Dry pressing forming method for advanced ceramics

1. Process principle of dry compression molding

Dry pressing forming is based on pressing powder supported blanks into a model under high pressure. Its essence is the process of particles approaching each other within the mold under external forces, and firmly connecting each particle with the help of internal friction force to maintain a certain shape.

1) Powder pressing process

The process of forming a powder material into a certain shape or shape (blank) through uniaxial compression in a steel mold is called compression molding.

(2) Flowability of powder

Although the powder is composed of solid grains, it has a certain fluidity due to its high dispersion, and its characteristics are expressed by the Angle of repose of the powder itself.

2. The process and application of dry compression molding

(1) The process of dry compression molding

The process of dry compression molding generally includes the following processes:

① Feeding: Fill the powder particles into the mold frame. In order to ensure the specifications and quality of the billet, the feeding should be pre uniform and quantified.

② Pressure forming: Using the relative motion between molds to apply pressure to the loose powder material, making the powder material compact into a dense body.

③ Demolding: Remove the formed blank from the mold cavity.

④ Ejection: Move the ejected formed billet onto the billet table or conveyor belt.

⑤ Clean the mold: If necessary, spray oil on the inner wall of the mold cavity to lubricate.

(2) Advantages and disadvantages of dry compression molding

Dry pressing is a commonly used forming method in the production of advanced ceramics and engineering ceramics, mainly because it has simple process, convenient operation, short cycle, high efficiency, and is convenient for automatic production. In addition, the green body obtained by this forming method has high density, precise size, small shrinkage, high mechanical strength, and good electrical properties.

(3) Application of Dry Compression Forming

Dry compression molding is a method of adding a certain amount of organic additives to the powder, then injecting it into the mold and relying on external pressure to form it.

4. Main defects of dry pressing forming method

For formed billets, if the ingredients are mixed and the molding operation is poor, the billets will produce defects or become scrap. And some defects are only exposed after drying or burning.

Spalling refers to the defect of layered cracks inside the pressed billet. This is often the main defect in molding, and spalling is not only affected by non-uniformity in compression molding, but also related to many factors.

(1) Influence of gas phase

Most of the gas in the blank is eliminated during the pressing process, and some is compressed. It should be emphasized that the reduction in the volume of the blank during pressing does not equal the volume of air removed from the blank.

(2) The influence of moisture on the green body

Too much water in the semi dry pressed billet will cause spalling, because the Compressibility of water is very small and elastic. Under high pressing force, water is squeezed into the pores from the gaps of particles. After the pressure is removed, it re-enters between the particles, causing them to separate and causing the volume of the green body to expand, resulting in spalling.

(3) The Effect of Pressurization Times on Spalling

As the number of loading and unloading increases, the residual deformation gradually decreases. So under the same conditions, the density of the green body after multiple presses is higher than that of one press.

(4) The impact of suppression time and pressure

Under the same conditions, gradually increasing the pressure, that is to say, extending the pressurization time, can also achieve results similar to a significant degree of shrinkage.

(5) Other factors

In addition to the above factors, there are also many factors that affect the generation of defects in the green body.

5. Mechanical equipment for dry pressing forming method

(1) Overview of hydraulic press

The hydraulic press was first invented by J. Brammer of England in 1795 by applying the Pascal principle. The working medium at that time was water. Since then, the hydraulic press was born. At that time, the equipment was mainly used for packing and pressing vegetable oil. Currently, depending on the fluid used, hydraulic presses include hydraulic presses and hydraulic presses.

(2) Advantages of hydraulic press

The hydraulic press adopts a hydraulic transmission system, which has the following advantages:

① The pressure of the liquid and the size of the working piston (plunger) can be selected within a large range, and hydraulic presses can easily obtain large pressing force to meet the requirements of pressing large-sized products.

② The use of hydraulic transmission can easily adjust and control parameters such as pressing pressure, rate, and holding time, and ensure its stability, which can well meet the requirements of the forming process.

③ The static pressure is applied to the pressed body, which works smoothly and is conducive to pressing and forming.

④ Easy to achieve automated operations.

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