High Pressure Hot Injection Molding
What Is High Pressure Hot Injection Molding
Although High Pressure Hot Injection Molding is also a grouting method, it is different from the previous grouting process. It utilizes the thermal fluidity characteristics of paraffin, cooperates with the blank, and uses a metal mold to form it under pressure.Hot die casting or hot pressure injection molding is a molding process that is widely used in the production of special ceramics. It is uniformly mixed to form a flowable slurry, which is injected into a metal mold under a certain pressure for molding, cooled and the wax slurry is solidified and then demolded to take out the formed body.
The Process Flow Of High Pressure Hot Injection Molding
- Add surface modifiers such as oleic acid, stearic acid, etc. to the ceramic powder, and mix it by ball milling to make it lipophilic and blend well with the wax liquid.
- Add the modified powder into the melted paraffin, stir and mix until uniform.
- Add the mixed slurry into the hot die casting machine, and inject it into the mold with appropriate pressure and temperature.
- Demoulding and trimming the blank properly.
- The green body is embedded in the adsorbent, and the temperature is raised to 900 ℃-1100 ℃ at an appropriate speed, so that the green body completely excludes paraffin and has a certain strength.
- Put the green body into the sintering furnace to sinter the final product.
The Process characteristics Of High Pressure Hot Injection Molding
- It can form complex-shaped ceramic products with high dimensional accuracy and almost no need for subsequent processing. It is the main molding process for making special-shaped ceramic products.
- Short molding time and high production efficiency.
- Compared with other ceramic molding processes, the production cost is relatively low, and the requirements for production equipment and operating environment are not high.
- It has strong applicability to raw materials, such as oxides, non-oxides, composite materials and various mineral materials.
Mainly used for the production of small and medium-sized and complex structural ceramics, wear-resistant ceramics, electronic ceramics, insulating ceramics, textile ceramics, heat-resistant ceramics, sealing ceramics, corrosion-resistant ceramics, and thermal shock-resistant ceramics.
The Preparation Of Wax Slurry
The purpose of this process is to add the prepared blank to the paraffin-based binder to make a wax plate for molding.
Weigh the paraffin (according to the proportion) to a certain amount (usually 12.5% to 13.5%), then heat and melt it into a wax liquid, and at the same time, dry the weighed material in an oven so that the water content is not more than 0.5%. 2%. This is because when the moisture content in the powder is greater than 1%, the moisture will prevent the complete infiltration of the dressing and paraffin, and the viscosity will increase, making it difficult to form. In addition, when heated, the water will form small bubbles and disperse in the slurry, so that the sintered product forms closed pores and deteriorates its performance.
When preparing wax slurry, adding a small amount of surfactant (usually 0.4% to 0.8%, such as beeswax, etc.) to the powder can reduce the content of paraffin and improve the molding performance. There are two specific mixing methods:
- One is to heat the paraffin to melt it, then pour the powder into it, and stir it while heating;
- The other is to heat the powder and pour it into the paraffin solution, stirring while adding. The preparation of wax slurry can be carried out in a rotary furnace, and then the slurry is poured into a container, and after solidification, a wax plate is prepared for molding.
The performance indicators of the slurry are as follows:
Stability means that the slurry still maintains its uniform and non-layered properties under the condition of long-term heating without stirring. It is usually expressed by the stability index:
- Vo “2
- V, where u-stability index;
- Vo – Slurry volume tested (CTI13);
- Vt – volume of wax liquid separated after heating (CIT13).
- The test conditions were: 100 mL of slurry was kept at 70 °C for 24 h. separated wax
- The liquid should not exceed 0.2 mL. At this time u>500.
Castability refers to the ability of the slurry to fill the mold cavity and retain the desired shape. It is a comprehensive index to measure slurry viscosity and solidification speed. Generally speaking, if the fineness of the powder is suitable, the powder is dry, and the amount of binder (paraffin and surfactant) added is suitable, the castability of the slurry is good.
To measure the castability of wax slurry, a mold can be used as shown in Figure 1-2-10. The mold cavity is a square cone with a bottom surface of 5 mm × 5 mm, and the cone height is 200 mm. Castability is expressed by the height H of the slurry casting cavity under a certain slurry temperature, a certain mold temperature and a certain pressure (the slurry temperature is 70 °C, the mold temperature is 25 °C, and the pressure is 5 atmospheres). Its H should be in the range of 70 to 120 mm. Below 70 mm, the viscosity of the slurry is too large and the castability is poor; if it is higher than 120 mm, the slurry is too thin, and of course it is difficult to form.
Hrinking percentage means that when the wax slurry is cooled and solidified from a molten liquid state to a solid state, there will be volume shrinkage. The size of its shrinkage is related to the expansion coefficient of the powder and paraffin, the particle size, particle shape, gradation of the powder, the mixing ratio of the binder in the wax slurry, and the molding temperature. The volume shrinkage of the slurry when it solidifies in the cavity is called cold shrinkage, which is generally about one percent.
Working principle of hot die casting machine
At present, there are generally two types of hot die casting machines used in production, one is manual and the other is automatic, but the basic principles are the same. Its working principle is to place the prepared slurry wax plate in the barrel of the hot die casting machine, heat it to a certain temperature to melt, and press the slurry in the barrel into the mold cavity through the suction casting port under the drive of compressed air. After the shape and size of the mold are maintained for a certain period of time, the pressure is removed, the slurry is cooled and formed in the mold cavity, and then the mold is demolded, and the green body is taken out. Some can also be processed, or turned, or punched.
High temperature wax removal
The green body formed by hot die casting must be dewaxed before firing. Otherwise, due to the loss, volatilization and combustion of paraffin wax at high temperature, the green body will lose its bond and be released, and its shape cannot be maintained. Wax removal is to bury the green body in loose and inert protective powder, which is also called adsor-bent. It is stable at high temperature and is not easy to bond with the green body. Generally, calcined industrial A1 is used. O. powder. During the heating process, although the paraffin will melt and diffuse, the green body is supported by the adsorbent. When the temperature continues to rise, the paraffin volatilizes and burns completely, and there is also a certain sintering between the powders in the green body. At this time, neither reaction nor bonding occurs between the green body and the adsorbent, and the green body has a certain strength. Usually, the wax removal temperature is about 900 to 1100 °C, depending on the properties of the green body. If the temperature is too low, there will be no certain sintering between the powders, and there will be no mechanical strength. Clean the surface of the blank. The green body after dewaxing should be cleaned of adsorbent on the surface, and then sintered.
Advantages and disadvantages of hot die casting
The hot die casting process is suitable for the production of small and medium-sized products with complex shapes and high precision requirements. The equipment is simple, the operation is convenient, the labor intensity is not high, the production efficiency is high, the mold wear is small, and the service life is long, so it is often used in the production of special ceramics. However, hot die casting also has disadvantages, such as complicated process, high energy consumption (requires multiple firings), long construction period, and it is not suitable for large and long products with thin walls because it is not easy to fill the mold cavity.