Plastic article or earthenware shaping or treating: apparatus – Means for molding powdered metal
Reexamination Certificate
2000-02-11
2002-04-16
Jenkins, Daniel J. (Department: 1742)
Plastic article or earthenware shaping or treating: apparatus
Means for molding powdered metal
C419S052000
Reexamination Certificate
active
06371746
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method of electric sintering and a mold for use in such method, and relates more particularly to the art of electric sintering utilizing plasma discharge, pulsating current, etc.
More specifically, the invention relates to an electric sintering mold having a clamping portion capable of clamping the powder material, the clamped material being sintered by the joule heat generated within the material in response to an externally supplied pulsating current and a pressure applied to the material from a pressurizer. The invention also relates to an electric sintering mold of a type including a die defining a cavity for receiving the powder material and a punch capable of advancing into the die cavity. The invention relates also to an electric sintering method using such mold. The invention further relates to an electric sintering apparatus including a die defining a cavity for receiving the powder material, a punch capable of advancing into the die cavity, a pair of electrodes capable of sending a current to the powder material received within the die, and a power supply unit capable of supplying a pulsating current to the pair of electrodes.
Description of the Related Art
In the art of electric sintering described above, for reducing the time required for sintering the powder material, the prior art has proposed a method of sintering the powder material by utilizing joule heat generated within the material in response to a pulsating current applied to the material in cooperation with a pressure also applied to the material from a pressurizer. Referring more particularly to this method, the powder material is charged in a die and then this die holding the material therein is clamped between a pair of upper and lower punches, and the material is pressurized and at the same time the pulsating current is applied to the layer of the powder material within the die, whereby joule heat is generated within the material, which heat, in cooperation with the pressure, sinters the material. With such electric sintering method, the time required for sintering the material may be reduced advantageously, in comparison with the more conventional method of sintering material in furnace atmosphere which requires hours until completion of sintering.
The sintering mold employed for such method as above requires high electroconductivity for allowing the externally supplied current to be smoothly conducted to the material via the mold and requires also sufficient mechanical strength under high temperature condition since the mold must be able to withstand the high temperature generated in the material held within the mold and must also be able to transmit the high pressure from the pressurizer to the material held within the mold.
Then, as material suitable for forming such mold satisfying both of the requirements of high electroconductivity and high mechanical strength under high temperature condition, the prior art has proposed e.g. graphite or WC—Co which is a superhard material.
In recent years, there is an increasing demand for forming products or components by means of sintering. In particular, such components as a piston head for an automobile engine has been manufactured by sintering. In this regard, with the conventionally proposed electric sintering method described above, if the material to be sintered is highly conductive material such as aluminum, a significant electric current density is required to obtain a large amount of joule heat. Hence, unless the electric power supply unit is capable of supplying an extremely large amount of current, it will take a long time for the material to reach its sintering temperature. With a typical power supply unit, the sintering operation takes as much as half an hour to be completed. In this manner, according to the conventional art, if improvement in the turn around time is desired, this is possible only with enlargement of the system and resultant increase of system costs. That is, in quest for more efficient sintering suitable for mass-produced articles, there has been the continuing need for minimizing their processing cycle. And, this should be made possible without inviting enlargement of the system, from the view point of manufacture costs.
In addition, the conventional electric sintering mold made of graphite, WC—Co or the like has the further disadvantage that the inner surface of the mold tends to erode gradually due to physical and/or chemical reaction occurring in the powder material when placed under the high temperature and pressure condition therein.
For this reason, in order for the mold to be usable for a plurality of times while maintaining its inner dimension, that is, as high as possible dimensional accuracy of the compact to be obtained therefrom, it would be needed to apply a mold releasing agent such as boron nitride (BN) powder or spray or carbon powder to the inner surface of the mold (generally, to the inner surface of the die and also to the pressing surface of the punch) for each run prior to charging of the material therein. More particularly, after completion of each sintering operation, before starting the next run, the operator must additionally carry out the troublesome maintenance operation of checking the inner dimension and the surface condition of the mold and then reapplying new releasing agent when he/she finds the mold unusable for the next run. In this respect, there remains room for improvement.
Moreover, even with use of such releasing agent, the conventional graphite or WC—Co mold still has a rather limited service life, which is unsatisfactory from the economical point of view. Presumably, this is because the releasing agent cannot fully block the physical and/or chemical reaction of the charged powder material occurring under the high temperature and pressure condition.
Accordingly, in view of the above-described shortcomings of the prior art, a primary object of the present invention is to provide a further improved electric sintering method and apparatus which enable highly efficient electric sintering operation by minimizing the time required for sintering operation without increasing the current capacity of the power supply unit, providing good releasing of the molded product from the mold after sintering without the need of applying a releasing agent prior to charging of the power material for sintering therein, and also by providing longer service life than the conventional graphite or WC—Co type electric sintering mold.
SUMMARY OF THE INVENTION
For accomplishing the above-noted object, according to one aspect of the invention, there is provided an electric sintering mold which contains metal boride having electroconductivity.
For example, this electric sintering mold of the invention may be provided in the form of a compact containing metal boride having high electroconductivity, plus other optional component such as refractory material (e.g. oxide such as SiO
2
, Al
2
O
3
, etc; carbide such as SiC; nitride such as SIALON, Si
3
N
4
, etc.). Then, with this mold, the electric current externally supplied thereto may be converted in a very efficient manner through this mold into joule heat to be generated within the powder material held therein. Further, as this mold has a higher mold-releasing performance than the conventional graphite or WC—Co molds, the invention's mold is free from the need of applying a releasing agent to the mold prior to charging of the power material therein. Moreover, even without application of such releasing agent at all, this mold can still provide greater durability, i.e. longer service life than the conventional molds described above.
According to another aspect of the invention, there is provided an electric sintering mold comprising: a die defining a cavity capable of receiving powder material therein; and a punch capable of advancing into the cavity of the die, the powder material held within the cavity of the die being subjected to a pressure from the punch and also to an externa
Kuroishi Nobuhito
Murata Masahiro
Shiomi Yasuaki
Sugai Atsushi
Tsuboi Shigeru
Jenkins Daniel J.
Kubota Corporation
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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