The present invention relates to a method of making cemented carbide by powder injection molding.

Cemented carbide is generally made by powder metallurgical methods comprising wet milling in an alcohol-water solution of a powder mixture containing powders forming the hard constituents and binder phase, drying the milled mixture to a powder consisting of agglomerates about 0.1 mm in diameter with good flow properties by means of spray drying, pressing the dried powder to bodies of desired shape and finally sintering.

An alternative method of making cemented carbide parts is by injection molding. Injection molding is a common production method in the plastics industry for the production of 'net-shape' or 'near net-shape' parts. A paste consisting of or containing thermoplastics or thermosetting polymers is heated to a suitable temperature and forced through a nozzle into a form with desired geometry. When used in powder metallurgy, injection molding is generally referred to as Powder Injection Molding (PIM). Because PIM is a more expensive method of making parts than conventional powder metallurgical technique, tool pressing, it is preferably applied to parts of complex shape in small or large series.

The four main process steps in Powder Injection Molding are:

• I. Intimately mixing of desired metallic or ceramic powders with organic binders such as polyolefines; polyethylene, polypropylene, copolymers of these and acrylates, acetates or polyacetals, in combination with or without waxes and surfactants. The mixture obtained is generally referred to as feedstock. The surfactant is used as a mixing aid. It makes it possible to increase the amount of powder that can be blended into the polymer matrix by enhancing the adhesion of the binder polymer and the metallic or ceramic powder. Additionally the surfactant acts as a dispersant i.e. is used as a remedy to obtain an even powder distribution through out the compound, which is the end product of the mixture of polymer constituents and the powder. In order to obtain the desired plastic properties the amount of binder must be 55-25% by volume and the amount of surfactant should correspond to at least one monolayer of the surfactant on the metal/ceramic powder. The mixing step is very critical because it is the base for the other process steps. The binder must have suitable properties during the molding and be easily burned out prior to sintering without leaving any unwanted carbon or other undesired residue. As a result of the mixing a compound is obtained. The mixing should be done in a twinscrew extruder at temperatures well above the melting interval for the polymer constituents with the to ensure that a thorough homogenisation is practicable.
• II. Molding a part into desired shape takes place in a conventional plastic injection molding machine. The feedstock is heated to about 100-240°C, depending on the polymer constituents used in the polymer matrix, and forced into a cavity of desired shape. After cooling the molded part is ejected out from the cavity and removed.
• III. Removing the binder from the molded part. The operation has to be performed in such a way that no cracks are generated in the part. Binder removal can be made in a number of ways. Generally the binder is removed by heating or by extraction in a suitable solvent or by a combination of both.
• IV. Sintering is performed essentially in the same way as for tool pressed parts.

A thorough mixing step is the foundation when striving to make a defect free product. Three different approaches are commonly used when mixing metal or ceramic powders with a binder:

• I. Simultaneous mixing of all binder components.
  In this case the binder components are simultaneously mixed with powder in a batchwise mixing equipment, of Brabender or Haake type, in molten state. Alternatively, the components are mixed in an extruder simultaneously.
• II. Dry premixing of powder and surfactant.
  In this case the surfactant is dryblended with the powder before mixing is performed in molten state, in a batchwise or continuous mixing equipment.
• III. Dry premixing of powder and surfactant as well as some binder with a surfactant.

In this case the surfactant is dryblended with the powder and the same or another surfactant is mixed with a part of or the rest of the binder constituents before mixing in molten state as in I and II.

As mentioned, cemented carbide powders are milled. Milling is considered necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. The milling operation is performed in mills of different sizes using cemented carbide milling bodies. The milling time is in the order of several hours up to days. The milling operation produces a slurry which subsequently is spray dried. As a result of the spray drying a powder is obtained essentially consisting of spherical agglomerates of around 0.1 mm size.

When mixing powder simultaneously, as in I, with all the binder constituents there is a risk of competing adsorption of the surfactant on all the different binder constituents as well as the powder constituents. This can lead to a poor wetting behaviour where it is most desired i.e. on the powder surface. As a result of this poor wetting behaviour the miscibility between a polar metal powder and a non polar binder is significantly reduced. In cases II and III this is improved although quite long mixing times are needed to ensure well dispersed surfactant and an equilibrial adhered surfactant to the powder. The disadvantage when mixing powders with a fine grainsize this way though is that the agglomerates are intact. Consequently, the binder adheres to the powder agglomerates and in that way helps to keep the agglomerates together. If agglomerates are broken up into particles during the mixing step there will be homogenizing problems due to poor wetting of the binder on the powder.

EP-A-443048 discloses a process wherein a carbide and a metal binder phase are wet milled and dried. Subsequently organic binder and surfactants are added and mixed and the mixture is injection moulded.

The object of the present invention is to provide a method of enhancing dispersion of the surfactant to ensure that the surfactant is adhering to particles and smaller agglomerates, rather than just to larger agglomerates.

It has now surprisingly been found that if the surfactant is added during the milling step of the cemented carbide powder a sintered structure with improved properties is obtained. The level of porosity in parts produced according to the invention compared to prior art is significantly reduced.

According to the present invention powders forming the hard constituents(<0.5 µm-10 µm) and powders forming the binder phase (3-20 percent by weight of the hard constituents) are milled together with the surfactant instead of the lubricating agent that is added to powders used for conventional tool pressing. The milling liquid shall in this case be able to dissolve or at least partially dissolve the surfactant. The amount of surfactant should be sufficient to cover all powder surfaces and the excess should preferably not be larger than what is soluble in or miscible with the rest of the binder components.

The surfactant can be a single fatty acid like hexadecanoic acid, tetradecanoic acid, 9,10 Octadecanoic acid, 9,12 Octadienoic acid or 9,12,5 Octadecatrienoic acid mixed with the powder in ethanol, acetone, benzene. Furthermore the surfactant can be some kind of organometallic compound, Zn-stearate, or corresponding alcohol to a fatty acid such as 1-hexadecanol. It can also be an amine such as octadecylamine. All these surfactants can be milled in ethanol.

As surfactants also Zn-salts of higher molecular nonsaturated fatty acids with melting point at 75-95 °C can be used or Zn-salts of high molecular mostly unsaturated fatty acids with a melting point of 97-105 °C, which can be milled together with benzene, ethanol, xylol.

Preferably, the surfactant is Zn salts of high molecular mostly unsaturated fatty acids or hexadecanol.

After milling for a sufficient period of time the slurry obtained is dried preferably by spray drying. The dried powder is then mixed with the rest of the binder at a temperature well beyond the melting point of these components. The binder components composition can be a wax mixed with a polyolefine i.e. paraffin wax or a microcrystalline wax mixed with EVA(Ethene Vinyl Acetate), EBA(Ethene Butyl Acrylate), EAA (Ethene Acrylic Acid), PE(Polyethylene), PP(Polypropylene) alone or in combination or solely mixed with the wax. Preferred binders are paraffine wax mixed with PP.

After mixing, molding into parts of a desired shape takes place in a conventional plastic injection molding machine. The binder is removed from the molded part preferably by extraction in a bath with para menta 1,8-dien or Methyl-Ethyl-Ketone and 2 propanole and after that by drying in vacuum or heating. Finally, sintering is performed essentially in the same way as for tool pressed parts.

The reason for the improvement observed is probably that a homogeneous coating of the powder particles with the surfactant is obtained. Wet mixing of powder and surfactant while milling the powder allows the surfactant to coat the powder particles and most of the agglomerates are breakable in this operation. This allows the surfactant to coat each particle rather than each agglomerate. When mixing the coated powder with the polymer constituents in a twinscrew extruder in a later step the risk of having agglomerates of the powder is minimized.

The invention has been described with reference to cemented carbide powders. It is obvious that it can be applied also powders of titanium based carbonitrides often referred to as cermets.

30 kg WC-powder with average grain size 1.3-2.9 µm and 3 kg Co-powder was mixed with 0.5 kg cetylalcohol and was wetmilled for 30 h in an alcohol-water solution 90:10. The slurry obtained was spraydried to a powder. The spraydried powder was mixed with PP(Polypropylene) and paraffine waxes and pelletized in an extruder. The pellets were fed into a conventional injection molding equipment and molded to a tangential insert at cylinder temperatures between 125-165°C. From the molded parts the binder was eliminated first by extraction in Methyl-Ethyl-Ketone and 1-Propanol and after that by heating to 400°C in flowing H₂ gas under atmospheric pressure. After this debinding step the parts were sintered according to standard practice. The sintered parts were found to have a porosity level of A00+B02+C00 according to ISO 4505, and 1-2 macropores/cm².

Example 1 was repeated with the exception that 0.7 kg stearic acid was milled with the powder in an ethanol solution and then spray dried and mixed as in Example 1. The sintered parts were found to have a porosity level of A00+B00+C00 and 1-2 macropores/cm².

24 kg WC-powder with average grain size 3 - 4 µm and 2 kg Co-powder was mixed with 0.17 kg Zn salts of high molecular nonsaturated fatty acids and was wetmilled for 22 h in an alcohol-water solution 90:10. The slurry obtained was spraydried to a powder. The spraydried powder was mixed with PP(Polypropylene) and paraffine waxes and pelletized in an extruder. The pellets were fed into a conventional injection molding equipment and molded to a Q-Cut insert at cylinder temperatures between 150-170°C. From the molded parts the binder was eliminated first by extraction in para menta 1,8-dien and after that by drying in vacuum at 50°C. After this debinding step the parts were sintered according to standard practice. The sintered parts were found to have a porosity level of A00+B00+C00 according to ISO 4505, and no macropores.

Example 1 was repeated with the exception that the cetylalcohol was added together with the powder during the mixing step. The molded parts were sintered together with those from Example 1. The sintered parts were found to have a porosity level of A00+B02+C00 and 8-10 macropores/cm².