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 in combination with liquid coating. 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.

Prior art

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)

Silicone release coating

• patent: US7052263B2 (expiration date 2023-4)
• 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 as photo-polymers can degrade silicone, which reduces the lifetime.

Continuous Liquid Interface Processing (CLIP)

• abbreviations; Continuous Liquid Interface Processing
• patent: WO 2014126837 A2 (expiration date 2034, not 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. CLIP does not have stair-stepping and can be used to build flexible parts.

Flex VAT

• Patent; US4575330 A (expired) The patent is now only used for marketing purposes; figure 4 technology etc. .
• Note; used to circumvent Envisiontec patent US7052263B2, also cheaper as Teflon film is easier to replace, the main disadvantage is that it is less accurate due to bulging of the film. CLIP also uses a Flex VAT but then with a Teflon AF film.

Transfer Substrate

• industrial usage: Admatec (film, WO2015107066), Lithoz (rotating disk, Lithography-based Ceramic Manufacturing (LCM), Patent unknown), Carima Master EV (video, ADD NUMBER), TNO (large film, EP20090164821, US8777602 B2)
• note: a liquid with a part gives a non-constant boundary condition, by the use of a transfer substrate the boundary condition is fixed, transfer substrates are ideal for filled slurries

Flow Photo-polymerization

• Patent; example here US 7709544 B2
• Description; due to high flow and oxygen inhibition the part does not polymerizes up to the end of the tubes,
• Note: tubes most likely fabricated with Teflon AF (add reference)

Claims

Flow Photo-polymerization

A transparent polygon scanner used in flow photo-polymerization. It is used to illuminate a Teflon or Teflon AF tube where a liquid flows through. Another option would be a glass tube. The glass tube might be coated internally with silicone.

Dual applicator bar

The Zephyr blade of 3D systems is symmetric so it can coat in two directions. If an array of transparent polygon scanners is combined in one unit with an applicator bar; this is no longer the case. The symmetry is broken. As a result, we claim the combination; applicator bar, array of transparent polygon scanners and applicator bar.

Conveyor belt

An array of transparent polygon scanners used in an conveyor belt printer. Examples are Google's Ara or TNO's Printvalley.

Through with protrusion

In bottom-up photo-polymerization see US4575330 A, Envisiontec came up with the idea to coat the glass with silicone, see US7052263B2 . If a single transparent polygon scanner is used, the scanner is not able to illuminate the substrate at once. This scanner must be moved in two directions. One of the key problems, in bottom-up photo-polymerization is the transport of fluid to form the next layer. The contact between part and substrate must be minimal to facilitate the flow of liquid. Therefore only the area which is illuminated by the transparent polygon scanner in a single snake like lane should be in contact by the part. There is trough for holding a liquid on top of a plate. This through is semi-transparent and has a flat protrusion which can be brought in contact with the part during illumination. The transparent polygon scanner moves along the surface of this protrusion to project a single lane. After projection, the part is separated. The through is moved and the part is again brought into contact. The transparent polygon scanner is used to form the next lane. This protrusion can be a flat glass substrate coated with a resilient layer, e.g. silicon see figure 1. This protrusion can also be a flex vat. This flex vat might be created with Teflon or Teflon AF.

Figure glassthrough

NOTES

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