Defects in Ceramic Forming Process
The initial model for the formation of ceramic products, also known as green body, is made by the cold isostatic pressing method. The final structural form of the product is directly influenced by the quality of the green body. The raw material particle size distribution, moisture content, binder and forming pressure system, forming mold, etc. are just a few of the factors that limit the green body forming process. Enhancing the quality control of the ceramic forming process during actual production will undoubtedly be a delightful assurance for subsequent processing. When isostatic pressing is used in actual production, there will always be a number of issues, such as cracking, breaking, and falling of blanks. The simplest approach is to choose only a few and discard the rest. However, based on production experience, the remaining blanks also have potential flaws, and these flaws will emerge during the subsequent processing, increasing the scrap rate and resulting in significant production waste.
As a result, it is important to take into account the shrinkage of the raw material during forming as well as the shrinkage of the raw material, and the sintering process when designing the mold. This will help to ensure that the green body is processed as little as possible or not at all, and that the product's size, shape, and other requirements can be met without further processing after sintering. This necessitates that the raw material particle size distribution be uniform, that no separation layer will form during charging, and that the charging meet the bulk density requirements of the filling; otherwise, the green body will have an uneven particle distribution or will take the shape of a dumbbell. The former contracts inequitably while sintering, so that the d eformation flaws are easy to form. The latter's unintended density results in both defects that are similar to the former and defects that form during trimming.
Secondly, to ensure the quality of the green body, the forming pressure system is crucial, particularly the pressure relief speed, which has a significant impact on the green body. The green body will strike the mold if the pressure relief speed is too high. Or the green body's excessively large elastic aftereffect will result in flaws like fracture and end drop.
Factors Affecting Ceramic Forming Process
Uneven Particle Size on the Surface
The primary manifestation of this type of flaw is the axial or radial layering of one side of the green body, which is impacted by the alternating distribution of small and coarse particles. When the material is charged in the mold, small particles are concentrated in some areas, and large particles are accumulated in some areas, and the same pressure is applied but the shrinkage is different during forming, this situation is typically attributed to the chromatography of the raw materials. As a result, it's important to minimize the occurrence of chromatography and prevent unilateral loading when loading materials.
The Appearance of Cracks, Fractures, or Poor Strength
This type of defect is influenced by a wide range of variables, including the pressure system, the quantity and type of binder in the raw material, the raw material's moisture content, the mold, and other elements. The key element for powder forming under a specific pressure are an appropriate moisture content and binder, and the particles can be compactly formed into a green body with a certain strength during forming.
Most experts agree that the ideal cold isostatic pressing moisture content is less than 0.2%. Long-chain organic polyvinyl alcohol compounds are suggested for use as the binder. Its value is less than 1%, which is ideal, but when it drops to less than 0.5%, the bonding force is weak and the green body is more likely to fracture and have poor strength. Furthermore, the aforementioned flaws can also be caused by unreasonable pressure regimes. When the green body's elastic aftereffect is strong, it will freely extend and collide with the mold to produce a defective green body.
When forming ceramics, it is best to keep the pressure constant for 60 seconds. If the time is allowed to go on for too long, it will have a negative impact on the powder. This will cause a shift in the powder's particle size distribution, which in turn will affect the sintering process that comes after. The pressure relief time is approximately thirty seconds, but this time can be extended as necessary for large or unusually shaped products in order to maintain the green body's high level of quality.
Dumbbell-Shaped Green Body
The green body has a dumbbell-like shape with thick ends and a thin middle, which could cause the middle part to be too small and therefore impossible to process. This flaw is primarily due to insufficient filling material that does not conform to the bulk density standard for filling. In order to achieve the desired filling effect, vibrating screens or other devices should be used in actual production, and the vibration time and other necessary parameters should be specified.
Unsuitable Mold Tooling
A steel mold can guarantee the integrity of the green body's core during the forming process, while elastic resin or materials with a similar elasticity can ensure the green body's external shape. As a result, the concentricity of the core and the shape of the mold both need to be satisfactory, and the finish needs to be in accordance with the specifications of the mold. Otherwise, after forming, water seepage and green body eccentricity are likely to happen. In extreme cases, the mold will be deformed, the core won't come out when it is removed, and when it is forcibly removed, it will result in cracks and other flaws in the green body.
Conclusion
In conclusion, manufacturing flaws are common in ceramics. To ensure the quality of the blanks, appropriate molds should be created, and to ensure a smooth forming process, appropriate pressure systems, raw material moisture content, and particle size distribution should all be considered. As a result, it's essential to strictly adhere to production procedures, tighten control over the production process, and reduce forming defect rates.