The Development of freeze casting technique for powder injection molding

Regarding ceramic production, the most common method found among industries is molding clay. Simply because it is easy and doesn’t depend on technology. However, in large scale production, most factories choose to form the products from slip, (The liquid resulted from the combination of clay) water, and certain chemical by pouring it into mold.

Ceramic products can be formed in several ways, for example, plastic forming, slip casting, pressure casting, injection molding, and machining. There are both advantages and disadvantages present in each method.

Plastic Forming is the oldest method used among industries. By mixing clay and other materials altogether, or using slip, then filter it before the forming. There are many kinds of plastic forming, for example, hand forming, which is the most flexible way. The forming can be done using either hands or equipment depending on preference on the products’ shape. Every shape can be formed by applying this method; however, products’ sizes tend to be uncertain. It also requires much time and highly skilled workers. This kind of forming is suitable for artwork or local handcrafts with low-medium production capacity.

Slip casting requires liquid to form ceramic work. Certain liquid used in this process is generally known as slip. This method isn’t complicated, suitable for forming large size products. However, it takes time during the forming, and results in products with uncertain density. This makes the sizes of them vary after sintering. Production personnel must be available in several numbers as well.

Pressure casting is adapted from the slip casting, well known as being time-saving method. However, the products resulted from this method come out to be various in size with uncertain density.

Injection molding is suitable for the products with complicated details, but the long sintering during the process causes cracks on the products’ surfaces. Also, there are some flaws from manufacturing process such as flow lines.

Machining. Any ceramic shapes could be formed using this process, but the cost for each product is quite high due to its specification.

Disadvantages of using slip

One of the problems in ceramic forming can be caused from slip. While the slip using in slip casting is supposed to easily flow due to its low solid loading, this attribute causes shrink and high risk of appearing crack after long sintering. On the contrary, slip with high solid loading doesn’t flow as smoothly, making it hard to pour into the mold, and followed by several flaws such as incomplete mold, appearing bubbles, flow lines, and etc.

Freeze casting

This process is developed from slip casting. Freeze casting or Freezegelation could be done by mixing the powder material with liquid, such as water, then pouring it into mold. The temperature must be below the freezing point in order to freeze the liquid. The powder material would be contained in liquid crystal. After the liquid is frozen, it must be removed from the mold before going through the sublimation. The result after displacing the liquid by putting the product inside vacuum sealed container will be the product with vesicles on its surface, and no stability. So, to stabilize the product, high temperature sintering is necessary and also help in forming metal form products with vesicles at around 67-57%.

Advantages

• 1.The product shrinks less because there is no burning in order to dry the product. Sintering rarely affects the size of the product since there are holes on the finish one to ensure that no internal connection can happen.
• 2.It is a clean technology with high efficient energy use.
• 3.The process saves both time and money without any limitation of the product’s size.
• 4.It is suitable for forming elaborate work which requires size accuracy.

Disadvantages

• 1.There is not enough information on the interaction of powder material and crystal in frozen liquid.
• 2.Lack of production equipment which can produce the product with constant vesicular structure.
• 3.The product before sintering is fragile and easily to break during the transfer. 

Development

Although some countries have already been successful in adopting this method in medical industry, there has not yet been any equipment or tools for applying this for commercial distribution.

Dr. Sompong Srimanosaowapak. Dr. Jarasporn Mongkolkhachit, and research teams from National Metal and Materials Technology Center have acknowledged the importance of learning and developing science of ceramic forming in order for Thai industry to be able to keep pace with others. Therefore, freeze casting technique has been studied for the promising future.

The research team has experimented on cylindrical ceramic tubes which is used to filter the coolant water. They are large, elaborate, and hard to produce with common manufacturing process. The imported ones cost 50,000 baht/each and by ordering domestic company, the price can go up to 80,000 bath.

In the study, researchers form cylindrical ceramic tubes using freeze casting technique, then compared them with those produced from slip casting. They discovered that the ones produced by using slip casting can yield higher compression, but more fragile, and with various flaws present. The products tend to easily break even from just simple transfer. On the other hand, the products produced by using freezing casting are more complete. They don’t easily break, and when going through sintering, they diagonally shrink less.

Boonrak Kanjanawarawanich 

National Metal and Materials Technology Center