Injection molding method for advanced ceramics

Grouting forming is a molding method that disperses ceramic powder in a liquid medium to form a suspension with good fluidity. The suspension is injected into a mold with a certain shape, and the suspension is solidified into a green body with a certain shape through the water absorption effect of the mold.

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1. Process principle of injection molding

Injection molding is based on the rapid absorption of moisture by gypsum molds. The principle of its molding process is not entirely clear in theory, but it is generally believed that the injection molding process is basically divided into the following stages:

(1) The first stage is from the mold absorbing water to the formation of a thin layer.

(2) The second stage is the gradual thickening of the mud layer after the formation of a thin mud layer, until it reaches the formation of the injection.

(3) The third stage is from the formation of the billet until demolding, which is the shrinkage demolding stage.

The process of slurry grouting essentially involves absorbing water from the slurry through the capillary suction of the gypsum mold, thus forming a mud layer on the mold wall. The formation rate of mud layers can be adjusted from the following aspects:

(1) The nature of mud: For mud, the high content of fine particles in the mud is not conducive to the rapid formation of the mud layer due to complete gelation.

(2) The injection molding time can be adjusted by controlling the water absorption capacity of the mold.

(3) Increasing the pressure difference on both sides of the mud layer can be used to shorten the molding time.

(4) Reducing the viscosity or relative density of water can also be used to increase the permeability of water, thereby shortening the molding time. In production, increasing the temperature of the slurry is commonly used to shorten the grouting molding time.

2 The Process and Application of Grouting Forming

(1) The process of injection molding

① The preparation of slurry is particularly important for injection molding, as its performance is crucial. There are the following requirements for the performance of the slurry:

a. The fluidity should be improved, which means that the viscosity of the slurry is small, which is beneficial for the slurry to fill all corners of the entire mold cavity;

b. Stability is better. The slurry can maintain stability for a long time and is not prone to sedimentation and layering;

C Thixotropy requires that the viscosity of small slurry has little change after injection for a period of time, and the green body after demoulding will not be softened by slight external force, which is conducive to maintaining the shape of green body;

d. The water content should be as small as possible under the condition of ensuring fluidity, which can reduce the forming time and drying shrinkage, thereby reducing the deformation and cracking of the green body;

e. The permeability is better, and the water in the slurry is easy to pass through the formed green layer, which can be continuously absorbed by the mold wall, causing the mud layer to continuously thicken;

f. The green body formed with good demolding performance is easy to detach from the mold and is not prone to chemical reactions with the mold wall;

g. The slurry should be as free of bubbles as possible and can be achieved through vacuum treatment.

② Pouring molding: Pour the slurry into a breathable, hairy and fine mold (mostly gypsum mold) with the required body size and shape, similar to the filtration process, where the same material accumulates at the interface between the mold and the slurry, forming a green mold.

(2) Application of Grouting Forming

The injection molding technology utilizes the water absorption of gypsum molds to inject the produced ceramic slurry into porous molds, and the liquid in the slurry is sucked out by the pores of the molds, leaving the green body in the molds.

3. Injection molding method

The injection molding method is divided into basic injection molding and accelerated injection molding methods. The basic grouting methods are divided into hollow grouting and solid grouting; Accelerated grouting methods include vacuum grouting, pressure grouting, and centrifugal grouting.

(1) Basic grouting method

① Hollow grouting, also known as single sided grouting, uses a gypsum mold without a mold core during grouting.

② Solid grouting, also known as double-sided grouting, uses a gypsum mold consisting of an outer mold and a mold core.

(2) Accelerated grouting method

① Vacuum grouting: Vacuum grouting is a commonly used accelerated grouting method, which forms a pressure difference between the inside and outside of the grouting mold to accelerate the water absorption capacity of the gypsum mold, thereby improving the efficiency and quality of grouting molding.

② Pressure grouting: Pressure grouting also forms a pressure difference between the inside and outside of the grouting mold to increase the driving force of the grouting process.

③ Centrifugal grouting: Centrifugal grouting is the process of rotating the mold while the slurry is injected into it.

4. Main defects of grouting molding

Due to factors such as the performance of the slurry, the properties of the gypsum mold, and the operating steps, the quality of the parts prepared by the injection molding method can be affected. Therefore, the green body after injection molding may have the following defects:

(1) Cracking

The main reason for this is the stress generated by uneven shrinkage. The specific reasons for its occurrence are as follows:

① The degree of dryness and wetness of each part of the gypsum mold varies, with different water absorption amounts and rates, resulting in varying degrees of dryness and wetness, uneven shrinkage, and cracking in each part of the green billet.

② The thickness of each part of the workpiece is inconsistent, with the most prominent changes at the junction of thickness and unevenness of shrinkage, leading to cracking.

③ During grouting, it is easy to form an interlayer containing air after the slurry is interrupted before grouting.

④ The quality of the mud is not good, and the aging time is not enough.

⑤ The gypsum mold is too wet or too wet.

⑥ Insufficient or excessive use of plastic clay.

⑦ The blank body has been stored in the mold for too long.

(2) Poor or slow formation of green bodies

The specific reasons for its occurrence are as follows:

① Insufficient or excessive electrolyte, or impurities in the mud that promote coagulation.

② The water content of slurry is too high or the water content of gypsum mold is too high and water absorption is Supersaturation.

③ The mud temperature is too low, usually not lower than 10-200C.

④ The temperature in the production workshop is too low, generally speaking, it is best to maintain it at around 22 0C.

⑤ The distribution of pores in the mold is uneven or the porosity is too low.

(3) Difficulty in demolding the billet

The reasons for its occurrence are as follows:

① The oil film attached to the surface of the new gypsum mold was not effectively removed when using it.

② The water content of the mud is too high or the mold is too wet.

③ The content of plastic clay in the mud is too high or there is improper gel release.

(4) Bubbles and pinholes

The reasons for its occurrence are as follows:

① The gypsum mold is too dry, wet, overheated, or old.

② There are bubbles in the mud that have not been eliminated.

③ When grouting, the slurry is added too quickly, sealing the air in the slurry.

④ The floating dust inside the gypsum model was not removed, and during firing, the floating dust evaporated into bubbles, forming pinholes.

⑤ Improper design of gypsum model hinders gas discharge.

(5) Mud wisp

The reasons for its occurrence are as follows:

① The viscosity and density of the mud are too high, and the fluidity is poor.

② Improper grouting operation, prolonged pouring time, rapid discharge of grout, lack of a certain slope or unclean return of grout

③ The temperature in the production workshop is too high, and the mud has wrinkled inside the gypsum mold, which was not removed during pouring.

④ It is related to the shape of the workpiece, and the mold with excessive slope and twists affects the flow of the slurry.

(6) Deformation

The occurrence of this defect is mainly due to the following reasons:

① The moisture content in the gypsum mold is uneven, resulting in premature demolding.

② The mud contains too much water and the use of electrolytes is not appropriate.

③ Improper design of parts and molds can easily deform the cantilever part.

(7) Collapse

The main reason for this defect is:

① The particles in the mud are too fine, there is too much water, the temperature is too high, and there is too much electrolyte.

② Caused by excessive humidity or unclean internal surface of gypsum molds.

5. Mechanical equipment for injection molding

(1) Vacuum degassing equipment

During the preparation, transportation, and storage of the slurry, it is inevitable to have gas mixed in, which can affect the density of the injection and the mechanical strength, electrical properties, and other properties of the product.

(2) Automatic grouting equipment

At present, the injection molding has been automated, and the production line uses a circular mechanized injection molding line with chain pullers. According to the production process of grouting, the forming process includes automatic grouting, feeding, pouring, residual grouting, drying, demolding, mold closing, model drying, and grouting stages.

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