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STEREOLITHOGRAPHY USING COMPOSITION
PROVIDING REDUCED DISTORTION
DESCRIPTION
Technical Field This inven~ion relates to the production of stereolithographically formed elements which are complexly-shaped thin-walled polymeric objects and which are incompletely cured as a result of having been produced by an exposure to ultraviolet light which is inadequate to completely cure the polymer constituting the objects.
Ba karound Art It is known, as illustrated in U.S. Pat. No.
4,575,330 to C. W. Hull, to form three-dimensional objects of complex shape using ultraviolet light to solidify superposed layers o~ liquid ultraviolet~ `o curable ethylenically unsaturated ma~erial at the surface of a liquid reservoir of such material. Thin walled objects are formed in this manner, apparently thick walls being hollow and dimensionally stabilized by thin internal webs. The ultraviolet dosa~e is limited to speed the action and to insure that only the thin lines struck by the laser beam on the surface of the unsaturated liquid will be solidified.
As will be evident, the thin walled object is incompletely cured and has inadequate strength and durability. Of particular significance is the fact that these optical fabrication processes are intended to form three dimensional models which conform as accurately as possible with what was intended, this usually being set forth in drawings which are cross-sectioned by computer to guide a laser beam in the production of the superposed layers. However, the incompletely cured products tend to be somewhat distorted, so the accuracy of the mod~l is less than desired.

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As a result, it is necessary to provide relatively low viscosity flowable liquid compositions which cure rapidly and which exhibit decreased distortion when only partially cured to possess some limited "green strength".
It is desired to point out that the ultraviolet lasers used have limited capacity, so a larger exposure dosage would slow the optical fabrication process. Moreover, a greater exposure would increase the thickness and depth of the exposed line which solidifies, thus reducing the dimensional accuracy of the optical fabrication process.
More particularly, the objects under consideration are formed by the ultraviolet polymerization of liquid ultraviolet-curable ethylenically unsaturated material at the surface of a liquid reservoir of such material using an ultraviolet dosage in the range of 0.1 to 10, preferably 0.2 to 5, Joules per square centimeter which is sufficient to convert the unsaturated liquid into a lightly cross-linked solvent- swellable three dimensional complexly-shaped polymeric thin-walled element constituted by heat-softenable solid polymer. A support is positioned beneath the surface of the reservoir to hold the polymerized layer which is formed. The liquid level is then raised (or the support lowered, and another layer is formed until the photoformed object is completed within the liquid reservoir.
The photoformed objects are thus only partially cured and are somewhat gelatinous and mechanically weak due to the low degree of cross-linking and the presence of unconverted monomers and oligomers (which are still unsaturated) within the partially polymerized polymeric structure of the solid object which is formed. The problem of this application is to ~8~79 have a low viscosity liquid composition which cures rapidly to speed the optical fabrication process and which~ at the same time, exhibits minimal distortion in its partially cured condition so that the gelatinous and weak solid products produced by ultraviolet exposure will accurately reflect the shape of the object which it is desired to reproduce.
It is desired to point out that the distortion which is encountered is a composite of the extent of shrinkage encountered during the cure, the strength of the partially cured polymer structure, and whatever mechanical stresses are imposed on the product before the object removed from the reservoir is further cured to strengthen the same.
After the partially cured object has been withdrawn from the reservoir, one can proceed in various ways to more completely cure (thermoset) these gelatinous and weak objects after they have been removed from the liquid ultraviolet-curable ethylenically unsaturated material in which they were formed, and one must be careful that the additional cure does not introduce additional distortion. Nonetheless, if the incompletely cured object is distorted, that distortion is retained when the cure is completed, and it is the minimization of the distortion in the incompletely cured object which is withdrawn from the reservoir which is the objective of this invention.
Disclosure of Invention In accordance with this invention, the ultraviolet-curable liquid composition which is used in the process of the previously noted Hull patent is formulated to include from 20% to 80%, preferably from 35% to 70%, of a resinous polyacrylate or polymethacrylate. This resinous polyacrylate or polymethacrylate is dissolved in a combination of 10% to 2 ~ 7 9 40%, preferably from 15~ to 30%, of a liquid polyacrylate or pol~methacrylate, and 10% to 40%, preferably from 15% to 30%, of a li~uid N-vinyl monomer.
It is important that one of the foregoing polyacrylates or polymethacrylates be a polyacrylate and that the other be a polymethacrylate. If both were polymethacrylates, the composition would cure too slowly. If both were polyacrylates, then severe distortion is seen. It is only when the resinous material is selected in one way while the liquid material is selected in the other way that it is found that good cure speed is accompanied by good distortion resistance.
More particularly, the above-described unsaturat~d liquid compositions are employed as a liquid reservoir which forms a thin liquid layer above a supporting platform, and the surface of this liquid reservoir is exposed to an ultraviolet dosage in the range of about 0.1 to about 10, preferably about 0.2 to about 5, Joules per square centimeter to solidif~ and partially cure the liquid at the surface. This process is repeated many times to superimpose one layer upon another and ultimately produce a three- dimensional object of partially cured polymer within the liquid reservoir.
~- The term "solidify", as used in its various grammatical forms, defines an at least partially cured, yet self supporting, state.
The energy that is utilized preferably is light in or near ultraviolet range, i.e~, light having a wavelength of about 200 to about 550, preferably about 250 to about 450, nanometers (nm). The term "near", as used in the phrase "near the ultraviolet range", refers to light at the lower end of the visible light spectrum.

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The specified ultraviolet exposure is sufficient to convert the unsaturated liquid into a lightly cross linked solvent-swellable three dimensional complexly-shaped polymeric thin-walled element constituted by heat-softenable solid polymer.
As a result, the photoformed objects are somewhat gelatinous and mechanically weak due to the low degree of cross-linking and the presence of unconverted monomers and oligomers ~which are still unsaturated) within the par~ially polymerized polymeric structure of the solid object which is formed. In this invention, the li~id compositions specified are found to cure rapidly to a dimensionally stable solid condition which exhibits reduced distortion.
The resinous polyacrylate or polymethacrylate provides a polymeric matrix which allows the ultimately fully cured solid object to have the structural strength which is desired and the two other components provide the liquidity needed to have the easily flowable liquid which is required by the process. The N-vinyl monomer is rapid curing to enable the rapid achievement of "green strength", and the specified liquid polyacrylates or polymethacrylates, which are preferably illustrated by trimethylol propane trimethacrylate and pentaerythritol trimethacrylate, or the corresponding triacrylates, serve to reduce the distortion encountered in going from the drawings which activate the computer-directed laser beam and the three dimensional model which is formed in the liquid reservoir.
The resinous polyacrylate or polymethacrylate which provides a polymeric matrix enabling the ultimakely fully cured solid object to have its structural streng~h is subject to considerable variation so long as it is of resinQus nature and contains an average of at least 2.0 acrylate or methacrylate groups 2~8l~7~

per molecule. These can be illustrated by epoxy diacrylates, such as Epon lO01 diacrylate or Epon 828 diacrylate, or one can use polyester diacrylates. Th~
corresponding methacrylates are also useful, but less preferred.
It is presently preferred to use polyurethane diacrylates, and especially those which ~mploy a polyester base. More particularly, one can take an hydroxy-functional polyester, preferably one having an average of from 2.1-5.0 hydroxy groups per molecule, and react it with monoacrylate monoisocyanate to form an acrylate-capped polyurethane polyacrylate. One such product which is available in commerce is identified as Uvithane 893, and this commercial polyacrylate polyurethane polyester will be used in the Example of this application.
The preferred N vinyl liquid monomer is N-vinyl pyrrolidone, ~-vinyl caprolactam is also useful.
As previously indicated, it is preferred to combine a resinous polyacrylate with a liquid polymethacrylate. While liquid trimethacrylates are preferred, dimethacrylates are also useful, such as 1,6-hexane diol dimethacryla e. Liquid polymethacrylates of higher functionality are also useful, such as pentaerythritol tetraacrylate. When the resinous material is a polymethacrylate, the liquid material must be a polyacrylate, and 1,6-hexane diacrylate and polyacrylates of high functionality, like pentaerythritol tetramethacrylate can be used in such instance.
From about 1% to about 10% of a photoinitiator effective on ultraviolet exposure to initiate the polymerization o~ ~crylate unsaturation is included in the reservoir, albeit the liquid compositions o~ this invention can be supplied without photoinitiator which ~O:~g~9 is added by the user prior to use. These photoinitiators are themselves well known and in common use. They are usually ketonic, and frequently aromatic, such as benzophenoneO It is presently preferred to use 5 diethoxy acetophenone which is a particularly effective photoinitiator. It will be appreciated that photoinitiators for the ultraviolet polymerization of (meth)acrylate functional compounds are well known and in common use.
As will be understood, after the 3-dimensional model (object) has been formed within the liquid reservoir, it is withdrawn and excess unsaturated liquid is allowed to drain therefrom, usually back into the reservoir from which it was withdrawn where it can be reused. If desired, one can wash the incompletely cured solid model with an alcoholic liquid before proceeding to complete the cure, but this is usually dispensed with and is not necessary.
The draining operation is a simple one which may be carri d out at room or slightly elevated temperature to reduce the viscosity of the unconverted liquid adhering to the photoformed object. It is convenient to allow the object to drain for periods of from 5 to 10 minutes, albeit ~his is not critical.
Completion of the cure can be carried out in various ways. Most simply, the drained ~and possibly washed) solid object is placed in an ultraviolet chamber and exposed uniformly to ultraviolet radiation to complete the cure. Other techniques are also possible for completing the cure, such as employing other more penetrating radia~ion or heat, and any of these may be used. It will be understood, however, that the speclfic technique employed to increase the strength of the incompletely polymerized object withdrawn from the 2 ~ 7 ~

polymerizable liguid reservoir is not itself the essence of this invention.
On the other hand, the compositions of this invention cure well when exposed to a temperature of about 250 F, and this is unusual. A thermal cure at a temperature up to about 325 F is thus surprisingly effectiYe and is a feature of this invention.
Preferably the baking temperature will not exceed about 300 F to insure avoidance of distortion during the cure.
The thermal cure is advantageous since heat permeates all portions of the three dimensional object whereas exposure of that object to ultraviolet light might not reach all portions of the object.
As previously indicated, the removal of polymerizable liquid clinging to the photoformed object can be aided by rinsing with an alcoholic solvent having the capacity to dissolve the polymerizable liquid, but which does not dissolve the partially polymerized solid formed from that liquid, albeit present practice does not usually employ such a wash.
A typical alcoholic solvent is represented by an alcohol, such as isopropanol. If desired, one may add a minor proportion of an ester solvent, such as butyl acetate. Other useful alcohols are illustrated by ethyl alcohol and butanol. When water-miscible solvents are used, wa er may be present in the solvent mixture.
The time of immersion in the alcoholic wash composition is of secondary significance, albeit it is convenient to immerse the object in the solvent at room temperature for from 5 to 30 seconds to dissolve the adhering polymerizable liquid. The partially cured object is then removed nd adhering solvent is allowed to drain therefrom.

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EXAMPLE 1: Liquid Unsa~urated Material An illustration of a (meth)acrylate-functional photocurable liquid which is useful to provide the reservoir of liquid ethylenically unsaturated material the surface of which is exposed to ultraviolet radiation in accordance with this invention was provided by mixing 60 grams of a polyacrylate-functional polyurethane polyester resin (a hydroxy-terminated polyester of ethylene glycol and adipic acid (number average molecular weight of lS00 daltons) conventionally reacted with isophorone diisocyanate and then conventionally capped with 2-hydro~yethyl acrylate [Uvithane 893 commercially available from Morton Thiokol can be used as the polyester resin]), 40 grams of trimethylol propane trimethacrylate, and 4 grams of a benzyl ketal-based photoinitiator (2-hydroxypropyl phenone [Darocur 1173 from EM Chemicals can be used]). Another photoinitiator which is fully useful in this Example in the same proportion is 1-hydroxycyclohexyl phenyl ketone available from Ciba-Geigy Corporation, Ardsley, NY, under the trade designation Irgacure 184.
The liquid reservoir of the above photopolymeriz~ble liquid was exposed to actinic energy : in the form of ultraviolet light from a Liconix model 4240 N, helium-cadmium light having an output of 15 milliwatts at 325 nm focused to 350 micron diameter.
The usually used dosage is about 0.5 Joules per square centimeter of surface which results in test specimens of about 20 mils thickness, i.e, the depth of solidi.fication is about 20 mils.
After draining for 10 minutes, the drained piece can be washed briefly in an alcoholic solvent mixture of isopropanol and bu~yl acetate in a weight ratio of 64/33 to facilitate complete removal of ~8~

polymerizable liquid, but in this Example washing was not employed.
The drained parts were then post-cured by exposure to ultraviolet light having a wavelength of about 200 to about 400 nm in a sealed chamber to provide a post-cure, and the exposure was continued until the parts are as fully cured as desired. ~s will be apparent, the drained parts must be rigid enough to avoid distortion when handled and subjected to additional cure. After such additional cure, the cured parts are expected to sustain some significant load without distortion.
Repeating the foregoing post-curing step, but using an oven at 330F (about 165 C) to provide the post-cure, a good cure is obtained in about 10 minutes.

EXAMPLE 2: Liquid Unsaturated Material The above Example l is also repeated utilizing a liguid unsaturated material produced by mixing 60 grams of a pol~methacrylate-functional resin (the dimethacrylate of Shell Chemical Company product Epon 1001 which is a diglycidyl ether of bisphenol A having a number average molecular weight of a~out 1000 daltons), 40 grams of trimethylol propane triacrylate, and 4 grams of a benzyl ketal-based photoinitiator (2-hydroxypropyl phenone [Darocur 1173 from EM Chemicals can be used]).
Corresponding results are obtained.

EXAMPLE 3: Liguid Unsaturated Material ~ further illustration of a (meth)acrylate-functional photocurable liguid which is useful to provide the reservoir o~ liquid ethylenically unsaturated material is provided by mixing 50 grams of a resinous diacrylate of a bisphenol A-based diepoxide (a diglycidyl ether of bisphenol A having a number average 201~7~

molecular weight of about 390 daltons reacted with 2 molar proportions of acrylic acid [the Celanese Corporation, Louisville, Kentucky, product Celrad 3700 can be used as the resinous diacrylate]), 15 grams of a dimethacrylate of a bisphenol A-based diepoxide, the Celanese product RDX 26936 which is the dimethacrylate of the same diglycidyl ether used in Celrad 3700 can be used as the dimethacrylate, 25 grams of liquid tetraethylene glycol dimethacrylate (the Sartomer Company, Westches~er, PA, product SR 209 can be used), 10 grams of liquid hexane diol dimethacrylate (the Sartomer product SR 239), ~ grams of a benzyl ketal-based photoinitiator (2-hydroxypropyl phenone [Darocur 1173 from EM Chemicals can be used as the photoinitiator])~ 2 grams of ethyl diethanol amine (from Aldrich Chemical Company, Incorporated, Milwaukee, WI), and 0.1 grams of the conventional stabilizer methoxy phenol (from Eastman Kodak Company, Rochester, NY).
This composition is fully useful in this invention and its use in the method provides reduced distortion in comparison with the use of a similar all acrylate-functional composition. An all methacrylate-~unctional composition cures too slowly to ~e practical.
EX~MPLE 4: Liquid Unsaturated Material Another illustration of a (meth)acrylate-functional photocurable liquid which is usaful to provide the reservoir of liquid ethylenically unsaturated material useful in this invention is provided by mixing 50 grams of a resinous diacrylate of a bisphenol A-based diepoxide (the Celanese product Celrad 3700 can be used as the resinous diacrylate), 21 grams of liquid tetraethylene glycol dimethacrylate (Sartomer 209), 17 qrams of methoxy hexane diol 20~8~7~

monoacrylate, 8 grams of liquid hexane diol dimethacrylate (Sartomer SR 239), 4 grams of an oligomer amine monoacrylate (Celrad 7100 from Celanese), 4 grams of a benzyl ketal-based photoinitiator (2-hydroxypropyl phenone [Darocur 1173 from EM Ch~micals can be used]), and 0.1 grams of methoxy phenol (from Eastman Kodak Company). This composition is fully useful in this invention and exhibits reduced distortion in comparison with the use of a similar all acrylate-functional composition~

EXAMPLE 5: Liquid Unsaturated Material ~nother illustration of a (meth)acrylate-functional photocurable liquid which is useful to provide the reservoir of liquid ethylenically unsaturated material useful in this invention is provided by mixing 3~ grams of a resinous diacrylate of a diglycidyl ether of bisphenol A having a number average molecular weight of about 390 daltons (the Cargill Incorporated, Carpentersville, IL, product PN
1570 can be used), 33 grams of a liquid dimethacrylate of a bisphenol A-based ethoxylate (Sartomer SR 348 can be used), 33 grams of the r~action product of isocyanatoethyl methacrylate from Dow Chemical Company, Midland MI, and 2-ethyl hexanol from Aldrich, 4 grams of a benzyl ketal-based photoinitiator (2-hydroxypropyl phenone [Darocur 1173 from EM Chemicals can be used]), 2 grams of ethyl diethanol amine (from Aldrich), and 0.1 grams of methoxy phenol (from Eastman Xodak Company).
This composition is fully useful in this invention and exhibits reduced distortion in comparison with the use of a similar all acrylate-functional composition.

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EXAMPLE 6: Liquid Unsaturated Material Still another illustration of a (meth)acrylate-functional photocurable liquid which is useful to provide the reservoir of liguid ethylenically unsaturated material is provided by mixing 34 grams of a resinous diacrylate of a diglycidyl ether of bisphenol A
having a number average molecular weight of about 390 daltons (the Cargill Incorporated, Carpentersville, IL, product PN 1570 can be used), 33 grams of a liquid dimethacrylate of a bisphenol A-based ethoxylate (Sartomer ~R 348 can be used), 33 grams of liquid tetraethylene glycol dimethacrylate (Sartomer SR 209), 4 grams of a benzyl ketal-based photoinitiator (2-hydroxypropyl phenone [Darocur 1173 from EM Chemicals can be used]), 2 grams of ethyl diethanol amine (from Aldrich), and 0.1 grams of methoxy phenol (from Eastman Kodak Company). This composition is fully useful in this invention and exhibits reduced distortion in . comparison with the use of a similar all acrylate-functional composition.

EXAMPLE 7: Liquid Unsaturated Composition A further illustration of a (meth)acrylate-functional photocurable liquid which is useful to provide the reservoir of liquid ethylenically unsaturated material is provided by mixing 30 grams of a diacrylate functional urethane polyester resin (Uvithane 783 from Morton Thiokol can be used), 30 grams of liquid trimethylol propane ethoxylate triacrylate (SR 454 from Sartomer), 40 grams o~ liquid tetraethylene glycol dimethacrylate, (Photomer 2050 from Henkel Corporation, Moxristown, NJ can be used), 4 grams of a benzyl ketal-based photoinitiator (2-hydroxypropyl phenone [Darocur 1173 from EM Chemicals can be us~d]), and 0.1 grams of methoxy phenol (from Eastman Kodak Company).

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This composition is fully useful in this invention and exhibits reduced distortion in comparison with the use o~ a similar all acrylate-functional composition.

EXAMPLE 8: Determination of the Minimum Solidification Dose (MSD) Five compositions were prepared by admixing the components set for~h in TABLE A for each composition in separate, suitable vessels.
TABLE A
Amount tarams) Component Composition: 1 2 3 4 5 Novacure 37001 60 60 -- 30 --HDDA3 20 -- __ __ 20 HDDMA4 __ 20 20 20 --`
TMPTAs 20 20 -- 20 20 Irgacure 1847 5 5 5 5 5 _ _ .

1 Novacure 3700 is the diacrylate ester of a bisphenol A based epoxy resin ~Molecular Wt. of about 500 daltons), supplied by Interez Corporation, Jef~ersontown, RY.
2 RDX 26936 is the dimethacrylate ester of a bisphenol A based epoxy resin (Molecular Wt. of about 530 daltons), supplied by Celanese.

3 HDDA is hexane diol diacrylate, obtained from Sartomer Company, West Chester, PA.

4 HDDMA is hexane diol dimethacrylate, obtained from Sartomer Company, West Chester, PA.

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5 TMPTA is the triacrylate ester of trimethyol propane, obtained from Sartom~r Company, West Chester, PA.
~ TMPTMA is the trimethacrylate ester of trimethyol propane, obtained from Sartomer Company, West Chester, PA.
7 Irgacure 184 is a Xetonic photoinitiator having hydroxycyclohexyl phenyl ketone as the active ingredient, obtained from Ciba-Geigy Corporation, Ardsley, NY.
The compositions were formulated so that, in addition to the photoinitiator, Composition 1 contained all polyacrylates, Compositions 2, 4 and 5 of the present invention each contained an admixture of polyacrylates and polymethacrylates, and Composition 3 contained all polymethacrylates.
The photoreactivity of a liquid composition can be measured by determinin~ the minimum solidification dose (MSD). This numerical parameter was determined by solidification of a single, free-floating layer of cured composition on the surface of a reservoir of the liquid composition. This layer was produced by means of rastering, i.e., scanning, a laser bea~ having an output of 16 milliwatts and a wavelength of about 325 nm in a preset pattern at a given speed to cause solidification of the liquid composition to a measurable depth. The rastering speed was varied for successively produced layers to apply varying incident radiation dosages, i.e., 0.12, 0.24, 0.48, 0.96, and 1.92 Joules/square centimeter (J/sqcm), to the liquid composition to vary the depth of the solidifi d layer.
The depth of solidification (cure) was determined for each solidified layer and is presented in TABLE B.

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TABLE B
De~th o~ cure_(mils!
osaae (J~sqcm) Compositions: 1_ 2 3 4 5 0.12 ~0.0 21.0 19.~ 20.0 21.0 0.24 23.0 24.0 23.0 24.0 24.0 0.48 28.0 29.0 27.0 27.0 28.0 O.g6 31.0 31.0 31.0 30.0 31.0 1.92 34.0 33.0 34.0 34.0 34.0 A conventional computer program was utilized to perform a least square regression analysis on the depth of solidification (cure) data set forth in TABLE B, a~ove, to determine the best fit of a straight line for a graph of the depth of solidification verses log of the dosage.
The straight line was extrapolated by the computer program to determine the point where the line intercepted the axis of the log of the dosage (i.e. the -depth of solidification was zero). The extrapolated value of dosage at zero solidification depth is a measure of the minimum dosage required to initiate solidification upon the surface of the liquid composition. This extrapolated dosage at zero solidification depth is termed the MSD, and is a characteristic parameter of a liquid composition. A
lower value of the MSD indicates a liquid composition that is more photoreactive than a liquid composition having a higher MSD value and thus less energy is xequired to initiate cure of this liquid composition.
The MSDs for the compositions set forth in TABLE A are provided in TABLE C.

TABLE C
Composition: l 2 3 4 5 MSD ~/sqcm): 2.55 l.00 3.62 1.95 1.45 - 2~8~

This measure of photoreactivity clearly indicates that the liquid unsaturated compositions composed of an admixture of polyacrylates and polymethacrylates (~ompositions 2, 4 and 5) require less energy to initiate cure than ~he other compositions containing either all polyacrylates (Composition 1~ or all polymethacrylates (Composition 3). Furthermore, the present liquid unsaturated composition can include an admixture of a liquid polyacrylate and liquid polymethacrylate (Compositions 2 and 4), an admixture of a resinous polyacrylate and a resinous polymethacrylate (Composition 4), or an admixture of a polyacrylate and a polymethacrylate can be present only after the liquid polyacrylate or polymethacrylate and resinous polyacrylate or polymethacrylate are combined (Composition 5). Superior results are obtained regardless of how the admixture of polyacrylate and polymethacrylate is achieved.

EXAMPLE 9: Determination of the Distortion Factor The distortion factor of the five compositions of EXAMPLE 8 were determined by employing laser beam scan parameters, e.g., laser output and raster speed, that would result in a layer 0.25 x 1.00 inches having a solidification depth of 20 mils. Cantilevered rectangular parallelepipeds having the dimensions of 0.25 inches x 1.00 inches x 260 mils deep were produced from the compositions. Each rectangular parallelepiped was made of 1 layer 20 mils deep and 24 layers each 10 mils deep. Thus, the laser beam solidified a bottom layer that was 20 mils deep. The bottom layer was coated with 10 mils of liquid composition that was solidified by exposure to the laser beam. However, the energy from the laser beam penetrated 10 mils into the bottom layer because of the selection of the scan parameters. This penetration improved adherence between the lay~rs. The steps of applying a 10 mil coating of liquid composition and exposing the lO mil coating to the laser beam, with penetration 10 mils into the previous layer, were repeated to produce the rectangular parallelepiped. When the cantilevered rectangular parallelepiped was completed, it was withdrawn from the reservoir of the liquid composition and then fully cured by subsequent exposure to ultraviolet light from a conventional medium pressure mercury lamp. The length of the top (formed by the last layer that was solidified) and the bottom (formed by the first layer that was solidified) of each cantilevered xectangular parallelepiped were then measured and the length of the top was divided by the length of the bottom to determine the distortion factor. A perfect rectangular parallelepiped will provide a distortion factor of 1.0, but in practice this ratio is greater than 1.0 because of the distortions caused by the stereolithographic process which produces a shrinkage of the bottom whicn causes the first layer to be smaller than the top.
Thus, this ratio of top length to bottom length provides the distortion factor. The closer the ratio is to 1 the less the distort$on. Less distortion is desirable because this indicates closer dimensional tolerances can be achieved. The distortion factors for the compositions set forth in TABLE A of EXAMPL~ 8 are provided in TABLE D.

Composition: _l 2 34 _5 Distortion Factor 2.2 1.4 NM1.4 1.5 _ _ _ _ _ NM: not measurable due to poor interlayer cohesion 20~8!17 These distortion factors indicate the compositions of the present invention (Compositions 2, 4 a~d 5) exhibit at least 46% less distortion than Composition l which only contains the polyacrylate component. The rectangular parallelepiped prepared from the all methacrylate composition (Composition 3) did not possess sufficient cohesive strength due to poor reactivity. This poor cohesive strength was exhibited as a separation of the discrete layers during photoformation. Therefore, a distortion factor for Composition 3 could not be measured.

EXAMPLE 10: Liquid Unsaturated Mate ial An illustration of a (meth)acrylate-functional photocurable liquid which is useful to provide the bath of liquid ultraviolet-curable ethylenically unsaturated material the surface of which is exposed to ultraviolet radiation in accordance with this invention is provided by mixing 60 grams of a polyacrylate-functional polyurethane polyester resin (Uvithane 893 may be used) 20 grams of trimethylol propane trimethacrylate, 20 grams of N-vinyl pyrrolidone and 4 grams of a benzyl ketal-based photoinitiator (Darocur 1173 available from EM Chemicals). Another photoinitiator which is fully useful in this example in the same proportion is Irgacure 184 available from Ciba Geigy Corporation.
The liquid bath of this photopolymerizable liquid was exposed to ultraviolet light using a Liconix model 4240 N, helium-cadmium light having an output of 15 milliwatts at 325 nanometers focused to 350 micron diameter. The usual dosage is about 3.0 Joules per square centimeter of surface which results in test specimens of about 20 mil thickness.
After draining for 10 minutes, the drained piece may be washed briefly in an alcoholic solvent mixture of 7 ~

isopropanol and butyl acetate in a weight ratio of 64/33 to ~acilita~e complete removal of polymerizable liquid, but in this example washing was not employed.
The drained parts were then exposed to ultraviolet light in a sealed chamber, and the exposure was continued until the parts are as fully cured as desired.
Repeating the foregoing, but using an oven at 300 F to provide the post-cure, a good cure is obtained in about 10 minutes.

EXAMPLE 11: Liquid Unsaturated Material The above example is also repeated by mixing 60 grams of a polymethacrylate-functional resin (the dimethacrylate of Shell Chemical Company product Epon 1001 which is a diglycidyl ether of bisphenol A having a number average molecular weight of about 1000) 20 grams of trimethylol propane triacrylate, 20 grams of N- vinyl pyrrolidone and 4 grams of a benzyl ketal-based photoinitiator (Darocur 1173 available from EM
Chemicals). Corresponding results are obtained.