Dry Stereolithography with Thermoplastic Photopolymers

Abstract

Dry stereolithography using solid thermoplastic photopolymer plates/sheets/films provides a new technique to make 3D printed objects. In this new additive manufacturing process, objects are built layer-wise using thermoplastic photopolymers and actinic radiation. The thermoplastic photopolymer compositions consist of a thermoplastic photopolymer layer sandwiched between a transparent flexible base without an anchoring layer and a release film. Uncrosslinked portions of the 3D printed object are removed by heat. Preferred method of radiation exposure is digital light processing (DLP).

Claims

1. A method of converting photosensitive layers of dry or solid thermoplastic photopolymer plate/sheet/film of same or gradually varying shape into a three dimensional object, the said method comprising steps of : removing release layer of dry photopolymer plate/sheet/film leaving behind photopolymer layer on a transparent support film; hot-laminating bubble-free photopolymer layer to a build pad; selectively exposing the photopolymer layer through the transparent support layer with prescribed radiation; removing the support layer; laminating a fresh dry photopolymer layer with the support layer in registration to the previous radiation exposed layer; selectively exposing the second photopolymer layer through the support layer to same or gradually varying shape with prescribed radiation; repeating the process until the desired three dimensional object is formed in layers of stack consisting of exposed and crosslinked photopolymer in three dimensions supported by unexposed and un-crosslinked photopolymer in three dimensions and removing the un-crosslinked portion of the stack with heat leaving behind the desired three dimensional object.

2. The method of claim 1, wherein successive layers are exposed first by the radiation source and then laminated to the previously exposed layers on the build pad in registration.

3. The method of claim 1, wherein the dry photopolymer plate/sheet/film is made with a transparent supporting layer without an anchoring or adhesive layer.

4. The method of claim 1, wherein the solid plate is an unprocessed or unexposed flexographic or letterpress thermoplastic photopolymer printing plate.

5. The method of claim 1, wherein the said solid thermoplastic photopolymer film is a dry film photoresist made of thermoplastic photopolymer layer.

6. The method of claim 1, wherein the preferred method of radiation exposure is digital light processing (DLP).

Description

DETAILED DESCRIPTION

[0016] Photopolymers are imaging compositions based on polymers/oligomers/monomers which can be selectively polymerized and/or crosslinked upon image-wise exposure by light radiation such as ultra-violet light. For final use, they are made into different forms including film/sheet, liquid, solution etc. which find outlets in printing plates, photoresists, stereolithography/3D printing etc. In stereolithography/3D printing, hitherto, photopolymers in liquid form, also called resins, have been used. This invention relates to use of photopolymers in solid or dry form,

[0017] U.S. Pat. No. 5,192,559, mentions use of dry film photoresists to build a 3D object. However, development is done with solvents or aqueous liquids and not heat. Whereas, this invention uses dry photopolymers which are thermoplastic and they melt upon heating.

[0018] This invention uses dry photopolymers in the form of thermoplastic sheets/plates/films, as a sandwich of a photosensitive polymer layer between a release film and a supporting transparent film. The photopolymer composition is of thermoplastic type, meaning, unexposed photopolymer melts upon heating. The release film may be typically polyethylene and the supporting film may be typically a polyester film. Examples of such photopolymers include dry film photoresists and solid printing plates made of thermoplastic compositions on a transparent polyester support without an anchoring layer.

[0019] In conventional stereolithography (SL), the first stage involves designing and inputting a precise mathematical and geometric description of the desired structure's shape into one of many computer-aided design (CAD) programs and saving the description in the standard transform language (STL) file format. In the second stage, the STL file is imported into SL machine-specific software which slices the design into layers and determines the placement of support to hold each cross-section in place while building the structure layer by layer. Finally, the machine language controls the SL machine to build a desired part and its support structures layer by layer. SL machines typically focus an ultraviolet (UV) laser onto a cross-section of a liquid photopolymer resin. The laser, in turn, selectively cures a resin to form a structure, layer by layer. Ultimately, the part is cleaned, the support structure is removed and the part is post-cured (typically exposed to UV) prior to completion.

[0020] The present invention is about an apparatus and method for forming an integral three-dimensional object from laminations of the same or gradually varying shape made of selectively radiation exposed dry photopolymer layers. The apparatus includes a station for storing and supplying a dry photopolymer plate/sheet/film with means for forming the material into a plurality of individually radiation exposed laminations in shapes required for assembly in a preselected sequence into the three-dimensional object. It also includes means for removal of release film and support layer and means for integrally bonding each layer.

[0021] Individual layers may be exposed via a laser spot scanning or through a photomask using area exposure technique but preferably digital light processing (DLP) technology to obviate the need of a photomask, wherein, DLP projector connected with a computer directly exposes each layer of photopolymer selectively.

[0022] A dry photopolymer form for this invention consists of three layers: 1) a release layer of polyolefin film 2) a photopolymer layer and 3) a support layer of a transparent polyester or similar film. Thickness of the photopolymer layer can range from 15 microns and above depending upon final finish desired in the 3D printed object. Hardness of cured dry photopolymer can range from Shore A hardness of 25 for a rubbery polymer to Shore D hardness of 95 of a hard plastic. In a typical process, the dry photopolymer plate/sheet/film is cut into a size little larger than the largest cross-section of the object to be printed. The release layer is removed and the photopolymer side layer is laminated to a platform or a build pad with a heated roller or a platen. It is then selectively exposed through the transparent polyester layer with UV radiation for, say, 20 to 30 seconds. During exposure, the polyester film prevents oxygen in the air to inhibit photopolymerization. After exposure, the polyester layer is removed and a new photopolymer layer with support polyester layer is laminated to the exposed layer. The process is repeated. The radiation exposed areas of photopolymer get partially crosslinked, whereas, the unexposed areas remain thermoplastic. The unexposed areas act as a support to the formed object. Alternately, successive layers are exposed first by the radiation source separately and then laminated to the previously exposed layers on the build pad in registration.

[0023] Upon completion of the process, entire assembly is removed from the platform and placed in a melting unit to melt-away, at a temperature of more than 150 degree C., unexposed and un-crosslinked portion leaving behind the intended three dimensional object. Aqueous or solvent washout may be necessary to remove any un-crosslinked residue left on the object after the melt out. The final object may be cured under UV light or thermal conditions to achieve full strength.

[0024] The photopolymers are typically negative acting, wherein, radiation-exposed areas become non-thermoplastic as a result of crosslinking or photopolymerization.

[0025] A photopolymer supply station with a feeding mechanism with means for removing the polyolefin release film, a registration mechanism, pre- or post hot lamination mechanism, a computer-assisted exposure system and a mechanism to remove the polyester support film may be built into the same apparatus. Exposure has to be done under safe light conditions as photopolymer layer is sensitive to visible and UV lights. A separate station to melt away unexposed polymer from the 3D object and clean it may be required.

[0026] Dry photopolymer plates/sheets/films are environmentally friendly and hence a 3D printer using dry photopolymer material would be specifically suitable for schools, homes and small offices. A 3D printer for dry stereolithography should be useful in markets such as footwear, jewelry, dentistry, hearing aid and similar objects.

COMPARATIVE EXAMPLE

[0027] Examples of commercial thermoplastic photopolymer printing plates include DuPont's Cyrel Fast FDT and Flint's nyloflex XFH. After exposure, unexposed photopolymer in these plates is melted away and absorbed in a special absorbent nonwoven web in a separate unit. Thermoplastic photopolymer compositions used in these thermal plates are well known in the art. They are generally based on styrene-isoprene-styrene and styrene-butadiene-styrene thermoplastic elastomers compounded with acrylic crosslinkers and photosensitizers. Other thermoplastic polymer bases are possible. Thermoplasticity is measured in terms of Melt Flow Index (MFI). These thermoplastic photopolymers should have a MFI of at least 4 grams/10 min. under a 2.16 kilogram weight at 140.degree. C., most preferably, greater than 10 grams/10 minutes at 140.degree. C.

[0028] However, the commercial photopolymer printing plates have a photopolymer layer integrally bonded to a flexible polyester film and hence may not be suitable for use in this invention. This bonded film is not easy to remove from the attached photopolymer layer.

[0029] In absence of availability of a commercial thermoplastic photopolymer plate without an anchoring layer on a polyester substrate, a commercial dry film photoresist, a photomask and UV light were used to build an object by dry stereolithography. In dry film photoresists, the polyester substrate is generally not anchored to the photopolymer layer. The photoresist used did not have truly thermoplastic photopolymer layer but a person skilled in the art has in-principle knowledge of formulating thermoplastic compositions suitable for thermal development such as those used in thermally developed printing plates as disclosed previously.