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POWDER INJECTION MOULDING (PIM)
Production of complex moulded parts from metal and ceramics
Injection moulding technology employing powdered materials is primarily used today for the manufacture of complex components for industrial use. Powder injection moulding is as an alternative to other moulding processes such as precision casting and axial or isostatic pressing. The manufacturing of a moulded part from the feedstock (powder/binder mixture) is in fact comparable to the injection moulding of plastics.
In recent years, the fields of application for injection moulded parts made from ceramic or metal powder have primarily been within the automotive, tools industry, textile and watchmaking industries as well as in magnet production, household goods, precision engineering, medical/dentistry technology and porcelain industry.
In order to process metal or ceramic powders, it is first of all necessary to mix, homogenise and granulate the material with a binder using a mixing unit. The resulting injection-mouldable feedstock is injected into the mould under pressure and subjected to heat. The binder is subsequently removed from the moulded part (green compact). What is left is the ‘brown compact’ which is then sintered.
High reproducibility within strict tolerances
Thanks to the use of powder injection moulding, volume production of components is possible, which can no longer be manufactured cost-effectively by means of machining or pressing technology.
Injection moulding technology allows virtually unlimited freedom in the design and manufacture of components.
The powder injection moulding manufacturing process comprises the initial injection moulding, debinding and sintering of the moulded parts. The component tolerances are determined by the following important factors:
- Binding agent content
- Powder characteristics
- Mixing process
- Injection moulding parameters
- Gravitational distortion
- Sliding properties on the sintering surface
Material: wide range of useable materials
In principle, all fine-grained, sinterable powders can be mixed with the appropriate binder and then processed on injection moulding machines. In addition to traditional oxide ceramics, it is also possible to use metals, carbides and even nitrides, for example.
As mixing and injection equipment can be subject to increased wear during the processing of powdered materials, it is recommendable to select a powder with the smallest possible particle size. Finer powders produce lower surface roughness, cause a low degree of wear during processing and result in a higher green strength. The range of properties of the various powder materials is indicated in the adjacent table.
Binder renders the powder injectable
The most important requirements with regard to the binder is its dimensional stability during debinding, good storage characteristics, lack of reaction with the powder materials, high green compact strength, good demoulding characteristics, thermal stability and easy and complete removal during debinding.
The adhesion between the binder and the powder particles should also be as high as possible so that the centrifugal forces arising during the injection process do not give rise to separation of the two components, resulting in unevenly filled parts. In order to achieve good injection moulding characteristics and isotropic sintering with a low level of shrinkage, the use of spherical powders is recommendable.
Feedstock: binder and powder in optimum relationship
During mixing, the binder and the powder are mixed to a homogeneous compound, the feedstock. There are feedstock suppliers for both metal and ceramic powders operating on the market. They offer a constantly growing selection covering a variety of materials.
Consequently, the materials required for MIM (Metal Injection Moulding) or CIM (Ceramic Injection Moulding) are available as ready-injectable feedstock and no longer need to be prepared in-house.
If the properties of the available materials are not sufficiently suitable for the required application, it is possible to develop customer-specific feedstocks and have them produced. Specialised material suppliers with comprehensive consulting services are also available for this purpose.
Debinding: removal of the binder from the green compact
During the debinding or dispelling process, the binder is removed from the green compact by catalysis, dissolving or thermal decomposition.
Debinding can be supported by a suitable furnace temperature and atmosphere that effectively promotes the sequence of chemical reactions. When the binder is removed, the moulded part becomes a porous and fragile moulding, known as a ‘brown compact’. In this condition, the part is only kept stable by a minimal residue of binder and Van der Waals forces#. The debinding furnaces must be specially adapted to the applicable requirements, depending on the binder system used.
The furnaces must feature very good gas circulation. The only exceptions are binder systems which require embedding of the parts during debinding. The waste gases generated must be disposed of (for instance through afterburning or the use of a catalytic converter) by means of suitable down-stream systems.
Sintering: Creating a mechanically stable bond
In order to securely bond the particles of the brown compact, the part is sintered by exposing it to a further increased heat supply (up to 2000°C). In terms of furnace atmosphere, temperature/time characteristic and pressure distribution, this process is similar to that used for pressed sintered parts. The final moulded part is produced through diffusion and/or the formation of liquid phases and grain growth.
Injection moulded, sintered parts can achieve a theoretical material density of up to 99.9 percent. In the case of a suitably selected powder, they shrink isotopically (i.e. equally in all directions during sintering). The sintered parts are characterised by their homogeneous properties. Faults occurring during the injection moulding process cannot be remedied through sintering.
It is usually only necessary to refinish contact surfaces or grind cutting edges by means of a refining process following sintering. Fine powders and suitable mould surfaces ensure outstanding surface qualities during the injection moulding of metals or ceramics.
Hard alloy applications
The fields of applications for hard alloy tools are becoming increasingly diverse. Optimum manufacturing of this type of part, such as radius cutters, can only be ensured through the use of powder materials.
Previously, some moulded part geometries placed certain restrictions on production. For this reason, reusable shafts upon which “disposable tips” could be screwed were commonplace. One-piece internal threads for milling tools were very difficult and expensive to produce, if possible at all.
With injection moulding technology, in contrast, the production of hard alloy cutters and the simultaneous demoulding of an internal thread for screwing the ‘disposable tips’ onto a shaft poses no problem. The production of this type of cutting tool is fully automatic, saving a great deal of time and expense. Owing to the very short sprues, considerable material savings are also achieved. A wide variety of cutting geometries can be produced in the mould by simply changing replaceable inserts.
Use of low and high-alloyed steels
In recent years, the production of stainless steel parts using the powder injection moulding process has become widespread for many applications. Owing to their toughness, stainless steels are very hard to machine during conventional production. These materials are also unsuitable for casting.
Through the use of powder injection moulding, stainless steel can easily be processed into complex components. The finished sintered parts have a surface quality which is extremely well suited for polishing or coating. Powder injection moulded stainless steel parts are used industrially in the following sectors:
- Watches
- Eyeglasses
- Locks
- Photo technology
- Weapons technology
- Medical technology
Technical ceramics applications
Ceramic parts for technical applications such as insulators for ignition electrodes, and also grinding discs for coffee machines can now be produced using injection moulding.
More functions can be integrated into the components thanks to the use of powder injection moulding. Material savings can also be achieved by means of component design especially adapted to powder injection moulding. This ensures the economical production of technical ceramic parts. The ceramic parts shown were produced from AI²O³, which is currently the most widely used ceramic material for technical components.
For the manufacture of the ceramic grinding mill, AI²O³ powder is mixed with a binder content of 14.5 percent by weight. The resulting feedstock is melted in the plasticising cylinder and fed into the mould using a staged injection profile. The injection parameters are specially adapted to the component being produced.
Removal of the parts is via a robotic system which sets down the parts in sequence on the debinding or sintering trays. The complete cycle for this component is approximately 15-25 seconds.
Another application example is the ferrule used for connecting fibre optic bundles for data transmission. The precision of these parts is crucial to the coupling of fibre optical cables. Through the use of powder injection moulding, the ferrules can be produced with greater precision than using other production techniques, significantly reducing post production costs.
Micro-injection moulding of powder materials
Particularly in high-tech industries such as medical technology and telecommunications, the demand for high strength, wear-resistant micro-components is increasing. Injection moulding is particularly well suited to the production of such miniature structures because it enables consistently high-quality serial production of the smallest possible components, right down to the nanometric range.
In order to produce high-quality moulded parts such as micro-gears, the entire production needs to run fully automatically without manual intervention.
The injection mould
Moulds featuring the equipment familiar from plastic injection moulding technology such as slides, core pulls, unscrewing units and internal pressure sensors can also be used for the injection moulding of powder materials. However, the mould cavities and injection unit must be protected against wear from the abrasive properties of the powder/binder melt by special hardening processes or alloys.
The mould operates with a three-platen mechanism to cut off the sprue. This sprue is much larger than the moulded part itself, but is 100% recyclable. During the first separation step, this three-platen technology is applied to cut off the sprue while it is still in the mould. Finally, during the second separation step. The finished moulded part is released and demoulded via ring ejectors.
Removal and handling
The handling system comprises an Arburg Multilift H removal robot, a vacuum gripper and integrated pneumatic kinematics. As the positioning tolerances are less than ±2 µm, the gripper head docks into place above a centring device on the moveable mould side. The regulated pneumatic actuators ensure perfect set-down by means of optimised gripper movements and low-stress component handling. The Multilift H operates with servo-electric axes in order to ensure the required degree of set-down accuracy.
Arburg of Germany, one of the world’s leading manufacturers of injection moulding machines, is a leading supplier of PIM processing equipment and technology. It can support companies interested in this technology to set up production facilities by providing comprehensive expertise, both regarding the machines and the complete processing procedure. At the Arburg PIM laboratory, customers can learn about the benefits of powder injection moulding in practice through the manufacture of sample parts.
- Arburg is represented in South Africa by Hestico.
NOTE
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Van der Waal’s forces are the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules.
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