**3. Overview of theoretical work**

96 Some Critical Issues for Injection Molding

Fig. 6. Exudation of the paraffin-wax binder during heating above the melting point of the paraffin, as observed with optical microscope. The photograph a) shows the state before the melting, b) shows the first molten paraffin exuding from the green body, c) shows the

Fig. 7. Green bodies with a complex shape can pose difficulties if they are debinded on a solid substrate. Small areas on which the green body rests on the substrate (1 and 2) can be deformed due to the large compressive stress. Suspended parts of the body can bend due to gravity or even crack at the point where the tensile stress is the highest (3). The wicking embedment can successfully reduce these flaws, since the support pressure is well spread

situation 1 minute after the b) and d) shows state 10 min after the b).

over the green body's surface.

Because of the complexity of the capillary system in the porous green body and the wicking agent the accurate and general theoretical model is difficult to obtain. Since the systems can be quite different, the extraction can also show different behaviour. The existing theoretical models predict different behaviors during the debinding and many even contradict each other. The basis of all models is Darcy's law and some form of capillary-pressure description. One of the first to theoretically describe the process of wick debinding for injection-molded samples was German (German, 1987), who in 1987 proposed a model, where he assumed that the binder is extracted from a molded compact as a continuous body in liquid form, leaving behind a binder-free region.

A partially debinded compact should, therefore, have a characteristic binder distribution with a binder-saturated region near the contact with the wicking powder and a region with no binder on the other side. A sharp border between these regions should be present – a sign of the trailing front of the molten binder. This model is simple and has frequently been used as a basis for research in wick debinding. Monte-Carlo simulations of binder removal based on German's assumptions have also been conducted (Shih & Houring, 2001; Lin & Houring, 2005). These simulations focused on binder penetration in the wicking embedment and examined the case where pieces are not completely surrounded by the embedment.

However, German's model has been criticized, on the basis of experimental data. Contradicting this model, many researchers observed that the binder is uniformly distributed inside the body at all stages of the debinding process (Liu, 1999; Bao & Evans, 1991; Vetter et al., 1994; Kim et al., 1999; Somasundram, 2008). There is also the question of how the air can enter behind the trailing front into the binder-free region if the molded pieces are completely surrounded by the wick (Somasundram, 2008). Furthermore, the debinding rate does not correspond well with some experiments (Vetter et al., 1994). It has also been observed that the permeability in a wick embedment can have important effects and can be a limiting factor, rather than the flow through a sample, as was suggested in German's model [Vetter et al., 1994; Somasundram, 2008]. With more precise examinations of the binder-removal rate it has been confirmed that wick-debinding must take place via more than a single mechanism.

One clearly observable effect, for example, is a rapid decrease in the binder content at the beginning of the process. This has been attributed to the pressure arising from the thermal expansion of the binder [Somasundram, 2008, Gorjan et al., 2010]. Before the debinding process, molded parts contain binder in the solid state, then during the melting a large, and relatively sudden, expansion of the binder occurs. For example, the density of the paraffin drops by around 15% during melting (Gorjan et al., 2010).

With further studies of the kinetics of capillary extraction it has been found that the molten binder inside the body exists in different states, a differentiation based on the position inside the body. It can behave as a 'mobile binder' located in the larger voids between the powder

Wick Debinding – An Effective Way

surface of the debinded parts.

porous alumina wicking agent.

wick-debinding reduces the possibility of flaws.

**4. Practice** 

of Solving Problems in the Debinding Process of Powder Injection Molding 99

In a practical use the wick-debinding process can offer significant benefits. Faster and safer debinding can be achieved in comparison with debinding a without the wicking agent. One of the most important factors in the debinding practice is to avoid the introduction of defects in green bodies. Potential defects include the loss of a compact's shape through distortion, warping, cracking and also the undesirable strong adhesion of the wicking powder on the

For example, in a low-pressure injection-molding, shaping technique it is almost impossible to debind samples without using a wicking agent. In HPIM the use of wick-debinding can be avoided, since the green body retains its strength after the wax has been melted due to the presence of polymer, which binds the particles together. Also in the case of HPIM, the

Fig. 8. Wick-debinding can significantly reduce the formation of flaws. Photograph a) shows the low-pressure injection molded sample, debinded without wicking embedment, while the photograph b) shows the sample which was debinded in the embedment of highly

A major practical problem of wick debinding is the danger of causing defects when the parts are removed from the wick embedment and cleaned afterwards. Because the debinded parts

particles, which can flow due to the pressure gradient caused by the wicking embedment and as an 'immobile binder' located on the surfaces of the particles and inside the smaller voids, which cannot be moved due to the capillary suction – it is too strongly bonded to the powder and trapped inside smaller pores (Gorjan et al., 2010).

There can also be shrinkage during the debinding, which is inversely related to the ceramic volume fraction, with less shrinkage in green bodies with a high solid loading. Very little or no shrinkage occurs at a volume fraction of around 64% [Wright et al., 1990, Gorjan et al., 2010]. In order to avoid a large shrinkage a green body must be made with high a green density. A high green density is also beneficial for the sintering process; however, a high solid loading is detrimental for the molding step. It is always necessary to make a certain compromise.

Capillary extraction effectively removes only a part of the binder, because there is always a certain amount of the binder "trapped" inside the finest pores of the green body. This residual binder must be removed in the form of a gaseous phase. In the case of oxide ceramics the removal of the remaining binder can be achieved by controllably burning the binder. If the temperature during wick-debinding is raised above approximately 200°C then an organic binder starts to decompose due to oxidation reactions.

All of the binder can be removed if the temperature is increased above approximately 600°C, where even carbon burns. However, when all of the binder is removed from the body, the body becomes extremely brittle and weak. In this state it would be impossible to remove it from the embedment and clean it without causing serious damage. One solution is to further heat the system to the temperature where first stage of sintering starts. Pre-sintered or 'biscuit sintered' parts can then be safely removed from the embedment and since they contain no binder they can also be sintered without problems. However, practical problems can accompany this procedure. For example, if alumina parts are debinded a high temperature is required for the pre-sintering and at this temperature the wicking agent also starts to lose the fine porosity and can stick strongly to the surface of ceramic parts.

Another, economically even more acceptable option is to heat the samples after the capillary extraction to the temperature where the organic binder starts to decompose and then hold the parts at this temperature. It has been observed that at this temperature some amount of paraffin wax cures into a hard, brown-colored, non-volatile resin, which remains in the parts and is stable at the dwell temperature of around 200°C. This curing effect drastically improves the strength of the samples, which increases with the dwell time (Gorjan et al., 2011).

Parts processed in this way can be made sufficiently strong for handling without any risk of damage. They are also appropriate for green machining, like cutting, boring and grinding. Since they contain a very small amount of the binder, rapid heating inside the sintering furnace can be applied and the curing of the binder does not influence the strength of the sintered ceramic parts.

A procedure of debinding, where the benefits of wick-debinding are fully used, has also been developed, while the main drawback, i.e., additional cleaning and handling operations are avoided. According to the patent the debinding and sintering can take place in a single furnace (Gorjan & Dakskobler, 2011). This can be achieved by using a high purity-carbon granulate, which completely burns after its role as the wicking agent is completed.
