**4. Injection molding**

The second step in the PIM process is molding the feedstock into the desired shape. The most popular method is to use a reciprocating screw, horizontal, hydraulic or electric machine in which a screw stirs the feedstock inside the barrel while it is melting. After melting the feedstock the screw acts as a plunger to generate the pressure to fill the die (Stevenson, 2009). Conventional screw-type injection molding machines consist of a clamping unit, injection unit and control system. The clamping unit houses the mold which is generally comprised of two halves. When the clamping unit is closed, material can be injected into the mold, when the clamping unit is open the molded part can be removed.

Powder Injection Molding of Metal and Ceramic Parts 79

The injection molding step can create undesirable features such as gate and ejection pin marks, which must be located in non-critical locations otherwise they must be removed after fabrication. Other design limitations include gradual thickness changes, minimized wall thickness, round corners to reduce stress concentrations and risk of crack appearance, and minimum undercuts on internal bores. Whenever possible, it is important to design PIM parts with one flat surface, which allows the use of standard trays during sintering,

Before sintering, the organic binder must be removed without disrupting the molded powder particles; this process is commonly referred as debinding. Organic polymers have to be removed completely from the "green part", since carbon residues can influence the sintering process and affect the quality of the final product negatively. Moreover, binder removal is one of the most critical steps in the PIM process since defects can be produced by inadequate debinding, like bloating, blistering, surface cracking and large internal voids. It has been shown that the rate of binder removal plays a main role in the defect production

The most commonly used debinding techniques include: thermal, solvent and catalytic. However there also exist some experimental techniques such as plasma debiding (dos Santos *et al*, 2004). The following sections have the aim to provide the reader with a description of the different debinding techniques showing their benefits and limitations.

Thermal debinding utilizes the mechanisms of thermal degradation of organic binders, which is based on the successive dissociation of polymers to produce light molecules that are later evaporated out of the surface of the molded part. Since the thermal degradation process is different for different polymers then thermal debinding time is greatly influenced by the type of polymer used. The binders developed in the original PIM process were a mixture of polyethylene or polypropylene, a synthetic or natural wax and stearic acid. Feedstock materials based on such binders can be removed thermally. However, it has been shown in the literature that POM and polybutyl-methacrylate (PBMA) have a much faster degradation than other polymers such as polypropylene (PP) and ethylene-vinyl-acetate (EVA) (Kankawa, 1997). It should also be noted that the choice of atmosphere under which thermal debinding is performed influences the rate of binder removal and some characteristics of the final piece such as density, carbon or oxygen

In general, it can be said that thermal debinding is an inefficient process that can result in a poor etching of the piece surface if not properly controlled. Additionally, increasing the temperature too fast may produce an excessive increase of vapor pressure in the core of the molded piece leading to defects. Consequently, in order to reduce the risk of cracks or shape deformation, low heating rates are generally used, resulting in a long debinding time, ranging from 10 to 60 h (dos Santos *et al*, 2004). However, thermal debinding is still selected due to its simplicity, safety and respect for the environment as compared to solvent and

due to structural changes in capillaries inside the green part (Oliveira *et al*, 2005).

otherwise special trays are required (Hausnerová, 2011).

**5. Debinding** 

**5.1 Thermal debinding** 

content (Quinard *et al*, 2009).

catalytic binder removal (Quinard *et al*, 2009).

The injection unit consists of a screw, a heating system and a nozzle. The screw transports the material inside a barrel, compresses it and removes any bubbles. The heating system brings the material to an appropriate temperature for easy flow. The nozzle is the conduct through which the heated feedstock is injected into the mold under pressure. The control system of a modern injection molding machine includes hardware and software where the processing conditions are set and saved to ensure the reproducibility of previously employed production cycles (Arburg, 2009).

Molding of the feedstock is comparable to the injection molding of plain thermoplastics and it has the following stages (Arburg, 2009; German & Bose, 1997):


The shaping equipment used in PIM is the same as the one used for plastic injection molding. Due to the size of molded parts, injection molding machines used for PIM are in the lower range, with clamp force typically less than 100 tons, 18 to 25 mm screws and shot size of less than 30 cm3 (Stevenson, 2009). However, the main important difference when dealing with injection molding of any powdered part is that many of the components of the molding machine are subject to a more intense wear, particularly screws, non-return valves, cylinders and molds (Rosato & Rosato, 1995).

Injection molding machines for processing of powdery materials are optimized with wearresistant components, through special hardening processes or utilization of alloys. For example, when dealing with stainless steel feedstock materials, hardening with carbon nitride is recommended by feedstock manufacturers. And when working with ceramic and hard metals, boride cladding or carbide hard facing are recommended. Since harder screws are more brittle, lower torque limits during startup are used to prevent screw breakage in PIM. The solid feedstock pellets cause the most abrasive wear in the feed section of the screw, thus the feedstock should be melted as early as possible in the injection cycle (Stevenson, 2009).

Screw geometry of PIM machine is adopted to lower the compression rate and extend the compression zone as compared to screws used for thermoplastics (Hausnerová, 2011). Compression ratios used in PIM tend to be in the lower range. Ratios between 1.2 and 1.8 are considered acceptable and a ratio of 1.6 is considered to be optimal for the removal of air between granules. It is also important to mention that when calculating the barrel capacity of the injection unit, the barrel rating must be scaled up to take into account the higher density of the PIM feedstock (Stevenson, 2009).

The injection molding step can create undesirable features such as gate and ejection pin marks, which must be located in non-critical locations otherwise they must be removed after fabrication. Other design limitations include gradual thickness changes, minimized wall thickness, round corners to reduce stress concentrations and risk of crack appearance, and minimum undercuts on internal bores. Whenever possible, it is important to design PIM parts with one flat surface, which allows the use of standard trays during sintering, otherwise special trays are required (Hausnerová, 2011).
