Coating apparatus – With means to apply electrical and/or radiant energy to work... – Electrostatic and/or electromagnetic attraction or...
Reexamination Certificate
1999-11-18
2001-10-09
Crispino, Richard (Department: 1734)
Coating apparatus
With means to apply electrical and/or radiant energy to work...
Electrostatic and/or electromagnetic attraction or...
C118S620000, C118S621000, C425S100000, C072S045000
Reexamination Certificate
active
06299690
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to metallic powders and, in particular, to the compaction of such powders to form metallic parts using powder metallurgy. 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.
2. Brief Description of the Background Art
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. During compaction, friction is generated between the metal powder particles themselves and also between the metal powder particles and the die wall, causing both adhesive wear on the die surfaces and lamination or breakage of the green compact after ejection from the die cavity. In order to decrease the friction between the metal powder particles and the die walls and to decrease the ejection force required to eject the green compact from the die cavity, dry lubricants have been historically added to the metal powder mixture. These are generally referred to as internal lubricants since they are admixed with the metal powder to be compacted.
It is well known that wet lubricants promote clumping of the metal powder and adversely affect the flow characteristics of P/M materials, and then they cannot be used successfully. On the other hand, dry lubricants have been used successfully since they are non-binding and do not affect flow characteristics. Due to the pressures and temperatures involved during compaction, dry lubricants typically melt and flow between the metal powder particles and lubricate the die walls. However, one disadvantage of using a dry lubricant in the metal powder formulation is that both the final density and the strength of the metallic part are less than theoretically achievable when no lubricant is admixed. In fact, the density of common lubricants used is usually lower than the density of the metal powders used.
Prior attempts at eliminating the addition of internal lubricants in the metal powder composition focused on spraying onto the die walls liquid lubricants, or dry lubricants that were dispersed in solvents. However, the poor distribution of liquid applied to the die walls limited the size and the shape of the green compact. Moreover, the use of dispersed dry lubricants poses numerous health, safety and environmental hazards due to the presence of volatile solvents.
Up to now, only a few systems have been developed to apply dry lubricants to die cavity walls. One system described in the prior art uses a tribogun to spray, directly from the outside of the die cavity, an electrostatically charged lubricant into the die cavity. Although this technique is simple, it can only be used for small dies and does not achieve uniform distribution of the lubricant in the die cavity. In another device, such as the one described in U.S. Pat. No. 4,840,052, a fluid mixture consisting of a lubricant and compressed air is used to lubricate the surfaces of die punches of a forging press before the part is made. In this case the lubricant coating applied with this device is localized and non-uniform. Another example of a device used to apply lubricant is the one described in U.S. Pat. No. 5,642,637 which is dedicated to die forging. In this case, the forging cavity was not coated and the lubrication, as in U.S. Pat. No. 4,840,052, was limited to the punch surfaces. Moreover, in this patent, the surfaces to be lubricated are not located in a die cavity.
The objective of the present invention is to overcome drawbacks and disadvantages of the prior art, and to provide an improved method of applying dry lubricant to die cavity walls in order to improve the manufacture of metallic parts by powder metallurgy. The apparatus of the present invention was developed to apply a constant, thin and uniform dry lubricant to the die cavity walls to make improved quality powder metallurgy parts.
SUMMARY OF THE INVENTION
The present invention describes a method for making a metallic part that eliminates or reduces as much as possible the ratio of internal lubricant to admixed metal powder compositions. The present invention is also intended to provide an environmentally safe method for making metallic parts. A further objective of the present invention is to provide a method for making a metallic part having an improved surface finish and green density. Yet another object of the present invention is to provide an apparatus capable of uniformly spraying a tribostatically charged dry lubricating material onto the die cavity walls to reduce ejection forces and wear on the compaction tool.
These objects and others are provided by a novel apparatus that can be used in the manufacture of a metallic part by powder metallurgy wherein the metal powder composition is compacted in a die cavity whose wall surfaces have been lubricated following a new method of tribocharging sprayed lubricants in dry form prior to compaction. The use of this apparatus and the new method allow a reduction or elimination of the amount of internal lubricant added to the mix, resulting in a metallic part having greater density, and a better surface finish. In addition, the method of this invention is environmentally safe since dry lubricants may be employed without being dispersed in volatile solvents.
The present invention utilizes a unit for measuring a precise quantity of dry lubricant, a flow path including tribocharging means for creating an electrically charged lubricating material, and a unit to move a part-shaped confining block or plug which is adapted for spraying the lubricant into the die cavity. The confining block or plug generally reproduces the shape of the part to be made but has slightly smaller dimensions compared to the part to be made, so that when the plug is positioned within the die cavity there is a narrow gap defined between the outer surface of the plug and the inner surface of the die cavity as defined by the walls thereof. Vent holes located in a closing plate to which the plug is fixed assure a preferential path for lubricant flow and avoid any gas turbulence in the die cavity during the coating process. In addition, but only if necessary, small metallic electrodes, metallic tape fixed on the plug, or metallic plating on the surface of the plug, can be used to repel the charged lubricating material from the plug towards the grounded die cavity as disclosed in U.S. Pat. No. 5,682,591, thereby enhancing the attraction between the lubricant and the die cavity walls.
More specifically, the present invention provides a method of lubricating a wall surface of a die cavity in which a powder will be compacted to form a three-dimensional article and from which a complete compacted article will be ejected, comprising the steps of
providing a plug member secured to a closing plate and having a three-dimensional shape generally conforming to that of the article, the plug member having a plurality of tubes extending therethrough to exit at one or more outer wall surfaces of the plug member, the tubes being spaced apart adjacent the periphery of the plug member;
providing a source of lubricant;
inserting the plug member into the die cavity, with the plug member defining a narrow gap between the outer wall surfaces thereof and adjacent walls of the cavity;
feeding lubricant using a pressurized inert gas from the source through tribocharging means to the tubes of the plug member to exit into the gap whereby the lubricant is attracted to the walls of the cavity;
permitting excess gas and lubricant to exit the gap via venting means in the closing plate to assure a preferred path of lubricant flow and to avoid gas turbulence in the die cavity; and
withdrawing the plug member from the die cavity, leaving a coating of lubricant on the walls of the die cavity.
In the method defined above, the die cavity and the metal powder composition
Mongeon Paul-Emile
Pelletier Sylvain
Ziani Abdelouahab
Crispino Richard
National Research Council of Canada
Rock H. Wayne
T. Yewebdar T.
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