The present invention relates generally to layerwise building, a lso referred to as 3 D- printing, of a n object more specifica lly to layerwise Selective Laser Sintering (SLS) of polymer powder. Therein, the invention relates to a process for refreshing used polymer, particula rly polya mide, conta ining powder from a SLS process for reuse in a next building process cycle. More pa rticularly the invention relates to a method for at least substa ntia lly regenerating polymer powder that has been wasted from a thermal fusion process, pa rticula rly polya mide powder that has been wasted from a selective laser sintering process that was used to create a n object.

A Selective Laser Sintering (SLS) process is a n additive ma nufacturing process in which layers of powder are fused together by irradiation with a laser. During the building process polymer powder, particularly polyam ide powder, of a carefully selected particle size distribution a nd polymer contents is introduced in the process reactor and the powder in the machine is heated to a tem peratu re close to its melting tem perature for a long period of time. Usually poly(a mide-12) ( PA12) is used in a SLS process. U nder these conditions solid state post-condensation is likely to occur in the polyam ide powder resu lting in chain-extension.

In a typical building process the majority of the powder will not form part of the u ltimate product, but rema ins in a powder form within the building platform . From a n economic point of view, however, it is desirable to reuse the unused powder as m uch as possible, however this is currently not possible as the powder significantly degraded d uring the former SLS process cycle. Particula rly, due to the cha in-extension reaction that occu rred in the powder during the prior building process, the molecular weight was increased, resulting in a higher melt viscosity of the polymer. A higher melt viscosity reduces the inter-particle fusion during later laser sintering, which deteriorates the performa nce a nd quality of the SLS process. A product ma nufactured from non- processed re-used polymer powder, for exa mple, will show more porous parts with inferior mechanical properties, like reduced stiffness a nd reduced tensile strength. In addition, the surface quality of the product will be poorer, also referred to as orange peel.

Current practice is to add old, used powder with fresh, virgin powder, referred to as blending. This reduces the amount of relatively expensive virgin material needed for a next process cycle while, when added in the right amount, a process powder comprising a blend of virgin powder with used powder, may still offer an acceptable melt viscosity for the sintering process to yield an acceptable product quality. Blending, however, still requires virgin powder and, due to the excess of waste powder that results from a SLS process, will only be capable of regaining use of merely part of all waste powder used in prior SLS processes.

According to DE 10105504 Al, a pre-treatment of waste powder or of a mixture of waste powder and fresh powder by liquefaction is proposed in order to reduce the effects of aging damages in the waste powder and, hence, to enable admixing more waste powder in the blend. However, the pre-treatment by liquefaction does not remove all aging damages of the powder. In particular, a high ratio of waste powder still causes unsatisfying surface properties of outer walls of the object by so-called pockmarks, which are also termed as sink marks or orange peel.

From WO 2005/097475 and DE 102004047876 Al, laser sintering methods and laser sintering devices are known, which reduce the problem of pockmarks by using a specific material that features an increased stability in the laser sintering process and, thus, less aging damages. As a result the waste powder may be re-used to give a more acceptable product quality. The powder according to DE 102004047876 Al comprises a mixture of diamide- and diamine-controlled polyamide and/or co-polyamide. The user, however, is forced to use this specific powder which has in turn also other properties that may cause it to behave differently than normally used habitual powder and, hence, may not fulfil all requirements of the SLS process. From US 2004/0138344 Al, it is known to increase the melting point and the enthalpy of fusion of polyam ide powder by contacting the polyam ide with water or water va pour. Thereby, some polyam ide ca n be rendered suita ble again for laser sintering. In DE 102008024465 a sim ilar process is described in wh ich waste powder is treated with water a nd/or water va pour at elevated temperatures, subsequently dried a nd afterwards re-used in a laser sintering process.

These prior art methods, that aim to refresh polya mide powder by the addition of free water or stea m, show a concomitant change in thermal properties. Both a change in Tm a nd Tc a re encountered, which have significant im plications for further processa bility in a further SLS building process. The use of stea m moreover creates a complex continuous process, where the addition of exact amounts of water is ha rd to realize. The use of free water or steam calls for a d rying step afterwa rds. The present invention has inter a lia for its object to provide a process of regenerating waste polymer powder that has been used in a building process for a n object by selective therma l fusion of polymer powder, pa rticularly polyam ide powder, such that it may be re-used in a sim ilar process with im proved processability. In order to achieve said object a method for at least substantially regenerating polymer powder that has been wasted from a therma l fusion process, particula rly polya mide powder that has been wasted from a selective laser sintering process that was used to create a n object, according to the invention, is characterized in that said polymer powder is introduced in a process cha mber, in that substantially all air is evacuated from said process cham ber to create a substa ntia lly dry, substa ntia lly oxygen free atmosphere, and in that sa id polymer powder is brought to an elevated temperature in said substantia lly dry atmosphere for a duration of time to allow a scission reaction of the polymer chain of said polymer. According to the invention, refreshing entails the reduction of molecular weight via the hyd rolysis of polymer concerned to provide a reduced viscosity com pared to the initia l va lue of the used materia l, hereinafter a lso referred to as Old' material. Because substa ntially all air has been expelled from the reaction cha mber before starting the scission reaction, so is substantially all free oxygen. It has been found that thereby a lso a premature degradation of the powder may be cou nteracted.

According to the invention no free water is added during the regeneration process of used polymer powder, thereby reducing the effect on the thermal behaviour to a m inim um. Instead, the hydrolysis reaction takes place at least substa ntia lly in the solid phase in order that the sha pe, size a nd/or size distribution of the powder particles will not, at least not significantly, change during the refreshing process. This a llows for the d irect re-use in powder form of the regenerated materia l after the refreshing process without a need of a drying step after the process or aga in creating a fine powder from it.

According to a preferred embodiment the method accord ing to the invention is characterized in that said process cham ber is flushed with a n inert dry gas to expel substa ntially all air. Flushing with a dry gas appears to be a convenient and effective method of creating a dry atmosphere in the reaction cham ber after it has been filled with powder, wh ile the use of an inert gas does not introduce any reactants in the following hyd rolysis process.

A specific embodiment of the method according to the invention is cha racterized in that said elevated temperature resides between approximately 120 °C a nd at least 5 °C below a melting temperature of said polymer. This tem perature ra nge offers an adeq uate reaction rate while the maxim um of at least 5 °C below a melting

tem perature of sa id polymer counteracts fusion of powder pa rticles a nd further degradation d uring the process. Specifica lly a n embodiment of the method according to the invention is carried out such that said elevated temperature resides between approximately 120 °C and at least 5 °C below a melting temperature of said polymer.

A further preferred embodiment of the method according to the invention is characterized in that the powder is brought to said elevated temperature in the presence of a carrier of water that releases water at least at said elevated temperature. By using such a carrier, containing water, the water content can be controlled and distributed in the container as desired in order to speed up and/or enhance the regeneration of the polymer powder in the reaction chamber.

In a particular embodiment the method according to the invention is characterized in that said carrier comprises a plurality of beads or grains that contain water, in that said beads or grains are mixed with said powder, and in that said beads or grains have a size that is considerably larger than a size of the powder particles. Using these bead shaped carriers a water delivery can be dispersed relatively well over the powder mass by using many beads or grains and mixing them with said powder. Because the beads or grains have a significantly larger size than the powder particles, the beads or grains can be filtered out relatively easily afterwards to offer clean, refreshed powder that is ready for re-use in a selective sintering process. In a specific embodiment, the method according to the invention is characterized in that said beads or grains comprise silica gel or ceramics.

Alternatively, a further particular embodiment of the method according to the invention is characterized in that said carrier comprises a solid body containing water, said solid body residing in the reaction chamber while the powder is brought to said elevated temperature. The solid body offers a localized water dispersion system in the reaction chamber. As a result the reaction can be accelerated at a single point of the reactor, for instance the middle of the powder mass. A further preferred embodiment of the method according to the invention is characterized in that said carrier is reversible carrier of water capable of releasing water at said elevated temperature and capable of absorbing water while being brought from said elevated temperature to room temperature. Using such a reversible carrier, like for instance the silicagel or ceramics beads or grains referred to hereinbefore, provides for the delivery of a controllable amount of water during the reaction process at said elevated temperature simply by controlling the amount of carrier(s) added to the powder. On the other hand, the reversible nature offers the carrier the capability of capturing the water again while the powder cools to ambient temperature at the end of the process to render a dry regenerated powder. No drying step is necessary afterwards although water has been admitted to enhance the reaction process.

Hereinafter the invention will be elucidated in greater detail with reference to a number of explanatory embodiments.

Explanatory Embodiment 1:

According to one embodiment, an autoclave reactor is filled with old, i.e. used, PA12 or PA11 polyamide powder that is to be refreshed such that it may be re-used in a selective (laser) sintering process. Hereafter the process chamber is closed and purged with nitrogen gas. Afterwards heating is applied, preferably under mixing, to raise the chamber temperature to temperatures of at least 120 °C. The maximum temperature of the chamber is kept 5 °C below the melting temperature of the polyamide.

When the desired temperature is reached it is maintained duringthe course of the process. Under these conditions the polyamide material undergoes a chain- scission reaction with water that is naturally present in the hydroscopic polyamide powder particles when stored under ambient conditions. Figure 1 shows a condensation reaction of PA12, where the reaction in the opposite direction is the hydrolysis reaction of PA12 according to this embodiment of the invention. The extent of the molecular weight increase of old material during the former SLS process determines the duration of the treatment and the process conditions needed. These process conditions particularly concern the temperature and the chamber pressure, which both may be varied and tailored to optimize the regeneration process. In this example a batch of approximately 50 kilograms of old PA12 powder has been processed for 24 hours at a temperature of 155-160 °C and at an initial process pressure of approximately 120-130 kPa. The resultant powder has a considerably lower average molecular weight than the initial powder, as shown in table 1, while Tc/Tm remained substantially unchanged which allows it to be re-used in a SLS process without further blending or other post-processing.

Explanatory Embodiment 2:

In another embodiment, an addition of water is used to enhance the reaction process. Also other components can be added (e.g. mono and di-acids) to regulate the reactivity of the end groups of the polymer chains and control molecular weight distributions.

This in turn allows for control over the reactivity of the chain-extension reaction during subsequent printing jobs.

Said addition of water is realized by mixing a plurality of silica gel beads or grains with a grain size that is considerably larger than the polymer powder particles. In this embodiment a polyamide powder is used with a steep particle size distribution around between 50 and 60 micron, while the silica gel beads or grains that are of the order of a few millimetre in size, particularly the beads or grains are of the order of 3,5 millimetre in this embodiment. Said silica gel beads or grains carry water and possibly also other components that could speed up or otherwise improve the reaction process. During the process these beads or grains will release said water and said possible other components. After the process the beads or grains can easily be removed physically from the powder, for instance by sieving, while having absorbed at moderate to ambient temperature all excess water that was unused during the regeneration process. A batch of approximately 250 kilograms of old, i.e. used, polyamide powder is mixed with approximately 10 kilograms of soaked silica gel beads or grains and then processed similar to the powder in the first example. The powder/beads or grains mixture is kept at an elevated temperature of about 140-150 °C for a duration of roughly 48 hours. Before the start of the process the reaction chamber is flushed with nitrogen gas to expel all resident air. Finally an elevated nitrogen pressure of about 120-130 kPa is left in the reaction chamber to maintain a substantially dry, substantially oxygen free atmosphere. Instead of nitrogen gas also noble gasses like helium and argon may be used to flush the process chamber.

The resultant powder has a considerably lower average molecular weight than the initial powder, as shown in table 1, which allows it to be re-used in a SLS process without further blending or other post-processing.

Process results:

To validate the performance of the re-generation process according to these

embodiments, the average molecular weight (M J of both the initial powder and the resultant powder is determined by size exclusion chromatography (SEC). A SEC (Waters) GPC system, equipped with model 510 pump and a model 410 differential

refractometer, is used to obtain data on the molecular weights of all the polyamide materials. The material is first dissolved in hexa-fluoro-isopropanol (HFIP). Before injection, the samples are filtered through a 200 nanometre PTFE filter (13 mm, PP housing, Alltech). For calibration PMMA standards were used (Polymer Laboratories, Mp = 580 Da up to Mp = 7.1 x 106 Da). Data acquisition and processing were performed using Waters Millennium 32 (v4.0) software.

The melt viscosity of the polyamide has been tested in an AR-G2 rheometer, while a parallel plate setup was used in the environmental test chamber for

compression-moulded samples of PA12 polyamide powder. The frequency sweep was performed from 500-0,05 rad/s at 200 °C and 1% strai n, the data given in ta ble 1 are the results at a frequency of 0,05 rad/s.

Molecula r weight a nd viscosity values of virgin materia l, blend materia l, old material a nd recycled material according to embodiment 1 a nd 2 according to the invention, referred to as refreshed-1 a nd refreshed-2 respectively, a re given in Table 1:

Table 1: Molecular weight and viscosity results of PA12

Sample: Mn (g/mol) Mw (g/mol) Viscosity (Pa.s)

Old 53186/63768 118832/134314 6624/8259

37941 91S17

Refreshed-1 33134 59669 2718

29233 .... .

Refreshed-l/old 62% 50% 41%

t%€tresft Q- oto :4

The Mw data with the corresponding rheologica l properties show that from blend (input in the SLS printer) to old powder (output of the SLS printer) a n increase in molecula r weight leads to the increase in viscosity after the SLS process. After 24 resp 48 hours refreshing process the molecular weight of the old materia l is reduced to 50% of the sta rting va lue and the melt viscosity is reduced to 40% of the starting viscosity of the old materia l. This a ppea rs an accepta ble value for re-use in a SLS process. Further optim ization possibilities of the refreshing process as described hereinbefore allow a fu rther reduction of the melt viscosity of old, i.e. used, material to values even closer to that of virgin material.

After a successful refresh cycle the powder ca n aga in be used in a SLS process to print, i.e. build, a next object. As in the initia l run, the quality of the unused powder will deteriorate again. The method according to the current invention may then aga in be used to im prove and upgrade the powder properties and generate a useable feedstock for further print cycles. This opens a path to close the loop of the expensive waste stream in current polymer powder based additive manufacturing systems.

Although the invention has been explained in further detail with reference to merely a few embodiments it will be understood that the present invention by no mea ns will be lim ited to only the em bodiments given. On the contrary ma ny va riations and other em bodiments a re conceivable to skilled person without depa rting from the scope and spirit of the present invention. As such the method according to the invention may also be applied on other polymers than polyam ide. In genera l the invention is a pplica ble to a ll poly-condensation polymers that may be broken down into smaller fractions by means of a hyd rolysis reaction. Other polymers that could be processed in a sim ilar way a re for insta nce polyesters, like PET, PBT, PTT, PEN, PLA, PGA and co-polymers thereof, polyam ides, like PA4.6; PA6; PA6.6; PA6.12; PA6.10; PA10.10; PA10.12; PA11; PA12.12 a nd co-polymers thereof, a nd polyester urethanes.

Instead of silicagel or ceram ics, a lso other reversible water absorbing materials may be used as a water carrier during the process, like for instance wood either as a solid body or as a plura lity of grains.