1. Standard viscosity

Also known as standard thick water usage or standard thick mixed water usage. It refers to the amount of water added to 100 parts of semi hydrated gypsum powder to achieve standard fluidity (specimen diameter 200 ± 5mm), expressed as a percentage. It is an important indicator for describing the performance of semi hydrated gypsum powder, and is the basis for comparing the quality of gypsum powder and describing other performance indicators of semi hydrated gypsum powder (comparing the performance of gypsum powder should be conducted at the same consistency). But this indicator is not the amount of mixed water used in our large-scale production model, the actual situation is larger than it. The standard consistency of α - hemihydrate gypsum is relatively small, usually between 45-55%, while the standard consistency of β - hemihydrate gypsum is relatively large, usually between 70-80%. By detecting the standard consistency of semi hydrated gypsum powder, the crystal form (type) of gypsum powder can be preliminarily distinguished and the quality of gypsum powder can be judged.

2. Fineness

It is also an important indicator for describing the performance of semi hydrated gypsum powder. It is related to the setting speed of semi hydrated gypsum in water, as well as the strength, surface hardness, and smoothness of the model. The methods for expressing fineness include mesh size (percentage of sieve residue), particle size distribution, and specific surface area. In general, the finer the fineness of gypsum powder, the larger the specific surface area of its particles, the larger the contact area with water, and the faster the dissolution rate in water. After combining with water, the reaction is complete, and the setting speed of gypsum slurry is accelerated. The more network structures are formed, the higher the model strength, but the corresponding water absorption rate decreases. The fineness of β gypsum powder is rough, generally between 80-90 mesh, and the model strength is relatively low; The fineness of alpha gypsum is fine, generally between 120-180 mesh, and the model strength is relatively high. At present, the gypsum powder used for making models in domestic ceramic production is generally between 100-120 mesh, with the highest not exceeding 180 mesh. The fineness of gypsum powder used abroad ranges from 180 to 250 mesh.

3. Condensation time

The setting time is an important performance indicator for semi hydrated gypsum powder. The setting time includes the initial setting time and the final setting time. The initial setting time is the time elapsed from when gypsum powder is sprinkled into water until the gypsum slurry loses its fluidity and begins to thicken under standard water usage; The final setting time is the time elapsed from the time gypsum powder is sprinkled into water to the time when the gypsum slurry solidifies under standard water usage. The standard for setting time of gypsum powder is: at standard consistency, the initial setting time is generally not less than 6 minutes, and the final setting time is not more than 25 minutes. The appropriate setting time is a necessary condition for the normal use of gypsum powder in production. In actual production, we prefer a longer initial setting time and a shorter final setting time. Short initial setting time or long final setting time are both not good and can affect production.

4. Strength

The mechanical strength of a model is an important performance indicator related to its service life. The strength indicators of the model (gypsum powder) include flexural strength, compressive strength, and tensile strength. The flexural strength is a commonly used main indicator to describe the strength of a model. Generally, the flexural strength is tested, including 2-hour wet flexural strength and 40 ℃ dry flexural strength. The 2-hour wet flexural strength refers to the strength of gypsum powder measured on a flexural tester after being placed in water for 2 hours; The flexural strength after drying at 40 ℃ is the flexural strength tested after the sample is dried to a constant weight in a 40 ± 4 ℃ electric blast drying oven. The wet flexural strength of β gypsum powder is generally above 2.7Mpa, and the dry flexural strength is generally above 5Mpa; The wet flexural strength of alpha gypsum powder is generally above 4Mpa, and the dry flexural strength is generally above 6Mpa.

5. Crystalline water content

Gypsum powder is a crystalline mixture that contains gypsum (CaSO4 · 2H2O), hemihydrate gypsum (CaSO4 · 0.5H2O), and anhydrous gypsum (CaSO4, also known as burnt gypsum) due to different roasting processes. During the frying process, if there is insufficient heat to produce tender gypsum, or if the heat is too high or the frying time is too long, resulting in overburning and anhydrous gypsum, it will affect the setting time of gypsum and affect normal production and use. Sometimes, due to prolonged exposure to moisture, gypsum powder can also affect its normal use. By measuring the content of crystal water, the quality of gypsum powder can be determined. The content of qualified gypsum powder crystal water is generally between 5.2% and 6.2%.

6. Surface hardness

The surface hardness of gypsum molds directly affects their service life and the quality of the body. Because the model is constantly squeezed, rubbed, and eroded by mud during use, it can cause wear and tear on the working surface, resulting in rough surfaces and the model being scrapped. There are two methods for measuring the surface hardness of a model: scratch hardness method and wear resistance method. Scratch hardness is measured using a scratch hardness tester after measuring dimensional stability, to determine the magnitude of scratches caused by a load of 50g. The smaller the value, the higher the surface hardness of the material. Wear resistance is measured by repeatedly grinding the surface of the specimen 30 times after measuring dimensional stability, and measuring its weight reduction rate. The smaller the value, the higher the surface hardness of the specimen. The scratch hardness of general gypsum molds at standard consistency is around 0.21mm, and the wear resistance is around 13.4%.

7. Water absorption rate

Water absorption rate is an important performance parameter of gypsum models. It is a reflection of the model's ability to eat pulp. The water absorption rate is the ratio of the weight of water absorbed by the gypsum mold to the weight of the dry gypsum mold, expressed as a percentage. The water absorption index is mainly directly related to the mixing amount (paste water ratio). The larger the amount of water mixed with gypsum powder, the more pores formed after drying the model, and the higher the water absorption rate. The water absorption rate of sanitary ceramic grouting models is generally between 40-48%. The types of pores in the model include open pores and closed pores. The water absorption rate of the model is mainly related to the opening porosity of the model, which is directly proportional.

8. Diffusion coefficient

The porosity of the gypsum model can only indicate the water absorption capacity of the model, and is not directly related to the speed of forming. The diffusion coefficient represents the area over which water in the mud diffuses in the model per unit time, denoted as Dg. There is a certain functional relationship between the weight of the formed billet and the diffusion coefficient of the model during the grouting process. The diffusion coefficient of the model is high, but the weight of the billet decreases instead; The diffusion coefficient of the model is small, resulting in a thicker body. This is because when the diffusion coefficient is high, a dense layer of billet is quickly formed on the inner surface of the model, which slows down the further migration and dehydration of water in the mud into billets; When the diffusion coefficient is smaller, the dehydration process is not intense, and the surface of the billet maintains its porosity, which does not affect the penetration of inner water and the formation of the billet. The diffusion coefficient of gypsum model affects the permeability of the outermost layer of the body, thereby affecting the speed of forming the body.

9. Expansion rate

The expansion rate is also an important technical parameter of gypsum powder. The expansion of gypsum is a harmful factor in production, and in severe cases, it can affect injection molding production, damage the model and mother mold, cause model leakage, deformation, and dimensional changes. In actual production, we hope that the smaller the expansion rate of gypsum powder, the better. Generally, high-quality grouting models use gypsum powder with an expansion rate of less than 0.2%, while the original master mold uses gypsum powder with an expansion rate of less than 0.06%.