Introduction to the Catalytic Debinding Process in the Production of MIM Stainless Steel Parts

 

Date：[2024/7/1]
 

The Catamold ® catalytic degreasing method is a one-step degreasing method for metal powder injection molding developed by Bloemacher from BSAF in Germany in the early 1990s. It is currently the most commonly used catalytic degreasing method by domestic metal injection molding manufacturers.

1. Working principle of acid removal (catalytic degreasing)

Catamold's ® The main component of the adhesive is polyoxymethylene resin. Due to the inherent chemical structure of polyoxymethylene resin, the adhesive is directly catalyzed by gas-phase decomposition.

The characteristic of polyoxymethylene resin chains is repetitive carbon oxygen bonds, as depicted in Figure 1. The oxygen atoms in polymer chains are sensitive to the action of acids, and when exposed to suitable acid catalysts, chemical reactions can cause macromolecules to split into CH2O (formaldehyde) units.

The catalyst commonly used in the degreasing process is currently high concentration gaseous nitric acid (which gasifies liquid nitric acid with a nitric acid content higher than 98.5%). The catalytic reaction of nitric acid is particularly suitable for the degreasing process of powder injection molding.

At 110 ℃, the degreasing rate is very high, which is much lower than the melting range of polyformaldehyde (150 ℃~170 ℃). In this way, the polymer is directly converted from solid to gas.

Due to Catamold ® The adhesive gas interface continues inward at a linear velocity of 0.5~2mm/h depending on the type of adhesive.

Small formaldehyde gas molecules (boiling point -21 ℃) can easily overflow through the porous outer layer of the formed parts without damaging the powder particle stacking structure.

At this point, the adhesive is still very strong, giving the parts continued hardness during degreasing, avoiding any plastic deformation, and achieving good tolerances.

After the complete removal of polyformaldehyde, there is still a certain amount of acid resistant adhesive component (usually 10% mass fraction of the adhesive content), which can provide a certain transport strength for the preformed powder.

This acid resistant organic component can be removed during subsequent sintering work (thermal removal).

Note: The degreasing process actually does not completely remove the adhesive, because the adhesive contains about 10% by mass of acid resistant components, which can ensure that the degreased parts will not collapse during the degreasing furnace, operation, or entry process. These acid resistant components will eventually be removed in the sintering furnace.

2. Catalytic degreasing furnace

The parts that need to be degreased are placed on the support plate of the MIM catalytic degreasing furnace grate. The furnace is equipped with a fan to ensure sufficient mixing and circulation of gas. The catalyst is quantitatively fed into the furnace body through a quantitative pump, and then vaporized on a ceramic plate.

Nitrogen serves as the carrier gas. Do not allow degreased metal parts to corrode, otherwise it will have a negative effect on the reaction occurring in the gas atmosphere.

The experimental degreasing furnace (50L) requires approximately 40g/h of nitric acid and 500L/h of nitrogen. Batch degreasing furnaces with a capacity of 420 liters can be purchased from some furnace manufacturers. Continuous degreasing furnaces can also be used for degreasing operations.

Before starting the degreasing process, a 1-hour cleaning operation should be carried out, followed by a uniform inert gas in the furnace, and the furnace and green body should be heated to 110 ℃.

If the degreasing time is too long, it will not damage the parts, while if the degreasing time is too short, it will produce waste. Therefore, it is recommended to significantly reduce the degreasing time by a 1-hour interval until weight loss begins to decrease, in order to determine a reasonable degreasing time.

During the experiment, especially compared to small parts, the degreasing time of a fully loaded furnace can be increased by 50%. Similarly, when the proportion increases, these safety measures should be taken again.

The discharged gas needs to undergo two-stage combustion treatment.

In the first stage burner, the reaction gas is burned in an oxygen deficient propane flame. The main method is to reduce the remaining nitric acid and the NOx component produced mainly by nitric acid to N2. Here, formaldehyde gas has a strong reduction reaction.

In the second stage burner, the designed and added air undergoes complete combustion. The emissions of gases entering the flue after combustion comply with current environmental regulations.

3. Part support

The form of support for the parts inside the degreasing furnace depends on the geometric shape of the parts.

Place the formed part in the best upright side position on the sintered tray. The spacing between parts should be large enough not to hinder the flow of gas.

Parts can be placed on porous plates or wire mesh, which allows for gas exchange at the bottom of the parts, thereby shortening the degreasing time.

remarks:

The degreasing furnace usually uses stainless steel trays, and the formed parts are placed on ceramic plates of certain sizes (such as 100mm * 100mm), which are neatly arranged on the stainless steel trays.

To achieve the best economic benefits, each furnace should strive to achieve the optimal loading capacity as much as possible.

4. Degreasing temperature

To maintain a safe difference with the boiling point of nitric acid, the actual lower limit value is 100 ℃, and the upper limit value is set based on the melting temperature of the adhesive (150 ℃~170 ℃).

In fact, the upper limit temperature is 140 ℃, and it is generally recommended to use 110 ℃~120 ℃.

5. Nitric acid

In principle, the degreasing speed accelerates with the increase of nitric acid flow rate.

But if the specified flow rate is exceeded, the concentration of oxidizing gas plus formaldehyde may cause spontaneous combustion in extreme cases. Based on experience, we recommend and regulate that the concentration of nitric acid is higher than 98.5%.

To ensure safety in production, degreasing furnace manufacturers generally mandate the concentration of nitric acid.

6. Degreasing rate

At a furnace temperature of 110 ℃, the typical speed at the degreasing front is between 1-2mm/h. If the loading capacity of the furnace increases, the degreasing time needs to be extended.

If the part stays in the furnace for more than the minimum degreasing time, it generally has no adverse effect on the part.

Tip: Due to the tendency of degreased parts to come into contact with moisture in the air and rust, we sometimes place the degreased parts directly in the furnace until they need to be removed from the furnace; Some companies also specialize in making nitrogen protection boxes for storing formed parts that have been degreased and ready for sintering.

7. Defatting operation control

There are two ways to detect the effect of degreasing.

7.1 Degreasing operations can be controlled by weighing and detecting one or more degreased parts. Calculate the degreasing rate by measuring the changes in the mass of the parts before and after degreasing.

When adjusting parameters, the degreasing rate can be tested each time by adjusting parameters such as degreasing time and temperature, until the data no longer changes, in order to find the optimal parameter combination.

7.2 Auxiliary testing method: Cut open the thickest part of the part wall and observe the fresh fracture surface to evaluate the degreasing condition. The fracture surface of the core that has not been degreased will show different colors. If degreasing is sufficient, the color of the fracture surface will be consistent.

Degreasing rate=(weight before defatting - weight after defatting)/weight before defatting

8. Continuous degreasing

The shorter degreasing time of Catalold ® molded parts makes it possible to use continuous degreasing and sintering.

Like batch furnaces, continuous furnaces can also use a mixture of nitrogen and catalyst gases, with the flow direction of the gas opposite to that of the parts, while being discharged and burned at the top.

At the entrance of the furnace, the parts need to pass through a preheated gate to enter, in order to prevent the catalyst from condensing on the surface of the parts.
Shenzhen Yujiaxin Tech. Co.,Ltd targeting Catamold ® The degreasing method has designed a continuous degreasing and sintering furnace system. The operation process is as follows:

>Place the MIM forming part in the first heating zone of degreasing and heat it to 86 ℃ under a nitrogen atmosphere to avoid condensation of nitric acid on the billet during the subsequent catalytic degreasing process.

>Then move the formed blank into the catalytic degreasing zone to decompose the polyformaldehyde resin into formaldehyde.

>After preliminary degreasing, the billet enters the sintering furnace through the first cleaning chamber, and residual binder is removed in the first heating zone of the sintering furnace.

>Subsequently, sintering is carried out under the action of nitrogen, hydrogen, argon, decomposed ammonia, and other mixtures.

9. Summary

One important feature of the Catamold ® method is the use of catalysts for degreasing, which eliminates the liquid phase during degreasing and avoids the weaknesses of MIM products that are prone to deformation and difficult to control size accuracy. This is a major breakthrough in the MIM industry, and due to catalytic degreasing, it greatly shortens the degreasing time, thereby reducing costs and enabling the production of large-sized MIM components.