Patent Description:
<NUM>-D printing, also known in the art as additive manufacturing, refers to a class of processes for the production of three-dimensional objects wherein multiple layers of a material known as a "build material" are applied to a bed or substrate on a layer-by-layer basis. Such processes are particularly useful in the manufacture of prototypes, models and molds; however, more recently these processes have been increasingly utilized for production parts, consumer products, medical devices and the like.

One recognized and widely practiced additive manufacturing process is known in the art as laser sintering. In general, laser sintering involves applying a layer of powdered or pulverulent polymer material to a target or build surface; heating a portion of the material; irradiating selected or desired part locations/shape with laser energy to sinter those portions and produce a part "slice"; and repeating these steps multiple times (the repetition often referred to as the "build") to create useful parts in the form of sequentially formed, multiple fused layers. Laser sintering is described for example in <CIT>; <CIT> and <CIT>. Additive manufacturing processes that utilize other irradiation energy sources such as infrared radiation are also known in the art. Fusion could be complete or partial fusion.

The prior art describes in detail the intimate interrelationship and tenuous balance between various temperatures used in laser sintering processes and the temperature-related and/or temperature-dependent characteristics and parameters of the polymer powder such as glass transition temperature, melt temperature, crystallinity and rate of crystallization (and of recrystallization after melting). In describing a laser sintering method and system, <CIT> notes that if the system maintains the bed of powder at a temperature that is too low (e.g., too near such powder's recrystallization point), then the fused powder may return to a solid state (or "recrystallize") too quickly, which may cause the formed object to warp or deform. This patent further notes that, if the system maintains the bed of powder at a temperature that is too high (e.g., too near such powder's melting point), then the remaining unfused powder may partially melt, which may increase the relative difficulty of separating the remaining unfused powder from the formed object. This difficulty of separating in turn reduces the recyclability of the material or the ability to reuse the material. Avoidance of curl by maintaining temperature at "maximum uniformity" just below the melting point of the polymeric material is also described in <CIT>. The above-referenced '<NUM> patent defines a "window of sinterability" temperature range and notes that a major practical consequence of the narrowly defined window requires that the part bed be maintained at a specified temperature and with a specified temperature profile so that each layer to be sintered lies within the confines of the selective-laser-sintering-window. As described, a different temperature, whether higher or lower, and/or a different temperature profile, results in regions of the just-sintered initial slice of powder which will either cause an already sintered slice to melt and be distorted in a layer of the part bed which has "caked"; or, will cause an already sintered slice to curl if the part bed temperature is too low. Additionally, the '<NUM> patent has related the rate of crystallization to the difference in temperature between the onset of melting and onset of recrystallization, while rate of crystallization is described as more correctly and closely related to fundamental polymer characteristics such as the crystallization half-time. Crystallization half-time is described as a more fundamental property of the polymer than the difference in melting and recrystallization endotherms, as this difference can be adjusted. In conclusion, the '<NUM> patent provides that "it has been discovered that the rate of crystallization of the semicrystalline organic polymer is a key property in controlling curl and achieving dimensional control in the sintered part. Materials that recrystallize relatively slowly after melting exhibit sufficient dimensional stability and create near-fully dense, distortion-free parts in the selective laser sintering process. "
<CIT> describes a build material for additive manufacturing applications comprising a semi-crystalline polyester obtained from the polycondesation of terephtalic acid and cyclohexanedimethanol.

While this conclusion may be indicative of the direction of development efforts in the field of polymer powder build materials for laser sintering, commercial product offerings to date have not relieved manufacturers from the burdens of meticulous process temperature control and the corresponding equipment and manufacturing costs, and the product quality issues such as warping and curling that can accompany failure to maintain such control.

In a first aspect, the present invention relates to a build material for additive manufacturing applications as set out in claim <NUM>. In a further aspect, the present invention relates to an additive manufacturing method for producing a three-dimensional object as set out in claim <NUM>.

In a further aspect, the present invention relates to a polymer article as set out in claim <NUM>.

Further aspects of the invention are as disclosed and claimed herein.

In the first aspect of the present invention, the build material of the present invention includes a build composition in powder form as set out in claim <NUM>. A suitable particle size range for the powder form of the build material is between <NUM> and <NUM> as measured by DLS and a suitable median for the volume particle size distribution (referred to as Dv[<NUM>]) is from <NUM> to <NUM> as measured by dynamic light scattering (DLS). The build composition is present in the build material in the amount of from <NUM>% to <NUM>% by volume of the build material based on the total volume of the solids fraction of the build material. Additional but optional ingredients in the build material include one or more of crystallizing agents such as nucleating agents; colorants; heat and/or light stabilizers; heat absorbing agents such as heat absorbing inks; anti-oxidants, flow aids, and filler materials such as glass, mineral, carbon fibers and like. In an embodiment where the build composition is present in the build material in the amount of <NUM>% by volume of the build material, the build material is the build composition in polymer form. Accordingly, in an embodiment, the build material may consist essentially of or consist of the build composition in powder form.

The build composition includes a semi-crystalline polymer which is a component of the build composition. "Semi-crystalline" is defined as a polymer with crystallinity level of <NUM>% or more as measured by DSC. Suitable semi-crystalline polymers for the present invention have a glass transition temperature (Tg) from <NUM> to <NUM> and a minimum crystallization half-time greater than <NUM> minutes and preferably are selected from the group consisting of polyesters, copolyesters, polycarbonates, and polyether ketones. Particularly suitable semi-crystalline polymers have a crystallinity of from <NUM>% to <NUM>%. The semi-crystalline polymer preferably has a minimum crystallization half-time (t ½) of from <NUM> to <NUM> minutes. The semi-crystalline polymer is present in the build composition in the amount of from <NUM>% to <NUM>% by volume of the build composition based on the total volume of the solids fraction of the build composition. In an embodiment wherein the build composition includes <NUM>% semi-crystalline polymer by volume based on the total volume of the solids fraction of the build composition, the semi-crystalline polymer is in the form of a powder and the build composition is a semi-crystalline polymer in powder form. Accordingly, in an embodiment, the build composition may consist essentially of or consist of the semi-crystalline polymer in powder form.

The semi-crystalline polymer in one embodiment is a crystallized amorphous polymer. "Crystallized amorphous polymer" is defined herein as a semi-crystalline polymer formed through inducement of crystalline structure in a polymer through solvent annealing crystallization. Crystallized amorphous polymers have a crystallinity level higher than that of the polymer before the crystallinity inducement process. Other methods for inducing crystalline structure in generally amorphous polymers known in the art, including for example solvent precipitation crystallization, thermal crystallization and strain crystallization and the like, may also be investigated for use by one of ordinary skill in forming a crystallized amorphous polymer.

Solvent annealing crystallization involves exposing a polymer to a low molecular weight (below about <NUM>/mol) solvent vapor or solvent liquid to swell the polymer without substantially dissolving it or causing the polymer pellets to stick together. Selection of a suitable solvent for solvent annealing crystallization will depend in part on the polymer type to be crystallized. For example, acetone or methyl acetate are suitable choices of solvent for copolyesters and may be used either in pure form or as part of an aqueous system. A partially miscible solvent system can also be used to expand the range of solvent choices. The polymer can be maintained at the crystallization temperature or a series of increasing crystallization temperatures below the melting temperature until the desired level of crystallinity has been achieved. Any residual solvent can be removed via thermal and/or vacuum treatments. The polymer may also be heated to a temperature at which crystallization is faster than at the solvent exposure temperature. Nucleation agents may also be incorporated via compounding or some other process to promote or control crystallization of the amorphous polymer.

As noted above, the semi-crystalline polymer component of the build composition of the present invention is characterized by (i) a glass transition temperature (Tg) from <NUM> to <NUM> and (ii) a minimum crystallization half-time (t<NUM>/<NUM>) of greater than <NUM> minutes. Glass transition temperature, as well known in the art, is the temperature at which the mechanical properties of a polymer fairly rapidly change glassy to rubbery due to the internal movement of the polymer chains that form the polymer. This change in behavior is typically measured by Differential Scanning Calorimetry (DSC) techniques known in the art and is evidenced for example by a sharp decline in modulus (stiffness) or increase in impact strength as the ambient temperature is increased. Glass transition temperature is measured according to the methods known in the art, such as ASTM E1356 - <NUM>(<NUM>). Particularly suitable semi-crystalline polymers for the build composition of the present invention have a glass transition temperature of from <NUM> to <NUM>.

Minimum crystallization half-time (t<NUM>/<NUM>), as the phrase is utilized herein, refers to the minimum length of time required to achieve approximately half of the maximum crystallinity achievable at a given crystallization temperature. t<NUM>/<NUM> depends in part on the crystallization temperature Tc, and t<NUM>/<NUM> is typically at its minimum, i.e., maximum crystallization rate, at a temperature approximately half way between the glass transition temperature (Tg) and the melt temperature (Tm). Minimum crystallization half-time is determined for the present invention using the small angle light scattering (SALS) technique described below wherein a helium-neon laser is used to measure the time at which the intensity of scattered light increases to half of the maximum scattered intensity achieved. A sample is first melted at a temperature well above the melt temperature to remove all preexisting crystallinity. Then, the sample is rapidly cooled to a predetermined temperature (Tcool) and the scattered light intensity is recorded as a function of time. The time at which the scattered light intensity increases to half the maximum value denotes the crystallization half-time reported. As crystallization rate varies with temperature, the temperature at which the crystallization rate is the highest in this range (corresponding to the temperature with the minimum crystallization half-time in the temperature range) was chosen to quantify the parameter for comparison purposes hereunder.

Crystallinity level is an indicator of the fraction of crystalline domains in a polymer. Crystallinity level is typically measured by using DSC and is the ratio between the enthalpy of fusion of the polymer and the enthalpy of fusion of a <NUM>% crystalline version of the same polymer. Crystallinity is measured according to methods known in the art, for example as described in ASTM F2625 - <NUM>(<NUM>).

The semi-crystalline polymer is a semi-crystalline copolyester including <NUM> mole % terephthalic acid residues, <NUM>-X mole % of a first glycol residue D1 and X mole % of a second glycol residue D2, wherein X, D1 and D2 are selected as follows:.

wherein the total acid residue content and total glycol residue content are each <NUM> mole %.

In an alternative the semi-crystalline polymer is a semi-crystalline copolyester including <NUM> mole % <NUM>,<NUM>-cyclohexanedicarboxylic acid (CHDA) residues, <NUM> - X mole % of a first glycol residue D1 and X mole % of a second glycol residue D2, wherein X, D1 and D2 are selected as follows:.

As described above, the build compositions are preferably in the powder form. As the build composition may include <NUM>% semi-crystalline polymer by volume based on the total volume of the solids fraction of the build composition, the semi-crystalline polymer may be in the form of a powder and the build composition is a semi-crystalline polymer in powder form.

Additional but optional ingredients in the build composition include one or more of crystallizing agents such as nucleating agents; colorants; heat and/or light stabilizers; heat absorbing agents such as heat absorbing inks; anti-oxidants, flow aids, and filler materials such as glass, mineral, carbon fibers and like.

When utilized in an additive manufacturing method such as for example a laser sintering process, an important advantage of the build materials of the present invention is that they remain amorphous from the period immediately after being melted/sintered by the laser until they eventually vitrify as they are cooled below the semi-crystalline polymer's glass transition temperature well after the build process is complete. This approach avoids the generation of mechanical stresses caused by crystallization. It also reduces the need for stringent control of temperature gradients across the surface or through the volume of the build. It also enables more of the sintering bed volume to be used and enable scaling up of the sintering bed sizes. It also enables the possibility of not cooling or rapidly cooling the build without the risk of curling or warping of sintered parts. In this regard, the build materials of the present invention address these issues with processing of semi-crystalline polymers and still maintain the advantage of being amorphous after being sintered.

Accordingly, a further aspect of the present invention relates to an additive manufacturing method for producing a three-dimensional object, said method including the steps of:.

wherein the temperature of said target surface varies more than <NUM> over said total time period.

As noted above, additive manufacturing methods, and in particular laser sintering processes, are generally known in the art and described for example in laser sintering <CIT>; <CIT> and <CIT>. An important and unexpected advantage of the method of the present invention is that the prior art's requirement of meticulous monitoring and control of the target surface and build environment temperature to avoid later and/or part warping is substantially reduced. Accordingly, the temperature of the part bed in the method of the present varies more than <NUM> over the said total time period for all the applying and directing steps.

In another aspect, the present invention is directed to a polymer article formed via an additive manufacturing process, referred to herein as an additive-manufactured polymer article. An important feature of the present invention resides in the fact that the polymer of an additive-manufactured polymer article formed from the build material of the present invention is amorphous.

The following examples represent methods and working instructions for illustration. Copolyesters in the below examples were manufactured via hydrolytic polycondensation according to standard methods. By way of background, copolyesters typically comprise one or more diacids and one or more diols. The total number of moles of all the diacids is equal to the total number of moles of all the diols. In the examples below, Copolyester A includes <NUM> parts terephthalic acid, <NUM> parts ethylene glycol, and <NUM> parts <NUM>,<NUM>-cyclohexanedimethanol; Copolyester B includes <NUM> parts terephthalic acid, <NUM> parts ethylene glycol, and <NUM> parts <NUM>,<NUM>-cyclohexanedimethanol; and Copolyester C includes of <NUM> parts terephthalic acid, <NUM> parts isophthalic acid, and <NUM> parts <NUM>,<NUM>-cyclohexanedimethanol. Copolyester A, B and C are not copolyesters according to claim <NUM>.

For the two experimental runs in this Example, <NUM> of pellets of Copolyester A and Copolyester C were each separately placed in a <NUM>-gallon polyethylene bucket. A sufficient amount of methyl acetate solvent was poured into the buckets to completely cover the pellets. The pellets were allowed to absorb the solvent for approximately <NUM> hours. Then, the solvent was drained and the pellets were purged with nitrogen in an oven at <NUM> for <NUM> hours. The solvent crystallized pellets were then purged in nitrogen and annealed at temperature close to the polymer's melting point as indicated in Table <NUM> below. Polymer melt point and glass transition temperature were measured using DSC according to methods described in ASTM E1356 - <NUM>(<NUM>) and ASTM F2625 - <NUM>(<NUM>).

Solvent annealing at multiple temperatures (temperatures increasing in a step-wise manner) was performed in this Example. More specifically, each of Copolyesters A, B and C and a polycarbonate commercially available from Makrolon under the trade name PC2608 were separately treated with solvents as indicated below and subjected to an annealing process as indicated below. Conditions and the melting temperature based on DSC for the different polymers, along with crystallinity as measured by DSC, are listed in Table <NUM> (melt point, glass transition temperature and crystallinity were measured using DSC according to methods described in ASTM E1356 - <NUM>(<NUM>) and ASTM F2625 - <NUM>(<NUM>)). To estimate crystallinity level according to ASTM F2625 - <NUM>(<NUM>), the theoretical heats of fusion of <NUM>% crystalline polymers of similar compositions are needed. Here, the theoretical heat of fusion for copolyesters is assumed to be <NUM> J/g (heat of fusion of PET or polyethylene terephthalate) and for polycarbonate between <NUM> to <NUM> J/g (<NPL>; <NPL>).

The minimum crystallization half-time of Copolyesters A, B and C were each measured using SALS with multiple runs using a range of temperatures. The minimum crystallization half-time values are set forth in Table <NUM> below.

In this example, Copolyesters A, B and C were each separately processed into a form suitable for use as build material for additive manufacturing. More specifically, the polymer was cryo-ground into powder with a particle size between <NUM> and <NUM>. A Voxeljet™ VX200 high speed sintering (HSS) machine was used to manufacture specimens from each powder sample that were suitable for tensile testing in accordance with the ASTM D638 Type <NUM> standard. In a first set of tests, the results of which are set forth in Table <NUM>, <NUM> and <NUM> below, bed temperature was set for each run as indicated in the tables below and the printed specimens were removed immediately from the print bed and tested for mechanical properties. In a second set of tests, the results of which are set forth in Table <NUM> and <NUM>, bed temperature was held constant at a set temperature of <NUM> for Copolyester A and Copolyester C for all the runs and printed specimens were either (i) removed from the print bed and tested for tensile properties (labeled "no cooling" in Table <NUM> and <NUM>) or (ii) allowed to cool overnight prior to tensile testing (labeled "overnight cooling" in Table <NUM> and <NUM>). Tensile testing of the specimens performed on a Tinius Olsen H5KS tensometer with a H500L laser extensometer at a strain rate of <NUM>/min, in accordance with ASTM D638. Tensile testing data, more specifically Young's modulus or modulus (Y), ultimate tensile strength (UTS), and elongation at break (EAB) for the specimens formed from annealed Copolyester A are reported in Table <NUM> and Table <NUM> below, from annealed Copolyester B are reported in Table <NUM>, and annealed Copolyester A are reported in Table <NUM> and Table <NUM>.

The data in Tables <NUM>, <NUM> and <NUM> indicates that, so long as the radiation source is able to supply sufficient heat to melt and sinter the material mechanical properties of the build materials are suitable for producing a wide variety of articles via additive manufacturing. Interestingly, it was possible to print parts without visible warping at a range of bed temperatures (including room temperature), which is not typically possible in nylon <NUM> due to the need to establish and maintain a specific bed temperature to avoid the negative impact of the relatively fast crystallization kinetics of nylon <NUM>. This ability to print at a range of bed temperatures highlight the build materials and build compositions of the present disclosure.

The data in Tables <NUM> and <NUM> demonstrate the manufacture of specimens with suitable-for-use properties and characteristics (such as dimensional stability) regardless of variation in the cooling processes. This is stark contrast to prior art polymers such as nylon <NUM>, for which the parts need to be cooled down slowly, with cooling steps sometimes longer than <NUM> hours, to prevent warping resulting from the fast crystallizing kinetics. The build materials of present disclosure allow for removal of parts from the printing equipment immediately after the printing process is complete, thus obviating the need for a long cooling step and significantly shortening the cycle time.

Claim 1:
A build material for additive manufacturing applications comprising a build composition in powder form, said build composition comprising a semi-crystalline polymer having a glass transition temperature from <NUM>° C to <NUM> measured according to ASTM E1356 - <NUM>(<NUM>) and a minimum crystallization half-time of greater than <NUM> minutes the minimum crystallization half-time being determined as described in the description,
wherein said semi-crystalline polymer is a crystallized amorphous polymer having a crystallinity of from <NUM>% to <NUM>% measured according to ASTM F2625 - <NUM>(<NUM>),
wherein said semi-crystalline polymer is a copolyester which comprises <NUM> mole % terephthalic acid residues, <NUM>-X mole % of a first glycol residue D1 and X mole % of a second glycol residue D2, wherein X, D1 and D2 are selected as follows:

<TAB>

and the total acid residue content and total glycol residue content are each <NUM> mole %
or
wherein said semi-crystalline copolyester comprises <NUM> mole % <NUM>,<NUM>-cyclohexanedicarboxylic acid residues, <NUM> - X mole % of a first diol residue D1 and X mole % of a second diol residue D2, wherein X, D1 and D2 are selected as follows:

<TAB>

and wherein the total acid residue content and total glycol residue content are each <NUM> mole %.