**Ceramic Injection Molding**

Zdravko Stanimirović and Ivanka Stanimirović *IRITEL A.D., Republic of Serbia* 

#### **1. Introduction**

130 Some Critical Issues for Injection Molding

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Powder injection molding (PIM), which encompasses metal injection molding (MIM) and ceramic injection molding (CIM), is a net-shaping process which enables large scale production of complex-shaped components for use in a diverse range of industries. It combines plastic injection molding techniques and performance attributes of ceramic and metal powders. Ceramic injection molding (CIM) uses ceramic powders such as alumina, zirconia, titania, ferrite powders, etc. It was introduced in 1940's, but for the next thirty years it was of minor interest to ceramic industry. In 1970's and 1980's CIM provided cost-effective fabrication method for mass production of ceramic parts for automotive industry. Today more than 300 companies practise PIM. Most of them practise MIM technology (>70%). Small percentage (5%) produce metals, ceramics and carbide components and about 25% practice CIM technology. This positive tendency can be attributed to unique properties of ceramic materials. They have excellent mechanical properties and low specific weight. Also, they are suitable for applications under extreme conditions (high temperatures, corrosive atmospheres, abrasive conditions, high loads at high temperatures). This combination makes them interesting for a wide variety of applications.

The ceramic injection molding process consists of four basic steps: feedstock preparation, injection molding, debinding process and sintering (Fig. 1).When powder technologies are in question, the key step in production process is choosing the adequate ceramic powder. Specific surface area, particle size, size distribution, particle shape and purity of the powder influence properties of the feedstock. Typical particle sizes in CIM are 1-2µm, but also much finer particles down to submicron or nano region are being used in advanced CIM. CIM uses a feedstock of composite granulate. A high concentration of ceramic powder is mixed with a thermoplastic binder system to form moderate viscosity feedstock - homogenous powder-binder mix that is free of agglomerates, has optimum ceramic/binder content and still maintains sufficient fluidity (Rak, 1999).

The feedstock is molded using injection molding equipment similar to that used for polymer injection molding. Injection molding involves concurrent heating and pressurization of the feedstock. It requires close monitoring in order to minimize molding defects. As a result, a green body is obtained (Fig. 2). After molding, the binder is extracted from the green body. Debinding usually takes place in two steps. Immersion is the first step. Soluble component of the binder is removed and system of pore channels develops to allow removal of the remaining component. The second step is thermal debinding and the insoluble component is being removed by thermal decomposition thus resulting in brown body (Fig. 2).

Ceramic Injection Molding 133

In the recent past extensive international research and development activities were performed. Ferrite ceramics, piezoelectric ceramics and alumina were recognized as the most commonly used ceramic powders in production of wide range of CIM components for various applications and for that reason they are described in this chapter. Also, as an insight in future trends of CIM technology development, advanced CIM technologies are

Ferrites are ceramic materials based on iron-oxide. They exhibit soft magnetism and therefore are being used in a variety of applications such as antennae, transformer cores, microwave waveguides, etc. There are three main types of ferrites: Mn-Zn ferrite, Ni-Zn ferrite and Mg-Zn ferrite. Ferrites have several advantages when compared to other materials: temperature and stability, high resistivity, wide frequency range and low loss combined with high permeability. Disadvantages are low saturation flux density and low tensile strength. Differences between soft ferrites and other magnetic materials are

There are several techniques available to forming ferrite specimens: grinding, extrusion, pressing and injection molding. Most ferrites are commercially produced by a dry pressing process. The powder flows into a die cavity and upper and lower punches at about 10 tons per surface square inch are being applied. Since the pressing is being done in vertical direction, resulting specimen geometries are limited to simple geometric shapes. Grinding is the most economical forming technique to produce non standard ferrite cores. It requires no tooling since cores are ground from isostatically formed sintered bars. Extrusion is an ideal

Parameter Ferrites Magnetic

range [ºC] 100-500 500 600

5 10 25

Resistivity [m] 10-108 10-5 104

density range [mT] 300-500 800-2400 1000-1200

However, in recent years ceramic injection molding technique (Rodrigez et al., 2003; Zlatkov et al., 2008) has been applied as an alternative forming process. Injection molded ferrite parts can be produced from very simple forms to quite complex shapes. Further processing is rarely required, but if necessary, this can be achieved using conventional tools. Parts produced through this process can have very intricate shapes and tight tolerances. Injection

Alloys

5-15000 5000-300000 5-150

8 80 4000

Iron Powder

> 25 30 100

presented in Table 1 (Z. Stanimirović & I. Stanimirović, 2010).

technique for forming long rods and bars.

Initial permeability range

Curie temperature

Loss factor [10-6] 10kHz 100kHz 160kHz

Saturation flux

Table 1. Differences between soft ferrites and other magnetic materials.

presented.

**2. Ferrite ceramics for CIM** 

Fig. 1. The ceramic injection molding process.

Fig. 2. CIM component: green body, brown body and sintered part, respectively.

After debinding process, the sintering process takes place. Sintering parameters depend on the type and electronic properties of the ceramic powder used and, as a result, CIM components are obtained. Quality control of ceramic components in the green, brown and sintered state is commonly carried out by visual inspection and weighting. In that way surface cracks, impurities, voids, pores, distortions, incomplete parts and skin marks can be detected. Measuring the density of sintered components is another indispensable method for characterisation of CIM parts. Additional processing after sintering is optional depending on the type of component and specific application and in standard applications is seldom required.

Fig. 1. The ceramic injection molding process.

is seldom required.

Fig. 2. CIM component: green body, brown body and sintered part, respectively.

After debinding process, the sintering process takes place. Sintering parameters depend on the type and electronic properties of the ceramic powder used and, as a result, CIM components are obtained. Quality control of ceramic components in the green, brown and sintered state is commonly carried out by visual inspection and weighting. In that way surface cracks, impurities, voids, pores, distortions, incomplete parts and skin marks can be detected. Measuring the density of sintered components is another indispensable method for characterisation of CIM parts. Additional processing after sintering is optional depending on the type of component and specific application and in standard applications In the recent past extensive international research and development activities were performed. Ferrite ceramics, piezoelectric ceramics and alumina were recognized as the

most commonly used ceramic powders in production of wide range of CIM components for various applications and for that reason they are described in this chapter. Also, as an insight in future trends of CIM technology development, advanced CIM technologies are presented.
