Patent Description:
The present invention relates to a method according to claim <NUM> for preventing oxygen inhibition of a light-initiated polymerization reaction used by a 3D printing system by purging the oxygen from the reaction surface using inert gas flow.

Many additive manufacturing, or so-called three-dimensional ("3D") printing, applications use ultraviolet ("UV") light-curable polymers. The UV curing process consists of three stages: photoinitiation, propagation, and termination. During photoinitiation, a photoinitiator produces free radicals when exposed to UV radiation. These free radicals react with nearby monomers and convert them into free radicals. Next, in the propagation stage, the free radical monomers bond with other monomers and turn those monomers into free radicals. In this way the monomers form a polymer chain. The process continues until it reaches termination. Termination can occur in many ways, including if two chains bond with one another, the free radical transfers to a monomer, or if the chain reacts with molecules from the environment and not a monomer.

There are two interactions between oxygen and the photopolymer that inhibit curing: quenching and scavenging. After the photoinitiator has been excited by exposure to UV radiation, it produces a free radical. Molecular oxygen easily reacts with this free radical, preventing it from reacting with monomers in the process of chain propagation. This is the quenching reaction. This reaction also produces an oxygen free radical. In the scavenging reaction, this oxygen free radical reacts with a free radical that is part of a propagating polymer chain. This reaction results in a less reactive free radical, which leads to early termination of the polymerization process. These two processes can be written as:.

Because of these phenomena, if a photopolymer is exposed to oxygen during curing in a 3D printing process it can result in uncured polymer residue on surfaces exposed to the air.

<CIT> describes enabling enhanced printing solutions by providing ultraviolet curing conditions without requiring complete evacuation of atmospheric oxygen. Increased ink coverage and adjusted surface appearance are also discussed.

Embodiments according to the invention are set out in the appended claims.

A UV curing system includes a gas diffusion system for introducing an inert gas into a workspace between a UV light source and a UV curable layer of a workpiece. A transparent cover separates the UV light source and the workspace and the inert gas (e.g., Ar, CO<NUM>, He, Ne, etc.) flows in from gas inlets and out through a diffuser towards the workpiece. A gas pressure homogenizer is used to ensure constant pressure throughout the system.

The diffuser is made of a transparent or diffuse material to allow UV light from the UV light source to pass through it. The diffuser includes an array of micro-holes for the inert gas to pass through towards the workpiece. The small diameter of the holes allows a closed-packed array thereof so that the gas is evenly distributed throughout the workspace (i.e., throughout the curing area). The small diameter of the holes also means that a larger area of the surface of the diffuser is free of holes making its optical properties more homogenous. This ensures a relatively even light distribution. The holes are covered with "bridges" of the UV-transparent material of which the diffuser is made. This ensures that all light passing through the diffuser passes through at least some thickness of the transparent material, further improving light distribution.

In an embodiment that is not as claimed, after the UV curable material has been deposited on the surface of the workpiece, and the workpiece introduced into the workspace of the UV curing system, the inert gas is pumped through the diffuser. This flow of gas purges the oxygen from the region of the workspace adjacent to the diffuser. The thickness of this region is related to the gas pressure as it is forced through the diffuser. With the workpiece maintained in the area of the workspace from which oxygen has been purged, the UV curing system then cures the layer of UV curable material through exposure to light from the UV light source.

A further embodiment of the invention provides for preventing oxygen inhibition of a light-initiated polymerization reaction by periodically emitting a UV light from a UV light source into a UV curing space in which a workpiece having a layer of UV curable material is disposed to facilitate, within the UV curing space, UV curing of the UV curable material, and purging oxygen from the UV curing space at times when the UV light source emits light onto the layer of UV curable material. Purging oxygen from the UV curing space includes introducing, via a gas diffusion system, an inert gas into a workspace between the UV light source and the layer of UV material of the workpiece. For example, the inert gas may introduced via one or more gas inlets of the gas diffusion system and through a plurality of micro-holes in a transparent diffuser separating the UV light source and the workspace towards the workspace. The UV light from the UV light source may be transmitted through bridges of a UV transparent material arranged over the micro-holes of said diffuser towards the layer of UV material of the workpiece. Thus, the inert gas and the UV light are each approximately evenly distributed throughout the workspace via the micro-holes.

In some embodiments of the invention, the inert gas flow is used to evenly heat the UV curable material during curing or to control the temperature of the UV curing space by controlling the inert gas temperature.

These and further embodiments of the invention are described below with reference to the accompanying drawings, in which the present invention is illustrated by way of example, and not limitation. The invention being defined by the appended claims.

Before describing the invention in detail, it is helpful to present an overview. Referring to the sequence of images shown in <FIG>, in many 3D printing processes in which an object <NUM> is undergoing fabrication, a materials printing system <NUM> is used to deposit UV curable material <NUM> on a surface <NUM>. This deposited material is then cured with a UV light source <NUM> to produce a new layer of the desired part <NUM>'. This process continues until the part undergoing fabrication is completed.

The present disclosure provides systems and methods for preventing oxygen inhibition of a light-initiated polymerization reaction at ambient conditions. Referring now to <FIG>, a UV curing system <NUM> is equipped with a gas diffusion system <NUM>. A transparent cover <NUM> is disposed between the UV light source <NUM> and the gas diffusion system <NUM>. The gas flows in from gas inlets <NUM> and out through a diffuser <NUM> at the bottom of the system. A gas pressure homogenizer <NUM> is used to ensure constant pressure throughout the system.

The diffuser <NUM> is made of a transparent or diffuse material to allow UV light to pass through it onto a workpiece <NUM>, and in particular onto a layer of UV curable material <NUM> disposed thereon. The diffuser <NUM> consists of an array of micro-holes <NUM>. The small diameter of the micro holes allows for a closed-packed array thereof so that the gas is evenly distributed throughout the curing area <NUM>. The small diameter of the micro-holes <NUM> also means that a larger area of the surface of the diffuser <NUM> is free of holes, making its optical properties more homogenous. This ensures more even light distribution. Of course, other arrangements and sizing of the micro-holes may be employed so as to optimize gas distribution and light distribution throughout the curing area. The micro-holes <NUM> are covered with "bridges" <NUM> of the material of which the diffuser is made. This ensures that all light passing through the diffuser must pass through some region of the transparent material. This further improves the light distribution.

Referring now to <FIG> and in an embodiment that is not as claimed, after the UV curable material <NUM> has been deposited on the print surface, the gas is pumped through the diffuser <NUM> via the gas inlets <NUM>. This flow of gas purges the oxygen from a region <NUM> adjacent to the diffuser <NUM>. The thickness of this region is related to the gas pressure as it is forced through the diffuser. Thereafter, as shown in <FIG>, if necessary the device under fabrication is raised so that the layer of UV curable material <NUM> disposed thereon is disposed within the oxygen-free region <NUM>, and the UV source <NUM> of the UV curing system <NUM> is activated, thereby curing at least a portion <NUM>' of the layer of UV curable material <NUM> of the part undergoing fabrication in a region exposed to the UV light <NUM>. In some instances, it will be unnecessary to move the device under fabrication because the layer of UV curable material <NUM> will already be within the oxygen-free region <NUM> when the gas is pumped through diffuser <NUM>. In embodiments of the invention, the gas pumped through diffuser <NUM> is preferably an inert gas (e.g., Ar, CO2, He, Ne, etc.) insofar as it does not interact with the photopolymer in UV curable layer <NUM> so as to inhibit curing thereof.

In some embodiments, the temperature of the feed gas may be controlled (e.g., through heating provided prior to gas inlets <NUM> and/or within the gas diffusion system <NUM>) to create a uniform reaction temperature in the vicinity of workpiece <NUM> (e.g., within a space within which curing of the layer of UV curable material <NUM> disposed on the surface of the workpiece <NUM> will take place). For example, the inert gas may be heated prior to its introduction into the gas diffusion system <NUM> so as to maintain a desired and uniform reaction temperature within the vicinity of the surface of workpiece <NUM> on which the layer of UV curable material <NUM> is disposed.

<FIG> illustrate one example of a gas diffusion system <NUM>. In <FIG>, a front cover <NUM> is in place, while in <FIG> it has been removed to show aspects of the interior of the gas diffusion system <NUM>. In this example, gas diffusion system <NUM> is a rectilinear box having a UV light source <NUM>, for example made up of one or more light emitting diodes (LEDs), mounted therein, inside a top of the box. A gas diffuser <NUM> forms a bottom face of the box. As mentioned above, the gas diffuser is made of a transparent (at the wavelengths of illumination necessary for curing of the photocurable material used to fabricate the part under construction) material to allow UV light from source <NUM> to pass through relatively unattenuated.

<FIG> highlight the construction of diffuser <NUM>. As noted above, bridges <NUM> (in the illustrated example, fashioned as ribs running longitudinally across an upper surface of the diffuser <NUM>) are disposed above gas flow holes <NUM> so that they are in an optical path between the UV light source <NUM> and the gas flow holes when the diffuser and UV light source are assembled in the gas diffusion system <NUM>. This ensures that across the entire curing area the UV light will pass through at least some thickness of the transparent material. This ensures better light homogeneity and more even curing of the photopolymer in UV curable layer <NUM>.

Returning to <FIG>, gas enters (e.g., by action of a pumping arrangement) the diffusion system <NUM> via one or more inlet holes <NUM> and exits through the diffuser <NUM>. A gas pressure homogenizer (not shown in this view) is used to ensure constant pressure throughout the system.

<FIG> and <FIG> illustrate the cooperating operations employed in the printing and curing process. Printing of a next layer (6a-<NUM>) begins with the deposition of a layer of UV curable material on the print surface of an object under fabrication. Towards the end of this deposition, gas is pumped <NUM> through the diffuser <NUM> via the gas inlets <NUM>, as shown in the gas pressure curve in <FIG>. The gas pressure increases to the level required for the curing process (6a-<NUM>) and purges the oxygen from a region <NUM> adjacent to the diffuser <NUM>. The thickness (H) of this region grows over time, as shown in the oxygen free area thickness curve in <FIG>, and is related to the gas pressure as it is forced through the diffuser. When a desired thickness H* has been attained, the device under fabrication is raised (if necessary) so that the layer of UV curable material disposed on the workpiece is within the oxygen-free region <NUM>, and the UV source <NUM> of the UV curing system <NUM> is then activated (6a-<NUM>), as shown in the UV source curve in <FIG>. This causes curing of at least a portion of the layer of UV curable material disposed on the workpiece in a region exposed to the UV light. At the conclusion of the curing (6a-<NUM>), the workpiece is repositioned for deposition of the next layer of UV curable material and the gas pressure reduced. Preferably, the gas pressure is maintained at a sufficient level to keep the diffusion system <NUM> filled in order to reduce the time necessary for the next cycle of curing. When the desired number of layers have been cured, the process ends.

Claim 1:
A method for preventing oxygen inhibition of a light-initiated polymerization reaction at ambient conditions, the method comprising:
depositing a layer of UV curable material (<NUM>) on a print surface of an object under fabrication (<NUM>);
the method being characterised in that
towards the end of depositing the layer of UV curable material (<NUM>) on the print surface, initiating a purging of oxygen from a region (<NUM>) adjacent to a diffuser (<NUM>), wherein purging oxygen from the region (<NUM>) adjacent to the diffuser (<NUM>) comprises pumping an inert gas through the diffuser (<NUM>);
upon finishing the deposition of the layer of UV curable material (<NUM>) on the print surface, raising the object under fabrication (<NUM>) so that the layer of UV curable material (<NUM>) is within the region (<NUM>) adjacent to the diffuser (<NUM>) while continuing to purge oxygen from the region (<NUM>) adjacent to the diffuser (<NUM>); and
after raising the object under fabrication (<NUM>), curing the layer of UV curable material (<NUM>) with UV light (<NUM>) while continuing to purge oxygen from the region adjacent to the diffuser (<NUM>), wherein the curing comprises passing the UV light (<NUM>) through the diffuser (<NUM>) and onto the layer of UV curable material (<NUM>).