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Thick coatings and dark adhesives/inks often suffer from "surface cured but interior uncured" – the surface is hard while the bottom layer remains tacky, resulting in poor adhesion and easy peeling. Ergu Optoelectronics' fifteen years of industry experience points out that the root cause lies in the exponential attenuation of UV light as it penetrates the coating. There are two types of solutions: multiple thin coats with layer-by-layer curing, which requires no equipment investment but lowers efficiency; or optimizing the light source configuration – for LED, long wavelengths (395 nm/405 nm) can enhance penetration, while mercury lamps, with their broad spectrum and high energy density, are also suitable for thick, dark systems. A mercury lamp + LED combination can leverage the advantages of both. We support sample testing with real measurements to determine the right solution based on data, rather than blindly increasing power.

For anyone working with UV curing, eight out of ten have been burned by the problem of "surface cured but interior uncured" – the surface looks cured and doesn't feel tacky to the touch, but after some time or when an adhesion test is performed, the coating scrapes off or cracks when bent, leading to entire batches being reworked. This problem occurs more frequently especially with thick coating applications, dark adhesives, or dark inks.

Many people's first reaction is "not enough power," so they increase light intensity and slow down the line speed. The result? The surface is nearly scorched, becoming brittle and cracked, while the bottom layer remains soft. If you don't find the root cause, the more you adjust, the messier it gets. To solve the problem at its root, you first need to understand a basic principle – how light travels through the coating.

The essence of incomplete bottom-layer curing: light runs out of energy halfway through

The principle of UV curing is not complicated: ultraviolet light penetrates the coating, activates the photoinitiators, generates free radicals, and then the resin begins crosslinking and polymerization, transitioning from liquid to solid. The problem is that as ultraviolet light passes through the coating, it is continuously absorbed and scattered by the resin, pigments, and fillers. The deeper the light travels, the weaker it becomes – this is the core of the Lambert-Beer law: the longer the path, the less remaining energy.

Which two scenarios are most likely to "fail"?

One is thick coatings. The thicker the coating, the longer the distance light must travel, and the more obvious the attenuation becomes. Once the light exceeds the effective penetration depth of the light source, the energy reaching the bottom layer is insufficient to excite enough photoinitiators, resulting in incomplete reaction, which naturally causes tackiness and incomplete curing.

The other is dark-colored or highly pigmented systems. Carbon black, colored pastes, and high-opacity titanium dioxide strongly absorb ultraviolet light – most photons are intercepted by the surface pigments before they can reach the bottom layer. Therefore, the curing depth of dark-colored systems is usually significantly worse than that of transparent systems, and poor bottom-layer curing is almost inevitable.

A coating with insufficient curing not only fails to meet surface performance standards, but also commonly leads to decreased adhesion, poor chemical resistance, and easy delamination during long-term use.

Two proven solutions, choose according to your needs

For the curing challenges of thick coatings and dark-colored systems, there are already two mature and verified solutions in the industry. Choose according to your production mode.

Solution 1: Multiple thin coats + layer-by-layer curing – zero equipment investment, just change the process

This solution has the lowest barrier – you don't need to spend money on new equipment; it can be achieved entirely through process adjustments. The idea is simple: split one thick coat into several thin coats, curing each layer before applying the next. This way, UV light can easily penetrate each thin layer to the bottom every time, building up to the target thickness layer by layer.

There are two key points to note when using this solution:

First, control the thickness of each individual coat. Different systems vary greatly – transparent clear coats can be applied slightly thicker each time; high-hiding dark-colored systems and highly filled systems have faster light attenuation, so each coat must be thinner. How thick is appropriate? It's best to do a small test with your own materials to confirm rather than guessing by feel.

Second, pay attention to interlayer adhesion. If the previous layer is over-cured and the surface crosslinking is too dense, the next layer will not adhere properly and will delaminate. Therefore, during layer-by-layer curing, the previous layer can be kept in a semi-cured state before applying the next layer. This way the bottom layer is set, and interlayer bonding is also guaranteed.

The advantage of this solution is that it requires no new equipment investment – existing mercury lamps and LEDs can be used. It is especially suitable for low-volume, high-mix production scenarios. The drawback is also obvious – more process steps reduce production efficiency, making it unsuitable for high-takt mass production lines.

Solution 2: Match the appropriate light source configuration – improve penetration from the root

If you don't want to change the production cycle or add process steps, optimizing the light source selection is a more efficient approach. Different light source characteristics have different emphases in penetration capability and applicable scenarios.

Let's start with UV LED. Wavelength is the core parameter affecting penetration depth. Short wavelengths (such as 365 nm) have higher photon energy, faster surface curing, and better surface hardness, but weak penetration – they are easily blocked by pigments and thick coatings, making them suitable for thin transparent systems. Long wavelengths (such as 395 nm and 405 nm) have slightly lower photon energy but stronger penetration, able to pass through the surface layer to deeper positions in the coating, excite the bottom photoinitiators to complete polymerization, and significantly improve curing depth. When using a long-wavelength solution, you need to pay attention to matching with the photoinitiator system – it must be paired with initiators that have long-wavelength absorption characteristics. It is recommended to coordinate with your material supplier to adjust the formulation. A more reliable approach is a combination of short and long wavelengths – short wavelengths for surface drying, long wavelengths for deep curing, covering both surface hardness and deep-layer curing.

Now look at traditional mercury lamps. The output spectrum of high-pressure mercury lamps is a continuous spectrum, covering multiple UV bands such as UVA and UVB, with high overall energy density. For some thick coatings and highly filled dark ink and coating systems, mercury lamps indeed have advantages in deep-layer curing, especially suitable for wood coating, industrial coatings, and other scenarios where substrate temperature rise requirements are not high, and the equipment procurement cost is also relatively controllable. Some complex processes can also use a combination of mercury lamps and LEDs, combining the spectral and energy characteristics of both to achieve a more balanced curing effect.

In addition to these two core solutions, you can also collaborate with material suppliers to optimize the formulation – adjust the photoinitiator ratio, select resins and fillers with low light absorption, and reduce light attenuation from the material side. Combined with equipment solutions, the effect is even better.

Full-category equipment support + real test validation to avoid detours

Ergu Optoelectronics has been in the UV optics field for fifteen years, with both UV LED curing systems and traditional mercury lamp curing equipment product lines. We have accumulated rich project experience in curing thick coatings and dark-colored systems, and can provide support from equipment selection to process validation.

On the LED side, we support multi-wavelength custom combinations of 365 nm, 385 nm, 395 nm, 405 nm, etc., designing wavelength ratios according to the customer's coating characteristics, pigment types, and formulation features to balance surface curing speed and deep curing degree. On the mercury lamp side, we can provide equipment selection with different power levels and spectral line characteristics to adapt to different thicknesses and coating systems. Both types of equipment support stepless power adjustment, allowing flexible adjustment of output energy according to actual line speed and coating thickness on the basis of the selected light source configuration, accommodating products of different specifications within the same category.

With 36 patented technologies, all core light source modules are independently designed – LED spectral purity is high and energy output is stable; mercury lamp optical structures are optimized for high light energy utilization. All equipment is produced under the ISO 9001 quality management system, undergoes optical calibration and 72-hour continuous aging tests before leaving the factory, ensuring stable operation after delivery.

To help customers accurately select equipment, we support sample testing and validation – you send us your workpieces and curing materials, and we compare curing effects under different light source types, wavelengths, and power levels, providing objective recommendations based on measured data to help you avoid selection mistakes and blind investment.

We always believe that solving curing challenges is not about "the higher the power, the better," but about finding the solution that best matches your process and materials. Ergu Optoelectronics is committed to giving customers objective and practical advice, not blindly promoting high-configuration, high-priced equipment, but recommending only solutions that truly fit production needs. In the future, we will continue to share curing process knowledge to help more companies solve practical production problems.