Soft capsules refer to sealed capsule formulations formed by quantitatively encapsulating oily drugs, drug solutions, or drug suspensions using gelatin as the main material in the capsule shell. Common preparation methods include drop and compression methods. This article briefly describes the preparation process and precautions of the soft capsule pressing method.
The basic principle of the pressing method is as follows: the rubber is passed through the middle of a pair of cylindrical molds, and the feeding pump injects a certain amount of medicine into the rubber through the spray body. The mold moves in opposite directions to press and cut the two rubber pieces from the rubber, forming a completely closed capsule. The main process steps of the pressing method include four steps: gelatinization, liquid preparation, pill making, and drying. In addition, to ensure product quality, a shot inspection process will also be added in production to remove non-conforming products.
(1) Gelatinization
① Feeding sequence
Usually, a certain proportion of gelatin, plasticizer, and water are added to a gelatinizing tank for heating and stirring. The gelatin absorbs water and dissolves to form a gel solution. The above materials can be added simultaneously, but it is recommended not to directly add gelatin to hot water. Otherwise, due to the dissolution of gelatin that comes into contact with hot water first, a dense gelatin layer will be formed, and the gelatin particles wrapped inside are difficult to dissolve. There will be many clumps in the obtained gel solution, which will affect normal production. In addition, pigments or shading agents such as titanium dioxide and iron oxide may also be added to gelatin. Colloidal grinding should be used to grind it together with some plasticizers and water. After sieving, it can be added to the gel solution to avoid the preparation of rubber with spots or pores.
② Glue temperature
It is generally advisable to control the temperature of the adhesive during production at 60-70 ℃. The main component of gelatin is a mixture of peptide molecules, and high temperatures can cause hydrolysis of peptide molecules, affecting the quality of the gelatin solution.
③ Vacuum degassing
There will be many bubbles in the newly melted glue, so vacuum degassing operation is necessary. Vacuum degassing generally requires a vacuum degree of -0.07~-0.08 MPa. At the beginning, a large number of bubbles will roll and release from the adhesive solution. Therefore, it is important to release the pressure in a timely manner to avoid pumping the adhesive solution into the vacuum pump and damaging the equipment. During the vacuum pumping process, attention should be paid to observing the state of the glue liquid until there are no obvious bubbles released on the surface, and the glue liquid flows in a honeycomb shape on the surface, indicating that the degassing is completed. The vacuum pumping operation can be stopped. The vacuum pumping time should not be too long, otherwise the glue liquid will lose a large amount of water, which will affect the subsequent shot pressing operation.
③ Glue insulation
The insulation temperature of the adhesive solution is generally controlled between 50-60 ℃. If the temperature of the adhesive solution is too low, it will lead to a decrease in the fluidity of the adhesive solution. Therefore, during the pelletizing stage, the adhesive solution still needs to be reheated, which takes a long time and may result in uneven properties of the adhesive solution.
The glue should not be used immediately after vacuum degassing is completed. It should be left to stand for more than 4 hours to fully discharge the bubbles in the glue. The upper limit of insulation time is evaluated based on specific projects, and the viscosity, freezing force, and moisture of the adhesive during the insulation process can be monitored to determine the stability of the adhesive quality during the insulation process.
(2) Liquid preparation
The content can be prepared according to the specific project requirements, and the preparation process can be controlled according to the relevant technical requirements of solutions, suspensions, and semi-solid formulations. If the raw materials are unstable due to light, heat, or oxygen, it is possible to consider avoiding light, cooling, or nitrogen filling. The liquid preparation process varies depending on the specific project, so it will not be discussed in detail here.
(3) Pill making
Pill pressing is the core work of soft capsule production. The prepared medicine is added to the hopper, and the glue is conveyed to the soft capsule machine through a rubber conveying tube to complete rubber preparation, quantitative filling of contents, and pressing of rubber and contents into capsules through molds. The main process parameters that need to be controlled are as follows:
① Rubber thickness
If it is a generic formulation, it is recommended to maintain the same thickness of the soft capsule shell as the reference formulation to ensure consistent disintegration time. A certain prescription can be used to apply wet rubber with different thicknesses, and after drying, the thickness of the dry rubber can be measured to determine the target rubber thickness range. In general, the thickness of rubber with a weight of 1g or more is controlled within 0.8-0.9mm, and the thickness of rubber with a weight of less than 1g is 0.7-0.8mm. Excessive thickness of the rubber may result in a higher spray temperature required, otherwise it may not have a good stitching effect. Thin rubber can affect the compressive strength of soft capsules, which is not conducive to storage and transportation. In addition, the thickness of the two pieces of rubber should be as consistent as possible, otherwise it will produce deformed pellets.
② Spray temperature
The spray temperature should be adjusted according to the condition of the rubber. If the moisture content of the rubber is too low or the rubber is too thick, the spray temperature should be appropriately increased. In addition, if the pressing speed of the shot is too high, it is also necessary to increase the spray temperature to ensure a better stitching effect. Usually, the temperature of the spray should be controlled between 35 ℃ and 40 ℃. If it is too low, it is difficult to sew, and if it is too high, it may cause the soft capsule suture line to twist and produce shaped pills.
③ Ball pressing speed
The increase in shot pressing speed may not only affect the spray temperature, but also affect the thickness of the rubber coating. A faster rotation speed can cause excessive stretching of the rubber during the shot pressing process, resulting in thinning of the rubber. Therefore, after adjusting the shot pressing speed, the thickness of the rubber should be monitored to ensure that it is within the target range.
④ Environmental temperature
The ambient temperature during the pressing process can also have an impact on the pressing effect. If the ambient temperature is too high, it may cause the pressed pellets to cool slowly and cause adhesion; If the ambient temperature is too low and the rubber is difficult to sew, it is necessary to increase the spray temperature appropriately.
⑤ Other
The soft capsule machine also has the temperature of the rubber conveying tube, the temperature of the rubber box, and the temperature of the cooling drum. The temperature of the rubber conveying tube and the temperature of the rubber box are generally 55-60 ℃ to ensure that the glue can flow smoothly and not cause blockage of the rubber conveying tube; The cooling drum is generally controlled below 20 ℃ to ensure that the rubber can be quickly cooled and formed.
In addition, attention should be paid to the type and amount of lubricating oil added during the pill making process, and corresponding quality standards should be established for the lubricating oil. There should be sufficient evidence to prove that the lubricating oil will not affect other components of the soft capsule. Commonly used lubricating oils include medicinal grade liquid paraffin and medium chain glycerides.
(4) Drying
The drying process is generally divided into two steps, namely fixed drying and static drying. The newly compressed soft capsules will be immediately transferred to a drying rotary cage for drying, reducing the moisture content to 25-30%. Under this moisture content, the shape of the soft capsules will be basically fixed. To improve production efficiency, transfer the shaped and dried soft capsules to a tray for further drying. To clean the lubricating oil on the surface of the soft capsule, an oil absorbing cloth can be added to the drying basket during the shaping and drying process.
Generally, the moisture content of soft capsule shells should be controlled between 8-12%. If the moisture content is too high, the finished capsule shell will be too soft and prone to mold; If the moisture is too low, the capsule shell will crack, which is not conducive to storage and transportation. The main factors affecting the drying effect of soft capsules are as follows:
① Environmental temperature and humidity
Research has shown that changes in drying conditions have an impact on the drying rate of soft capsules. The temperature and sound of the drying environment are high, and the soft capsules dry quickly in the early stage of drying. However, in the later stage of drying, due to the surface drying and hardening of the capsule shell, it affects the internal water conduction, resulting in a decrease in drying rate. As the environmental humidity decreases, the drying rate shows an accelerating trend. In general, the humidity in the environment should be controlled below 60%, otherwise it is extremely difficult for the capsules to dry.
② Dry wind speed
Increasing the wind speed in a dry environment can increase the water transfer rate and improve drying efficiency.
③ Drying time
Under strict control of environmental temperature and humidity, the moisture content of dried soft capsules depends on the length of drying time. During the process investigation stage, capsules with different drying times can be taken to measure the moisture content of the capsule shell, and a drying curve can be drawn to determine the drying endpoint.
3) Pill testing
The pill inspection process is to eliminate unqualified products in dry soft capsules, such as small and large pills, deflated pills, bubble pills, and shaped pills. The general method of pill inspection is to place the soft capsules on the lamp inspection table and manually pick them.
In the process of preparing soft capsules, we need to evaluate the output material properties of each process to ensure the production of qualified products.
During the gelation stage, the appearance, moisture content, viscosity, and freezing force of the adhesive solution are generally used as evaluation indicators. The appearance of the adhesive should be delicate and smooth without detailed bubbles; The moisture content should be controlled between 40% -50%. If the moisture content is too low, it will cause difficulties in stitching during the pressing process. If the moisture content is too high, it will make it difficult for the rubber to form; Viscosity and freezing force are the main testing indicators for gelatin quality. If the viscosity is too low, the soft capsule shell is prone to saponification. These two indicators should be monitored during the glue insulation process to prevent excessive decrease in glue quality.
During the pill making stage, the main focus is on the appearance, quantity, and stitching of the soft capsules. The appearance is focused on whether there are bubbles on the surface of the capsules and whether the seams are flat; To determine the dosage, the soft capsules need to be cut open, and the weight of the capsule skin should be measured after wiping off the contents. It is also recommended to use ether to clean the contents in some formulation quality standards. The filling amount is the difference between the total weight of the capsule and the weight of the capsule skin. After the sample is taken out, it should be measured immediately. If the waiting time is long, the evaporation of water in the soft capsule will affect the accuracy of the filling amount measurement results; In cases where immediate measurement is not possible, the soft capsules should be stored in a closed container and placed in a low-temperature environment to prevent moisture from evaporating; The stitching situation can be observed using a microscope. The measurement method is to cross cut the newly made soft capsule, cut off a circle of rubber in the middle, observe the thickness of the rubber and the thickness of the seam under the microscope. The mirror image of the microscope is shown in Figure 5, measure and calculate the seam rate. Generally, the seam rate should not be less than 30%.
Suture rate=seam thickness/capsule thickness * 100%
The main equipment required for the production of soft capsules
The main equipment for producing soft capsules includes the following key parts:
1. Gelatin tank: used to mix gelatin and other excipients, heat and dissolve them to form a gel solution.
2. Vacuum stirring tank: After gluing, the glue solution needs to be further stirred and vacuumed in this tank to remove bubbles and ensure the smoothness and aesthetics of the soft capsule.
3. Colloidal mill: used to refine the glue solution, ensuring its uniformity and appropriate viscosity.
4. Soft Capsule Pressing Machine or Dropping Machine: This is the core equipment used to combine glue and fillers (such as medicine or nutritional supplements) to form soft capsules through dropping or pressing.
5. Baking disc dryer or shaping cage: used to dry and shape fresh soft capsules to prevent adhesion.
6. Rotary dryer: helps with rapid drying and solidification of soft capsules.
7. Lamp inspection table: used to check the quality of soft capsules and remove unqualified products.
8. Cleaning equipment: For example, organic solvents such as ether and 95% ethanol are used to wash soft capsules to remove surface residues.
9. Packaging equipment: divided into inner packaging and outer packaging, used for sealing and packaging qualified soft capsules to prevent contamination.
10. Refrigerator: Maintaining the appropriate temperature of the glue and filling material is crucial for temperature control during the production process.
11. Insulated glue storage bucket: used to store and maintain the temperature stability of the glue solution.
12. Raw material bucket: used to store fillers, such as medicinal liquids or nutritional supplements.
13. Hot water exchanger: used to control the temperature of the glue tank and maintain suitable heating conditions.
14. Water ring vacuum pump: used for vacuuming the glue tank.
15. Hot water circulation pump: maintains the circulation of the hot water exchanger to ensure a constant heating efficiency.
16. Ingredient tank: used to prepare fillers and ensure accurate ingredients.
17. Colloidal mill: mentioned again, used to ensure the uniformity and suitable physical state of the filling material.
All these devices together constitute a complete soft capsule production line, ensuring an efficient, hygienic, and quality controlled production process from raw materials to finished products. During the production process, strict adherence to GMP (Good Manufacturing Practice) standards is also necessary to ensure product quality and safety.
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