Injection molding of advanced ceramics

Injection molding is to mix ceramic powder and organic binder, and inject ceramic powder into the metal mold cavity through an injection molding machine in the temperature range of 130-300 °C. blank. The advantages of the ceramic injection molding process are mainly reflected in the following aspects:

• It has superior molding ability and can produce precision parts with complex shapes.
• Since the raw material is finer powder particles, the sintering density is high, the relative theoretical density of solid phase sintering can reach more than 95%, and the liquid phase sintering can reach more than 99%, and its microstructure is fine and uniform, with Excellent mechanical properties.
• Injection molding is in the state of isostatic pressing during the injection process, and the obtained molding blank has a uniform density. Uniform shrinkage of sintering is guaranteed. Therefore, the parts produced by the injection molding method have high dimensional accuracy and small tolerances.
• The product utilization rate of injection molding is high, and the sintered parts can be used without or only need a small amount of subsequent mechanical machining, which reduces the production cost and can achieve higher production efficiency.Zirconia ceramic parts

The process principle of injection molding The principle of ceramic injection molding is as follows: firstly, the ceramic powder is mixed with an appropriate amount of binder to prepare a feed suitable for injection molding process requirements. When the temperature rises, the feed will have better fluidity. At this time, under a certain pressure, the injection molding machine will inject the fluid feed into the cavity of the mold to make a blank: after it cools, take out the The solidified shaped blank is degreased at a certain temperature, the binder contained in the blank is removed, and then sintered to obtain a ceramic part of the desired shape and size.

Process and application of injection molding

Injection molding is the ratio and mixing of ceramic powder and thermoplastic organic carrier. After granulation, it is injected into the injection machine and injected into the mold cavity. Slowly heat and pressurize in the plastic furnace to remove the organic carrier (this process is the plastic discharge). Finally, a ceramic green body is obtained.

Preparation of feed

By “feed” is meant a mixture of powder and binder.

a. Powder properties

Because the size of the parts prepared by injection molding shrinks greatly after sintering, in order to prevent their deformation and control the accuracy of the size, the content of the powder in the feed must be increased, that is, the packing density of the powder must be increased.

b. Choice of binder

The powder particles and the binder are mixed to form agglomerates of a certain size and shape, and the binder plays a key role in the injection molding method. For ceramic injection molding. The ideal binder requires the following:

• Make the feeding material have good fluidity to satisfy its defect-free filling process.
• Good wettability between the binder and ceramic powder particles, good adhesion to the powder particles, and no chemical reaction between the binder and the powder particles.
• The binder generally adopts a multi-component system. Multi-component organics are easier to meet the fluidity of the feed required for injection molding than single-component binders. In addition, the binder composed of multi-component organic matter is more favorable for degreasing.
• The adhesive should have higher thermal conductivity and lower thermal expansion coefficient, which can reduce the defects caused by heat concentration and thermal shock. V) Each component in the binder must be chemically miscible and compatible without phase separation.
• The binder must also be non-toxic, non-polluting, non-volatile, non-moisture-absorbing, non-deteriorating by circulating heating, easy to remove, non-residual, and have a long storage life.

c. Mixing process

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

• I) Diffusion mixing mechanism:The diffusive mixing mechanism believes that the driving force for mixing comes from the chemical sites of two different substances, and this mechanism is dominant in mixing between small molecular substances, such as liquid substances, because it can be performed quickly and efficiently.
• Ⅱ) Convective mixing mechanism:The convective mixing mechanism is believed to cause laminar flow in high-viscosity fluids through the action of external forces, such as mechanical force, thermal force, etc.
• Ⅲ) Dispersion and mixing mechanism:The dispersive mixing mechanism means that the size of the agglomerated particles becomes smaller under the action of higher external force. The strong Brownian motion under the action of strong external force makes the system get a uniform mixing state.

Preparation of blanks

In the preparation process of the blank, the feedstock is first obtained according to a certain ratio, and then dried and solidified after heating and mixing, and then crushed and granulated, so that the plasticized granular blank can be obtained.

Injection process

The injection process is the key sequence of the whole process. The ceramic injection molding process is formed by virtue of the properties of high molecular polymers that melt at high temperatures and solidify at low temperatures.

Degreasing process

Degreasing is the process of removing organic substances in the molding body by heating or other physical methods and producing a small amount of sintering. Degreasing is the most difficult and important step in injection molding, and it is also the longest step in the injection molding process.

Application of injection molding

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, cemented carbides, permanent magnet alloys, as well as alumina, Ceramic materials such as zirconia and silicon nitride.

Molds for injection molding

The molds used for injection molding are usually metal molds. The following points should be paid attention to in the design of the mold:

1. The shrinkage of the green body inside the metal mold is very small, usually 0.1% -0.2%, so the size of the blank and the metal mold is basically the same.
2. The metal mold must have no cooling groove for cooling and heating, so that the metal mold can maintain a certain temperature, and the cooling groove is connected to the temperature adjustment structure.
3. The thinnest injection port in the metal mold is easily worn due to the high-speed and high-pressure molding of the blank. Some use a layer of nickel-chromium alloy plating on the inner wall of the cylinder to improve its wear resistance.
4. Mechanical equipment for injection molding The injection molding machine is generally composed of an injection mechanism, a clamping mechanism, a hydraulic mechanism, and an electronic and electrical control mechanism. 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.