are critical in crosslinking photopolymer chemistry. Too much, however, results in undesired heat transfer and can even hinder crosslinking. This applies to both conventional mercury lamps and UV LED systems. Refer to UV Curing Science for more information. Any light energy that does not go into the chemical reaction will be absorbed by the parts or substrate being converted as well as surrounding machine components. Heat can be transferred away from the manufacturing line to the air within or outside the plant via blowers and exhaust ducting. While conventional lamps emit heat generating infrared wavelengths, UV LED systems have their own heat generating component - irradiances up to ten times that of conventional lamps. Systems that emit infrared, high irradiance, and high energy density have the potential to transfer more heat energy than those without infrared or those with a lower irradiance or lower energy density. While more power may be necessary for high speed lines or to cure at a greater working distance, it is not always necessary for slower line speeds and close working distances. In addition, slower line speeds and close working distances will naturally facilitate greater heat transfer as a result of proximity and exposure time. In some cases, a chilled roller, chilled drum, or chilled plate can be placed behind the part or substrate to capture unwanted heat energy and carry it away from the process. When it comes to applications with heat sensitive materials, more power may be necessary for fast line speeds, but it can be problematic at slower line speeds. Heat is manageable, but it's best to mitigate it from the beginning by properly matching the UV system to the formulation and application. For shrink film applications in narrow web flexo, it is technically correct that LED lines can be run without chilled rollers; however, the process window is narrow and not something that operators necessarily want to monitor. As a result, it is generally recommended that chilled rollers be used with shrink film applications.  

  • Mounting Space:  For retrofits, the mounting location and machine design determine the allowable space for the UV lamp head. For new machines, the lamp head mounting can be designed into the machine mechanics. Focal lengths for conventional mercury systems typically intend for the lamp to be mounted 50 mm (2") from the cure surface while LED systems without optics should be mounted as close to the cure surface as possible (typically 10 to 15 mm). Water-cooled systems are generally more compact than air-cooled systems. Air-cooled systems that draw from or exhaust to the space in or around the manufacturing line also require a minimum clearance around both the air inlets and outlets to ensure adequate circulation. Ducted systems must allow for routing of the ducting through the machine and up to the roof or side wall. Both conventional mercury and UV LED systems should be mounted such that they can be easily accessed for maintenance and service. While UV LED systems do not require bulb, reflector, and magnetron changes, they do need to be inspected periodically to ensure that the emitting window remains clean. Heads that are awkwardly mounted, hard to access, and difficult to remove from a press will seldom be inspected and cleaned. This should be avoided by taking the time to design brackets that make it easy and quick to access the head. 

  • Optics & Shielding:  Lamp proximity and orientation to unwanted cure surfaces influences the need for optics and shielding. Care should be exercised to ensure that UV rays are blocked from digital inkjet print heads, ink and coating pans, image transfer plates, blankets, rollers, and heat sensitive materials on the machine. In addition, all light should be shielded from any direct line of sight with the operator. Industrial applications that require lamp heads to be mounted further from the cure surface (several inches or more) may require optics and/or side reflectors to direct or contain the light closer to the cure surface. Optics and reflectors are either built into the curing device or are an external add-on. For UV LED curing systems, optics can be micro, macro, or secondary.  

  • Part Profiles:  Free radical UV chemistry requires direct line of sight between the UV system and the cure surface as well as sufficient exposure time. Cationic chemistry only requires UV light to initiate the reaction and will continue crosslinking once the UV source is removed. Refer to UV Curing Science for more information as well as the pros and cons of each. For fee radical chemistry, applications with drastic part surface profiles and shadow areas can be challenging. When the lamp head is mounted in a location that allows it to fully clear the shapely part being cured, the deepest draw of the part may result in some cure surfaces being too far away from the lamp head. This means that the irradiance levels will vary unevenly across the cure surface and may be blocked from some shadow areas. The lamp must be powerful enough to ensure that all surface areas are exposed to the necessary minimum irradiance threshold value while not being too powerful so as to cause heat damage or fry the chemistry at surfaces closer to the lamp. In many industrial applications, multiple lamps installed in different locations are often used to ensure sufficient UV coverage, or the parts are individually rotated or robotically manipulated. Conventional mercury systems with their 50 mm (2") focal length currently lend themselves a bit better to these industrial applications with complicated part profiles and cure distances of several inches or more.

  • Regulatory Climate:  Every industry operates within its own unique regulatory climate. Some policies are legally enforced while others are self-imposed due to prevailing consumer demands and perceptions (both warranted and unwarranted). Regulatory pressure has several origins: governmental bodies, standards organizations, large industry players, consumers and their NGO advocates (medical, food, environment, trade, etc.), and supply chain stresses among others. In some cases, regulatory policy may be decades in the making and in others it may materialize within a year or two. Some polices only apply to small geographic regions while others have a global impact. Regardless, it is important to understand what external pressures influence manufacturing processes, what policy changes are anticipated in the coming years, whether grandfather clauses will provide cover or transition periods to certain processes, and how technology can be utilized to establish and maintain regulatory compliance. Regulatory policy can sometimes be a moving target, and no one has a crystal ball. Predicting and planning for regulatory changes is a challenge even for those who focus on it full-time. As a result, the best recommendation is to work closely with trade organizations within a particular industry as well as RadTech so that knowledge and recommended courses of action can be collectively pooled. With respect to application design, regulatory policy should be considered when evaluating UV curing sources, formulations, and application methods. Refer to UV Curing Associations and UV Curing Trade Journals for more information.

  • Source Type:  While conventional curing technology has been used for decades across many industries, UV LED technology will increasingly become the preference. For industrial, functional, high speed, and wide web applications, there is still much development work that must take place before UV LED technology is a viable alternative. But the work is happening, and the change will come. Despite the inability to use the exact same LED system interchangeably across the various applications (see next section), there do exist unique UV LED solution sets that meet the needs of many. While OEMs may perform the LED curing device and formulation matching for the end user ahead of purchase, it is always recommended that end users confirm through testing or by referencing previous installations that the correct system was indeed matched to the specific needs of the process. An incorrectly matched system may mean too little or too much curing power. Too much power also means unnecessary energy consumption. When it comes to UV LED curing, do not assume that all the formulations you need already exist. If you are starting the search yourself, follow the guidelines in the various sections of this website as well as the various publications available for download (refer to UV Curing Publications) and work closely with UV LED curing system suppliers, formulators, OEM machine builders, integrators, and end users in order to ensure greater overall UV LED curing success. When in doubt, seek assistance from subject matter experts such as Eminence UV.
     

 
 
Types of UV Curing Applications

UV curing technology (arc, microwave, LED, and spot cure) is used across a wide range of graphic and industrial applications. These include print for communication, decoration, and function; adhesives for structural bonding, laminating, pressure sensitive, and resealing; and industrial coatings for hardness as well as chemical, scratch, mar, anti-corrosion, and environmental resistance and protection. Many of the application and material handling methods available span multiple industries. A short list includes the following. 

Print Processes - Graphics, Labels, & Flexible Packaging

  • Cast & Cure

  • Coating (over print varnish, haptic, laminating)

  • Digital Inkjet

  • Flexo

  • Gravure

  • Letterpress

  • Lithography

  • Offset - Sheetfed

  • Offset - Web

  • Screen

Print & Decorative Assembly Processes - Rigid Parts & Packaging

  • Cast & Cure

  • Digital Inkjet

  • Dry Offset

  • In-Mold-Decoration

  • In-Mold-Electronics

  • In-Mold-Label

  • Pad

  • Screen

Field Repair

  • Automotive Refinishing

  • Floor Refinishing

  • Patch Repair

​Industrial Applications - Functional, Hardness, & Chemical, Scratch, Anti-Corrosion, and Environmental Protection

  • Automotive

  • Aerospace

  • Composites

  • Fiber Optics (strengthening, bundling, coloring)

  • Medical Device

  • Metal (coil, cans, pipe, & tube)

  • Plastics

  • Web Converting

  • Wire

  • Wood

Electronics Applications

  • Conformal Coating

  • Encapsulation

  • Flexible Printed Electronics

  • Photoresist

Product Assembly

  • Sealing & Coating

  • Structural Bonding
  • In-Mold-Decoration

  • In-Mold-Electronics

  • In-Mold-Label

Adhesive Processes

  • Laminating

  • Pressure Sensitive

  • Structural Bonding

Marking & Coding Applications

  • Product Identification

  • Serialization

  • Direct Mail

Material Handling Methods

  • Conveyor (chain-on-edge, flatbed, overhead, racked, rotated)

  • Carousel

  • Indexing Table

  • Sheetfed

  • Web (narrow, mid, wide)

  • X-Y Table

Application Methods

  • Printing (inkjet, flexo, gravure, letterpress, offset, pad, screen)

  • Coating (curtain, flow, roll, gravure, mayer-rod)

  • Dispensing

  • Spray

  • Sputter

  • Vapor Deposition

 
Ink, Coating, and Adhesive Film Build Conversion Guide

For usage rates and quality control purposes, inks, coatings, and adhesives are often communicated in terms of actual thickness or weight per area, especially in industrial applications. Printed inks, however, are more typically qualified by color density which is a light reflectivity measurement obtained through either a densitometer or a spectrophotometer. Color density is a better indicator than weight or thickness for determining whether more or less ink should be applied. This is because the amount of ink that must be deposited on a substrate to achieve a desired look depends on the ink formulation (pigment load, additives, viscosity, etc.), the application method, and the substrate. For applied films, a porous substrate requires the application of more material since it is readily absorbed into the substrate as opposed to non-porous substrates which allow the film to lay on top with little to no penetration. It is possible to measure film build with a wet film gauge; however, this requires making contact with the uncured material. A wet film gauge doesn't necessarily reflect total applied material but rather film build above the substrate prior to cure. 

 

The following table provides a quick means of converting between more commonly used units of measure for applied material thickness. The conversions are courtesy of Applied Molecules.
  

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Typical Film Build Ranges by Press Type

Material film laydown varies by application method. The following table provides a rough guide of typical film builds (in microns) associated with various printing technologies. Formulations used for litho, offset, letter press, rotogravure, and screen are typically referred to as pastes while those associated with flexo and digital are much lower in viscosity. Since there are always exceptions to every rule, this chart is primarily meant to provide context to those looking to expand into new decoration and protection methods or communicate across the various printing methods.

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Plastic Film and Sheet Thickness Conversion Charts

The following tables provide a list of plastic film and sheet gauges with corresponding measurement thicknesses in mils, microns, millimeters, and inches. Plastic film is considered anything thinner than 0.010 inches (0.25 mm), and plastic sheets are defined as thickness greater than 0.010 inches (0.25 mm). While thinner sheets can be supplied in continuous lengths wound on a core, they are more typically supplied cut to size. 

 

mil is a US unit of measure that equates to an imperial value of one thousandth of an inch (0.001 inch). A mil is an entirely different unit of measure than a metric millimeter. One mil is equivalent to 0.0254 millimeters, and one millimeter is equivalent to 39.37 mils. By comparison, the commonly used unit of micron is one millionth of a meter (0.000001 meter) or one thousandth of a millimeter (0.001 mm).

 

Film gauge is a nominal communication value correlated to the substrate thickness in inches as illustrated in the following tables. For example, a film gauge of 30 is 0.00030 inches thick. Not all film gauge sizes are available for purchase through suppliers as not all sizes are commonly used. The gauges provided in the charts are simply meant for communication and conversion purposes.

 

A ranking in magnitude of one unit in each measurement thickness is as follows:

 

1 Gauge < 1 Micron < 1 Mil < 1 Millimeter < 1 Inch

19.04.09 Film Gauge Convesion Chart Colo
19.04.09 Film Gauge Convesion Chart Colo