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What is Ceramic Injection Molding Technology (CIM)?

Ceramic Injection Molding (CIM) is a new process for preparing ceramic parts developed by combining the polymer injection molding method with the ceramic preparation process. CIM technology is similar to metal injection molding (MIM) technology. They are the main branches of powder injection molding (PIM) technology and are developed based on relatively mature polymer injection molding technology. For mass production of ceramic products with high dimensional accuracy and complex shapes, the use of ceramic powder molding is the most advantageous.

The ceramic injection molding technology is to add thermoplastic resin, thermosetting resin, plasticizer, and anti-friction agent to the ceramic powder to make the ceramic powder into a viscous elastomer, and then inject the heated and mixed chain slurry into the metal mold from the nozzle. Commonly used thermoplastic resins are polyethylene, polystyrene, and polypropylene, and the additional amount is 10%~30%. CIM technology can achieve the appearance that the traditional dry pressing process cannot achieve, and the product can almost not be restricted by appearance.

Features of Ceramic Injection Molding Technology (CIM):

• The molding process has the characteristics of a high degree of mechanization and automation, high production efficiency, short molding cycle, and high blank strength, and its process can be precisely controlled by the program, which is easy to achieve large-scale and large-scale production.
• It can nearly net shape various 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 cost.
• The formed ceramic products have extremely high dimensional accuracy and surface finish.

• One-time equipment investment and processing costs are high, and it is only suitable for mass production.

The Process Flow and Process of Ceramic Injection Molding (CIM):

• Feeding preparation:
  The feed is a mixture of powder and binder. The injection process requires the injection feed to have good fluidity, which requires the selection of powders that meet the requirements and an appropriate binder system, and a certain loading ratio at a certain temperature. Injection molding feeds, to ensure the smooth progress of the subsequent process, and its products may go from the laboratory to the high-tech market. So, feed preparation is critical in the whole process.
• Injection molding and mold design:
  Improper control of the injection molding process can lead to the formation of many defects in the product, such as cracks, pores, welds, delamination, powder, and binder separation, etc., which cannot be detected until debinding and sintering. CIM often uses multi-cavity molds. The dimensions of each cavity are different, and the wear of the cavity in use will lead to different sizes of parts. In addition, the use of injection return material can affect viscosity and rheology. Therefore, controlling and optimizing molding parameters such as injection temperature, mold temperature, injection pressure, and pressure holding time is crucial to reducing the fluctuation of green body weight, preventing the separation and segregation of components in the injection material, and improving product yield and material utilization. The mold design of CIM technology mainly considers the flow control of the feed material in the mold cavity during injection molding. Because most CIM products are small-sized parts with complex shapes and high precision requirements, it is necessary to carefully design and arranges the position of the feed port, the length of the runner, and the position of the exhaust hole. Of course, mold design requires a clear understanding of feed rheological properties, cavity temperature, and residual stress distribution. In addition, computer simulation technology will play an important role in powder injection molding mold design.
• Degreasing process:
  Degreasing is the process of removing organic matter from the molding body and producing a small amount of sintering by heating and other physical methods. Compared to batching, molding, sintering, and post-processing of ceramic parts, debinding is the most difficult and important factor in injection molding. Incorrect process methods and parameters of the debinding process cause inconsistent product shrinkage, resulting in deformation, cracking, stress, and inclusions. Debinding is important for subsequent sintering, and cracks and deformations that occur during debinding cannot be compensated for by sintering. Binder and degreasing are linked together to determine the way of degreasing. In addition to traditional thermal degreasing and solvent degreasing, the current degreasing process also includes catalytic degreasing and water-based extraction degreasing developed in recent years.
• Sintering:
  The degreasing ceramic green body is densified and sintered at high temperature to obtain dense ceramic parts with desired appearance, shape, dimensional accuracy, and microstructure. Since the ceramic injection molding blank contains pores left by degreasing, the product shrinkage rate is relatively large during sintering, usually reaching 13%-15%. The research focus of CIM technology is the precision control of sintered dimensions. In addition, sintering equipment is also the key to sintering technology.

Several Elements of the Ceramic Powder Injection Molding Process:

• Selection of raw material powder:
  The cheap and high-quality powder is one of the key factors in the CMI process. The physical and chemical properties of the selected ceramic powder, such as particle morphology, size, distribution, and specific surface area, have a great influence on the rheological properties of the mixed melt. The influence of the characteristics of ceramic powder on the injection molding melt is reflected in the solid phase volume fraction, powder particle size, and particle size distribution. In addition to the general powder injection molding requirements for raw material powders, such as powder with no agglomeration, cleanliness, and no impurities, etc., CMI has some special requirements for powder performance. Requirements, improve the stability of the molding process and optimize the particle size distribution of the ceramic powder, increase the volume fraction of the solid phase, or reduce the viscosity of the suspension.
• Selection of binder:
  Binder is the core and key in CMI technology. The uniform mixing of the binder and the powder can improve the fluidity of the powder and enable the powder to be filled into the desired shape. It has an important impact on the entire process. Therefore, the composition and configuration of the binder are the most confidential technical know-how in injection molding.

  Adhesives for CIM must have the following properties:

  • Flow characteristics: The fluidity is related to the molecular weight and distribution of the binder. Generally, low-molecular-weight binders have lower viscosity and better flowability, while high-molecular-weight binders have higher viscosity and higher flowability. In general, we believe that the viscosity of the binder at the injection molding temperature should be less than 0.1 Pa.s, and the fluctuation of the viscosity with temperature should not be too large.
  • The relationship between the binder and the powder: The binder must be able to wet the powder well and have good adhesion to the powder. The wetting angle between the binder and the powder should be small. To improve the wetting properties of the binder, some surfactants, such as stearate and titanate, are added. At the same time, the binder generates capillary force to adsorb the particles by wetting the particles, so that the green body is not deformed. Furthermore, the binder should be inert concerning the powder.
  • The binder is composed of multi-component organic substances: To meet the fluidity requirements of the feed, it is difficult to achieve a single type of organic binder, and the binder composed of multi-component organic polymers is more favorable for degreasing at the same time, the compatibility between the organic polymers of each component of the binder is better.
  • The adhesive has higher thermal conductivity and lower thermal expansion coefficient: Higher thermal conductivity dissipates thermal energy in a larger area to avoid defects due to thermal stress, while a lower thermal expansion coefficient can Reduce the thermal shock of the body and reduce defects. In addition, the binder must also be non-toxic, non-polluting to the environment, non-volatile, non-hygroscopic, non-changing in cyclic heating performance, etc.

• Mixing process:
  Before injection molding, the ceramic powder and the binder must be fully mixed evenly. After the binder formulation is selected, the addition amount should be limited to the minimum required. The mixing sequence is to first add a binder with a high melting point and a large particle size, mix and dissolve, then add the ingredients with a low melting point, add powder, and finally add a plasticizer, generally mixing for more than 30 minutes. There are three types of mixers suitable for preparing mixtures for ceramic injection molding, the rolling mills and Banbury mixers are batch operated, and the extruders are semi-continuously operated. Extruders are generally single-screw or twin-screw, with the latter being more efficient. A distinguishing feature of many screw extruders is the variety and variation in the geometry of the runners to avoid unmixed material.
• Injection molding process:
  Injection molding refers to a process in which pellets are heated and softened by an injection molding machine and then injected into a mold, and then cooled and re-solidified in the mold to obtain the desired shape. The injection molding machine consists of an injection device, a clamping device (mold installation part), a hydraulic device, and an electrical control device. Injection molding ceramic materials require the corresponding parts of the injection machine to be resistant to wear, especially the screw, check valve, and barrel of the injection machine.
• Degreasing process:
  Degreasing is required before sintering. Degreasing is the process of removing the binder components in injection molding by physical or chemical methods. It is the longest and most critical step in the ceramic injection molding process. With the increase and improvement of the binder system, a variety of CIM degreasing methods have been formed, including solvent degreasing, siphon degreasing, catalytic degreasing, water-based extraction degreasing, supercritical extraction degreasing, microwave degreasing, etc.
• Sintering process:
  After degreasing, the size of degreasing is almost the same as that of the preform, it is a porous molded body with low density, so high-performance dense products need to be obtained by high-temperature sintering. Various sintering methods and densification measures in powder metallurgy are suitable for CIM. The sintering speed is related to viscous flow, coagulation, volume diffusion, surface diffusion, etc. The smaller the particle diameter, the lower the melt viscosity, and the higher the surface tension, the faster the sintering rate. And after sintering, the product generally has a shrinkage rate of about 13-20%.

Application of CIM Ceramic Injection Molding:

Injection molding technology has been applied to the preparation of various high-performance ceramic products. Such as ceramic medical devices in the biomedical field, ceramic brackets, and ceramic dental posts for dental orthodontics and restoration. Zirconia ceramic ferrules and ceramic sleeves for optical communications. Alumina insulating ceramic components are used in the semiconductor and electronics industries, such as integrated circuit high-encapsulation tubes, small vacuum switch ceramic tubes, and small ceramic sliding shafts. Ceramic knives, ceramic bracelets, and ceramic cases are used in modern life and watchmaking. Turbine rotors, blades, aircraft, spacecraft bearings, rocket nose cones, etc. in the aerospace industry. Automotive engines, valves, pistons, turbocharger rotors, nozzles, etc. in the automotive industry.