Advanced Ceramic Injection Molding - A Great Process

Injection molding was first invented in 1850 and was first used for metal casting. After entering the mid-20th century, it was widely used in plastic molding, and then studied and applied in ceramic molding. Ceramic injection molding (CIM) is a technology based on plastic injection molding technology that integrates material rheology, polymers and debinding processes. The earliest use of ceramics was in the manufacture of spark plug insulators.

I. Overview
The preparation process of ceramic injection molding mainly includes the following steps: first, the preparation of injection feed, mixing ceramic powder with a suitable organic carrier at a certain ratio at a certain temperature, and then drying and granulating; then adding the injection feed to the injection machine, injecting it into the mold at a certain temperature and pressure and cooling and curing it, then removing the organic matter in the blank by heating or other physical and chemical methods, and finally sintering and densifying to obtain various ceramic products.

Compared with traditional molding methods, ceramic injection molding technology has the following advantages:
(1) It can nearly net-form small ceramic parts with complex geometric shapes and special requirements, so that the sintered ceramic products do not need to be machined or less processed, thereby reducing the expensive ceramic processing costs;
(2) The green density of the molded products is uniform and the strength is high. The product density can reach 99.8%, the sintered body has excellent performance and the consistency of product quality is good;
(3) The molding process has a high degree of mechanization and automation, high production efficiency, short molding cycle, high strength of the blank, and convenient management and control during the production process, which is easy to achieve large-scale and large-scale production;
(4) The molded ceramic products have extremely high dimensional accuracy and surface finish, and the surface finish can reach 5μm.
Therefore, CIM technology has become one of the high-precision and high-efficiency molding methods in the existing ceramic molding technology, and has been widely used and studied at home and abroad.

II. Feeding
Feeding is to mix the powder and the binder into a uniform and stable suspension at a certain temperature by kneading, uniform stirring, extrusion and other methods. The preparation of feeding occupies a very important position in the entire ceramic powder injection molding process. The main defect of the mixing process is the unevenness of the mixture, including the separation of powder and binder and the segregation of powder in the binder caused by the particle size, which will lead to the reduction of density and structural deformation of the final ceramic parts.

Powder is the basic raw material for ceramic injection molding. In ceramic injection molding, the bulk characteristics of ceramic powder particles are very important. In the natural stacking state, the larger the pores between particles, the pores are filled with binders during the preparation and feeding process, the solid content of the powder is reduced, and the sample size shrinks greatly after the sintering binder volatilizes, and the sample accuracy and structural shape are difficult to control.

In theory, the lower the porosity of ceramic powder in the natural stacking state, the better, so that less binder is removed in the degreasing stage, the green body after degreasing is denser, and the sample volume changes less in the sintering stage, which is easy to maintain shape. Ceramic powders are generally required to have small particle size and regular shape. The feed prepared in spherical or near-spherical shape has good fluidity and high solid content, but the friction between powder particles is poor, which can easily cause sample deformation in the degreasing stage. Therefore, the powder shape is not spherical.
Binders play a vital role in the injection molding process. Binders are usually composed of primary polymer components, and then various additives such as dispersants, stabilizers and plasticizers are added. The basic purpose of the binder is to maintain the shape of the molded part before sintering and to provide a certain strength to the molded part.

III. Injection Process
The injection process includes three stages: injection, pressure holding and cooling. That is, the powder is first heated and softened and then injected into the mold, pressure is held in the mold for a period of time, and finally cooled to prepare the desired shape of the blank. Each stage in the injection process is crucial. Improper control will cause many defects such as cracks, delamination, separation of powder and organic binder in ceramic parts. The parameters involved and the causes of defects are as follows:

(1) Injection speed: If the injection speed is too high, the molten feed will be locally ejected and refluxed in the mold cavity. This will produce ripples on the surface of the product and defects such as weld lines and pores inside. The injection speed is often controlled by the injection pressure. Too high an injection speed requires high injection pressure, which often leads to uneven pressure distribution inside the product. Too low local pressure will cause warping defects. When the injection speed is too low, the growth of the condensation layer near the mold wall will cause under-injection defects, prolonging the production time and reducing productivity.
(2) Injection temperature: A lower injection temperature will cause under-injection and material shortage defects. In order to facilitate injection molding, a higher injection temperature is often required to reduce the feed viscosity while avoiding the decomposition of the binder components.
(3) Injection pressure: Too low injection pressure will lead to material shortage, and turbulence will occur during the filling process, resulting in powder-binder stratification. Conversely, too high injection pressure often requires a larger clamping force, resulting in higher equipment requirements. Studies have shown that every 2MPa change in injection pressure is equivalent to the change in feed viscosity when the temperature changes by 1°C.
(4) Holding pressure: It is about 50%~65% of the injection pressure. The main function of holding pressure is to reflux and compensate for shrinkage. Too high holding pressure will cause excessive filling and stress concentration; too low holding pressure will lead to failure to compensate for shrinkage in time, resulting in a larger shrinkage rate of the green body.
(5) Holding time: It is close to the time required for the gate to solidify.
(6) Mold temperature: A small temperature difference between the mold and the feed material can reduce heat loss and defects such as under-injection and lack of material, but too high a mold temperature will prolong the holding time.

IV. Degreasing process
The degreasing process is the most important stage in injection molding, which determines the quality of the final product to a certain extent. Because most of the defects in ceramic materials are formed in the degreasing stage, such as cracks, pores, deformation, blistering, etc., and the defects generated in the degreasing process cannot be compensated by the later sintering stage. Therefore, people have been improving and looking for new degreasing processes to reduce the defects formed in the degreasing process of ceramic injection molding, so that ceramic injection molding can play a greater role.

4.1 Thermal degreasing Thermal degreasing is an early developed and most widely used degreasing process. It is particularly suitable for precision ceramic parts with relatively small cross-sectional dimensions. However, its degreasing rate is very slow and the degreasing time is very long; especially for thick-walled ceramic parts, thermal degreasing is prone to defects such as blistering, swelling, and deformation, resulting in the size of ceramic parts being limited, generally controlled to within 10 mm. Microwave heating degreasing developed in recent years is a volumetric heating method, with a fast and uniform heating process, and only takes half the time of conventional degreasing.

4.2 Solvent degreasing Solvent degreasing (also known as dissolution extraction degreasing) is a method in which a low molecular weight solvent (such as acetone, heptane, hexane, etc.) diffuses in the body, contacts and dissolves the binder, forms a binder-solvent solution, and finally diffuses to the surface of the body. Compared with thermal degreasing, solvent degreasing is efficient and the time required is greatly reduced, but it has high requirements for equipment, complex processes, and most solvents are harmful to the human body and the environment.

4.3 Siphon degreasing Siphon degreasing refers to placing the formed body on a porous substrate and then heating the body until the viscosity of the binder is low enough to allow capillary flow. At this time, the binder is sucked out into the suction material under the action of capillary force. The siphon degreasing speed is fast, but the organic carrier powder will adhere to the ceramic body and is difficult to remove.

4.4 Catalytic degreasing Catalytic degreasing was first developed by the famous German chemical company BASF. Its principle is to use catalysts to decompose organic carrier macromolecules into smaller volatile molecules that can quickly diffuse in the blank. Catalysts usually use nitric acid, oxalic acid, etc. Studies have shown that when nitric acid is used as a catalyst, the removal rate is 0.7-1.5 mm/h, and the order of removal rate is Si3N4>ZrO2>SiC; when oxalic acid is used as a catalyst, the removal rate is 0.9-1.5 mm/h, and the order of degreasing is ZrO2>SiC>Si3N4.

4.5 Supercritical degreasing Supercritical degreasing uses advanced supercritical technology to heat and pressurize the fluid to above its supercritical point to dissolve and remove part of the binder. Generally, CO2 fluid is used, which is convenient to source and simple to operate. Supercritical CO2 fluid has the property of dissolving non-polar molecules or low molecular weight organic matter (such as paraffin) but not polar molecules or high molecular weight organic matter (such as polypropylene and polyethylene). Therefore, low molecular weight organic matter can be extracted first, and then the rest can be removed by rapid heating, thereby improving the degreasing efficiency.

4.6 Water-based extraction degreasing Water-based extraction degreasing is an improvement on solvent degreasing and is widely used in ceramic powder injection molding. The binders used are divided into two categories: one is water-soluble, such as polyethylene glycol (PEG), polyethylene oxide (PEO), etc., which can be directly removed by filtering water; the other is the part that is insoluble in water, such as polyvinyl butyral resin, which is generally removed by heating. Water-based extraction degreasing has the characteristics of fast degreasing rate, little damage to the blank, and is friendly to the human body and the environment. It is an important research direction in the degreasing system.

As an emerging precision manufacturing technology, ceramic injection molding technology has its incomparable unique advantages. In particular, the continuous expansion of industrialization around the world in recent years has further demonstrated the attractive development prospects of CIM technology. The organic combination of the excellent physical and chemical properties of ceramic materials and precision injection molding will surely make CIM technology play an increasingly important role in high-tech fields such as aerospace, national defense and military, and medical equipment, and become the most advantageous advanced preparation technology for precision ceramic parts at home and abroad.