1. Judging from the appearance
The quality of gypsum powder can be preliminarily judged by appearance observation and hand feeling. Just look at the appearance and color of the gypsum powder, and the feel is the roughness (fineness) of the gypsum powder when you rub it with your hands.
In terms of appearance, gypsum powder with high whiteness has high fineness and mesh, good quality and higher price; while gypsum powder with gray and black impurities in it is of poor quality. Rub the gypsum powder with the thumb and forefinger of the hand, and the gypsum powder with a rough feel is rough in fineness, poor in quality and lower in price; while the stone tone powder with a fine hand feels fine in fineness, better in quality, and lower in price. high.
2. Determination of crystal water content
The method for detecting the crystallization water content of gypsum powder is as follows: weigh the sample mass G (generally 2-5 g) that is cooled to room temperature after drying at 40 ℃, place it in a platinum crucible (or porcelain crucible) of constant weight, and weigh it. Take its quality as G1; burn for 2h at 230 ± 5°C, take out and move in the desiccator, take its quality after cooling to room temperature as G2, then its crystallization water percentage is (Gt-G2)/GX100 %.
3. Determine the setting time of gypsum powder
The setting time includes initial setting time and final setting time. The quality standard of the setting time of gypsum powder is: under the standard consistency, the general initial setting time is not less than 6min, and the final setting time is not more than 25min. Appropriate setting time is a prerequisite and necessary condition for the normal use of gypsum powder in production. The initial setting time is too short or the final setting time is too long. If the initial setting time is too short in use, less than 6min, it proves that the gypsum powder is not cooked, or contains raw stones in the damp, which affects the injection molding operation, which is manifested as barrel injection during production, delaying production; if the final setting time is too long , far greater than 25tnin, it means that the gypsum is over-burned during frying, and it contains over-burned gypsum, which is easy to leak slurry during injection molding, and even the gypsum slurry does not solidify, which affects production.
In both cases, gypsum powder cannot be used in production. Therefore, the determination of the setting time can determine the quality of the gypsum powder or determine whether the gypsum powder can be used in production.
4. Determine the diameter of the test cake
According to the ratio of gypsum and water in production, the quality of gypsum powder can be judged and whether it can be used in production by observing and measuring the diameter (fluidity) of the test cake. The diameter of the test cake is related to factors such as the amount of water, the ratio of paste-water and the quality of gypsum powder. The amount of water should be fixed in the experimental detection, and the instruments and experimental methods used should be consistent. Under the ratio of large-scale production gypsum and water, if the diameter of the test cake of qualified gypsum powder normally used in production is D, the diameter of the test cake of the gypsum powder to be inspected is d. If d is much smaller than D, even if it does not flow and lay flat, it means that the quality of gypsum powder is not good, it is not cooked, or contains raw gypsum in the damp, which affects the injection molding operation or even cannot be used; if d is much larger than D, the slurry is very If the fluidity is strong, it means that the gypsum powder is over-burned, and the setting time is too long to solidify during injection molding, and it cannot be used. In this way, ordinary P gypsum powder and high-strength a gypsum powder can also be distinguished. Under the same ratio of paste and water, if the slurry has strong fluidity and the diameter of the test cake is large, it is proved to be a gypsum powder; on the contrary, if the slurry has poor fluidity or even no flow, and the diameter of the test cake is small, it is proved to be a gypsum powder of 3 .
5. Production pilot test
According to the ratio of gypsum and water in production, ensure that the instruments, operators, equipment and process methods used are consistent, select a small tire master mold for trial injection of production models, and pour the models with qualified gypsum powder normally used in production. process to compare. If the setting time of the gypsum powder to be tested is normal and reasonable, and the operation is smooth without filling the bucket, it means that the gypsum powder is qualified gypsum powder; if the setting time of the gypsum powder to be tested is abnormal, the setting time is too short, or the setting time is too long. Even if it does not solidify, such gypsum powder is unqualified and cannot be used in production.
