The UV resin coating selector

A practical guide to calm beads, clean process windows and films that behave

By Guri Dhillon, Founder, XRD Nano Ltd, United Kingdom

Abstract

Choosing a coating method for UV resins is a balance of viscosity, coat weight, web speed and cure strategy. This selector links viscosity at 25 °C to the methods that naturally work, the coat weights they tend to deliver, and the speeds they tolerate. It then offers simple rules for common nano imprint and micro optics tasks, with a few shop floor notes that save days of trial and error.

Introduction

Begin with one number: viscosity at 25 °C. From there the choices fall into place. Pick the method that fits the thickness. Set flow and speed so the bead stays calm. Check surface energy so the film wets and stays put. The rest is quiet observation and small, honest adjustments.

Quick selector

(25 °C, PET unless noted. For most UV acrylates the density is about 1.0 grams per cubic centimetre, so 1 gram per square metre is about 1 micrometre wet. If shrinkage is 3 to 6 per cent, trim the final cured thickness accordingly.)

Viscosity 5 to 30 mPa s

• Micro gravure or reverse gravure at 50 to 120 lines per inch: 0.5 to 8 g per m² → 0.5 to 8 µm, 20 to 150 m per min

• Slot die with a low flow lip: 1 to 20 g per m² → 1 to 20 µm, 10 to 120 m per min

• Curtain coating, well de aerated: 5 to 30 g per m² → 5 to 30 µm, 80 to 300 m per min

Viscosity 30 to 150 mPa s

• Slot die, standard head: 2 to 60 g per m² → 2 to 60 µm, 10 to 120 m per min

• Comma bar or knife over roll with a 0.1 to 0.6 mm gap: 5 to 80 g per m² → 5 to 80 µm, 5 to 80 m per min

• Direct gravure at 80 to 160 lines per inch: 3 to 25 g per m² → 3 to 25 µm, 20 to 120 m per min

Viscosity 150 to 1000 mPa s

• Comma bar or knife over roll: 10 to 150 g per m² → 10 to 150 µm, 5 to 60 m per min

• Slot die with a heated manifold: 5 to 80 g per m² → 5 to 80 µm, 5 to 60 m per min

• Mayer rod number 3 to number 20: 3 to 60 g per m² → 3 to 60 µm, 5 to 40 m per min

Viscosity 1000 to 10000 mPa s

• Screen or stencil: 15 to 300 g per m² → 15 to 300 µm, batch or low speed

• Knife over plate or knife over roll with gentle heat: 30 to 300 g per m² → 30 to 300 µm, 2 to 30 m per min

What to use in common nano imprint and micro optics work

• Ultra thin release or primer, 0.5 to 2 µm: micro gravure or a low flow slot die

• Lens cup or base layer, 3 to 15 µm: slot die for flatness, or direct gravure when tool life and cost matter

• Backfill or levelling, 20 to 80 µm: comma bar or knife over roll; use slot die with mild heat when uniformity must be spotless

• Lab step and repeat on wafers or slides: spin coat 0.2 to 30 µm for 5 to 100 mPa s, or a simple doctor blade for quick benches

• Patterned or localised films: screen for higher viscosity deposits, or flexo for 1 to 10 µm when a light micro texture is useful

Setup tips that save days

• Rheology: if viscosity is below 30 mPa s and you see ribbing or orange peel, add a gentle associative thickener at about 0.05 to 0.2 per cent, or raise solids slightly

• De aeration: vacuum de gas, then a 50 to 100 micrometre inline mesh filter just before the head; microbubbles become pinholes under UV

• Surface energy: PET should read at least 44 to 46 dynes by corona or plasma; primers or adhesion promoters help cup layers

• Bead stability in slot die: start with a capillary number near 0.001 to 0.01, where capillary number equals viscosity times speed divided by surface tension; adjust speed and viscosity to hold the bead; narrow the lip for coats below 5 µm

• Gravure cells: for coats below 5 µm choose shallow microcells; deep cells invite surplus laydown and misting

• Curtain coating: keep viscosity low, flow steady and edges guided; excellent for 5 to 30 µm at high speed

• UV curing in air: to tame oxygen inhibition use nitrogen below 500 parts per million oxygen, or add amine synergists, or over varnish with a very thin topcoat and co cure

Fast decisions for the shop floor

• Need below 5 µm and mirror uniformity, choose slot die. Micro gravure works when budget or tooling favours it

• Five to fifty µm with a balance of cost and precision, choose slot die if flatness matters, comma bar if simplicity wins

• Above fifty µm or filled and thixotropic, choose comma or knife with gentle heat

• Very low viscosity at very high speed, choose curtain coating at 5 to 30 µm

• Patterned or localised films, choose screen for thick films or flexo for thin films

• R and D on wafers and slides, choose spin coat for the fastest design of experiments

Handy conversions

• Target 8 µm final with 5 per cent shrinkage: wet about 8.4 µm, which is about 8.4 g per m²

• Need 3 g per m² on a 400 mm web at 60 m per min: flow about 0.003 kg per m² times 0.4 m times 60 m per min, which is about 72 g per min to the head, plus recycle

• Switching method: if slot die leaves edge beads on an 800 mm web, try tapered shims or a micro manifold; if beads remain, micro gravure often damps edges naturally

Your turn

On your line this week, which two numbers drive the choice more than any others, viscosity or target thickness, and why

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 writes on Tuesdays about nano imprinting resins and machines and on Fridays about the quieter humour of the industry.