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Catalytic de-binding process and principle of metal powder injection molding

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2019-03-22 13:40:00
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Catamold? catalytic de-binding method is a one-step de-binding method for metal injection molding developed by Bloemacher of BSAF in Germany in the early 1990s. It is a catalytic de-binding method most commonly used by Chinese metal injection molding manufacturers.

1.Acid removal (catalytic de-binding) works

The main component of Catamold? adhesive is acetal resin. Due to inherent chemical structure of acetal resin, adhesive is directly decomposed by catalytic gas phase. Polyacetal resin chain is characterized by repeated carbon-oxygen bonds, as depicted in picture 1. Oxygen atoms of polymer chain are sensitive to action of acid, and when exposed to an acid catalyst at a suitable temperature, chemical reaction will cause the macromolecule to split into CH2O (formaldehyde) units.

For catalyst used in the de-binding process, a high concentration of gaseous nitric acid (gasification of liquid nitric acid having a nitric acid content of 98.5%) is currently widely used. This catalytic reaction of nitric acid is particularly suitable for de-binding process with metal injection molding. 

At 110 ℃ 130 ℃ ∽, de-binding rate is very high, temperature is much lower than melting of the poly formaldehyde interval (165 ℃ ~ 180 ℃).In this way, the polymer is converted directly from a solid to a gas. Because Catamold ? kinds of different, adhesive - gas interface to within 0.5 ~ 2 mm/h linear velocity to proceed. Small formaldehyde gas molecules (boiling point - 21 ℃) can be easily by porous outer overflow has formed parts, without disrupting the powder particles stacking structure (picture 2).At this point, adhesive is still very strong, so that part in de-binding is a continuation of the hardness, to avoid any plastic deformation, and get a good tolerance.

2. Catalytic de-binding furnace

As shown in picture 3, parts to be de-binding are placed on support plate of catalytic de-binding furnace. The furnace is equipped with a fan to ensure sufficient mixing and circulation of gas, and nitric acid catalyst is metered into furnace body by a metering pump and then vaporized in the de-binding furnace and used as a carrier gas with nitrogen. Metal parts with de-bind parts cannot be allowed to corrode, otherwise they will counteract reactions occurring in gas atmosphere.

A test de-binding furnace (50 L) requires about 40 g/h of nitric acid and 500 L/h of nitrogen. Before starting de-binding, it is necessary to carry out cleaning operation of nitrogen gas as an inert gas for one hour, and simultaneously heat the furnace and green part to 110 ° C ∽ 130 ° C, and then pump  nitric acid cyanogen into furnace. The de-binding time is too long and will not damage parts, and de-binding time is too short, so de-binding is not clean and waste is generated. Therefore, it is recommended that de-binding time be greatly reduced by the gear per hour, until the weight loss begins to decrease to determine a reasonable de-binding time.

During test, especially for small parts, de-binding time of the full load furnace can be increased by 50%. Similarly, when the ratio increases, these security measures should be taken again.

Exhausted gas is subjected to two-stage combustion treatment. In first stage burner, reaction gas is combusted in an oxygen deficient propane gas flame. Mainly reduce remaining nitric acid and NOx component produced by nitric acid to N2. Here, the formaldehyde gas has a strong reduction reaction.

In second stage burner, added air is completely combusted. exhaust gases entering the flue after combustion comply with current environmental regulations.

3. De-binding temperature

In order to maintain a safety difference with boiling point of nitric acid, actual lower limit value is 100 ° C, and upper limit value is set according to melting temperature of adhesive (150 ° C to 170 ° C). In fact, upper limit temperature is 140 ° C, and it is generally recommended to use 110 ° C ~ 130 ° C.

There are two ways to degrease effect.

a. De-binding can be controlled by weighing one or more degreased parts. The de-binding rate is calculated by weighing change in mass before and after de-binding of part. When adjusting parameters, you can adjust de-binding rate by adjusting parameters such as de-binding time and temperature until data no longer changes and find best combination of parameters.

b. Auxiliary test method: Cut thickest part of part thickness and observe fresh fracture surface to assess de-binding condition. Different colors are displayed on fracture of core which is not de-bind. If de-binding is sufficient, color of fracture is same.

Keywords in the article: metal injection molding
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