EP0392405B1 - Process for dewaxing and improving the properties of injection molded metallic articles - Google Patents

Abstract

Process for dewaxing injection-moulded metal parts consisting of a metal/binder mixture, characterised in that a metal oxide is added to the metal/binder mixture.

Description

The present invention relates to a method for dewaxing injection-molded metal parts from a metal / binder mixture (metal injection molding).

The use of Metal Injection Molding (MIM) allows the production of intricately shaped, smallest parts that cannot be manufactured with the classic pressing and sintering technology without post-processing.

Alloys commonly used for the MIM are Fe, Fe-Ni, Fe-P and stainless steel.

The basic work involved in this method is disclosed in U.S. Patents 4,197,118 and 4,113,480.

Here, finely divided metal powders - often carbonyl iron powder (CEP) and mixtures with other alloy powders - are mixed with a binder and shaped into the green body using injection molding technology. Subsequent sintering achieves final densities of around 94%.

The binder is intended to give the mixture the viscosity necessary for spraying and to hold the green body together. The subsequent removal of the binder has proven to be the time and quality determining factor of the process.

As a binder, many-component systems made of low-molecular, thermoplastic plastics, waxes, resins and special additives, but also water-soluble binders based on cellulose are used.

Which binder system is used depends primarily on the powder particle size and morphology. The proportion by weight of the binder in the finished mixture is approximately between 7% and 20% (US Pat. No. 3,989,518, GB 808 583).

In practice, thermoplastic binders have become increasingly popular. Polyethylene and its low molecular weight waxes should be mentioned here in particular.

There are several ways to remove the binder. Patents GB 779 242, US 3,989,518 and US 4,431,449 describe the thermal decomposition of the different binder.

However, extraction by dissolving in various solvents (US 4 197 118, US 4 404 166) and under pressure (DE 31 20 501) is also found.

But here, too, the cheaper method of thermal decomposition, which can be controlled more technologically more quickly, has become established in practice.

Document DE-A-1 483 604 describes a process for the production of moldings from powders of metals or metal alloys, including pure iron powder, in which the powders used contain an easily reducible metal oxide, including iron oxide in the form of Fe 2 O 3 , are treated in order to influence the carbon content of the sintered molding. An influence of the treatment on the necessary debinding time or on the sintered density of the molded parts was not found.

The time required for the dewaxing process can vary widely and can be up to several days.

Such long times were necessary in order to prevent the green body from heating up too quickly and to prevent an excessive increase in pressure inside due to the liquefaction and evaporation of the thermoplastic binder.

Dewaxing too quickly often resulted in severe deformation of the green body, cracking or blistering.

In order to further improve the MIM process, great efforts are currently being made to shorten the dewaxing cycle.

The object of the present invention is accordingly to shorten the necessary dewaxing cycle by changing the metal / binder mixture and to improve the density of the parts thus produced and sintered.

This object is solved by the features of claims 1 to 6.

The present invention relates primarily to the metallic part of the metal / binder mixture. A four-component plastic mixture (a long-chain polyethylene and 3 polyethylene waxes with different melting points) has proven to be a practical binder.

However, other binder systems can also be used, e.g. Polystyrene and polypropylene.

It was surprising that the addition of iron oxide made dewaxing easier. To carry out the process according to the invention, it is advantageous to use very pure iron as the metal of the metal / binder mixture and iron oxides.

The iron oxide is an iron oxide produced via the carbonyl process.

Here iron pentacarbonyl (Fe (CO) ₅) is burned with an excess of oxygen. This gives a very finely divided iron oxide (5 to 80 nm) with high specific surfaces of 5 to 120 m² / g.

It has proven to be advantageous to mix the oxide very intensively with the metal so that there is an intimate bond between the metal and the oxide. This is preferably achieved by grinding in a rotating or vibrating grinding mill.

The oxide is added to the metal at 2 to 30% by weight, preferably at 4 to 10% by weight.

The surface area of the oxide is 10 to 120 m² / g, preferably 70 to 110 m² / g.

When the thermoplastic binder was removed from the metal / oxide / binder mixtures produced in this way, the time required was clearly reduced.

For example, parts made of metal / binder mixtures without oxide showed blisters and cracks after a temperature treatment of 36 hours.

In contrast, parts which were produced from the metal / oxide / binder mixture according to the invention by grinding 4 to 10% by weight of carbonyl iron oxide did not show any cracks or bubbles even after temperature treatments of about 14 hours.

In addition, the effectiveness of temperature treatment increases with increasing oxide content up to a certain limit.

The carbonyl iron powder OM from BASF used as a metal component has a carbon content of about 0.9%.

After removal of the wax from a part produced without oxide, a carbon content of 1.2% was found even after 36 hours, which is attributed to the presence of a residual binder.

For the parts containing about 5% by weight carbonyl iron oxide, the carbon content had dropped to 1% after just 14 hours.

The specific surface area of the oxide showed a similar influence on the removal of the binder.

The effect of the specific surface was particularly evident in the density of the sintered parts. With increasing specific surface area of the oxide, the density of the sintered parts increases.

Carbonyl iron oxide with a surface area of 110 m² / g has proven to be outstanding, with which the highest density of the sintered part was determined in the shortest necessary times for removing the binder.

Examples

The mixtures required for the spraying process were produced as follows:

Metal / binder mixture

Commercial carbonyl iron powder OM from BASF was placed in a 4-liter Sigma mixer while the gearbox was running. The mixer was heated and was at a temperature of 170 ° C. That from the four above The pre-mixed binder system was then slowly added to the carbonyl iron powder.

From the point at which the mass had a pasty consistency - this point in time was very precisely reproducible - the mixture remained in the running mixer for 20 minutes and was then removed from the mixer in the warm state.

The chunks resulting from the cooling mass could then be processed into granules in a granulating machine. This granulate was injection molded into small round parts with a wall thickness of 2 mm using a conventional injection molding machine.

Metal / oxide / binder mixture

The process from mixing in the Sigma mixer to spraying was identical to that for the metal / binder mixture. Before the mixing process, the metal powder was ground together with the carbonyl iron oxide in a 200 liter mill. The mill was filled with 60 kg of the product and 200 kg of grinding media (Cylpebs) and operated at 15 rpm.

example 1

The parts produced from the metal / binder mixture were heated in nitrogen to remove the binder at a linear temperature profile from room temperature to about 480 ° C. and held at the final temperature for 1 hour. The furnace was continuously flushed with 120 l / h nitrogen.

• a) Aufheizrate ca. 32°C/h
  Massenverlust ca. 6 %
  C-Gehalt ca. 1,2 %
• b) Aufheizrate ca. 13°C/h
  Massenverlust ca. 8 %
  C-Gehalt ca. 1,2 %.

The binder content was 8% by weight.

• a) Heating rate approx. 32 ° C / h
  Mass loss approx. 6%
  C content approx.1.2%
• b) Heating rate approx. 13 ° C / h
  Mass loss approx. 8%
  C content approx. 1.2%.

All parts were cracked and blistered.
Some parts were also badly deformed.

The sintering process was kept the same for all examples. The mixture was heated to 900 ° C. in a hydrogen atmosphere and the temperature was held for 15 minutes. The mixture was then heated to 1200 ° C. and held for 4 hours. The oven then cooled to room temperature.

The parts treated according to Example 1 reached a maximum density of about 7.2 g / cm³.

Example 2

• a) Aufheizrate ca. 32°C/h
  Massenverluste ca. 7 %
  C-Gehalt ca. 1 %.

5% by weight of the carbonyl iron oxide with a surface area of 110 m 2 / g were ground together with the carbonyl iron powder in the mill as described and then processed as described above. The parts were also heat treated under nitrogen.

• a) Heating rate approx. 32 ° C / h
  Mass losses approx. 7%
  C content approx. 1%.

No bubbles or cracks were observed in any part. The parts were easier to handle after waxing.

The maximum density of the sintered parts reached was about 7.6 g / cm³.

Claims (2)

1. A process for dewaxing injection-molded metal parts made from a metal/binder mixture, which comprises adding to the metal/binder mixture from 2 to 30% by weight of an iron carbonyl oxide which has a virtually spherical habit and is prepared by burning iron carbonyl and has a specific surface area of from 10 to 120 m²/g, and subsequently carrying out a temperature treatment.
2. A process as claimed in claim 1, wherein the iron carbonyl oxide is bonded particularly strongly to the metal particles by grinding together with metal powder (particles).