UV Resin shrinkage and the science of curing wavelengths

A reflection on precision, chemistry and the quiet art of control in polymer replication

By Guri Dhillon

Founder, XRD Nano Ltd, United Kingdom

Abstract

Shrinkage is one of the oldest and most persistent challenges in UV curable resin technology. This article looks at why it happens, how curing wavelength influences dimensional stability, and what can be done in formulation and process control to achieve replication accuracy at the sub-micron level.

Introduction

The story of UV-curable resins is a meeting of chemistry and engineering that began in the 1960s. What started as a quick solution for coatings and adhesives has grown into a cornerstone of modern nanoimprint lithography, micro-optical replication and advanced surface engineering.

Even after decades of progress, the problem of shrinkage still sets the limits of precision. In optical replication, a contraction of just a few percent can change how a surface bends light or how cleanly it separates from the mould.

The Physics of Shrinkage

When a liquid photopolymer becomes a solid, the molecules pull closer together as the network forms. This tightening creates internal stress and a measurable reduction in volume. The shrinkage can range from one to six per cent, depending on the resin chemistry and curing conditions.

Shrinkage is shaped by the type of monomer, its functionality, the efficiency of the photoinitiator and the way the resin adheres to the substrate. Highly functional acrylates cure quickly but tend to contract more. Softer, less functional monomers cure more gently but take longer to harden. In precision optics, even two per cent shrinkage can alter the focal length of a microlens or distort a diffractive surface.

The Influence of Wavelength

The colour of light used for curing plays a major role in how a resin behaves. Each wavelength interacts differently with the photoinitiator and the resin’s depth profile.

Light at 365 nanometres penetrates deeply and cures evenly, making it suitable for thicker coatings or applications that demand optical clarity. At 385 nanometres, there is a good balance between surface and bulk cure, offering faster throughput without creating too much internal stress. At 395 nanometres, the surface hardens quickly, but if the exposure is not well tuned, deeper layers can lag behind, creating a stress gradient.

To manage this, many systems now use a staged cure: a soft first exposure that holds the geometry in place, followed by a stronger cure that completes polymerisation. This step-by-step approach helps maintain fidelity and reduces warping or distortion.

The overlap between LED emission spectra in the 365 to 395 nanometre range and the absorption of the photoinitiator determines how evenly the resin cures. This overlap, in turn, decides how much shrinkage or residual stress remains in the final part.

Strategies for Control

Reducing shrinkage begins with the right formulation. Low-shrinkage urethane acrylates, flexible monomers such as isobornyl acrylate, and hybrid radical cationic systems all help create a more stable polymer network.

Curing under nitrogen and keeping the temperature constant improve uniformity. Modern measurement tools such as interferometry and digital holography now make it possible to watch the surface deform in real time as the resin cures. This kind of feedback connects material design, optical modelling and process control, creating a more predictable outcome.

Future Outlook

The next generation of resins aims to achieve less than two per cent shrinkage while keeping transparency, adhesion and processing speed intact. Multi-spectral LED curing and software-driven control of energy delivery are opening new frontiers in precision replication.

The future of UV curing lies not just in faster or stronger materials but in understanding how light and matter interact. The goal is not only to harden a resin but to let light teach matter to hold a shape. That quiet dialogue between photons and polymers remains one of the most elegant forms of engineering.

About the Author

Guri Dhillon is the founder of XRD Nano Ltd, a British company working at the intersection of optics, engineering and UV resin chemistry. He spends his time developing machines, materials and methods that shape the next generation of authentication and optical technologies.