Method and device for producing powdery substances from plastic

Abstract

The invention relates to a method for producing powdery plastic particles having as spherical a structure as possible, wherein a starting product (30) made of plastic, in particular a viscous to solid starting product (30), is brought into contact with a smooth surface (24) of a body (20), which is moved at a speed v of at least 5 m/s relative to the starting product (30). In the contact area (34) between the starting product (30) and the body (20), the starting product (30) is heated locally, and is flung in powdery form out of the contact area in the direction of movement of the body (20).

Claims

1. A method comprising: producing powdery plastic particles having a substantially spherical structure, including: contacting a stationary, plastic starting product in non-powdery form with a surface of a body; and moving the body with a speed of at least 5 m/s relative to the starting product, thereby locally heating the starting product in a contact area between the starting product and the body, and transporting resulting powdery plastic particles out of the contact area substantially in a direction of movement of the body.

2. The method according to claim 1, wherein the body defines a cylinder rotatable about an axis thereof, and the contacting step includes contacting the starting product with a cylinder barrel of the cylinder or a circular cylindrical area of the cylinder.

3. The method according to claim 1, wherein the surface of the body defines discontinuities defining one or more of projections or recesses.

4. The method according to claim 3, wherein the discontinuities, in the direction of movement of the body, are shorter than 10% of a distance between two adjacent discontinuities.

5. The method according to claim 4, wherein the discontinuities in the direction of the movement of the body are shorter than 5% of said distance between two adjacent discontinuities.

6. The method according to claim 1, wherein the speed is at least 10 m/s.

7. The method according to claim 1, wherein the contacting step includes pressing the starting product against the body with a force of least 1 N.

8. The method according to claim 1, wherein an average grain size of the powdery plastic particles is larger than a maximum roughness of the surface of the body.

9. The method according to claim 8, wherein the average grain size of the powdery plastic particles is at least ten times larger than the maximum roughness of the surface of the body.

10. The method according to claim 1, wherein the surface of the body is curved and the transporting step defines a wedge-shaped exit area for receiving transported powdery plastic particles extending from the contact area.

11. The method according to claim 1, further comprising collecting the plastic particles with a collecting vessel located behind the body in the direction of movement of the body.

12. The method according to claim 1, comprising selecting one or more of a material of the body or the speed such that substantially no amount of the starting product adheres to the surface during the producing step.

13. The method according to claim 1, further comprising movably guiding the starting product within a guide tube surrounding the starting product, and supporting the starting product to the surface of the body from an end of the guide tube proximate to the surface of the body without the guide tube contacting said surface.

14. The method according to claim 1, wherein the contacting step includes delivering the starting product into contact with the body with an extruder defining an exit area thereof configured for discharging the starting product therefrom located close to the body.

15. The method according to claim 1, wherein the starting product defines a viscous to solid starting product.

16. A device comprising: a body having a surface, wherein the body is movable with a speed v of at least 5 m/s; a viscous to solid plastic starting product in non-powdery form in contact or contactable with a contact area of the surface; wherein the device is configured to produce powdery plastic particles having a substantially spherical structure, by performing the following steps: contacting the starting product with the surface of the body; and moving the body with the speed of at least 5 m/s relative to the starting product, thereby locally heating the starting product in the contact area between the starting product and the body, and transporting resulting powdery plastic particles out of the contact area substantially in a direction the movement of the body; and a collecting vessel located sufficiently close to the contact area to substantially collect the powdery plastic particles.

17. The device according to claim 16, further comprising a guide tube configured to movably guide the starting product therein and defining a free end sufficiently proximate to the contact area of the surface to support the starting product from the end of the guide tube to the surface without the guide tube contacting said surface.

18. The device according to claim 16, further comprising an extruder defining an exit area thereof located sufficiently close to the contact area to deliver plastic material forming the starting product therefrom into contact with the surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments, which are not to be interpreted as limiting, are described in greater detail in the following description, with reference to the Figures, which are understood not to be limiting.

(2) FIG. 1 schematically shows a first embodiment of a device designed for producing powder,

(3) FIG. 2 schematically shows a second embodiment,

(4) FIG. 3 schematically shows a third embodiment, and

(5) FIG. 4 schematically shows a fourth embodiment, which is similar to the third variant but has a directly attached extruder.

DETAILED DESCRIPTION

(6) In the following section, the embodiment according to FIG. 1 will be described in detail.

(7) The other embodiments will be described in ways they differ from the first embodiment.

(8) FIG. 1 shows a body 20, embodied here as a cylinder. It rotates about an axis 22, which in this case is the cylinder's axis. The rotary drive for the body 20 (not shown) is supplied in accordance with the related art, for example at about 30,000 rpm. The radius of the cylindrical body is in the order of about 20 mm. Consequently, a surface of the body 20, in this case the cylinder barrel (surface), moves at a speed of about 63 m/s.

(9) This surface 24 is at least substantially smooth except for a plurality of discontinuities 26. These discontinuities are realized as notches or grooves extending parallel to the axis 22. They have a depth of 0.5 mm for example and extend over the entire axial length of the cylinder. They are distributed substantially evenly about the circumference, for example 4-8 such discontinuities 26 are provided on the cylinder barrel shown. They have a width of about 0.5 mm.

(10) The body 20 moves in the direction of the arrow 28. This indicates the direction of movement of the body 20. Undisturbed regions of the surface 24 of body 20 are located between two adjacent discontinuities 26 in the direction of movement. The length thereof in the direction of movement is considerably greater than the width of a discontinuity 26 measured in the direction of movement, in this case about forty times greater.

(11) A rod made from a solid starting product 30 is in contact with the surface 24. It is pressed against this surface 24 with a f force, see arrow 32, and propelled towards it. The arrow 32 also schematically represents a feed device. This device ensures a constant supply of fresh material of the starting product 30, so that the illustrated contact between the starting product 30 and the surface 24 is substantially consistently maintained.

(12) This contact takes place in a contact area 34, this size of which is substantially determined by the cross-section of the starting product 30 and is typically smaller than said cross-section. In the variant shown, the starting product 30 is a round rod. But it may also have a different shape that is suitable, for example, it may be embodied as a flat profile with a rectangular cross-section. In such a case, the long side of the rectangle extends parallel to the axis 22.

(13) Considerable frictional heat is produced in the contact area 34. This results in melting of a very small, localized portion of the material of the starting product 30 close to the contact area 34. In this process, material is constantly separated, that is to say torn away from its bond with the rest of the starting product 30, and shaped. Surprisingly in view of the prior art, it was found that spherical particles are formed. These are accelerated very rapidly and exit the contact area 34 as a jet 36. They travel as far as a collecting vessel 38. As the figure shows, the plastic particles 40 leave the contact area 34 substantially tangentially and substantially perpendicularly to the starting product. They leave the contact area 34 in a wedge formation. The starting product 30 may be aligned with the surface 24 in such manner that the force vector (see arrow 32) passes through the axis 22. Drive motors that can be used are known from the related art.

(14) In the variant according to FIG. 2, the body 20 is formed by a band which passes and runs around two rollers 42. Again, the arrow 28 indicates the direction of movement. At least one of the rollers 42 is driven. The starting product 30 may be pressed into the area of the band-like body 20 that is not supported (e.g., directly over a roller 42), as represented by the dashed lines, but it may also contact the band on the outside of a roller 42, see solid lines.

(15) In the variant according to FIG. 3, an orbital disc is used as body 20. Here too, the direction of movement is indicated by the arrow 28. The starting product 30 is located inside a guide tube 44. The tube is stationary. The starting product 30 is supported movably inside the guide tube 44. The guide tube 44 has a free end 46 which is located in close proximity to the surface 24. Consequently, the starting product 30 is not guided, e.g., laterally unsupported, only for the shortest possible distance between the free end 46 and the surface 24. It sustains the reaction forces itself in this small area. Otherwise, the forces are absorbed (at least on part) by the guide tube 44. The design according to FIG. 3 is therefore suitable, in at least some embodiments, for less solid starting product 30.

(16) Finally, FIG. 4 shows the interaction between the apparatus and an extruder 48. The extruder delivers warm plastic material as starting product 30, which is guided through a guide tube 44 which is in contact with a rotating disc, which forms the body 20 as in the variant according to FIG. 3. In this variant, the guide tube 44 may be eliminated if the starting product 30 is sufficiently stable.

(17) The body 20 is made from metal, for example, e.g., from stainless steel. It may also be manufactured from a ceramic or other suitable material.

(18) In a method for producing powdery plastic particles, a starting product 30 of plastic, for example, a viscous to solid starting product 30, is brought into contact with a smooth surface 24 of a body 20 which is moved at a speed v of at least 5 m/s relative to the starting product 30. In the contact area 34 between starting product 30 and body 20, the starting product 30 is heated locally and is flung in powdery form out of the contact area 34 in the direction of movement (substantially) of the body 20.

(19) The higher the heat deflection temperature (HDT) is, see DIN EN ISO 75-1 to 3, the better the sprayability of the powdery form. The heat deflection temperature may be above 100° C. This makes it possible to spray as well without cooling. The effect of this is that threads do not form and the undesirable coarse material remains in a reasonable proportion to the desirable quantity of fine material, that is to say, the powdery plastics obtained.

(20) The lower the heat deflection temperature is, the smaller the contact area may be, that is to say the “heating area,” relative to the surface of the body, also called the “transport area.”

(21) The lower the heat deflection temperature is, the slower the speed v can be used. In this way, it is possible to prevent too large a quantity from melting, which cannot be transported away.

(22) The contact area in at least some embodiments is not be coated or covered with the plastic while the method is proceeding, but rather remains largely free thereof. Coating of the contact area of the roller for example is not to be considered a problem in and of itself. But if the charge becomes excessive, threads may form.

(23) With a soft starting product and otherwise identical parameters, the size of the projections and/or recesses—the latter also being called depressions—has a greater effect on the grain size distribution than with a harder starting product. The wider the depressions are counter to the direction of rotation of the roller, the coarser the powder is.

(24) The following is true for a soft (relatively) starting product: The smaller the part of the smooth surface of the body is, that is to say, the part without projections and/or recesses, the less the body—the roller for example—becomes coated.

(25) The ratio between smooth surface part and depressions may be selected such that the starting product which is melted by contact with the smooth surface part is transported away by the depressions and can be separated subsequently.

(26) The higher the heat deflection temperature is, the more spherical the plastic particles are. This temperature may be above 110, for example, above 125, above 150 in at least some embodiments, and even above 175° C. in at least some embodiments.

(27) Even fibrous powder can be flowable by avoiding sharp edges and corners. The size of the parts is less important. This was demonstrated experimentally with TPU (thermoplastic polyurethane), which could not be screened at 125 μm, and hardly at all at 500 μm. Accordingly, particle sizes from >500 μm up to 20,000 μm (coarse material) were present. A flow behavior was revealed that exhibits only minimal tendency to break off even without additives.

(28) Under otherwise identical conditions, as the starting product cools progressively, that is to say, as the temperature of the starting product falls, the grain distribution becomes coarser. This is counteracted by means of higher speed v, for example, the rotating speed of the roller 20. When the speed v was increased from 60 to 160 m/s, in a test a powder was obtained with a 3+ finer grain distribution, e.g., which is three times as fine. With increases in the range from 50 up to 250 m/s for soft materials (heat deflection temperature <100° C.) with N2 cooling, the ratio appears to be almost linear.

(29) The starting product may be cooled, for example, to a temperature below minus 50° C., or less than minus 100° C. in at least some embodiments, e.g., approximately to the temperature of liquid nitrogen. In at least some embodiments, it is not the body 20, for example the roller, that is cooled, but the starting product. In this way, the determining process of the method, that of the formation of the thinnest possible melt layer is achieved before it is transported away by the depressions.

(30) A purely mechanical removal, such as occurs for example when metal is filed with a file or a metal workpiece is ground with a grinding disc, is less desirable. The differentiation from mechanical removal is defined in that for a very short period (<1 sec) the material is fused and/or melted.

(31) The roller 20 may be charged with multiple starting products spread around the circumference thereof, e.g., contacted by three starting products each offset by an angle of 120 degrees. This is possible because most products are separated quickly from a roller that constitutes the body and guided by airflows.

(32) The starting product can be moved against the body, such as with the roller with pressure or a certain force. In this context, the rate can be measured such that the starting product does not melt over too large an area, which results in coarse material, but at the same time it is hard enough to support the feeding effect of the roller. The force may be in the range from 1000 N to not more than 100,000 N, depending on the material.

(33) The more dimensionally stable the starting product is at elevated temperatures, the greater the pressure or force may be. Then, an effect is initiated which looks like “smoke.” This typically consists of particles in the range below 30 μm. Softer materials tend more often to be “entrained” by the roller 20.

(34) Starting product which is dimensionally stable at elevated temperatures, e.g., with a heat deflection temperature >100° C., may, in at least some embodiments, be placed against the middle of the roller. Material with a heat deflection temperature below 100° C. may be applied to the roller in the case of warm spraying (i.e., without cooling), or with cold spraying, may be treated similarly to material with a heat deflection temperature >100° C. A warm spraying process is understood to mean spraying without cooling.

(35) Spraying with velocity v below 50 m/s is possible, but not as economically advantageous, because the contact area for melting would have to be large, and consequently, too little feed power would be available.

(36) The depressions may be in the shape of a spherical segment. They are also called dimples. Depressions of such kind are also found on golf balls, for example. A roller with dimples enables finer atomization than other geometric configurations of the depressions due to the constantly changing centrifugal forces within the circle of the dimples.

(37) While the method is being carried out, the roller 20 takes on an end temperature specific to the material, but, however, this should always be below the melting point of the starting product. It is usually not necessary to provide external cooling of the roller.

(38) With starting products that are dimensionally stable at elevated temperatures (>100° C.), the fineness of the X10, X50, X90 fraction only varies slightly, in the range from 10-20%, as the circumferential speed v of the roller increases. The larger the value v becomes, the finer the fractions become. The quantity of powder/h increases with speed v if the pressure is increased at the same time. However, the pressure does not increase linearly, but rather in the range from ½-⅓.

(39) Both cooled starting product and starting products which have a heat deflection temperature of >100° C., yield a grain distribution in the range from X10:25 X50:60 X90:95 X99:115 with individually economical settings in terms of pressure, circumferential speed and cooling in each case, the powder having been screened at 125 μm.

(40) The higher the heat deflection temperature is above 100° C., the less cooling is necessary for the starting product. A low heat deflection temperature of the starting product may be replaced to a certain degree by cooling to lower temperatures. In general, it is advantageous if the starting product has low thermal conductivity.

(41) It is advantageous if the starting product is porous, for example has small air inclusions or exists as foam. Porosity, etc., encourages the spraying process because the air inclusions have the effect of reducing the effective area in the contact area, and therewith the area that is melting at the same time, thus providing the conditions for generating smaller plastic particles. At the same time, the pressure or force exerted by the starting product on the roller can be increased as a consequence of its lower friction.

(42) Starting product that has a softening temperature higher than 100° C. and is generally suitable for warm spraying, that is to say, without additional cooling, becomes coarser and more fibrous if liquid gas is used for cooling. In this case, the cooling prevents the formation of the a melt film, and the material is rather removed by abrasion instead. Although it may be discerned that it was briefly molten, this phase was too short to lead to the formation of economically useful quantities of spherical particles in a range below 125 μm.

(43) Porous starting product does not reduce the bulk density pSch of the end product during spraying. (Porous PEKK yielded a bulk density of 265/Solid PEKK yielded a bulk density of 270 with similar grain size distribution). See Table 1:

(44) Maximum Circumferential Speed v in m/s

(45) TABLE-US-00001 Heat deflection cooled with N2 temperature in ° C. warm to below O° C. to −150° C. >100 200+ 200+ (then coarser and more fibrous than warm) <100 50-100 100-200+ (coarser and more fibrous than cold)

(46) The method not only enables the production of powders from starting products which are very difficult or impossible to grind. It also yields powder with a desirable spherical shape as close to spherical as possible. It has the effect of increasing flowability as well as bulk density and tamped density (see nid=106849980X).

(47) The method is also suitable for filled starting products which also lend themselves very poorly to grinding because of their filling, or in which the filling would be destroyed. These include for example fibers such as GF (glass), CF (carbon), but may also be iron components, magnetite or the like. This enables plastic parts made of reinforced material or also conductive materials, e.g., sintered (SLL, SLA), to be processed as well.

(48) The method has previously been performed successfully with the following starting products: PP, HDPE, POM, TPU, PEEK, PEKK, PEI, PPS. Previously sprayed powders with filling: PPS+glass fiber, PEKK+carbon fiber, PEKK+magnetite.

(49) When sprayed, TPU—which is known to tend to clump after conventional grinding and is first allowed to rest for up to 48 hours before satisfactory pouring behavior is obtained—does not exhibit this complication when the methodsherein are used, and it is immediately flowable and capable of being processed. This is without any extra additives. Thus, if any additives are desirable subsequently, the quantity of the additive can be reduced, resulting in improved melting behavior and better properties in the end product. This represents a significant advantage for SLL, SLA as well as for slushes.

(50) For products with a low heat deflection temperature (below 100° C.), additional cooling can be carried out using liquid gas, CO2 or N2.

(51) For products with a high heat deflection temperature (above 100, above 150° C., etc.) a high energy input may be needed in order to melt enough starting product (also called material) during the short contact time. It may therefore be desirable to attempt to minimize additional cooling.

(52) In the method, the starting product may be only melted very briefly and only at temperatures close to the (lower) melting point. Consequently, the chemical property of the material is only minimally inhibited. This was demonstrated for PEKK by means of DSC. Hardly any polymer degradation occurs. By the nature of the method, the powder is amorphous. If crystallinity is desired, it must be adjusted subsequently, as those of ordinary skill in the art should understand.

(53) In order to obtain more spherical plastic particles and so increase the bulk weight, the powder produced can be melted in freefall in a gravity chamber immediately following the method and using its heat and by introducing additional heat. In this process, the outer shell of the plastic particles is melted and improved spherical structures form. The gravity chamber may be flared conically to prevent sticking to its edge. In addition, the temperature to which the air in this chamber is heated may be at a temperature 25% higher than the actual melting temperature of the starting product, so that the chamber does not have to be extended to gain the necessary exposure time. By sufficient air routing, the plastic particles have enough space to melt singly and not stick together, unless this effect were intended in a deliberate attempt to reduce the fine content <5 μm by agglomeration. After the selected fall distance, depending on the thermal capacity of the starting product it is advisable to cool the powdery material obtained promptly with liquid gas N2 or CO2 so that the material can be screened.

(54) While the above describes certain embodiments, those skilled in the art should understand that the foregoing description is not intended to limit the spirit or scope of the present disclosure. It should also be understood that the embodiments of the present disclosure described herein are merely exemplary and that a person skilled in the art may make any variations and modification without departing from the spirit and scope of the disclosure. All such variations and modifications, including those discussed above, are intended to be included within the scope of the disclosure.