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Catalytic Debinding Process for Metal Injection Molding Stainless Steel Parts

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There are many processes for debinding: thermal, solvent, catalytic...Etc. Catamold ? catalytic debinding method is a debinding method for metal injection molding developed in the early 90 s by German BSAF Bloemacher. It is the most common catalytic degreasing method for current domestic metal injection molding manufacturers.

Working principle of acid removal (catalytic debinding)

Catamold ?'s main composition is poly formaldehyde resin adhesive. Due to the inherent chemical structure of polyformaldehyde resin, the adhesive is directly catalyzed by gas phase decomposition. Polyformaldehyde resin chains are characterized by repeated carbon-oxygen bonds, as depicted in figure 1.The oxygen atoms of polymer chain are sensitive to acid action, and when exposed to appropriate acid catalyst, the chemical reaction causes macromolecule to split into CH2O (formaldehyde) units.

Catalyst used for debinding is concentrated gaseous nitric acid (gasify liquid nitric acid with a content higher than 98.5%).This catalytic reaction of nitric acid is particularly suitable for powder metal injection molding. Under 110 ℃, the debinding rate is very high, the temperature is much lower than the melting of polyformaldehyde interval (150 ℃ ~ 170 ℃).In this way, the polymer is converted directly from a solid to a gas. Small formaldehyde gas molecules (boiling point - 21 ℃) can be easily spillover by porous outer, without disrupting powder particles stacking structure (figure 2). At this time, the adhesive is still very strong, so that parts in debinding is a continuation of hardness, to avoid any plastic deformation, can get a good tolerance.

After the complete removal of polyformaldehyde, there is still a certain amount of acid-resistant adhesive component (generally 10% of adhesive content), which can provide a certain delivery strength for the preformed powder. This acid-resistant organic component can be removed (thermally) in subsequent sintering operations.

Note: the adhesive is not completely removed during debinding because the adhesive contains about 10% acid-resistant components. These components ensure that the debinding parts will not collapse during transportation. These acid-resistant components are eventually removed in sintering furnace.

Catalytic debinding furnace

Placing parts on the support plates inside debinding furnace. The furnace is equipped with a fan to ensure adequate mixing and gas circulation. The catalyst is fed quantitatively into furnace by a quantitative pump and then vaporized on a ceramic plate. 

Nitrogen is the carrier gas to ensure metal parts anti-resistance, or it will react on gas atmosphere. Before starting Before debinding, cleaning for 1 hour, then unified inert gas in furnace, at the same time heat furnace and green parts 110 ℃.  Too long debinding will not damage parts, but too short time will lead to defects. Therefore, it is recommended to decrease debinding time by a large margin every 1 hour until the weight loss begins to decrease to determine a reasonable debinding time. The exhaust gas is treated by two stages of combustion. In the first stage burner, the reaction gas is burned in anoxic propane. It is mainly to produce the remaining nitric acid and the NOx component produced by nitric acid into N2. In this case, formaldehyde gas has a strong reduction reaction. In a second stage burner, air is designed and added for complete combustion. Emissions from the flue after combustion comply with current environmental regulations.

Parts support

Part support method in debinding furnace depends on part geometry. The formed part is placed on sinter tray in the best vertical side position. The spacing between parts should be large enough not to impede gas flowing. Parts can be placed on porous plate or screen. This allows part’s bottom side to exchange gas, which can shorten the debinding time.

Remark: It generally use stainless steel tray in debinding furnace. The formed parts are placed on ceramic plates (such as 100mm*100mm), and then arrange ceramic plates neatly on the stainless steel tray. In order to get the best economic benefits, each furnace achieves the best loading as far as possible.

Debinding temperature

To maintain the security with nitric acid boiling point, actual limit temperature is 100 ℃, ceiling is set based on the melt temperature of binders (150 ℃ ~ 170 ℃).  In fact, the maximum temperature is 140 ℃, but generally take 110 ℃ ~ 120 ℃. 

Nitric acid

In principle, debinding speed increases with the flow rate of nitric acid. But if exceeded the specified flow, oxidizing gas plus formaldehyde concentration, in extreme cases, may cause spontaneous combustion, according to experience, we recommend with the regulation: concentration of nitric acid is higher than 98.5%. In order to ensure the safety of production, debinding furnace manufacturers will generally force the concentration of nitric acid.

Debinding rate

Under the temperature 110 ℃, a typical speed in forefront of debinding is 1 ~ 2 mm/h. If the furnace load is increased, the debinding time should be extended too. Generally there is no adverse effect on parts if remain them in the furnace longer than the minimum debinding time. 

Tip: Since the debinding parts are easy to rust in contact with moisture in the air, we sometimes place the debinded parts directly in furnace until they need to be sintered. There are also some enterprises will specifically make a nitrogen protection box to store the debinded parts.

Control of debinding operation

The debinding effect can be detected in two ways.

Debinding can be controlled by weight. The debinding rate is calculated by weighing the quality change of parts before and after debinding. When adjusting the parameters, time, temperature and other parameters can be adjusted to test the debinding rate each time until the data is no longer changed to find the best solution. 

Auxiliary detection method: cut the thickest wall of part to observe the fresh broken surface. The core of broken surface without debinding will show different colors. If the core is fully debinding, the color will be consistent.

Keywords in the article: debinding metal injection molding
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