Injection molding method for advanced ceramics Injection molding is the process of mixing ceramic powder and organic binder, and then injecting ceramic powder into a metal mold cavity through an injec

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Injection molding is the process of mixing ceramic powder and organic binder, and then injecting ceramic powder into a metal mold cavity through an injection molding machine at a temperature range of 130-300 ℃. After it cools down and solidifies, the formed green body can be taken out. The advantages of ceramic injection molding process are mainly reflected in the following aspects:

(1) Has superior forming ability and can produce precision components with complex shapes.

(2) Due to the use of finer powder particles as the raw material, the sintering density is high. Solid phase sintering can achieve a relative theoretical density of over 95%, while liquid phase sintering can achieve over 99%. Its microstructure is fine and uniform, and it has excellent mechanical properties.

(3) Injection molding is in an isostatic pressure state during the injection process, resulting in a uniform density of the formed billet, ensuring uniform shrinkage during sintering. Therefore, the injection molding method produces components with high dimensional accuracy and small tolerances.

(4) Injection molded products have high utilization rates, and sintered parts can be used without or with only a small amount of subsequent mechanical processing, reducing production costs and achieving high production efficiency.

1. Process principle of injection molding

The principle of ceramic injection molding is as follows: first, mix ceramic powder with an appropriate amount of binder to prepare a feed suitable for the injection molding process requirements. When the temperature increases, the feed produces good fluidity. At this time, under a certain pressure, the injection molding machine injects the flowable feed into the mold cavity to make a blank. After it cools down, the solidified formed blank is taken out and degreased under certain temperature conditions to remove the adhesive contained in the blank. Then, sintering is carried out to obtain the desired shape and size of ceramic parts.

2 The Process and Application of Injection Molding

(1) The process of injection molding

Injection molding is the process of matching and mixing ceramic powder with thermoplastic organic carriers. After granulation, it is added to the injection machine and injected into the mold cavity. After filling, cooling, demolding, and other processes, the required green body is obtained. It is then slowly heated and pressurized in an exhaust furnace to remove the organic carrier (this process is called exhaust molding), and finally the ceramic green body is obtained.

① Preparation of feed

The so-called "feed" refers to a mixture of powder and binder. 

a. Performance of powder

Due to the significant size shrinkage of the sintered parts prepared by injection molding, in order to prevent deformation and control the accuracy of size, it is necessary to increase the powder content in the feed, which means that the filling density of the powder must be increased.

b. Selection of adhesive

The mixture of powder particles and binder forms agglomerates of a certain size and shape, and the binder plays a crucial role in injection molding methods. For ceramic injection molding, the ideal binder requires the following:

I) Ensure good flowability of the feed to meet its defect free filling process.

II) The adhesive has good wettability with ceramic powder particles, has good adhesion to powder particles, and there is no chemical reaction between the adhesive and powder particles.

III) The binder generally adopts a multi-component system. Multicomponent organic compounds are more likely to meet the flowability of the feed required for injection molding than single component binders. In addition, multi-component organic binders are more advantageous for degreasing.

Ⅳ) The binder shall have high thermal conductivity and low thermal expansion coefficient, which can reduce the defects caused by the concentration of heat on Thermal shock.

V) The components in the binder must be chemically miscible and compatible, without phase separation.

The adhesive must also be non-toxic, pollution-free, non volatile, non hygroscopic, non deteriorating during cyclic heating, easy to remove, residue free, and have a long storage life.

c. The mechanism of mixing process is very complex, and it is generally believed that the mechanisms of mixing process include diffusion mixing mechanism, convective mixing mechanism, and dispersion mechanism.

I) Diffusion mixing mechanism

The diffusion mixing mechanism believes that the driving force of mixing comes from the chemical sites of two different substances. This mechanism plays a dominant role in mixing between small molecule substances, such as liquid substances, because it can proceed quickly and efficiently.

II) Convective mixing mechanism

The convective mixing mechanism is believed to cause laminar flow in high viscosity fluids through external forces such as mechanical and thermal forces.

III) Dispersion and mixing mechanism

Dispersion mixing mechanism means that the size of agglomerated particles becomes smaller under the action of higher external force, and the strong Brownian motion under the action of strong external force makes the system get a uniform mixing state.

② Preparation of billets

② Preparation of billets

In the preparation process of the billet, the feeding material is first obtained according to a certain ratio, heated and mixed, dried and solidified, and then crushed and granulated to obtain plasticized granular billet.

③ Injection process

The injection process is the key T-sequence of the entire process. The process of ceramic injection molding relies on the characteristics of polymer melting at high temperatures and solidification at low temperatures for molding.

④ The degreasing process is the process of removing organic substances from the formed body through heating or other physical methods and generating a small amount of sintering. Degreasing is the most difficult and important step in injection molding, and it is also the longest time-consuming step in the injection molding process.

(2) At present, the materials suitable for injection molding mainly include powder metallurgy materials such as iron based alloys, Fe Ni alloys, tungsten based alloys, titanium alloys, hard alloys, permanent magnet alloys, as well as ceramic materials such as alumina, zirconia, and silicon nitride.

3. Mold used for injection molding

The molds used for injection molding are usually metal molds. In terms of mold design, the following points should be noted:

(1) The shrinkage of the green body inside the metal mold is very small, usually between 0.1% and 0.2%, so the dimensions of the green body and the metal mold are basically the same.

(2) The metal mold must have no cooling grooves for cooling and heating to maintain a certain temperature, and the cooling grooves are connected to the temperature regulating structure.

(3) The thinnest injection part inside the metal mold is prone to wear and tear due to the high-speed high-pressure forming of the billet. Some use plating a layer of nickel chromium alloy on the inner wall of the cylinder barrel to improve its wear resistance.

4. Mechanical equipment for injection molding

Injection molding machines are generally composed of injection mechanisms, clamping mechanisms, hydraulic mechanisms, and electronic and electrical control mechanisms. Injection molding machines are divided into two categories due to the different internal structures of the plasticizing mechanism: plunger injection molding machines and hydraulic screw injection molding machines.

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