WO2006060906A1

WO2006060906A1 - Apparatus and method for lubricating a wall surface of a die cavity

Info

Publication number: WO2006060906A1
Application number: PCT/CA2005/001851
Authority: WO
Inventors: Paul Caron; Patrick Lemieux
Original assignee: Imfine Inc
Current assignee: Imfine Inc
Priority date: 2004-12-06
Filing date: 2005-12-06
Publication date: 2006-06-15
Anticipated expiration: 2007-06-06

Abstract

The invention is an improvement in an apparatus for lubricating the wall surface of a die cavity of a compaction press in which a powder is to be compacted to form a three-dimensional article and from which the article will be ejected. The compaction press comprises a die with opposite top and bottom faces, provided with the die cavity spanning from the top face to the bottom face thereof. The compaction press further comprises a lower punch and an upper punch which movable within the die cavity for compacting the powder. The apparatus for lubricating the wall surface of the die cavity comprises supply means for supplying tribocharged particles of a lubricant material on the walls of the cavity prior to filling the powder, and venting means for venting excess gas and lubricant from the die cavity. The improvement comprises, as the supply means, an injector sealingly connectable to one of the top and bottom end of the die cavity for injecting the lubricant material into the die cavity; and, as the venting means, a vent passage located on an opposite end of the die cavity with respect to the injector such that the lubricant material injected into the die cavity travels in a one way direction from the injector to the vent passage.

Description

APPARATUS AND METHOD FOR LUBRICATING A WALL SURFACE OF A

DIE CAVITY

FIELD OF THE INVENTION

The present invention relates to metallic powders and, in particular, to an apparatus used for the compaction of such powders to form metallic parts. However, this invention is not limited to the powder metallurgy field and can be applied in the pharmaceutical field for instance or any other fields requiring the lubrication of a die cavity prior to shaping.

BACKGROUND OF THE INVENTION

The present invention is an improvement to US Patent No. 6,299,690 which patent includes a clear and complete description of the background art.

As mentioned in that patent, in powder metallurgy ("P/M"), metal powders are compacted in a die cavity to form a green compact which is then heat treated or sintered at relatively high temperatures to create metallic bonds between particles to form a metallic part. Referring to figures 1a) to 1d), a press commonly used in P/M comprises a die 1 provided with a die cavity 2 for receiving the metal powder 3, a movable lower punch 4 for supporting the powder 3 within the cavity and an upper punch 5 for compacting the metal powder 3 with the lower punch 4. A typical compaction cycle comprises the following steps, : a filling step (fig. 1a), a die closure step (fig 1 b), a compression step (fig.1c) and an ejection step (fig. 1d). During the compaction, friction is generated between the metal powder particles themselves and also between the metal powder particles and the die wall, thereby causing both adhesive wear on the die surfaces and sometimes lamination or breakage of the green compact after ejection from the die cavity.

US patent No. 6,299,690 discloses a method of lubricating the wall surfaces of a die cavity used in powder metallurgy. The method disclosed therein involves spraying the wall surfaces with dry tribocharged particles of a lubricant material. The apparatus used to carry out the method disclosed comprises a plug member, which has a three-dimensional shape conforming generally to that of the article to be formed. The plug member is slightly smaller than the article so that when the plug member is inserted into the die cavity, there is a small, but uniform, gap created between the outer wall surfaces of the plug member and the walls of the die cavity. The plug member is secured to a dust-proof closing plate, devised to seal the die cavity . The closing plate is provided with vent holes and the plug member has a plurality of spaced-apart tubes extending therethrough, and exiting at the bottom wall surface of the plug member. By means of an inert gas, nitrogen or dry air under pressure, the lubricant material, which consists of dry lubricant particles, is fed from a reservoir of dry lubricant through the tubes of the plug member to be sprayed into the gap. During their transport, the lubricant particles are tribocharged. Consequently, as they enter the gap they are electrostatically attracted to the walls of the die cavity and adhere thereto. Any excess gas and lubricant exits the gap through the vent holes in the closing plate. A uniform thin coating of lubricant is created on the walls of the die cavity. The green density of the article formed in the die is greater due to the fact that there is less lubricant material in the mixture of powder, and the ejection force required to remove the formed article from the die cavity is less than when dry lubricant is only mixed with the powders in a larger quantity as in methods and apparatus known prior to US

6,299,690.

One limitation however with the apparatus disclosed in US 6,299,690 is that the plug member has to be removed from the die cavity once the die wall has been coated with the lubricant material. In a continuous compaction process where the goal is always to increase the rate of articles compacted in a minute, such removal step of the plug member is time consuming and not sought-after. Also, to be able to remove the plug member, a clearance of at least the height of the plug member is required between the top of the die and the upper punch in its raised position. If the clearance between the upper punch and the die does not provide enough space to move the plug in and out between the upper punch and the die, a telescopic actuator and plug member must be developed, and those telescopic devices are less reliable, more fragile and need more frequent maintenance. The system of US Patent No. 6,299,690 has the disadvantage of slowing the press movement due to the long plug movement into the die cavity and due to the delay imposed to the upper punch movement to keep the clearance required to extract the plug member.

Consequently, there is still presently a need for an improved apparatus and method for lubricating die walls.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus and a method for lubricating die walls that satisfies the above-mentioned need.

Another object is to provide an apparatus and a method for lubricating die walls with a uniform film of dry lubricant while at the same time avoiding turbulent gas flow in the cavity, and that without requiring the utilization of a plug member.

In accordance with the present invention, these objects are achieved with an apparatus for lubricating a wall surface of a die cavity in which a powder will be compacted to form a three-dimensional article and from which the article will be ejected, the die cavity being part of a press comprising:

- a die with opposite top and bottom faces, provided with said die cavity spanning from the top face to the bottom face, the cavity having die walls defining the shape of the article and opposite top and bottom open ends;

- a lower punch; and

- an upper punch, said lower and upper punch being movable relative to and within the die cavity for compacting the powder,

the apparatus for lubricating the wall surface of the die cavity comprising:

- supply means for supplying dry tribocharged particles of a lubricant material on said walls of the cavity prior to filling the powder, said supply means including a pressurized dry gas; and - venting means for venting excess gas and lubricant from the die cavity;

wherein the improvement comprises:

- said supply means comprising an injector connectable to one of the top and bottom end of the die cavity for injecting the lubricant material into the die cavity; and

- said venting means comprising a vent passage located on an opposite end of the die cavity with respect to said injector such that the lubricant material injected into the die cavity travels in a one way direction from the injector to the vent passage.

As can be appreciated, since the tribocharged particles flow in a one way direction within the die cavity, any unwanted turbulence is avoided even for very long die cavities. The present invention also does not have the disadvantage of importantly slowing down the press movement due to a long plug movement into the die cavity and due to the delay imposed to the upper punch upward movement to clear the space above the die top so as to be able to extract the plug member. Rather than using a very long plug and venting by the top closing plate, the apparatus of the invention provides for a unidirectional injection of the tribocharged particles within the die cavity by simply injecting the triborcharged particles from one end of the cavity and venting the excess gas and lubricant particles from the opposite end. The apparatus of the present invention provides an alternative solution to the lubrification of very deep cavities with presses that do not allow sufficient space between the upper punch and the die to be able to use a very long plug member and insert it into the die.

Even if sufficient space would be available for a plug member as described in the US Patent No. 6,299,690, the apparatus of the present invention still have the advantage of providing an accelerated die wall lubrication step since it requires shorter movement and is thus more rapid.

Furthermore, and as for the apparatus disclosed in US Patent No. 6,299,690, the present invention provides the following advantages: - eliminatation or reduction as much as possible of the ratio of internal lubricant to admixed metal powder compositions;

- an environmentally safe method for making metallic parts.

- the metallic parts made with the apparatus of the present invention have an improved surface finish and green density;

- a uniform spraying of the tribostatically charged dry lubricating material onto the die cavity walls, thereby providing a reduction of the ejection forces and wear on the compaction tool.

The present invention however provides the following further advantages over the apparatus disclosed in US Patent No. 6,299,690:

- the space and time required to complete the lubrication cycle is greatly reduced; and

- it enables the formation of a uniform spraying even for very long parts or very deep die cavity without taking too much space between the upper punch and the die top.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will become apparent upon reading the detailed description and upon referring to the drawings in which :

Figure 1a to 1d are schematic views of a powder metallurgy compaction press known in the prior art, showing a typical compaction cycle.

Figure 2 is a schematic side view of an apparatus for lubricating die walls according to a first variant of a first preferred embodiment of the invention.

Figure 3: Figure 3a is a schematic side view of a second variant of the apparatus of figure 2 shown without the injection cover plate, and which variant is used for forming annular shaped article; figure 3b is a close up view of the encircled zone in figure 3a Figure 4 is a schematic side view of a third variant of the apparatus of figure 2, shown with only the top portion of the lower punch and core rod.

Figure 5: Figure 5a is a schematic side view of an apparatus for lubricating die walls according to a second preferred embodiment of the invention, shown without the injection cover plate and with only the top portion of the lower punch and core rod; figure 5b is schematic side view of the lower punch shown in figure 5a; figure 5c is a top view of the lower punch of figure 5a; figure 5d is a side view of the core rod shown in figure 5a.

Figure 6: figure 6a is a schematic side view of an apparatus for lubricating die walls according to a third preferred embodiment of the invention; figure 6b is a side view of the die shown in figure 6a.

Figure 7 is a schematic side view of an apparatus for lubricating die walls according to a fourth preferred embodiment of the invention.

Figure 8 schematic side view of an apparatus for lubricating die walls according to a fifth preferred embodiment of the invention.

Figure 9 is a graph showing the ejection shearing stress of different mixes compressed at 827 Mpa (60 tsi) with (B) and without (A) die wall lubrication applied with the embodiment shown in figures 2, 6 or 7.

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to such embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the appended claims

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description, similar features in the drawings have been given similar reference numerals and in order to lighten the figures, some elements are not referred to in some figures if they were already identified in a precedent figure. Referring to either one of figures 2 to 8, the apparatus 10 is for lubricating the wall surface 12 of a die cavity 14 in which a powder will be compacted to form a three- dimensional article and from which the article will be ejected. The die cavity 14 is part of a typical press which comprises a die 16 provided with said die cavity 14 spanning from the top face 18 to the bottom face 20 of the die 16. The cavity 14 has die walls 12 defining the shape of the article.

The press further comprises a lower punch 22 and an upper punch, such as the one shown in figure 1a to 1d). As in a typical P/M press, the lower and upper punch are movable relative to and within the die cavity for compacting the powder.

As in US Patent No. 6,299,690, the apparatus for lubricating the wall surface of the die cavity 14 comprises supply means for supplying tribocharged particles of a lubricant material on the walls 12 of the cavity 14 prior to filling the powder; and venting means for venting excess gas and lubricant from the die cavity 4. The supply means includes a pressurized gas (not illustrated).

The apparatus according to the invention is an improvement of the apparatus disclosed in US Patent No. 6,299,690 in that the supply means comprises an injector 30 sealingly connectable to one of the top and bottom end of the die cavity 14 for injecting the lubricant material into the die cavity 14; and the venting means comprises a vent passage 42 located on an opposite end of the die cavity 14 with respect to the injector 30 such that the lubricant material injected into the die cavity 14 travels in a one way direction from the injector 30 to the vent passage 42.

The injector 30 and the vent passage 42 can take various shapes as long as they are located on opposite sides of the die cavity 14 so as to cause the lubricant material to travel in a one way direction within cavity 14.

The description of the preferred embodiments of the present invention is generally the same as in US Patent No. 6,299,690, except that small gaps or vents are left between the lower punch 22 and the die cavity 14 by different ways to eliminate the excess of gas and lubricant (as in the embodiments shown in figure 2 to 6) or to inject the lubricant material ( as in the embodiments shown in figure 7 and 8). Six examples with drawings explain the five embodiment with variants of producing vents or gaps between the lower punch 22 and a die 16 while keeping the lower punch 22 engaged in the die 16 for its alignment. These five embodiments of the invention will now be described in more details.

First embodiment (Figures 2 to 4)

Figures 2 to 4 show three variants of the first preferred embodiment which uses as the vent passage 40, vertical grooves 42 at the bottom end of the wall cavity 14.

In each of these variants the vent passage 40 is located at the bottom end of the cavity 14; and the injector 30 comprises a cover plate 32 for covering the top end of the die cavity 14.

The cover plate 32 comprises an inlet 34 for receiving, via tubing 36, the tribocharged particles from a source of tribocharged particles and an outlet 38 in fluid communication with the inlet 34 and facing the die cavity 14 to inject the tribocharged particles within the die cavity 14. More preferably the cover plate 32 contains a conical gap cavity 39 between the inlet 34 and the outlet 36 to deflect spray coming from the tubing 36 along the die walls 14.

In each of these variants, the venting means, that provides the vent passage 40, comprises the lower punch 22 in a partially introduced position within the die cavity (also referred to as a lower punch level) associated with the vertical grooves 42 provided at the bottom end of the wall cavity 14. These substantially vertical grooves 42 originate above the lower punch level and end at the bottom end of the die cavity, such that the grooves define the vent passage between the lower punch and the die wall.

By partially introduced position, it is meant that top face of the lower punch extends just above the bottom face of the die, as in Figures 2, 3, 4, 5a, 6 and 7.

In the first variant of that embodiment (figure 2), the lower punch 22 is solid, that is to say not hollow, whereas the lower punch 22 of the second and third variant (figures 3 and 4) is hollowed, the latter variants being used to form an article with an annular-shaped portion, such as ring shaped parts.

In the second and third variant, the top face 24 of hollow lower punch 22 is provided with a hole 26 sized to receive in a close-fit relationship a core rod 50 used for forming the annular three-dimensional article. The core rod 50 is slidably received in the lower punch 22 for forming the ring-shaped piece. In order to improve the venting of excess gas and lubricant in these variants, the core rod 50 has a plurality of substantially vertical continuous grooves 52 located in a median section 54 thereof and the lower punch 22 has an inner wall 28 spaced from the core rod 50 so that a gap 29 is present between the inner wall 28 and the core rod 50. Therefore, an additional vent passage is obtained by positioning the median section 54 of the core rod 50 at the level of the top face 24 of the lower punch 22 such that the die cavity 14 and the gap 29 are brought in fluid communication via the vertical grooves 52 of the core rod 50.

As can be appreciated, a shoulder-shaped relief 23 defining the entry for the core rod 50 could be machined in the top part of the lower punch 22 for receiving the core rod 50 in a close-fit relationship.

Referring to figure 4, in the third variant of the first embodiment, the part has multi-levels. The same principle is applied in combination with the use of a short plug member or confining block . Spraying tubes 36 are fixed trough a die cover plate 32.

Second embodiment (Figure 5)

Referring to figure 5, the second preferred embodiment is also devised to form ring-shaped parts. In that case, the lower punch 22 has a non planar top face 24 and the venting means, providing the vent passage 40, comprises the lower punch 22 in a partially introduced position within the die cavity 14, as in the case of the first embodiment described above, such that the recesses 27 provided on the top face 24 of the punch 22 define the vent passage 40. By non planar top face 24, it is meant a top face that is provided with ridges 25 and recesses 27, or a concave or convex top face, or an irregular top face.

Also, in this embodiment the core rod 50, which has a given diameter, presents along its length a median section 54 with a generally uniform diameter smaller than said given diameter. The median section 54 divides the core rod 50 into a top part 56 and a bottom part 58, each of the top and bottom part (56, 58) having respectively a lower end 57 and an upper end 59 adjoining the median section 54 with a shoulder-shaped relief. On its side, the lower punch 22 has an inner wall 28 spaced from the core rod 50 so that a gap 29 is present between the inner wall 28 and the core rod 50. With that structure an additional vent passage is obtained by positioning the lower end 57 of the top part 56 of the core rod 50 at the level of the top face 24 of the lower punch 22 such that the die cavity 14 and the gap 29 are brought in fluid communication via the recesses 27 of the top face 24 of the lower punch 22 and the median section 54 of the core rod 50.

Third embodiment (Figure 6)

As can be appreciated, in the third embodiment, rather than providing vertical grooves on the lower part of the die wall 12, as the vent passage, like in the first and second embodiments, the vent passage 40 consist of at least one tunnel 60 through the die. More specifically, the tunnel 60 has an entry 62 in the die cavity 14 and an exit 64 in an outer wall surface of the die 16 such that the excess gas and lubricant exit the die cavity 14 via the tunnel (s) 60.

The tunnel (s) 60 is preferably obtained by providing the bottom face 20 of the die 16 with at least one groove 66 spanning from the die cavity 14 to the outer wall of the die 16 and by covering the die with a die extension 68 having at least one groove 69 on an upper face thereof complementary to the at least one groove 68 of the die 16 to form the tunnel (s) 60.

In other words, the tunnel(s) 60 may be formed by a die extension 68 (or machined plate) that is screwed under the die 16 before the final rectification to be sure for the alignment and clearance of the punch 22 in the die 16. The lubricant particles are injected through the cover plate 32, as in the first and second embodiments

Fourth embodiment (Figure 7)

The fourth embodiment is also a variant of the third embodiment in that it also comprises at least one tunnel 60 extending through a bottom part of the die 16.

However, inversely to the third embodiment, the lubricant particles are injected through the tunnel(s) 60 and the excess gas and lubricants are vented through a vent passage 40 located at the top end of the die cavity 14. The vent passage 40 is provided with a vacuum shoe 41 positioned above the die cavity 14 during the time lower punch 22 is positioned for the shooting of the lubricant spray (mix of air or nitrogen or inert gaz and tribocharged particles,.

Fifth embodiment (Figure 8)

In the fifth embodiment, the lubricant is injected from the bottom of the die 16. More specifically, the lower punch 22 divides into a head section 72 and a bottom section 74. The head section 72 has a diameter sized to snugly fit in the die cavity 14 and is provided with a plurality of substantially non ending vertical grooves 75 originating below a top end of the head section 72 and ending at a bottom end thereof, whereas the bottom section 74 has a diameter smaller such that a gap 76 is present between the die wall 12 and the bottom section 74 of the lower punch 22 when the lower punch 22 is in a fully inserted position within the die cavity 14 with the head portion 72 substantially levelled above the top face 18 of the die 16. In that case, the vent passage 42 extends from the gap 76 and through the groove 75.

In that particular case, the lubricant particles are injected from the bottom end of the die cavity 14 and the excess gas and lubricant is vented from the top end of the cavity 14 through the vent passage 42 obtained when the head section 72 of the lower punch 22 is levelled above the top face 18 of the die 16. As in the fourth embodiment, vacuum is done above the die cavity with a vacuum shoe 41 to orientate the lubricant dust to enter the gap 76 all around the lower punch 22. The vent passage 42 can also be obtained by levelling down the die 16 so as to let the lower punch 22 head section 72 exit the die 16 slightly more than when a part is ejected during the regular process. The lower punch 72 then exposes its grooves 75 and the lubricant over spray can be vented by the vacuum system 41. After the spraying step, the lower punch 22 is levelled down or the die 16 is levelled up until the regular feeding position is reached. If the punch 22 is well< centered, lubricant coverage will not be or will be very partially swiped by the return of the punch to its initial powder feeding position. During its return, the lower punch can 22 bring powders with it, if the press feed shoe take rapidly the place of the vacuum shoe 41. This operation is called suction filling. It could be more rapid than "drop filling " or " gravity filling" operation, i.e., filling operation when the lower punch is already at its lowest position.

Although preferred embodiments of the present invention have been described in detail herein and illustrated in the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments and that various changes and modifications may be effected therein without departing from the scope or spirit of the present invention. For instance, if sufficient precision in the press movement, the lower punch can simply be disengaged to vent excess lubricant by the opposite side it entered in the cavity of the die.

It is worth noting that the plug described in US Patent No. 6,299,690 is no longer needed or if it is still needed to avoid turbulence like in the third variant of the first embodiment (figure 4), it does not need to go close to the lower punch surface as when lubricant and gas excess is vented by the top of the die by holes in the closing plate, like in US Patent No. 6,299,690. Vacuum hoses or dust collector intakes are placed under the die in the first to third preferred embodiments, close to the die cavity, to collect the excess of lubricant exiting the die cavity by the bottom. In the fourth and fifth embodiments, a vacuum shoe above the cavity collect the excess lubricant sprayed from the bottom of the cavity. EXAMPLE:

Cylinders of a diameter of 1 cm and a length of 3,7 cm were pressed with a die like in the first variant of the first embodiment (like in figure 2). A uniform lubricant layer of a thickness of 25-100 μm was obtained. The powder filling height was 2.1 times the thickness of the part formed like in regular powder metallurgy of iron or steel powders i.e 8 cm. The die cavity sprayed was 1 cm in diameter and 9 cm height. If the technique used would have been the same as US Patent No. 6,299,690 , the plug member would have needed to be 8 cm long. The coating head above the die cavity to move the plug member in and out of the cavity would have needed to be 8 cm height plus the thickness of the cover plate, the sailing and fixtures to move the plug member in and out of the cavity. Additional space would also have been needed for pneumatic or electric actuators or cylinders to move the plug member in and out. Pneumatic cylinder would have needed to be as long as the plug member displacement, i.e. 8 cm. This space is needed in addition of the plug member. The total height of the coating head would thus have needed to be approximately 20 cm. Hopefully, pneumatic cylinder of approximately 10 cm can be positioned to clear the upper punch (each side) height or thickness.

With the technique of the present invention shown figure 2, the coating head height or thickness was 1 cm for the cover plate plus the pneumatic actuators height. As the displacement of the cover plate needed to be minimal, i.e. only to slightly compress a foam above the cavity to be gas sealed tight, pneumatic actuators installed were 2.5 cm height. The total coating head height was 3.5 cm height. Same result i.e. a white uniform coating of dry powder lubricant of a thickness 25-100 μm was also obtained i.e. like in the third embodiment, with the lower part of the die as in figure 6. Same results were obtained like in the figure 7, i.e. by positioning the tubes in the hole of the figure 6. No cover plate was used when the configuration of the figure 7 was tried. Excess lubricant and gas was delivered freely above the cavity. In all cases, ejection forces were lower than when using a reference mix, i.e. a powder mix including a large amount of lubricant but without spraying lubricant on the die wall. Figure 9 is a graph of ejection shearing stress or the force, needed to eject the powder compacted part from the die applied on the lower punch , divided by the surface of the powder compacted part in contact with the die wall during ejection. The X axis represents the time needed to eject the part form the die at a constant speed. It can also be the position of the part in the die during ejection. Mixes used contained 0.2% by weight of graphite. The base iron powder was a water atomised powder grade commercially available. It was the ATOMET 1001 grade from Quebec Metal Powders Ltee. Mixes contained different amount of lubricant mixed with the powder. A reference mix containing 0.75 % by weight of Ethylene bi stearamide (Acrawax C Atomized from Lonza), and a mix specifically developed to maximize density with the use of die wall lubrication, containing 0.4% of a newly developed Quebec Metal Powder proprietary lubricant. All parts were pressed with the same compacting pressure. The compacting pressure is the force applied on the upper punch needed to press the powders divided by the surface of the part, or the die aperture surface. The compacting pressure was 60 tsi or 827 Mpa.

On figure 9, curve A is for the pressing of the reference mix (0.75% EBS) without die wall lubrication. Part density was 7.22 g/cc. As seen maximum ejection shearing stress is ~ 1.3 tsi. Curve B is for the pressing of the mix containing only 0.4% of internal lubricant with die wall lubrication illustrated in the figure 1 , 5 or 6. Parts obtained had a density of 7.41 g/cc. Maximum ejection shearing stress was only -0.9 tsi.

Claims

WHAT IS CLAIMED IS:

1. In an apparatus for lubricating a wall surface of a die cavity in which a powder will be compacted to form a three-dimensional article and from which the article will be ejected, the die cavity being part of a press comprising:

- a lower punch; and

- an upper punch, said lower and upper punch being relative and movable within the die cavity for compacting the powder,

the apparatus for lubricating the wall surface of the die cavity comprising:

- supply means for supplying dry tribocharged particles of a lubricant material on said walls of the cavity prior to filling the powder, said supply means including a pressurized gas; and

- venting means for venting excess gas and lubricant from the die cavity;

wherein the improvement comprises:

- said supply means comprising an injector sealingly connectable to one of the top and bottom end of the die cavity for injecting the lubricant material into the die cavity; and

2. The improvement as claimed in claim 1 , wherein :

- the vent passage is located at the bottom end of the cavity; and

- the injector comprises a cover plate for covering the top end of the die cavity, the cover plate comprising:

- an inlet for receiving the tribocharged particles from a source of tribocharged particles; and

- an outlet in fluid communication with said inlet and facing the die cavity to inject the tribocharged particles within the die cavity.

3. The improvement as claimed in claim 2, wherein the venting means comprises:

- the lower punch in a partially introduced position within the die cavity, the lower punch when in said partially introduced position having a top face extending just above the bottom face of the die, hereafter referred to as a lower punch level; and

- a plurality of substantially vertical grooves in a bottom portion of the die wall, said grooves originating above said lower punch level and ending at the bottom end of the die cavity, such that the grooves define said vent passage between the lower punch and the die wall.

4. The improvement as claimed in claim 3, wherein the lower punch is hollowed and the top face thereof is provided with a hole sized to receive in a close-fit relationship a core rod used for forming an annular three-dimensional article, the core rod having a plurality of substantially vertical continuous grooves located in a median section thereof and the lower punch having an inner wall spaced from the core rod so that a gap is present between said inner wall and the core rod; and wherein

- an additional vent passage is obtained by positioning the median section of the core rod at the level of the top face of the lower punch such that the die cavity and the gap are brought in fluid communication via the vertical grooves of the core rod.

5. The improvement as claimed in claim 2, the lower punch has a non planar top face and the venting means comprises:

- the lower punch in a partially introduced position within the die cavity, the lower punch when in said partially introduced position having said top face extending just above the bottom face of the die such that the recessed portion of the non planar top face define said vent passages between the lower punch and the die wall.

6. The improvement as claimed in claim 5, wherein the lower punch is hollowed and the top face thereof is provided with a hole sized to receive in a close-fit relationship a core rod of a given diameter used for forming an annular three- dimensional article, the core rod having along its length a median section with a generally uniform diameter smaller than said given diameter, said median section dividing the core rod into a top part and a bottom part, each of said top and bottom part having respectively a lower end and an upper end adjoining the median section with a shoulder-shaped relief and the lower punch has an inner wall spaced from the core rod so that a gap is present between said inner wall and the core rod; and wherein

- an additional vent passage is obtained by positioning said lower end of the top part of the core rod at the level of the top face of the lower punch such that the die cavity and the gap are brought in fluid communication via the recesses of the top face of the lower punch and the median section of the core rod.

7. The improvement as claimed in claim 2, wherein the vent passage comprises at least one tunnel through the die, said at least one tunnel having an entry in the die cavity and an exit in an outer wall surface of the die such that the excess gas and lubricant exit the die cavity via said at least one tunnel.

8. The improvement as claimed in claim 7, wherein the bottom face of the die is provided with at least one groove spanning from the die cavity to the outer wall of the die and the apparatus comprises a bottom die extension secured to the bottom face of the die, the die extension have at least one groove on an upper face thereof complementary to said at least one grove of the die to form said at least one tunnel.

9. The improvement as claimed in claim 1 , wherein :

- the vent passage is located at the top end of the cavity; and

- the injector comprises at least one tunnel through the die, said at least one tunnel having an entry in an outer wall of the die and an exit in a bottom part of the die cavity such that the tribocharged lubricant material is injected in the die cavity through the injector and excess gas and lubricant exit the die cavity via the vent passage at the top end of the die cavity.

10. The improvement as claimed in claim 1 , wherein :

- the lower punch has:

- a head section with a diameter sized to snugly fit in the die cavity, said head section being provided with a plurality of substantially vertical grooves originating below a top end of the head section and ending at a bottom end of the end section; and

- a bottom section with a diameter smaller than said diameter of the head section such that a gap is present between the die wall and the bottom section of the lower punch when the lower punch is in a fully inserted position within the die cavity with the head portion substantially levelled above the top face of the die such that a vent passage from the gap through the grooves is created, and wherein:

- the injector is located at the bottom end of the die cavity.

11. A method for lubricating a wall surface of a die cavity in which a powder will be compacted to form a three-dimensional article and from which the article will be ejected, the die cavity being part of a press comprising:

- a lower punch; and

- an upper punch, said lower and upper punch being movable within the die cavity for compacting the powder,

the method comprising the steps of:

a) supplying dry tribocharged particles of a lubricant material on said walls of the cavity prior to filling the powder; and

b) venting excess gas and lubricant from the die cavity;

characterized in that:

- in step a) the lubricant material is supplied to one of the top and bottom end of the die cavity; and

- in step b), the excess gas and lubricant is vented from an end of the die cavity opposite said one of the top and bottom end such that the lubricant material supplied to the die cavity travels in a one way direction.