Ceramic injection molding
The ceramic injection molding ( English ceramic injection molding , CIM) is a near net shape molding process for technical ceramics according to the principle of the plastic molding. It offers almost unlimited possibilities in terms of design. In this way, demanding geometries such as thin wall thicknesses and the smallest bores can be implemented in a final shape (including threads).
The ceramic injection molding process is suitable for the mass production of complex parts and is characterized by high productivity and good automation options.
process
By adding organic binder systems, a working mass , the so-called feedstock , is produced in ceramic injection molding . The precise setting of the flowability of the ceramic feedstock via the binder system enables excellent process reliability during shaping. In injection molding, the tool cavities completely reproduce the component contour. The same amount of material is always injected when the cavity is filled . The binder system is removed from the molded component before the sintering process , the final temperature treatment. This takes place in a two-stage debinding process.
Manufacturing process
Material preparation
The usually highly pure raw material powders are mixed homogeneously with an organic binder system under the influence of temperature (preferably) in kneaders. This is followed by further homogenization on so-called shear rollers. At the end of the preparation process, granules are present, similar to those in plastic injection molding.
Shaping
In ceramic injection molding, a working mass, the so-called feedstock, is produced by adding organic binders to the ceramic raw material, a fine powder. This mass can be further processed in a similar way to plastic injection molding. The individual chemical components of this binder system must ensure that
- By heating the feedstock in the injection molding machine, the material becomes sufficiently flowable and can thus be injected into one or multiple cavities under high pressure and
- the blank solidifies again in the mold and has sufficient strength for removal from the tool and further handling outside the machine.
The ceramic feedstock must therefore be precisely adjusted in terms of flowability via the binder system so that excellent process reliability is achieved during shaping. The cavity is then filled: the same amount of material is always injected so that constant compression is guaranteed. The advantage is that the shrinkage during sintering can be limited to a minimal fluctuation.
This is followed by the thermal treatment, which includes the stages “debinding” and “sintering”.
The binder system must be removed from the molded component before the actual sintering process. This takes place in a debinding process, which can be one or two stages. If an injection-molded component were to be fed directly into the sintering process, the risk increases that the binder expulsion would destroy the component during the heating phase. This debinding process can be designed very effectively, especially for small, filigree and very complex components.
Debinding
Debinding is the preparation for the fire. In addition to the ceramic powder mixtures, the green compacts produced after injection molding also contain moisture and organic binders or other auxiliaries. In the first step of the two-stage debinding process, a water-soluble component of the binder is dissolved out in an aqueous medium. This creates an open porosity through which the residual binder is thermally expelled in the second step. The thermal debinding (also heating or annealing) of the molded products is carried out with a temperature-time program up to 400 degrees Celsius. The aim is to remove the organic binders contained in the working masses as gently as possible. The debinding is followed by sintering as a manufacturing step.
Sintering
Sintering is the final temperature treatment during which the actual ceramic material is created from the raw material. In this way, ceramic components can be manufactured very precisely from ceramic green compacts in a highly automated process. The ceramic bond and thus the high strength are only obtained by firing at high temperatures. The firing only allows the actual ceramic material to develop. The processes involved in sintering ceramic bodies are very complex (multi-component systems) and take place at different speeds depending on the composition, purity, grain size, packing density and firing atmosphere. In the event of fire, the products solidify and compact, which is also reflected in a decrease in porosity. This process causes a decrease in volume, the so-called burning shrinkage. This can be very different for different ceramic materials. Similar to drying, ceramic products require defined times and a suitable atmosphere for firing. Failure to comply leads to increased internal stresses, defects in the workpiece or inadequate properties. Specially supported sintering processes. With some ceramic materials, special material variants can be produced using specially supported sintering processes:
- Hot pressing (HP) is used to manufacture components with an almost theoretical density and is a sintering process that is supported by a pressing process.
- Hot isostatic pressing (HIP) gives preferably small components the highest density by using isostatic gas pressure of up to 3,000 bar at the respective sintering temperature of up to 2,000 ° C.
Follow-up processes / finishing
The tightest tolerances can be achieved by removing finishing processes. Ceramic components are linearly up to 30 percent smaller when fired. Due to this high shrinkage, the tightest tolerances cannot be created straight away in production. In the final machining or even hard machining, these high demands in terms of accuracy up to the 1/1000 mm range can be created.
Because of the great hardness of the ceramic materials, diamond tools are mainly used for the final machining. The follow-up processes include:
- Grinding : Ceramic has a very high hardness of all materials and can therefore only be ground with the hardest materials. Diamond disks or pastes as well as corundum , silicon carbide and boron carbide are suitable.
- Cut-off grinding : In this manufacturing process, the shape of the workpiece is changed by cutting off certain material particles from the original workpiece.
- Honing : Honing is also known as cross grinding. Excellent geometrical accuracy, high surface quality with high cutting performance and thus cost-effective production are combined in one operation.
- Lapping : Lapping is a production or finishing process in which the machining takes place with loose grain dispersed in a liquid or paste. Non-directional machining marks are characteristic of lapped surfaces.
- Polishing : The components are often lapped or ground before polishing. Only the highest surface qualities up to a mirror finish are produced by polishing.
- Ultrasonic vibratory lapping : Ultrasonic vibratory lapping is a cutting manufacturing process with a geometrically undefined cutting edge that is not used for fine machining, but for the production of geometrically complex components.
- Water jet cutting: The separation or cutting of the ceramic blank is done with water jet cutting using a water jet under very high pressure; 3,000 bar and more can be achieved. Water jet cutting is especially useful for flat and not too thick structures, e.g. B. substrates, suitable.
- Sandblasting : Blasting (also sandblasting) is used to remove burrs or process surfaces. With this treatment method, surfaces can e.g. B. be specifically "matted".
- Lasers : By means of laser processing, z. B. Apply labels to ceramic components very quickly and durably. Cutting or drilling ceramic is also possible, but a tedious process and is used relatively rarely. One of the advantages of laser processing is the possibility of making the smallest and very precise holes in ceramic.
- Glazing : Glazes offer effective protection, especially against mechanical effects and contamination. In addition to white and colorless glazes, there are a large number of customer and product-specific colored variants. They ensure that the products concerned have an attractive, design and company-specific unique position.