**4. Practice**

98 Some Critical Issues for Injection Molding

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

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

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

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.

the strength of the samples, which increases with the dwell time (Gorjan et al., 2011).

granulate, which completely burns after its role as the wicking agent is completed.

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

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

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

powder and trapped inside smaller pores (Gorjan et al., 2010).

an organic binder starts to decompose due to oxidation reactions.

compromise.

sintered ceramic parts.

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 surface of the debinded parts.

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 wick-debinding reduces the possibility of flaws.

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 porous alumina wicking agent.

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

Wick Debinding – An Effective Way

cooled enough after a thermal regeneration.

of Solving Problems in the Debinding Process of Powder Injection Molding 101

Fig. 10. Comparison of the properly debinded part (left) with the deformed part (right), which was deformed and had granulate wicking agent adhered to the surface. The defect was caused when the part was embedded into a too hot wicking agent, which had not been

Fig. 11. Wick-debinded parts are loaded on a tray for the sintering process. Successfully wick-debinded samples contain an open porosity and are ready for a fast sintering cycle, in

which complete burnout of the residual binder takes place.

can be quite fragile, a gentle and manually intensive operation is required. If the debinded compacts are strong, then a more robust handling such as sieving can be applied. During this handling the breaking of parts can occur.

In the practice a wicking embedment must also satisfy some additional considerations besides having good capillary-extraction characteristics. It is the most practical if it is in the form of granules with a size of 50–200 μm. This size of granules ensures uniform contact with a green body and has, at the same time, good flowability. This flowability is crucial for easy handling. Smaller pores are powders tend to form dust, which is undesirable. Also, the granules are easier to clean from the surfaces of the parts after the debinding process. Each individual granule contains a very fine porosity, which is crucial for a highly efficient capillary extraction.

The correct temperature regime must be used in order to achieve debinding. A slow heating rate must be applied in order to give the wicking agent time for extraction. Typical debinding cycles last from 20 hours to several days.

The adhered wicking agent causes problems, because it would lead to a rough surface after the sintering. Therefore, it should be thoroughly cleaned from the debinded parts.

Fig. 9. Alumina wicking agent in the granular form. Photograph a), taken with optical microscop, shows granules of the wicking agent. Photograph b), taken with scanning electrone microscope show the surface of one granule. It can be seen, that the granula contains very fine porosity, which is a condition for efficient capillary extraction.

can be quite fragile, a gentle and manually intensive operation is required. If the debinded compacts are strong, then a more robust handling such as sieving can be applied. During

In the practice a wicking embedment must also satisfy some additional considerations besides having good capillary-extraction characteristics. It is the most practical if it is in the form of granules with a size of 50–200 μm. This size of granules ensures uniform contact with a green body and has, at the same time, good flowability. This flowability is crucial for easy handling. Smaller pores are powders tend to form dust, which is undesirable. Also, the granules are easier to clean from the surfaces of the parts after the debinding process. Each individual granule contains a very fine porosity, which is crucial for a highly efficient

The correct temperature regime must be used in order to achieve debinding. A slow heating rate must be applied in order to give the wicking agent time for extraction. Typical

The adhered wicking agent causes problems, because it would lead to a rough surface after

the sintering. Therefore, it should be thoroughly cleaned from the debinded parts.

Fig. 9. Alumina wicking agent in the granular form. Photograph a), taken with optical microscop, shows granules of the wicking agent. Photograph b), taken with scanning electrone microscope show the surface of one granule. It can be seen, that the granula contains very fine porosity, which is a condition for efficient capillary extraction.

this handling the breaking of parts can occur.

debinding cycles last from 20 hours to several days.

capillary extraction.

Fig. 10. Comparison of the properly debinded part (left) with the deformed part (right), which was deformed and had granulate wicking agent adhered to the surface. The defect was caused when the part was embedded into a too hot wicking agent, which had not been cooled enough after a thermal regeneration.

Fig. 11. Wick-debinded parts are loaded on a tray for the sintering process. Successfully wick-debinded samples contain an open porosity and are ready for a fast sintering cycle, in which complete burnout of the residual binder takes place.

Wick Debinding – An Effective Way

676, ISSN 02728842

2219

0972764208, State College, Pennsylvania

(Januar 2010), pp. 3013 -3021, ISSN 0955-2219

10.1111/j.1551-2916.2011.04872.x, ISSN 0002-7820

,Vol. 29, No. 9 (June 2009) pp. 1595-1602, ISSN 0955-2219

Heinemann, ISBN 0 7506 4445 1, Oxford

14 (July 2001), pp. 3802 – 3809, ISSN 0009-2509

(December 2001), pp. 457-480, ISSN 09218831

of Solving Problems in the Debinding Process of Powder Injection Molding 103

German R. M. (1987). Theory of thermal debinding, *International Journal of Powder Mettalurgy.* Vol. 23, No. 4, (October 1987), pp. 237 – 245, ISSN 0888-7462 German R. M. (1990). Debinding Practice, In: *Powder Injection Molding*, german R. M., pp. 281 – 316, Metal Powder Industries Federation, ISBN 0-918404-95-9, New Jersey, 1990 German R. M. (2003). Overview of Process Variations, In: *Powder Injection Molding – Design* 

Gorjan L.; Dakskobler A.; Kosmač T. (2010). Partial wick-debinding of low-pressure powder-

Gorjan L.; Dakskobler A. (2011). *Postopek toplotne obdelave oblikovancev s sintranjem : patentna prijava 201100196*. Ljubljana: Urad RS za intelektualno lastnino, (2011) Gorjan L.; Dakskobler A.; Kosmač T. (2011). Strength Evolution of Injection-Molded Ceramic

Kim S. W.; Lee H. W.; Song H. (1999). Effect of minor binder on capillary structure evolution

Lin T. L.; Houring L. W. (2005). Investigation of wick debinding in metal injection molding:

Novak S.; Dakskobler A.; V. Rubitsch. (2000). The Effect of water on the behaviour of

Richardson J. F.; Parker J. H. (2002). Flow of Fluids through Granular Beds and Packed

Somasundram I. M.; Cendrowitz A.; Wilson D. I.; Johns M. L. (2008) Phenomenological

Somasundram I. M.; Cendrowitz A.; Johns M. L.; Prajapati B.; Wilson D. I. (2010). 2-D

Shih M. S.; Houring L. W. (2001). Numerical simulation of capillary-induced flow in a

Vetter R.; Sanders M. J.; Majewska-Glabus I.; Zhuang L. Z.; Duszczyk J. (1994). Wick-

1994, Vol. 30, No. 1 (Januar 1994), pp. 115-124, ISSN 0888-7462

*and Applications*, German R. M., pp. 9 – 20, Innovative Material Solutions, Inc., ISBN

injection-moulded ceramic parts. *Journal of European Ceramic. Society*, Vol. 30, No. 15

Parts During Wick-Debinding. *Journal fo American Ceramic Society*, doi:

during wicking, *Ceramics Internetional*, Vol. 25, No. 7, (September 1999), pp. 671 -

numerical simulations by the random walk approach and experiments, *Advanced Powder Technology.*, Vol. 16, No. 5 (September 2005), pp. 495 - 515, ISSN 0921-8831 Liu D. M. (1999). Control of yield stress in low-pressure ceramic injection moldings, *Ceramics International*, Vol. 25, No. 7 (September 1999), pp. 587 – 592, ISSN 02728842 Loebbecke B.; Knitter R.; Hausselt J. (2009). *Rheological* properties of alumina feedstocks for

the low-pressure injection moulding process, *Journal of European Ceramic. Society*

alumina-paraffin suspensions for low-pressure injection mouldnig (LPIM), *Journal of European Ceramic. Society*, Vol. 20, (November 2000), pp. 2175 – 2181, ISSN 0955-

Columns, In: *Coulson &Richardson's Chemical engineering, vol. 2: Particle technology & separation processes*, Richardson J. F., Parker J. H., pp. 191 – 234, 2002, Butterworth-

study and modelling of wick debinding, *Chemical Engineering Science*, Vol. 63, No.

simulation of wick debinding for ceramic parts in close proximity, *Chemical Engineering Science*, Vol. 65, No. 22 (November 2010), pp. 5990-6000, ISSN 0009-2509

powder-embedded porous matrix, *Advanced Powder Technology*, Vol. 12, No. 4

Debinding in Powder Injection Molding, *International Journal of Powder Metallurgy*,

After single or multiple uses the wicking agent accumulates a certain amount of organic phase – binder degradation products. This phase decreases the porosity of the wicking agent and thus its capillary-extraction ability. However, it can be regenerated by heating it to around 600°C, where all the organics burn. In practice, a wicking granulate with different amounts of residual organic phase can be used for debinding different parts. Small parts are debinded in the embedment, which is rich in organics, whereas the large parts are debinded using freshly regenerated granulate with a maximum capillary-extraction ability. As a result, the embedment can thus be consequently used for ever smaller parts.
