Thermal vs UV Recombination: Choosing the right path for Optical films

In optical films and holography, “recombination” sounds esoteric, but it is simply the point where ideas in the design office become real square metres of structure.

It is also where most projects quietly succeed or quietly die.

Two families still dominate: thermal recombination and UV recombination. Both can produce good work. Both have long production histories. But they behave very differently once you start asking for tighter tolerances, higher brightness and more demanding substrates.

This note looks at the differences in a practical way, from the viewpoint of someone who must make or buy a real product, not write a textbook.

What we mean by recombination

Recombination is the process of taking a small, precise master and replicating it across a larger area, often in a step-and-repeat sequence, to create:

• large-area holographic or diffractive masters

• tools for roll embossing or casting

• optical films that will later be metallised, laminated or transfer coated

You are not yet producing packaging runs. You are building the “mother” from which production tools and films will follow.

Thermal recombination in outline

In thermal recombination, the work is done by heat and pressure. A thermoplastic layer is heated until it can deform; a nickel shim is pressed into it at controlled temperature and pressure; the structure is held while it cools; then the shim is separated; and the step-and-repeat sequence continues.

Strengths

• A mature, widely understood technology

• Reasonable tooling costs for simpler, lower-resolution structures

• Adequate for decorative effects and coarser relief where absolute fidelity is not critical

Limitations

Thermal recombination places a great deal of strain on the substrate. The polymer must often, accept the relief and then cool without moving too much. In practice:

• Internal stress builds in the structure

• Cooling is rarely uniform over large areas

• Fine features and higher aspect ratio relief are more prone to rounding and drift

• Later thermal steps can disturb what was originally embossed

The shim also undergoes epeated cycles of heat and mechanical load, which affect tool life when long campaigns are required.

UV recombination in outline

UV recombination removes the thermal drama. Instead of softening the substrate, a UV-urable resin is introduced between the nickel shim and the target surface, typically glass or coated film, and then cured with controlled UV light.

In simple terms:

• A thin, defined layer of UV resin is applied

• The nickel shim contacts the liquid, imprinting the structure

• UV light cures the resin in place

• The shim is separated, and the sequence repeats

The substrate remains close to ambient temperature and is not expected to flow.

Strengths

• Low thermal load on the substrate, with reduced shrinkage and better dimensional stability

• High fidelity for fine and deep microstructures, as a liquid is shaped and then immobilised rather than a solid being forced to move

• Good compatibility with rigid carriers such as glass, which is helpful for metrology and longterm stability

• Process windows that can be tuned through resin chemistry, lamp spectrum and process speed

UV recombination is particularly strong for high-brightness diffractive structures, microlens and other micro-optical films, and for masters expected to generate several generations of tooling.

A practical comparison

Fidelity and stability

Thermal processes can give acceptable replication, but thermal history, cooling rate and residual stress all leave their mark. Fine detail is more vulnerable to rounding and slow drift.

UV processes shape a low-viscosity resin and then immobilise it. The system is less at the mercy of bulk heat in the carrier. Stability over time is usually better, especially for demanding structures where brightness, angular profile or focus matter.

Substrate constraints

Thermal recombination requires a substrate that can withstand repeated heating and cooling without distortion or crazing. Glass is not a realistic option in this regard.

UV recombination works well with glass and with coated films designed for replication. This expands the range of useful combinations and makes serious metrology easier.

Process control

Thermal processes depend on careful management of temperature gradients, nip pressure, dwell and cooling. Once tuned, a line can be stable, but it tends to be sensitive to environmental drift and material variation.

In UV recombination, the critical variables are resin formulation, coat weight, shim contact and UV dose. These are often easier to measure and control tightly with the right sensors and recipes. When coupled to sensible inline or offline inspection, a UV recombiner offers a clear feedback loop between structure and process settings.

Tool life

Thermal tools experience repeated heat cycles and mechanical force, which encourages wear, especially on finer relief.

UV recombination runs at milder temperatures with gentler contact and is generally kinder to shims. Tool life and consistency are often improved over long campaigns.

Investment and know-how

Many older installations were built around thermal processes, so the equipment and skills may already exist, and new investment is incremental.

A modern UV recombiner is a more specialised asset. There is a learning curve in understanding the interplay between resin, lamp and mechanics, or a need to work with a partner who already has that knowledge. Once in place, however, the system can be easier to keep within a narrow performance window.

Where thermal still makes sense

Thermal recombination remains a sensible choice when:

• The structures are relatively coarse and primarily decorative

• The product will later see higher temperatures, where a UV-cured layer would be a weak link

• Existing thermal tooling is being reused for modest upgrades rather than an entirely new generation of optical products

In these situations, a well-run thermal line can continue to perform acceptably.

Where UV recombination has become the default

UV recombination is increasingly the natural choice when:

• Higher optical performance is required, not simply more of the same

• Consistent behaviour over large areas matters as much as attractive samples

• A structure must scale from laboratory or pilot work to industrial volume

• Customers or regulators expect tighter documentation and traceability of process conditions

Much of the more interesting work in micro-optical films and high-performance diffractive structures now assumes a UV replication backbone for these reasons.

How this relates to XRD Nano

At XRD Nano, we have deliberately centred our development on UV-based recombination because it gives clients more headroom in both performance and stability. The combination of tuned UV resins, controlled recombiner mechanics and, increasingly, AI-based inspection allows converters,

security printers and material innovators to move up a level in what their optical films can achieve without relying on heroics.

For organisations currently using thermal recombination, the question is not whether thermal is “wrong”, but where UV would materially improve yield, brightness or stability, and where existing assets remain adequate. In many cases, the answer is a staged approach: maintain thermal capacity where it fits and introduce UV where the next generation of products demands tighter control.

The choice between thermal and UV is ultimately a design decision, not an ideological one. It is a matter of which route gives the film, and the business behind it, the precision, stability and economics required for the next decade rather than the last.

By Guri Dhillon

Co-Founder, XRD Nano Limited

About XRD Nano

XRD Nano is a London-based engineering company developing UV nanoimprint equipment and advanced chemistries that make it easier to manufacture better optical films for anticounterfeiting and the holographic industry with less drama on the production line.

More at www.xrdnano.com