Ceramic dry press molding is also called press molding. It is characterized by a low binder content, only a few percent (generally 7% to 8%), can be directly calcined without drying, and the body shrinks small, and can be automated.
Dry pressing is to add a small amount of binder to the powder and granulate it according to the granulation method mentioned above. Then, the granulated powder is placed in a steel mold and pressed on a press. A shape of the blank. It is suitable for pressing products with a height of 0.3-60 mm and a diameter of 5 to 500 mm.
Dry pressing process principle
The essence of dry pressing is that under the action of external force, the particles are close to each other in the mold, and the particles are firmly connected by internal friction to maintain a certain shape. This internal friction force is applied to a thin layer of peripheral binder of the particles adjacent to each other. In either case, when the particles are in contact, R will be greater than the ruler:, R. Corresponding to the micropore radius or microporosity, as the micropore pressure will bring the particles closer together, also known as “adhesion”.
1. spherical contact; 2. cusp contact
The dry compact can be thought of as a three-phase dispersion consisting of a liquid phase (binder) layer, air, and billet. If the particle size and granulation of the billet are appropriate and the stacking density is relatively high, the air content can be greatly reduced. As the pressure increases, the blanks will change shape, slide against each other, the gaps are filled, and the contacts are gradually increased and tightly attached to each other. Due to the closer proximity between the particles, the force between the colloidal molecules and the particles is enhanced, so that the green body has a certain mechanical strength. If the billet particles are properly graded, the binder is used correctly, the pressurization method is reasonable, and the dry pressing method can obtain a relatively desirable green body density.
Pressurization mode and pressure distribution
In dry press molding, there are two types of pressurization methods: simple pressurization and double-sided pressurization. Due to the different pressurization methods, the pressure is different in the friction, transmission and distribution between the mold and the powder, so the density of the green body is also different.
Single side pressurization
When pressing on one side, the pressure is only pressurized by the plug. At this time, due to the frictional resistance between the powder and the powder and the mold wall, a pressure gradient is generated. The lower the pressure, the lower the pressure distribution. , D is the diameter. As can be seen from the figure, the larger the L/D value, the greater the pressure difference within the blank. The compacted body has the highest density at the upper and near mold walls, while the lower near mold wall and the central portion have the lowest density. Pressure distribution of single flour compact
1. Low mode, L/D-0. 45; 2. High mode, L/D=1. 75
Compared with single-sided pressurization, double-sided pressurization is simultaneous pressure on the upper and lower sides. At this time, the various frictional resistances do not change, but the effective transmission distance of the pressure gradient is short, and the energy loss due to frictional force Also reduced. In this case, the density of the blank is relatively more uniform. It can be seen from Fig. 1-2-16 that the smaller the height of the product, that is, the smaller the L/D, the better the uniformity of density. Pressing on both sides, the density of the center of the blank is small. Regardless of single-sided or double-sided pressurization, if the mold is applied with a lubricant, the pressure gradient will be reduced.
Pressurization speed and holding time
Practice has shown that the pressurization speed and the dwell time have a great influence on the performance of the blank, that is, it has a great relationship with the transfer of pressure and the elimination of gas. If the pressurization is too fast, the dwell time is too short and the gas is not easily discharged. Similarly, when the pressure has not been transferred to the desired depth, the external force has been removed, and it is obviously difficult to obtain a satisfactory green body quality. Of course, if the pressurizing speed is too slow and the dwell time is too long, the production efficiency is lowered, and it is not necessary. Therefore, the pressurizing speed and the dwell time should be adjusted according to the size, thickness, and shape of the blank. Generally, for products with large size, thick wall thickness, high height and complicated shape, the pressure should be slow, the middle can be fast, the latter should be slow, and there is a certain pressure holding time, which is beneficial to gas elimination and pressure transmission. If the pressure is large enough, the dwell time can be shorter. Otherwise, the pressurization speed is not good, no gas is discharged, and bubbling, interlayers, cracks, and the like may occur. For small-sized sheet blanks, the requirements are not critical in this respect, and the pressurization speed can be appropriately increased to increase the production efficiency.
Advantages and disadvantages of dry pressing
Dry pressing is a common molding method in the production of special ceramics, because it has simple process, convenient operation, short cycle and high efficiency, which is convenient for automatic production. In addition, the green body has a large density, accurate size, small shrinkage, high mechanical strength, and good electrical properties.
However, dry press forming has difficulty in the production of large blanks. Firstly, the mold is worn out, the processing is complicated, and the cost is high. Secondly, the pressurization can only press up and down, the pressure distribution is uneven, the density is uneven, the shrinkage is uneven, and cracking occurs. Layering and other phenomena. With the development of modern molding methods, this shortcoming is overcome by isostatic pressing!