Method and device for surface finishing of an article produced by 3D printing
阅读说明:本技术 用于通过3d打印生产的制品的表面精整的方法和装置 (Method and device for surface finishing of an article produced by 3D printing ) 是由 A·阿里恩蒂 M·弗拉多尔 于 2018-06-23 设计创作,主要内容包括:一种用于通过3D打印获得的工件的表面精整的方法和设备,其中将待处理的工件和液体工艺增塑剂在气密密封的处理腔室中一起加热至低于工艺增塑剂的沸腾温度的工作温度。空气蒸气混合物在腔室中维持持续循环,以保持温度和浓度均匀并且使工件与空气/蒸气混合物接触,从而避免在工件表面上形成冷凝物并且允许工艺增塑剂蒸气被工件表面吸收而没有冷凝物形成。具体取决于所期望的渗透深度,蒸气暴露时间是固定的。设备包括具有加热装置(15)的腔室(11)、用于使空气/蒸气混合物在腔室(11)中循环以保持其中均匀的温度和浓度条件的装置,以及用于通过冷凝从空气/蒸气混合物中分离工艺增塑剂蒸气的单元(24)。还设置有用于过滤残留的空气/蒸气混合物的单元(39)。(A method and apparatus for surface finishing of workpieces obtained by 3D printing, wherein the workpiece to be treated and a liquid process plasticizer are heated together in a hermetically sealed treatment chamber to an operating temperature below the boiling temperature of the process plasticizer. The air vapor mixture is maintained in the chamber for a continuous cycle to maintain uniform temperature and concentration and contact the workpiece with the air/vapor mixture, thereby avoiding formation of condensate on the workpiece surface and allowing process plasticizer vapor to be absorbed by the workpiece surface without condensate formation. The vapor exposure time is fixed depending on the desired depth of penetration. The apparatus comprises a chamber (11) having heating means (15), means for circulating the air/vapour mixture in the chamber (11) to maintain uniform temperature and concentration conditions therein, and a unit (24) for separating process plasticizer vapour from the air/vapour mixture by condensation. A unit (39) for filtering the residual air/vapour mixture is also provided.)
1. A method for the surface finishing of a workpiece made of plastic material obtained by 3D printing, characterized in that it comprises the following steps:
a) placing at least one workpiece to be treated in a hermetically sealed chamber equipped with a heating device;
b) feeding a controlled amount of liquid process plasticizer in the chamber;
c) closing the chamber air-tightly and activating the heating device to raise the temperature in the chamber to an operating temperature below the boiling temperature of the process plasticizer, thereby forming a mixture of air and the vapor of the process plasticizer in the chamber;
d) maintaining a continuous circulation of the air/vapor mixture in the chamber as the heating means is activated to maintain a uniform temperature and process plasticizer vapor concentration within the chamber and to ensure that the workpieces are simultaneously heated so that the surface temperature of the workpieces thereof is substantially equal to the temperature of the mixture and no condensation of process plasticizer vapor occurs on the surface;
e) maintaining the workpiece at the working temperature for a predetermined time sufficient to allow direct contact between the supply plasticizer vapor and the surface of the workpiece and to absorb the vapor up to a desired depth below the surface of the workpiece, the working temperature being just above the glass transition temperature of the mixture between the plastic material and the absorbed process plasticizer vapor;
f) after the predetermined time has elapsed, stopping the heating and drawing the air/vapor mixture from the chamber to a vapor separation device by condensation;
g) removing the processed workpiece from the chamber.
2. A method according to claim 1, characterised in that the working temperature is not more than 10 ℃, preferably not more than 5 ℃ higher than the glass transition temperature of the mixture of plastic material and process plasticiser.
3. The method of claim 1 or 2, wherein the workpiece is exposed to the process plasticizer vapor for a period of time of 20 to 80 minutes.
4. The method according to any of the preceding claims, wherein the process plasticizer is provided in a packaged dose having a volume adapted to the volume of the treatment chamber, the amount of process plasticizer being in the range of 2 to 10ml, preferably 2 to 5ml, per litre of volume of the chamber.
5. The method according to any of the preceding claims, characterized in that after the separation step of the process plasticizer by condensation, the residual air/vapor mixture is filtered in a liquid capable of absorbing or neutralizing the residual process plasticizer.
6. The method according to any of the preceding claims, characterized in that an inert gas is contained in the chamber, mixed with the vapour of the process plasticizer.
7. The method according to any of the preceding claims, characterized in that the working pressure in the process chamber is 125 to 150 kPa.
8. The method of any of claims 1 to 6, wherein the working pressure in the processing chamber is below atmospheric pressure.
9. The method of any preceding claim, wherein the temperature in the process chamber is rapidly increased above the operating temperature and then decreased to the operating temperature.
10. Method according to any one of the preceding claims, characterized in that the workpiece to be treated is made of ABS and the process plasticizer is acetone, the working temperature being between 30 and 40 ℃.
11. Method according to any one of the preceding claims, characterized in that the treatment time is comprised between 30 and 50 minutes.
12. An apparatus for surface finishing a workpiece made of plastic material obtained by 3D printing, characterized in that it comprises:
-a hermetically sealable chamber (11), said chamber (11) being intended to contain at least one workpiece requiring such a treatment;
-means (20, 22) for feeding a controlled amount of liquid process plasticizer to the bottom of the chamber;
-heating means (15, 17) placed at least in the bottom of the chamber for heating the liquid process plasticizer, thus forming a mixture of the air and process plasticizer vapor when the chamber is hermetically closed, and for raising the temperature of the air/vapor mixture to below the boiling temperature of the process plasticizer;
-means (18) for uniformly circulating said air/vapour mixture within said chamber (11) during the heating step and during the step of maintaining at said working temperature for a predetermined time sufficient to allow direct contact of said plasticizer vapour with the surface of said workpiece and to absorb said vapour up to a desired depth below the surface of said workpiece;
-means (24) for separating the process plasticizer vapour from the air/vapour mixture, said means (24) being capable of being placed in communication with the chamber once the predetermined exposure time has elapsed.
13. The apparatus according to claim 12, characterized in that the means for feeding a controlled amount of process plasticizer comprise a housing (20) for a package of metered amounts of the process plasticizer and a conduit (21) for communicating the housing (20) with the chamber (11).
14. The apparatus according to claim 12, wherein the packaged dose is in the form of a pierceable capsule (C), means for piercing the capsule (48) being provided at the bottom of the housing (20) to allow the contents of the capsule to flow into the chamber through the conduit (21).
15. Apparatus according to claim 14, characterised in that optical detection means (22) are provided for detecting predetermined identification marks on said capsule, so as to be able to pierce it.
16. Apparatus according to any one of claims 12 to 15, characterized in that a fan (18) is provided in said chamber (11), said fan (18) being powered by external motor means (19) to create a circulation of the air/vapour mixture and a uniform distribution of said mixture inside said chamber.
17. The apparatus according to any one of claims 12 to 16, characterized in that the means (24) for separating the process plasticizer vapor from the air/vapor mixture comprises a thermoelectric condenser.
18. The apparatus of claim 17, wherein the thermoelectric condenser is of the peltier cell type cooled by a cooling fluid in a closed loop.
19. Apparatus according to claim 17 or 18, characterized by comprising a body (27) having an hermetically sealed inner portion containing fins (28) thermally connected to the hot side of the peltier unit, said inner portion being configured to be positioned in communication with the chamber (11) by means of a pump (25) to circulate the air/vapour mixture in the inner portion of the body (27).
20. Apparatus according to claim 17, characterized in that said condenser (70) is a peltier unit condenser (72) and is in direct communication with said chamber (11) through openings (73, 74) made on one side of the chamber interior, the opposite side being in contact with the cold side of said peltier unit (72), means (78) for circulating said air/vapour mixture in said condenser being placed at one of said openings (73), the hot side of said peltier unit being in contact with heat exchanger means (71) to remove the heat generated.
21. The apparatus of claim 20 wherein said air/vapor mixture circulation means (78) in said condenser also serves as air/vapor mixture circulation means in said chamber (11) when said condenser is not in operation during said process plasticizer heating step and during said step of maintaining said operating temperature constant while exposing said workpiece to said process plasticizer vapor.
22. Apparatus according to any one of claims 12 to 21, characterized in that additional heating means (17) are provided at the side walls of the chamber (11).
23. The apparatus according to any one of claims 12 to 22, further comprising a filtering device (39), the filtering device (39) being configured to be positioned in communication with the chamber after the air/vapor mixture has been processed in the separator device (24).
24. The apparatus according to claim 23, wherein the filtering means comprises a bubbler means (40) immersed in a liquid capable of absorbing or neutralizing residual process plasticizer contained in the air/vapour mixture after treatment in the separator means (24).
25. An apparatus according to any one of claims 12 to 24, further comprising externally mounted cooling means (16) for cooling the chamber (11).
26. The apparatus according to any one of claims 12 to 25, characterized in that the chamber is in the shape of a tank (11) within an outer casing (10) containing and supporting the tank, the tank being hermetically sealable by a cover (12), an insulated inner casing (54) being provided in the outer casing (10) delimiting a gap (55) with the tank (11), the means (20, 22) for providing a controlled amount of the process plasticizer and the separator means (24, 70) of the air/vapor mixture being contained in the outer casing (10).
27. The device according to any one of claims 12 to 26, further comprising microprocessor means (53) for managing the operation of the device.
Technical Field
The present invention relates generally to the field of printing three-dimensional objects (hereinafter simply referred to as "3D printing") and in particular to a method for surface finishing of articles/objects (hereinafter also referred to as "workpieces") obtained by 3D printing processes, in particular those involving the use of polymeric materials for the manufacture of workpieces (parts). The invention also relates to an apparatus for surface finishing a workpiece obtained by 3D printing.
Background
As is well known, 3D printing processes allow the reproduction (replication) of three-dimensional objects from corresponding models made by 3D modeling software. Machines, devices and overall systems based on such processes are rapidly spreading on the market thanks to their reduction in cost and to the increasing diversification of the kinds of products put on the market, from professional and/or industrial equipment to desktop, office and domestic machines.
The use of 3D printers has expanded rapidly from rapid prototyping to a wide range of other application areas: from construction to entertainment, from biomedicine to aerospace, and the like.
In general, 3D printing processes involve layer-by-layer deposition of suitable materials to obtain three-dimensional objects. Deposition printing or FFM/FDM (fused filament process/fused deposition modeling) is a more widely used technique, primarily due to its lower cost, in which a polymer filament is heated to melt and passed through a nozzle, which is under the guidance of software while moving, depositing material to form overlapping layers. The most commonly used materials in this technology are ABS (acrylonitrile-butadiene-styrene) and PLA (polylactic acid). According to other techniques, powdered polymer or metal materials or liquid polymer materials are used.
The thickness of the layer is typically 50-100 microns, but thicknesses in the range of 10 microns can be achieved, but result in longer manufacturing times. It is clear that the quality of the finish can be improved by reducing the layer thickness, however, the object surface is relatively striated, uneven and porous, and therefore the objects may show undesirable characteristics (e.g. they may be saturated with liquid) or have an appearance that is not suitable for their final destination (e.g. in the case of decorative objects). In other cases, the surface irregularities do not meet the desired requirements and/or dimensional tolerances (e.g., where the object being manufactured by 3D printing is a mold). In all these cases, and in other cases not mentioned in relation to the quality of the manufactured piece, a surface finishing treatment has to be used.
A known method for surface finishing of articles of plastic material, such as by injection moulding, involves immersing the article in a solvent compatible with the plastic, or exposing the article to a solvent vapour for a predetermined period of time sufficient to solubilise a limited surface of the plastic material which can flow sufficiently over the surface of the article to smooth any roughness present thereon (see for example US 5448838).
The method has also been extended to the surface treatment of articles made of plastic material obtained by 3D printing. See, for this subject, WO03/089218, WO2007/044007, WO2008/088761 and WO 2010/002643. The 3D workpiece is exposed to the vapor of a suitable solvent compatible with the plastic after production in the 3D printer. Upon contacting the workpiece, the vapor condenses thereon and softens the material to cause the material to flow over the surface, thereby smoothing the surface. The workpiece is gradually heated until the surface reaches the temperature of the boiling solvent, and the workpiece is continuously covered on its surface with condensed solvent until said temperature is reached. The workpiece is kept exposed to the solvent vapor condensing thereon until the desired degree of finishing is achieved, and then removed from the vapor to allow it to dry by re-evaporation of the solvent.
The exposure time is determined by observing the condensation of the solvent on the workpiece and can be removed from the vapor chamber when the condensation is complete, indicating that the surface temperature of the workpiece has reached the boiling temperature of the solvent. In addition, the exposure time takes into account the type of solvent and material, the shape characteristics of the workpiece, and the solvent vapor concentration.
This method must condense the solvent on the surface of the workpiece as a necessary condition. However, prolonged exposure to condensed solvent, in addition to preventing vapor penetration into the surface, can lead to corrosion and surface runoff and loss of material, and in addition, runoff can also occur in a non-uniform manner due to the condensate droplets that may have preferential flow paths due to workpiece shape, thus leading to localized condensate buildup. Furthermore, with the above known method, it is not possible to control the depth of penetration of the solvent below the surface of the workpiece.
As a solvent suitable for this application, halogenated solvents (such as trichloroethylene, fluorocarbons, and mixtures thereof), ketones (such as acetone, methyl ethyl ketone, and the like), and the like have been proposed to be selected depending on plastic materials and operating conditions.
WO2010002643 also discloses an apparatus for surface finishing of 3D printed workpieces. The apparatus involves exposing the article to solvent vapor to remove surface roughness and porosity and achieve a smooth glossy surface. The device is formed by a metal containment box structure with a gas impermeable lid, in which a heating chamber is placed. In the chamber, the solvent is first evaporated, and then the 3D workpiece to be processed is disposed. The workpiece is cold or may have been pre-cooled. The containment box structure also contains a drying chamber separate from the heating chamber into which the 3D workpiece is transferred at the end of the solvent treatment to remove the solvent from the workpiece. The drying chamber is maintained at a temperature below room temperature to speed up the solvent removal process. A cooling coil is provided above the heating chamber to condense solvent vapour which is sent to a solvent collection tank for re-use before passing through the water separator.
The apparatus according to the above-mentioned patent application operates at atmospheric pressure and the heating chamber remains open during processing of the workpiece to allow the operator to keep the workpiece suspended in the solvent vapor atmosphere throughout the process and to control the outcome of the process. Fluorocarbon was used as the solvent.
The sequence of operational steps includes first heating the chamber, in which the liquid solvent has been charged to produce a solvent vapor, and then immersing the workpiece in an atmosphere of solvent vapor. The solvent vapor is at the working temperature and the workpiece is first at room temperature, whereby the vapor must condense on it and gradually heat up until a thermal equilibrium condition between the working temperature and the workpiece surface temperature is reached. In this case, the condensation of the vapor on the workpiece is stopped, and the process is ended. Typically, the treatment duration (which may be repeated if deemed undesirable) is less than a few minutes, and in some cases is about 30 seconds.
The above-mentioned equipment is very complex and expensive, and uses very expensive solvents; it is therefore only of interest for special industrial applications. Furthermore, since a minimum level of 0.5 inches of solvent must be maintained at the bottom of the chamber in which the evaporated solvent evaporates, the foreseeable amount of necessary solvent may be high, on the order of a few liters. Moreover, the need to carry out the treatment with the vapor chamber open entails risks of contamination of the working environment and of health of the operator immersing the work pieces in the vapor chamber, and therefore a suitable suction system must be provided. Finally, there is also operational complexity in transferring the workpiece to the drying chamber, taking into account the semi-fluid state of the workpiece surface at the end of the solvent exposure period.
Therefore, there is a great need to be able to surface finish plastic material workpieces obtained by 3D printing in an efficient, fast and safe manner, and at a cost that can be tolerated even for home and office use.
Disclosure of Invention
It is therefore an object of the present invention to provide a method for surface finishing a workpiece made of plastic material obtained by 3D printing, which allows to obtain a finishing degree and quality at least comparable to those obtainable by the known methods, without the inconveniences due to the formation of condensates on the surface of the workpiece.
A particular object of the invention is to provide a method of the above-mentioned type which allows to adjust the treatment intensity as desired, in particular to control the penetration depth of the vapor to which the workpiece is exposed.
Another important object of the present invention is to provide an apparatus for surface finishing a workpiece made of plastic material obtained by 3D printing, which can operate according to the above-mentioned method, has small dimensions, consistent with the working space of most 3D printers.
It is a further object of the present invention to provide an apparatus of the above-mentioned type which is easy to use for professionals (architects, designers, design rooms, etc.) and even for individuals who are enthusiastic about 3D printing without adequate surface finishing techniques.
It is a further object of the present invention to provide an apparatus of the above type which does not require the workpiece to be displaced in an intermediate step of the process.
It is a further object of the present invention to provide an apparatus of the above type which can be used without the need for additional equipment (e.g. fume hoods) for its use.
It is a further object of the present invention to provide an apparatus of the above-mentioned type which can be safely used by an operator, never requiring the operator to be in direct contact with the process material in liquid or vapor form.
It is a further object of the present invention to provide an apparatus of the above type which is remotely controllable and programmable by a user.
These objects are achieved by the method and the apparatus for surface finishing of workpieces made of plastic material obtained by 3D printing according to the present invention, the essential features of which are set forth in
According to an important feature of the method of the invention, one or more workpieces obtained by 3D printing are placed in a hermetically sealable chamber equipped with heating means. In the chamber, a controlled amount of liquid process plasticizer is fed, which collects in the bottom region of the chamber. Upon activation of the heating means, the liquid plasticizer is heated and a mixture of air and the process plasticizer vapor is formed in the chamber, the mixture being maintained in circulation to achieve uniform distribution throughout the chamber and uniform contact with the workpiece surface, the workpiece surface being progressively heated until an operating temperature below the boiling temperature of the process plasticizer is reached. The workpiece is maintained in contact with the process plasticizer vapour for a predetermined time to allow the vapour to be absorbed by the workpiece surface up to a desired depth (in relation to the contact time and to obtain a corresponding softening of the plastic material), the working temperature being slightly above the glass transition temperature of the mixture between the plastic material and the process plasticizer vapour absorbed therein. Once the predetermined time has elapsed, the air/steam mixture is bypassed to a separation unit outside the chamber which separates the process plasticizer from the air/steam mixture by means of condensation. At the end of the separation step, the treated component is then removed from the chamber.
By operating in such a way that by maintaining a substantially equal temperature between the surface of the workpiece and the air/vapour mixture in contact therewith (also called thermal equilibrium conditions), condensation of vapour and therefore softening due to solubilisation of the surface layer is avoided, and vapour absorption and therefore softening due to plasticisation of the surface is promoted.
According to a preferred embodiment of the method of the invention, the working temperature is not more than 10 ℃, preferably not more than 5 ℃ above the glass transition temperature of the mixture and the workpiece exposure time to process plasticizer vapour is in the range of 20 to 80 minutes.
According to another feature of the method of the invention, the process plasticizer is provided in an amount suitable for a prepackaged dose of the volume of the processing chamber. According to a particular embodiment of the invention, the amount of process plasticizer is in the range of 2 to 10ml, preferably 2 to 5ml, per litre of chamber volume.
According to another important feature of the invention, the device comprises:
a) a hermetically sealable chamber for containing at least one workpiece requiring such treatment;
b) means for feeding a controlled amount of liquid process plasticizer to the bottom of the chamber;
c) heating means placed at least in the bottom of the chamber for heating the liquid process plasticizer, thereby forming a mixture of the air and process plasticizer vapor, and for raising the temperature of the air/vapor mixture to an operating temperature below the boiling temperature of the process plasticizer and slightly above the glass transition temperature of the plastic material;
d) means for uniformly circulating said air/vapor mixture within said chamber during the heating step and during the step of maintaining at said operating temperature for a predetermined time sufficient to allow direct contact of said plasticizer vapor with the surface of said workpiece and to absorb said vapor up to a desired depth below the surface of said workpiece;
e) means for separating the process plasticizer vapor from the air/vapor mixture by condensation, the separation means being external with respect to the chamber and capable of being placed in communication with the chamber once the predetermined contact time has elapsed. In a preferred embodiment of the apparatus according to the invention, a feeder of prepackaged doses of process plasticizer in the form of a pierceable capsule is provided, comprising a housing at one side of the treatment chamber, and means for cutting or piercing the capsule at the bottom of the housing and letting its contents flow into the chamber through the conduit. Preferably, detection means are provided to enable cutting or puncturing of the capsule. Preferably, the detection means are of the optical type. In this way, the feeding operation of the process plasticizer is simplified as much as possible, thereby avoiding any risk of contact with the user and achieving a very compact layout of the apparatus.
According to a preferred embodiment of the device of the invention, a condenser of the thermoelectric type using a Peltier unit is employed at the end of the heating step in order to separate the process plasticizer from the air/vapor mixture coming from the treatment chamber. The cooling of the peltier unit can be performed using a cooling liquid in a closed (closed) loop or by air. The condenser group is very compact and, because the condenser body can be hermetically sealed, any risk of environmental damage is avoided.
In another particular embodiment of the invention, the peltier unit condenser communicates directly with the treatment chamber through at least one inlet opening of the mixture of rich process plasticizer vapor and at least one return opening of the mixture of lean process plasticizer vapor facing the inside of the chamber, and means are provided at the at least one opening for circulating the mixture through the condenser. Advantageously, the same circulation means are also provided for maintaining a uniform circulation of the air/vapour mixture inside the chamber during the treatment of the workpiece, when the condenser is not active.
According to a further embodiment of the apparatus of the present invention, all the components of the apparatus (the treatment chamber, the process plasticizer feeding unit, the condenser group and the cooling fluid circuit, and the residual mixture filtering unit) are contained in an external casing of smaller dimensions with respect to the external casing of a common 3D printer, and the interior of the casing is easily accessible through an upper opening sealable with a cover.
Drawings
Further features and advantages of the method and apparatus for surface finishing of parts made of plastic material obtained by 3D printing of the invention will become apparent from the following description of an embodiment thereof, given by way of example and not limitation, with reference to the accompanying drawings, in which:
figure 1 is a perspective view of an apparatus according to the present invention;
FIG. 2 is a cross-sectional view of the apparatus taken along the vertical plane of trace A in FIG. 1 and in the direction of arrow II, this section being referred to as the front section;
FIG. 3 is a cross-sectional view of the apparatus taken along the vertical plane of trace B in FIG. 1 and in the direction of arrow III, this section being referred to as a side section;
FIG. 4 is a cross-sectional view of the apparatus taken along the vertical plane of trace C in FIG. 1 and in the direction of arrow IV, this section being referred to as the rear section;
FIG. 5 is an exploded and enlarged perspective view of a condenser unit within the apparatus according to the present invention;
FIG. 6 is a perspective view of the condenser unit of FIG. 5;
FIG. 7 is an enlarged view of an apparatus for feeding a process plasticizer bladder according to an exemplary embodiment of the present invention;
FIG. 8 schematically illustrates process plasticizer circulation in an apparatus of the present invention;
fig. 9 is a perspective view of another embodiment of an apparatus according to the present invention with the outer housing partially removed to show the internal components.
FIG. 10 is a cross-sectional view of the device of FIG. 9 made according to the vertical plane of trace D in FIG. 9 and in the direction of arrow X;
fig. 11 is a perspective view of the condenser unit as viewed from the inside of the chamber;
fig. 12 is a perspective view of the condenser unit as viewed from the outside, i.e., the side opposite to fig. 11;
FIG. 13 is an exploded perspective view of the condenser unit of FIGS. 11 and 12;
FIG. 14 is a flow chart illustrating a method according to the present invention;
fig. 15 shows two SEM images of a section of a 3D printed ABS sample fractured by means of liquid nitrogen before and after treatment according to the method of the invention;
fig. 16 shows, in a front view, two SEM images of a face of the same sample before and after treatment according to the method of the invention.
Detailed Description
The method of the invention can be used to treat objects or workpieces made of plastic material (hereinafter referred to as "3D workpieces" or "workpieces") obtained by 3D printing, in particular by using the above-cited printing process known as FFF/FDM, in which polymer filaments are melt heated and passed through nozzles under the guidance of modeling software while depositing material to form successive layers overlapping each other to form the desired object. Objects made in this way are characterized by a surface with a substantially stepped, wrinkled and striped appearance, and this often constitutes a drawback from an aesthetic and/or functional point of view, which must be eliminated.
To this end, according to the invention, the 3D workpiece is suspended or placed in any other way in a hermetically sealable chamber, into which process plasticizers compatible with the plastic material constituting the 3D workpiece are also fed.
In the context of the present description and the appended claims, the term "process plasticizer" means a vaporizable or vaporizable substance which is capable of being absorbed in the form of a vapor in a plastic material and of forming an intimate mixture therewith in order to bring about its glass transition temperature (T [) ]G) To a certain extent in order to soften the surface layer of the 3D workpiece and follow the release from evaporation from the 3D workpiece surface when the 3D workpiece is cooled. Without wishing to be bound by any particular mechanism of action, it is contemplated that the process plasticizer is not chemically bound to the polymeric material, but rather it forms an intimate mixture with the material or one of its components to reduce TG. Softening of the surface layer of the 3D workpiece causes a structural rearrangement that produces the desired surface smoothness depending on the depth of vapor penetration, and in addition, a structural change of the inner layer that promotes interpenetration of the inner layer and homogenizes the morphology of the 3D workpiece.
Clearly, there may be several substances available for use as process plasticizers for each type of polymeric material (amorphous or semi-crystalline) that makes up the 3D workpiece, and some of them will be preferred. For example, in the case of 3D workpieces made of ABS, the above-mentioned types of substances may be low-boiling ketones such as acetone and methyl ethyl ketone, or may even be halogenated compounds such as dichloromethane and fluorocarbons, although ketones are preferred in the method of the invention: at least because they are water soluble.
As used in the context of the present specification and the appended claims, the term "surface finishing" refers to a treatment with the vapour of a process plasticizer compatible with the material constituting the 3D workpiece, which can be spread (extended) to a layer of material below the surface layer at a depth, typically up to 3mm, but even more if desired, whereby an increase in the mechanical resistance (strength) of the surface is obtained in addition to a smooth surface as required.
After the workpiece to be treated is placed in the chamber and the process plasticizer is fed into the chamber in a liquid state, the chamber is closed air-tightly and heat is supplied to the inside of the chamber to heat the 3D workpiece and the process plasticizer until an operating temperature is reached which is suitably lower than the boiling temperature of the process plasticizer at the operating pressure. The working pressure is substantially equal to atmospheric pressure plus the vapor pressure of the process plasticizer at the working temperature. One important condition that needs to be met is: the heating rate of the 3D workpiece is the same as the rate of temperature rise of the process plasticizer, and the temperature within the chamber will be as uniform as possible during the heating step and once the working temperature is reached to ensure that a sufficient thermal equilibrium is reached between the surface of the workpiece and the air/vapor mixture in the process.
The operating parameters are the working temperature and the treatment duration, i.e. the exposure time of the 3D workpiece to the process plasticizer vapor. During the period that the 3D workpiece is always exposed to the process plasticizer vapors, the 3D workpiece must be in thermal equilibrium with these vapors and must be maintained at such temperatures to prevent condensation of the process plasticizer on its surface.
The working temperature depends on the material constituting the 3D workpiece, in particular on its glass transition temperature, and the glass transition temperature of the liquid used as process plasticizer, as well as the weight ratio of polymer and absorbed process plasticizer.
In particular, the operating temperature (T)es) Must be above the glass transition temperature of the plasticizing polymer, i.e. the mixture consisting of polymer and process plasticizer absorbed therein, and is therefore also dependent on the weight concentration of process plasticizer in the mixture. There are empirical formulas well known to those skilled in the art, such as Focus' equation (1/T)mis=wp/Tp+wpl/TplWherein, Tmis、TpAnd TplIs the glass transition temperature of the plasticized polymer, the neat polymer, and the process plasticizer; w is apAnd wpiIs the weight fraction of pure polymer and absorbed process plasticizer; and wp+wpi1) to obtain a rough estimate of the glass transition temperature of the polymer mixture.
For the purposes of the invention, i.e. to keep the surface of such a workpiece softened during processing to allow the structural rearrangement of the surface layer of the workpiece, the working temperature TesSpecific to the glass transition temperature T of the Polymer/Process plasticizer mixturemisSlightly higher, i.e. some degrees celsius higher, usually not more than 10 c, and in practice preferably not more than 5 c, is sufficient.
Unlike the solubilization process (process) of the surface layer of the 3D workpiece, which is carried out according to known methods due to the action of the solvent condensed on the surface of the surface layer of the 3D workpiece, the absorption of the process plasticizer vapor according to the invention is a relatively slow process (process), and the depth of penetration of the process plasticizer in the polymer and thus the depth of the softening effect depends on the duration of the exposure to the vapor at the working temperature, which time ranges from 20 to 80 minutes.
In an exemplary embodiment of the invention, wherein the material constituting the 3D workpiece is ABS and the process plasticizer is acetone, the working temperature is comprised between 30 ℃ and 40 ℃, and the exposure time of the 3D workpiece to acetone vapor ranges from 20 to 60 minutes, preferably 30 to 50 minutes, while the working pressure ranges from 125 to 150 kPa.
As already mentioned, the exposure time of the 3D workpiece to the process plasticizer vapor determines the depth of vapor penetration and therefore the effect produced by the treatment. As an indication, in the above described embodiments, relatively short exposure times (approximately 20 to 30 minutes) produce penetration depths of 100 to 200 μm, whereas relatively longer exposure times (approximately 50 to 60 minutes), penetration depths may reach 2 to 3 mm. The effect is different in the two cases. In the first case, the 3D workpiece surface layer is softened and the result is a flow of the surface material, thus obtaining the desired effect of smoothing out surface irregularities (steps, porosity, streaks, etc.). In the second case, in addition to the surface smoothing effect, a relatively deep softening and material flow effect occurs, involving a layer of a certain depth below the surface in relation to the steam exposure time. In this case, the involved layers become softer and interpenetrate in a single layer, increasing the mechanical resistance (strength) of the 3D workpiece surface.
Due to this feature of the method according to the invention, the duration of exposure of the 3D workpiece to the process plasticizer vapor can be adjusted and, as a result, the desired type of surface finish can be selected: ranging from simply smoothing and polishing the surface with a short exposure time to increasing the mechanical resistance (strength) of the 3D workpiece with a relatively long processing time. In this regard, it is noteworthy that with known methods it is not possible to succeed with this degree of exposure time, since continued exposure of the 3D workpiece surface to condensed solvent can lead to unacceptable material loss.
The required amount of process plasticizer is preferably provided as a pre-packaged dose. The volume of this dose of process plasticizer necessary in the method according to the invention is adapted to the volume of the chamber in which the working cycle is carried out. For example, a dose of 60 to 80ml of acetone is sufficient for use in a process chamber as follows: the processing chamber had a volume of 27 liters and contained a 3D workpiece of ABS to be processed. Typically, the chamber will require 2ml to 10ml, preferably 2ml to 5ml, of process plasticizer per litre of volume to achieve the desired workpiece surface finish.
Once the set time of exposure to the process plasticizer vapor has elapsed, the heating is stopped and the step of circulating the air/vapor mixture between the processing chamber and the air/vapor separation unit is initiated without removing the processed workpiece from the chamber. In the separation unit, most of the process plasticizer vapor present in the air/vapor mixture is separated by condensation, while the work piece in the treatment chamber cools down, releasing the absorbed process plasticizer vapor. The remaining portion of the process plasticizer which cannot be condensed is removed by filtration by bubbling a lean (lean) air/vapor mixture through a liquid device for its absorption or neutralization depending on the chemical properties of the process plasticizer.
In an equivalent alternative embodiment of the method according to the invention, the 3D workpiece and the liquid process plasticizer present in the treatment chamber are heated rapidly to a temperature above the working temperature (in the case of ABS/acetone, to a temperature above 50 ℃, preferably not above 70 to 80 ℃ to avoid any risk of damaging the 3D workpiece to be treated), then cooled naturally to the working temperature, or the heat is taken away by a cooling device to speed up the process. Also in this case, it is important to maintain a maximum uniform temperature inside the chamber by continuous circulation of the mixture formed by air and process plasticizer vapour.
In another embodiment of the invention, the catalyst may be in a gas phase such as CO2Or N2The surface finishing treatment of the 3D workpiece is performed in an inert atmosphere, such as an atmosphere. This solution may be advantageous when it has to be absolutely certain that no combustion reaction is physically possible.
According to yet another embodiment of the method of the present invention, the surface finishing treatment of the 3D workpiece may be performed at a working pressure below atmospheric pressure. This may be necessary when the plastic material forming the 3D workpiece requires or is compatible with process plasticizers having a low vapour pressure and/or a higher boiling point at atmospheric pressure, whereby a correspondingly higher working temperature would be required. For example, in the case of 3D workpieces made of polylactic acid, it may be advantageous to use 1, 2-dichloroethane as a process plasticizer, which has a boiling point of 84 ℃ at atmospheric pressure and has a vapour pressure at an operating temperature of 30 ℃ to 40 ℃ which is too low to generate an air/2, 2-dichloroethane vapour mixture suitable for achieving softening of the workpiece surface. Instead, such operating temperatures may be used when operating at reduced pressures of about 270 mmHg.
According to the present invention, there is also provided an apparatus for surface finishing a workpiece [ "3D workpiece" ] made of plastic material obtained by 3D printing. With reference to fig. 1 and 2, the device according to the invention, generally designated 1, comprises an
The
The
As shown in fig. 2, the heating member 15 is fixed to the outside of the bottom of the
At the bottom of the
In a particularly preferred embodiment of the present invention, a process plasticizer feed system is provided that uses metered amounts of plasticizer in prepackaged dosage forms. The plasticizer feed system, shown schematically in fig. 3, is placed outside the
In fig. 7, an enlarged axial cross-sectional view of the process plasticizer feed system is shown. A seal 56 is provided at the bottom of
The feed system also includes a capsule identification (recognition) system designed to identify the capsule as new and having an authenticated source, including an
The plant also comprises a recovery system by condensation of the process plasticizer evaporated at the end of the treatment cycle. More specifically, the process plasticizer recovery system comprises: a
As shown in fig. 5, in the present embodiment of the invention, the
As shown in fig. 6, the cooling fluid circulates in a closed (closed) circuit comprising a circulation pump 31, a
In another embodiment of the invention, two additional peltier units (not shown) with associated cooling coils may be provided on the other two opposite sides of the
Two connectors 37 (only one of which is visible in figure 5) extend from the upper and lower faces of the
The process
At the end of the condensation step, there is still some process plasticizer in the
Referring also to fig. 8, the
With reference to fig. 9 to 13, a further embodiment of an apparatus for the surface finishing of an article obtained by 3D printing is described, in which the method according to the invention can be carried out. This embodiment is designed to further increase the compactness of the device. In fig. 9-12 and 1-7, like parts are indicated by like reference numerals.
With particular reference to figures 9 and 10, the device according to the invention is generally designated 1, the
The
Like the embodiment described with reference to fig. 1 to 7, the
Also, a process plasticizer feed system is provided that uses a dose of plasticizer in the form of a prepackaged capsule. In fig. 9 and 10 there is shown a
The apparatus also includes a recovery unit for evaporated process plasticizer by condensation, which begins to operate at the end of the exposure of the work pieces to the process plasticizer vapor in the
The hot side of the peltier unit is air cooled by the
At the end of the condensation step, there is still some process plasticizer in the
It is worth noting that in the apparatus of this embodiment of the invention, the
It is also worth noting that in the present embodiment of the invention, the basic function of maintaining the movement of the hot air/vapour mixture within the
Hereinafter, the apparatus operation will also be described with reference to the process plasticizer cycle schematically shown in fig. 8 and the flow chart of fig. 14.
The process basically comprises four main operating steps. These steps are as follows:
evaporation of process plasticizer: in this step (starting the treatment cycle of the 3D workpiece previously positioned in the tank 11), the heating member 15 raises the temperature of the process plasticizer in the liquid phase (which is also previously flushed into the tank 11), thus correspondingly increasing the amount of plasticizer vapor in the air/vapor mixture present in the
Once the equilibrium operating temperature is reached, which is maintained in the
Recovery of process plasticizer: once the exposure step to process plasticizer vapor is complete, the process plasticizer vapor will be circulated through the condenser unit and then converted to a liquid. In this step, due to physical and technical limitations, it is possible to convert only a certain percentage of the process plasticizer into a liquid, not 100% thereof.
Filtration: the process plasticizer, which is still present in the air/vapor mixture in the gas phase, is filtered, in particular in the
The process plasticizer circulation in the apparatus according to an embodiment of the invention shown in fig. 1 to 7 is shown in detail in fig. 8.
The device operation is discontinuous. After positioning the component to be treated in the
The process plasticizer in the
The results of the treatment can be tracked through a
At the end of the 3D workpiece treatment step, i.e. when the 3D workpiece surface reaches the desired degree of finish, a mixture of air and process plasticizer vapour and liquid plasticizer are present in the
At the cold side of each peltier unit, the operating temperature of the condenser is about-15 ℃, which is reached within about 5 minutes after the unit is started up. The plant according to fig. 9-13 operates in a similar manner, except that the
When about 70% of the plasticizer vapour is condensed, the two
The condensed process plasticizer collected in the
The operation of the above-mentioned apparatus is governed by a program executed by a microprocessor housed in the
The program managing the working cycle of the apparatus according to the invention receives pressure and temperature signals from suitable sensors placed in the tank 11 (for example, the
It will be appreciated that under the operating conditions of the method according to the invention, the formation of condensed process plasticizer on the surface of the workpiece to be treated is avoided. This is primarily due to the constant thermal equilibrium maintained between the workpiece surface and the air/vapor mixture circulating in the
In the following example, the performance of an apparatus operating according to a method of surface finishing a workpiece obtained by 3D printing is shown. Each of these tests was printed by zotex M200 (zorrax M200)3D printer using a dedicated filament Z-ABS, unless otherwise noted. Unless otherwise noted, the printing option used is a 20% mesh pattern preset in the Z-suite software. The processing according to the method of the invention is carried out with the following optimized parameters: exposure time of process plasticizer vapor: 50 minutes; working temperature: 35 ℃ is carried out.
Example 1
The relationship between penetration and mechanical and aesthetic properties of process plasticizers, in particular acetone, in polymers, which are strictly correlated with exposure time, was studied. Samples printed at maximum fill using zotex M200 (zorrax M200) (4 large-sized parallelepipeds manufactured by ABS zotex, size 80x10x4 mm) were processed at different times (30, 60 and 90 minutes). After processing in an apparatus operating according to the method of the invention, the SEM image of the workpiece is visible after capturing the low temperature fracture of the sample. In this way, it was observed that acetone penetrated up to a length equal to 1.5mm (90 minute test). Basically, the acetone penetrates deeper and deeper over time, thus forming a greater concentration gradient on the axis of penetration (meaning the content of acetone in the polymer).
The test results are shown in table 1 below.
TABLE 1
Minute (min)
Penetration (mm)
30
Negligible
60
1.1
90
1.5
SEM captured images of the samples show that the roughness of the ABS samples was significantly reduced after treatment. With reference to fig. 15 (magnification 100 times) and 16 (magnification 5000 times), front views of a portion of a parallelepiped surface and a fracture section made (obtained) by liquid nitrogen are shown, respectively, before and after treatment. The roughness appears clearly in the left image and is negligible in the right image.
Example 2
In order to test the mechanical properties of the samples processed in the apparatus operating according to the method of the invention, IZOD testing was performed on simple printed samples. The results obtained are shown in table 2. Zotex (zorrax) and mekerbot (Makerbot) are the two filaments used in this test. All samples were printed with zotex M200.
TABLE 2
Filament
Untreated (kJ/m)2)
Treated (kJ/m)2)
Improvement (%)
Zotex, 45 ° angle
7±0.6
11.7±1.1
+57
Zoltwix, 0 degree
9.8±0.3
16.2±0.1
+65
Merketable, 45 ° angle
9.6±0.8
12.2±1.4
+27
Meclobote, 0 degree
14.4±1.3
15.8±1.1
+2.5
Zotex, vertical
1.3
4.3±0.5
+230
Merketable, vertical
5.0±0.5
16.8±0.4
+236
It is important to note that the orientation of the printing affects the impact strength and, in general, the mechanical properties of the workpiece being printed. It was found by NMR analysis that merbort ABS has a higher butadiene content (50% styrene phase in zotex and only 35% styrene phase in merbort) and this explains why this treatment has less impact on the mechanical property improvement of the merbort workpiece. In fact, the plasticizer used with the treatment according to the invention interacts with the styrene phase of the ABS, while it is inert with respect to the butadiene phase.
The samples treated with the same time period were also subjected to an IZOD test for investigating the permeability of the above plasticizer. The results obtained by three tests performed on each sample are shown in table 3 below.
TABLE 3
Untreated
30minutes
50minutes
90minutes
11.8±1.2kJ/m2
16.1±0.3kJ/m2
17.5±0.8kJ/m2
20.1±2.0kJ/m2
From the above description it is clear that the device and the method according to the invention fully achieve the above objects.
In general, the described apparatus has the following advantages with respect to other apparatuses that perform surface finishing processes: neither adding nor reducing material to the workpiece being processed. The surface finishing according to the method of the invention is in all respects a rearrangement of the surface layer of the workpiece being treated. Eliminating non-uniformities and making the surface more uniform. There is no material removal relative to techniques such as sanding, barrel polishing, etc. No material was added for coating and the like.
With respect to the apparatus performing surface chemical finishing, since there is substantially no formation of condensation during the treatment of the 3D workpiece, a more precise control of the finishing process is obtained without loss of plastic material due to loss and/or condensation build-up, and since the penetration depth of the process plasticizer vapor can be controlled, an increase of the surface mechanical resistance (strength) of the 3D workpiece can be obtained.
With respect to the apparatus for surface finishing of 3D workpieces according to the known art, the apparatus of the invention has the following advantages: relatively small size and weight, and can be automated and safely used even in non-industrial environments. From the user's point of view, an important advantage is the ease of use and the almost complete lack of contact with the process plasticizer, both in liquid form during the process preparation step and in vapor form during and after the process.
The compactness of the device also stems from the use of thermoelectric condensers. The advantage of this solution over condensers such as those used in distillation processes is the type of cooling provided by the thermoelectric elements rather than by the liquid in contact with the surface where condensation occurs. This allows a more compact design also because there is no need to cool the liquid at the condensation temperature. In contrast to thermoelectric condensers, such as those used in dehumidifiers, the condenser used herein has the feature of being completely insulated from the outside, airtight for the purpose of not spreading the vapor into the environment at the condensing machine.
Another advantage of the condenser used in the device of the invention lies in the use of constituent materials, which are metallic and are both thermally and electrically conductive, allowing the peltier unit to be cooled rapidly and at the same time preventing the accumulation of electrostatic charges which could ignite flammable vapours.
Finally, it is worth noting that, although the present description refers to a workpiece made of plastic material obtained by 3D printing, the basic concept of the present invention is intended to be applicable to objects, articles and products made by different techniques, but having the same problems and being able to be treated with compatible technological plasticizers (meaning used herein) to obtain a better surface finish and an increased surface mechanical resistance (strength).
It will be appreciated that although reference has been made to a preferred process plasticizer feed system using pre-packaged doses in the form of a pierceable capsule, controlled doses of process plasticizer can be fed through a metering pump and associated plasticizer tank in a conventional manner.
These and other variants and/or modifications can be made to the method and apparatus for surface finishing a component made of plastic material obtained by 3D printing, without departing from the scope of the invention set forth in the appended claims.
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