Coating of liquids in 3D printing

This page describes the coating of liquids in 3D printing. At the end, the page tries to describe how they could be used in combination with a transparent polygon scanner. The text is not very polished as its sole purpose to increase more room for transparent polygon scanners. The expiration dates of patents are estimates. There are several ways to extend patents. This is complicated as regulation varied over the past 20 years.

Literature

The following literature is key in this field.

LIQUID FILM COATING; Stephan F. Kistler, Peter M. Schweiz

It describes coating as in general. Liquid film coating has been key to industrial processes for a long time.

Dip & Blade Coating

• patent: US5651934 (expired, 2008)
• description:In the first implementations of Stereo Lithography Apparatus (SLA). Parts were dip coated. To speed up the process, deep dip & blade coating was invented.

Dip & blade coating was used in SLA 250 or SLA 500 systems of 3D systems. Later, an update was made available to use an active blade. Up to 2017, systems sold by 3D systems were still using an active blade retailed as "Zephyr blade"

• process settings; blade clearance, blade gap, number of sweeps, velocity per sweep, layer thickness & illumination time, deep dip distance
• properties; leading edge hump, scoop out, flow back and lift force
• blade design; symmetric to allow sweeps in both direction, multi-dent or tri-dent
• physics; the key equation are provided by the theory of boundary layers, see https://en.wikipedia.org/wiki/Boundary_layer
• end users; 3D systems (no longer used), R&D

Slot Die Coater

• literature; Flows in thin Film Coating, Christian Kuschel
• end users; none in 3D printing

Applicator Bar

• patent: EP 0807014 B1 (expired 2015 or 2016, not sure)
• process settings; blade clearance, velocity per sweep, blade gap
• blade design; symmetric to allow sweeps in both directions
• sensors; differential pressure sensor to measure internal pressure applicator bar versus external pressure, laser to measure internal liquid height
• physics; boundary layer theory, law of communicating vessels, Wilhelmy plate method
• end users; Prodways, 3D Systems (sold as Zephyr recoater)
• literature; Process Errors and Aspects for Higher Resolution in Conventional Stereolithography, Carsten Tille // Recoating issues in stereolithography, Renap and Kruth // A study of recoating in stereolithography, D T Pham and C Ji

Curtain coating

• patent: EP0928242 (expired 2016 or 2017, not sure)
• challenges: teapot effect, start-up distance, curtain guides, wind shield, curtain edges must be outside vat, heel, air entrapment
• end users; Materialise
• literature; A study of the behaviour of a thin sheet of moving liquid, D.R. Brown (1960) // HIGH-SPEED CURTAIN RECOATING FOR STEREOLITHOGRAPHY (M. Gilio, J.-P. Kruth, and P. Vanherck)

Silicon release coating

• patent; US7052263B2 (expiration date 2022, not sure)
• materials; silicone, silicon with Teflon
• notes; there are some key differences between European and US version, tilting or movement of build plate can be beneficial, silicone can be coated with Teflon via heat treatment.

The Teflon is used to protect the silicone.

Continuous Liquid Interface Processing (CLIP)

• abbreviations; Continuous Liquid Interface Processing
• patent; WO 2014126837 A2 (expiration date 2034, note sure)
• materials; Teflon AF
• note; CLIP (Continuous Liquid Interface Processing) also uses a non common acrylate, i.e. a polyurethane acrylate. Polyurethane can be strongly inhibited by oxygen (J. Mater. Chem., 2011, 21, 14325). Carbon 3D fixes this via post-processing.

Transfer Substrate

• industrial usage: Admatec (film, WO2015107066), Lithoz (rotating disk, ADD NUMBER), Carima (add number), TNO (large film, EP20090164821)

NOTES

SLA is a trademark. Please use the term photo-polymerization in RepRap circles.