The feeding system of the domestic tablet press is generally divided into three kinds: the boot feeding system of the single stamping machine, the moon-grid feeding system of the ordinary medium-low speed rotary tablet press, and the forced feeding system of the sub-high-speed and high-speed tablet press. 1 The structural characteristics of the boot-type feeding system of a single stamping machine and its pressing process compare the main structure of the boot-type feeding system. The feeder is boot-shaped outside, and is driven by a CAM to swing from side to side on the die plane to complete the action of feeding, scraping and pushing.
The working process of the boot feeding system
(a) Feeding. The upstroke lifts, the feeder moves to the die hole, the downstroke drops to the appropriate depth (so that the weight of the particles contained is equal to the weight of the piece), the feeder swings on the die, and the particles fill the die hole;
(b) Scraping. The feeder is removed from the die hole so that the particles in the die hole are in line with the upper edge of the die hole.
(c) Tablet pressing. Upthrust down, and press the particles into sheets;
(d) Push film. The upper punch is lifted, and the lower punch then rises to the edge of the die hole. At this time, the feeder moves to the die hole and pushes the pill away into the receiver. At the same time, the lower punch is lowered, and the die hole is filled with particles for a second feeding powder, and so on.
2 The structure characteristics of the moon-grid feeding system of the ordinary medium and low speed rotary tablet press and its pressing process are compared with the moon-grid feeding system of the ordinary medium and low speed rotary tablet press, as shown in Figure 3. The moon-grid feeder is fixed on the mold plate of the turntable, and the bottom surface of the feeder maintains a certain gap (0.05~0.1mm) between the bottom surface of the feeder and the surface of the mold plate. When the rotating mold ring passes under the feeder, the drug particles in the grid fall into the mold hole, multiple curved grid plates cause the drug to be filled multiple times, and the last grid of the feeder is equipped with a scraping plate. It is close to the working plane of the turntable, and the excess particles on the surface of the turntable and the die ring can be scraped away, and then the downward flushing has a drop, so that the powder in the die hole can be shaken after the powder board is scraped. The pressing process of the moon-grid feeder is shown in Figure 4.
(1) Filling. When flushed under the feed hopper, the particles are filled into the die hole;
(2) Scrape flat. The excess particles are scraped off by the scraping plate when the front is flushed to the upper plane of the filling rail;
(3) Tablet pressing. When the front flushes above the lower press wheel and the upper flushes below the upper press wheel, the distance between the two flushes is minimum, and the particles are pressed into sheets;
(4) Push the film. After pressing the tablet, the upper and lower punch rise and fall along the guide rail respectively, and the tablet is pushed out of the die hole when the lower punch reaches the top of the exit rail, and is imported into the container by the scraper plate, and so on.
The forced feeding system is a feeding system widely used in the sub-high-speed and high-speed rotary tablet press in recent years (as shown in Figure 5). The forced feeding system is mainly composed of three parts: hopper, feeding motor and forced feeder. The hopper mainly plays the role of storing materials, and its lower part is connected with the forced feeder. The feeding motor is the power source of the forced feeder, frequency conversion speed regulation; The forced feeder can be divided into two parts: gear box and feeder. The gear box transmits the power of the feeding motor to the two impeller of the feeder, and the two impeller rotate in reverse to force the material to be filled into the middle die hole. The pressing process of the forced feeding system is basically the same as that of the moon-grid feeding system, which is also divided into four steps: filling → scraping → pressing → pushing.
4 Comparison of the weight difference of the three feeding methods The current feeding method is to use the volume of the material to control the weight of the piece, g=ρv(where g is the weight of the material filled, ρ is the density of the material, v is the volume of the material filled), now the volume of the filling is certain, that piece of weight is determined by the density of the material, The density of the material is determined by the particle uniformity and looseness of the material. The boot feeding system of the single stamping machine and the moon-grid feeding system of the medium and low speed rotary press are naturally fed by the weight of the material, and the powdered material is generally not suitable for these two ways, because the fluidity of the powder is generally much less than that of the granular material. However, if the granular material is uneven or has poor fluidity, it will also affect the weight difference of the tablet. Therefore, these two feeding methods will be more demanding on the type of material and the condition of the particles. The forced feeder of the sub-high-speed and high-speed tablet press can forcibly fill the middle die hole, so it can press powdered materials, and the requirements for granular materials are relatively looser than the first two types of feeders, and the weight difference is more stable.
5 Performance comparison of various feeding methods 5.1 boot-type feeding system The powder in the boot-type feeder of the single stamping sheet machine is easy to cause the powder to be stratified by the swing of the feeder, and the pressing sheet is one-sided pressure (upthrust pressure), so the pressure distribution is uneven, easy to crack, and the noise is large. In addition, the production efficiency is relatively low, the general single stamping machine output of 60-80 pieces /min, so it is only suitable for laboratory and large-size tablet production.
5.2 Moon-grid feeding system. At present, the moon-grid feeding system of the ordinary medium-low speed rotary tablet press is widely used in production. This feeding method must first make the material into particles with uniform size and good fluidity, rely on the self-weight of the material particles, and fill the drug multiple times through a number of curved grid plates, so the filling is full, stable, and the difference in the weight of the piece is small. The up-and-down impact of the rotary tablet press can be pressurized at the same time, and the pressure distribution is uniform. Moreover, most of the domestic low-speed tablet press are designed to place two sets of feeding system before and after the turntable, so that the turntable rotates one week, each set of mold can be filled twice, pressing two pieces, its production efficiency is greatly improved, the output of ordinary ZP35D tablet press can reach 1900~2200 pieces /min, Therefore, this feeding method of tablet press can be used for large-scale mass production by pharmaceutical manufacturers. However, this feeding system also has some drawbacks:
(1) Strict requirements for material particles. Before pressing, the material should be pelleted, the particle flow is better, the particle size should be uniform, and the production cost will increase;
(2) The filling time should not be too short. Because this feeding method is that the material particles are filled into the die hole by their own weight, the filling should ensure a certain time, so that the rotary speed can not be raised too high, otherwise the filling is insufficient, and the difference in sheet weight can not reach the standard.
5.3 Forced-Feed system Forced-feed system can use either pressed granular material or pressed powder type material. Because the material is forced to fill, the filling time can be greatly shortened, and the corresponding rotation speed of the turntable can also be increased a lot, so this feeding method is generally used in sub-high-speed and high-speed rotary tablet press, and the maximum speed of the turntable of the high-speed tablet press can reach 80 RPM. With the increasingly mature design and production technology, this tablet press is also favored by some large and medium-sized pharmaceutical manufacturers.
Although there are many advantages of the forced feeding system, there are still some disadvantages:
(1) Powder dumping phenomenon. Because the rotation speed of the turntable is increased, the excess material flowing out of the feeder will be thrown out, resulting in a waste of material and an increase in dust;
(2) Hopper leakage is not smooth. Because of the particularity of the hopper structure (the upper mouth is large, the lower mouth is small, and there is a slope), some materials with poor fluidity (mostly powder materials without pelleting) will not flow smoothly, resulting in unstable piece weight differences.
5.4 Advantages and disadvantages of various feeding methods Based on the above analysis, the advantages and disadvantages of several feeders can be summarized (as shown in Table 1). Table 1 Advantages and disadvantages of several feeders
Feeding system Boot-type feeding system Moon-grid feeding system forced feeding system adapted sheet press Single stamping machine Medium and low speed and sub-high speed rotary sheet press Sub-high speed and high speed rotary sheet press Feeding method Natural feeding natural feeding forced feeding sheet weight unstable stable stable suitable material type Granule granule Granule granule or powder Low production efficiency the highest.
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