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1、REACTION AND TRANSPORT KINETICS OF A WATER-SOLUBLE MIM BINDER SYSTEM Dr. Steffen Krug Polymer-Chemie GmbH, Metal Powder Compounds Division Am Gefach 55566 Bad Sobernheim / Germany Abstract A water soluble binder system is used as an organic vehicle for metal powder injection molding. The binder solu
2、bility and transport kinetics through the porous metal powder assembly are studied for various powder morphologies and for powder loadings from 54-64 vol %. The binder removal rates are measured. Corresponding reaction-rate and diffusion coefficient can be calculated from the measurements using a sh
3、rinking-core reaction kinetic model. The binder removal kinetics are described and compared with those of catalytic and thermal binder systems. Binder saturation during water extraction and environmental compatibility of the commercially available binder system polyMIM are discussed. Introduction Me
4、tal injection molding has evolved to become a versatile mass production method for a wide range of complex-shaped metal components1. The powder is first incorporated into a polymer by high shear mixing at a volume loading typically between 50 and 70 vol %. The melted suspension is injected into a ca
5、vity where the polymer solidifies. The task that remains is to remove the organic components before sintering the metal powder assembly. Binder removal is a critical processing step and plays a central role in MIM part production. The main reason for this is that the risk of introducing defects into
6、 the components1 is particularly high during this step. In the past binder removal involved reheating the molding to cause thermal, degredative or evaporative loss of the organic vehicle2. This processing step has to be very long in order to avoid sample distortion and defects. Therefore feedstock d
7、evelopment is the MIM processing step with the greatest improvement potential. In the past decades considerable progress has been made in feedstock development and binder removal. The feedstock or binder systems are classified by their debinding techniques. The conventional thermal debinding and vac
8、uum debinding processes involve one step only: controlled heating in an inert or reducing gas atmosphere. The more advanced debinding techniques require a two-stage process. During the first stage a major binder fraction is removed in order to create an open pore structure within the metal powder as
9、sembly. Solvent extraction, water debinding, and chemical degradation techniques are most common. A certain binder fraction remains rigid during the first step in order to provide mechanical strength during the chemical and physical removal of the main binder content. In a second processing step the
10、 remaining binder fraction is removed thermally. The advantage of the two-step binder systems is that the thermal binder fraction is greatly reduced, thus minimizing the risk of defects such as cracking, bloating or viscose- and gravity-driven part deformation1. Various theoretical approaches are av
11、ailable for the description and calculation of binder removal3-6. If the reaction is quantitatively localized a shrinkage-core reaction kinetic model can be applied7-8. In this study a water soluble feedstock system polyMIM was used to explore the reaction and transport mechanism during water debind
12、ing. Experimental details The feedstock used is commercially available and distributed as polyMIM by Polymer-Chemie GmbH in Germany. Two different MIM compounds are used in this study. A martensitic prealloyed stainless steel (polyMIM 17-4PH B110 E) and a nickel alloy steel (polyMIM FN02 341) blende
13、d from carbonyl iron and carbonyl nickel powders. To measure the binder removal rates 20 mm x 60 mm (0.79 in. x 2.36 in.) rectangular samples of 2 mm (0.079 in.), 3 mm (0.12 in.), 5 mm (1.97 in.) and 8 mm (0.31 in.) thickness are used. For this purpose the samples are weighed before and after water
14、debinding at a preciseness of 0.01 g (3.5 x 10-4oz). The binder removal time is the dwelling time of the samples in the water bath. After water binder removal all samples are dried in an air-circulated oven at 80 C (176 F) 2 C (35.6 F) for 4 h before the weight loss of the samples is measured. The s
15、tandard water debinding temperature is 40C (104 F) 2C (35.6 F). Additional tests are made at 25C (77 F) and 60C (140 F). Particle distribution was measured with a Fritsch Particle Sizer, analysette 22. Results and Discussion Binder Removal Mechanism During water binder removal the water dipole molec
16、ule dissolves the water soluble binder fraction. Starting from the molding surface, the water penetrates gradually into the molding. As the water diffuses into the powder assembly, it dissolves and extracts the polymer molecules. The dissolved polymer is transported from the inter-particle spaces into the water bath. This mechanism is driven by capillary forces and concentration gradients of dissolved binder in the molding and the water bath. Binder removal takes place until an
