Rigid packaging materials and materials filled into packaging materials have a certain linear expansion or body expansion coefficient when the temperature changes. Therefore, when designing products with rigid packaging materials, a certain amount of space will be left at the top of the container—— Head gap, because the head gap is filled with air, and air has a certain degree of compressibility. When temperature changes cause the volume of the material to change, it will inevitably lead to changes in the head gap in the closed container, which will correspondingly generate pressure in the container. Variety. For each material, each packaging material, different specifications, and different sterilization modes, the amount of headspace required will be different. This is only a qualitative analysis. The specific quantitative settings are still Technical personnel on the production line are required to set the conditions according to the determined conditions.
The biggest characteristic of glass bottle packaging materials is that the ambient temperature cannot be suddenly cooled or heated during the sterilization process of the product. For a single bottle, the temperature difference in various parts of the body cannot exceed a certain value (the specific temperature difference value depends on the material and structure of the glass bottle) Please refer to the corresponding manual for the difference). This is because the glass material has almost no ductility and toughness within the sterilization temperature range of the product. If there is a temperature difference throughout the bottle, the thermal expansion and contraction of the glass material will cause damage to the various parts of the bottle. The linear expansion of the glass is inconsistent, which generates internal stress in the bottle material. Since the ductility of the glass material is very small, that is, it is very brittle, excessive internal stress will cause the bottle to crack, which is what the saying goes, it is better to break than to bend. .
This characteristic of the glass bottle material determines the sterilization mode and the structural requirements of the sterilization retort used in the sterilization process: the temperature change process at each bottle must be uniform and continuous, and the temperature change speed must be consistent with the structure of the glass bottle. Adaptation, in short, the internal stress caused by temperature changes on the glass bottle body cannot be greater than the strength of the glass bottle can withstand.
When the glass bottle is sealed, part of the gas in the bottle needs to be discharged, so that the glass bottle with the cap installed will have a negative pressure compared with the atmospheric pressure. There are two purposes for doing this. First, during the sterilization process, the pressure inside the glass bottle will not rise too high during the high temperature stage (of course, if the pressure inside the bottle rises too high, it can be balanced by increasing the pressure inside the sterilization retort. But this increases the consumption of compressed air). Another purpose, which is also the main purpose, is that during the storage of glass bottles and cans, once the filling material deteriorates, gas will be generated due to bacterial activity. As a result, the pressure in the glass bottle will increase, and the dented bottle cap will bulge, thus achieving the purpose of reminding the eater that the food in the bottle has deteriorated before opening the bottle cap.
Although the glass bottle is relatively brittle, it still has a certain strength. Therefore, during the sterilization process, the range of temperature changes that the glass bottle adapts to is still relatively large, which reduces the requirements for pressure control accuracy during the sterilization process. . However, it is worth noting that during the entire sterilization process, the pressure in the sterilization retort is appropriately greater than the pressure in the glass bottle. The resulting pressure difference between the inside and outside of the glass bottle can enhance the sealing effect at the bottle cap.
02Three-piece tinplate can
As the name suggests, this type of can is composed of three pieces of material: the bottom, the body, and the lid. The three-piece can made of tinplate can withstand sudden changes in temperature and can also withstand a certain pressure difference between inside and outside (the ability to withstand the pressure difference between inside and outside is determined by the thickness of the plate, the shape of the tank, the size of the tank and other factors).
During the steam sterilization process, when the sterilizing retort is in the steam exhaust stage, steam quickly enters the sterilizing retort, and the temperature inside the retort will suddenly rise from normal temperature to over 100°C, and the temperature change will be quite dramatic. When the sterilization and heat preservation is completed, the traditional sterilization process is to first release the steam in the sterilization retort, causing the pressure in the retort to decrease. However, the inside of the can is still at the sterilization temperature and has a high pressure. At this time, the can will bear the greatest of internal pressure.
For tinplate cans with larger diameters, during the high-temperature sterilization stage of the sterilization process, the pressure inside the cans (to be precise, it should be pressure) is not increased compared with cans with small diameters, but due to the area of the end cap It has increased a lot, so the total pressure endured by the end cap is also much greater. Therefore, it is not suitable to use pure steam sterilization mode for larger diameter tinplate cans.
In the normal pressure environment after filling and sealing, making the cans present a negative pressure will help reduce the pressure inside the cans during the high-temperature stage of the high-temperature sterilization process.
03Two-piece can
Two-piece cans are distinguished by material, including aluminum alloy and tinplate. Two-piece aluminum alloy cans, because the ductility of the aluminum alloy material is very good, the can material can be stretched very thin, and its tensile strength will also be greatly improved, but such a thin aluminum alloy material has its bending resistance The strength will drop sharply, so when filling materials in aluminum alloy two-piece cans, it is necessary to inject liquid nitrogen into the can before sealing. After sealing, the liquid nitrogen gas vaporizes to make the pressure inside the can higher than that of the can. A certain value of external atmospheric pressure, in layman's terms, causes the aluminum alloy two-piece can to bulge in order to increase the strength of the can itself.
It is worth noting that when high-temperature sterilization is performed on nitrogen-filled cans, the pressure inside the cans will rise very high, which requires that the sterilization environment pressure (pressure inside the sterilization retort) where the cans are located should also be increased accordingly, so that The pressure difference between the inside and outside of the can is reduced to the range allowed by the tank material.
In some cases, it is believed that reducing the pressure in some sterilization retort and increasing the pressure difference of cans during high-temperature sterilization does not cause any problems with cans. This can not only reduce the consumption of compressed air, but also reduce the manufacturing cost of sterilization retorts. Cost (reducing the maximum allowable working pressure of the sterilization retort, correspondingly reducing the wall thickness of the tank). This is a misunderstanding: Instead of thinking like this, why not work hard to reduce the wall thickness of aluminum alloy can materials? How to improve the economic efficiency of manufacturers that use aluminum alloy can products is a very simple economic account.
The pressure inside the aluminum alloy two-piece can (relying on this pressure to give the can a high compressive strength) is determined by factors such as the filling amount of nitrogen, the height of the head gap after the material is poured, and the ambient temperature. How much pressure is needed is affected by the mechanical properties of the aluminum alloy can body material, the maximum extreme impact force that the can product may withstand during storage and transportation, and other factors. Choosing these parameters carefully and carefully will beneficially improve the economic benefits of the enterprise.
For two-piece ferroalloy cans, due to the mechanical properties of the ferroalloy material, the finished can still has a certain bending strength, and in most cases there is no need to fill it with nitrogen.
04 aluminum foil box
The classification of aluminum foil boxes as hard packaging materials is not based on the bending strength of the packaging materials, but on the anti-deformation ability of the packaging materials. We can understand that aluminum foil boxes are the most fragile hard packaging materials. Package. The materials that make up aluminum foil boxes are almost inelastic, which means they are prone to plastic deformation. During the heating, insulation, and cooling stages of the sterilization process for materials filled in aluminum foil boxes, as the temperature changes, the volume of gas in the aluminum foil box must be kept constant, which ensures that the aluminum foil box is not affected by deformation.
During the sterilization process of the aluminum foil box, as the temperature in the sterilization retort changes, the temperature of the materials in the aluminum foil box also changes accordingly. It is worth noting that the lag time of the temperature change in the aluminum foil box is related to the filled content. Relevant to material properties. As the temperature changes, the gas state in the aluminum foil box also changes at the same time. If you want to keep the shape of the aluminum foil box from changing, you must keep the volume of the gas in the aluminum foil box constant. Keep the volume of the gas in the aluminum foil box constant. , the pressure of the gas in the box should change accordingly. Since the aluminum foil material of the aluminum foil box can hardly withstand a slight pressure difference on both sides, that is to say, the pressure inside the aluminum foil box is almost equal to the pressure inside the sterilization retort outside the aluminum foil box. Therefore, the pressure inside the sterilizationretort of the aluminum foil box during the sterilization process Pressure adjustment must strictly follow temperature changes.
With the above analysis of the changes in the state of the aluminum foil box during the sterilization process, we can draw a conclusion: as long as the pressure inside and outside the aluminum foil box is equal, the shape of the aluminum foil box will not change. This is very important for the sterilization process of aluminum foil boxes. The formulation will have a vital influence on the design of the sterilization retort structure.
It is relatively easy to determine the sterilization process parameters of a single aluminum foil box product and ensure that its shape remains unchanged. However, in actual production, aluminum foil box products are sterilized in batches during the sterilization process. To ensure that the sterilization quality of each batch of aluminum foil box products (mainly referring to the shape of the aluminum foil box) is consistent, it is necessary to design the structure of the aluminum foil box itself, the structure of the sterilization cage, the sterilization mode and structure of the sterilization kettle, etc. In terms of style, we should work hard on the following items:
a) Based on the above analysis, the heat transfer characteristics of the materials packed in the aluminum foil box should be as consistent as possible. To achieve this, when designing the product formula, the heat transfer method of heat through the aluminum foil box to the material during the sterilization process should be taken into consideration. In specific production practices, whether it is a product manufacturer that uses sterilizing retorts or an equipment manufacturing enterprise that manufactures sterilizing retorts, it is necessary to carefully analyze the thermodynamic characteristics of specific sterilized products. This is the best sterilization process for formulating sterilized products. Very helpful;
b) The speed of temperature change must be strictly controlled. Since the aluminum foil box material itself is particularly fragile, it can hardly withstand any pressure difference between the inside and outside. The pressure inside the aluminum foil box is determined by the temperature of the materials inside. During the heating, insulation, and cooling stages of the sterilization process, the heat transfer of all products The speed must be synchronized within the required range, so that the pressure in all aluminum foil boxes in the sterilization retort will be within the same required pressure range, so that all aluminum foil boxes will not be affected by deformation due to the difference in pressure within their own boxes. . When considering this issue, one thing should always be remembered: During the sterilization process, all products in the sterilization retort are in the same pressure environment, and the pressure in the aluminum foil box will change as the materials in the box change. The pressure of the external environment of the aluminum foil box can be adjusted at any time. The basis for adjusting the external pressure of the aluminum foil box is based on changes in the pressure inside the aluminum foil box. The required result is to maximize the internal and external pressure difference that the aluminum foil box material can withstand. Extremely close to zero;
c) At the same time, the temperature of the aluminum foil box products in the sterilizer should be controlled within the required range. In specific practical operations, it is very difficult to achieve absolutely consistent temperatures everywhere in the sterilizing retort. This requires taking certain measures to keep the temperature of the sterilized products in the sterilizing retort at a minimum. temperature difference;
d) In production practice, based on the limitations of the existing sterilizing
retort equipment structure, most of them adopt staged heating. That is, because the phenomenon of uneven heat distribution during the heating process cannot be eliminated, the heating is stopped and the temperature is stopped every time the temperature rises. Keep warm at this temperature for some time. In this way, products with a relatively fast temperature rise can be delayed and products with a relatively slow temperature rise can be heated up, so that the temperature of all products in the sterilization
retort is maintained within a required temperature difference range. Of course, if the temperature rise rate of all products and the temperature at the same time during the temperature rise process are kept consistent from the internal structure of the sterilization r
etort, continuous heating is the best way to heat up.
2. Flexible packaging materials
01Soft packaging vacuum high-temperature cooking bags
The sterilization of flexible packaging products is relatively easy, because the requirements for pressure control of flexible packaging products are relatively broad, that is, as long as the pressure in the sterilization retort is higher than a certain value, which is what we usually It's talking about constant pressure sterilization. How to determine the pressure set in the sterilization retort during the sterilization process is an issue worthy of serious consideration.
The following aspects need to be considered when setting pressure:
a) As long as the pressure is higher than the vaporization pressure of the sterilization temperature by a certain value, the specific value of the vaporization pressure depends on the specific product. Generally, the set pressure is about 0.05 higher than the vaporization pressure of the sterilization temperature. ~0.1MPa is enough;
b) The set pressure in the sterilization retort is higher than the lower limit of the vaporization pressure, which saves the consumption of compressed air;
c) The set pressure in the sterilization retort is higher than the upper limit of the vaporization pressure. The smaller the expansion of the gas contained in the product material will be, the texture of the product material will be denser, which will be beneficial to the product during the sterilization process. The heat is transferred to;
d) When designing the tank structure and sterilization mode of the sterilization retort, it should be considered that the pressure in the sterilization retort should not fluctuate too much during the entire sterilization process, and the lower limit of pressure reduction should not be exceeded during the pressure fluctuation process. The minimum value is set to avoid adverse effects on the material quality of packaging materials and sterilized products. This point should attract the attention of product sterilization process developers, because in specific production practices, sometimes the reduction and fluctuation of the pressure in the sterilizationretort occur instantaneously and are often difficult to observe in real time;
e) Another factor that needs to be considered when setting the pressure in the sterilizationretort is the cavitation margin of the circulating water pump. This factor mainly involves the inlet diameter and flow rate of the circulating pump, the opening position of the water outlet in theretort, etc. It is worth noting that once cavitation occurs, it will cause noise, vibration, flow fluctuations and other phenomena of the water pump.
02Soft packaging inflatable packaging materials
Inflatable flexible packaging, most of which are self-standing bags with aluminum foil. For this type of packaging material, since the packaging material contains air, the volume of the air will change with changes in temperature. If you want to keep its initial state (volume) unchanged, or minimize the amount of change, you need to change the packaging The stress of the environment in which the material is exposed. In other words, the pressure in the sterilization kettle needs to change with the temperature, so that the deformation of the packaging material can be kept within the required range.
One issue that needs attention here is that during the process of heating and cooling, all products in the sterilization retort need to be within a temperature range as small as possible, because the temperature of the gas in the product directly affects the volume of the gas. All products in the sterilizing
retort container are in the same external pressure environment. If the temperature difference between different places in the sterilizing
retort is large during the heating and cooling process, it will inevitably lead to differences in the internal temperatures of the products in different places. Increase, which makes it more difficult to ensure the deformation of the aluminum foil bag (this is similar to the characteristics of the aforementioned aluminum foil box, but the requirements are relatively lower than that of the aluminum foil box).
04PE bottle packaging materials
One of the main characteristics of plastic bottles is that their strength will be low after being heated, they have a certain degree of plasticity, they have heat shrinkage, and they have a certain elasticity.
1. When the volume of the gas in a closed space remains constant under normal pressure and the temperature rises from 30°C to 121°C, the pressure in this closed space will increase to 1.2587 times the normal pressure. Of course, if water is present, the pressure of saturated steam needs to be added.
2. When hot filling at 75°C, even if the plastic bottle maintains its shape during sterilization, cooling, etc., after the material in the plastic bottle is cooled to normal temperature of 30°C, due to the change in water density,:
● For a 360 ml plastic bottle, the volume of water will shrink by 7.416 ml;
●For a 1500 ml plastic bottle, the volume of water will be reduced by 30.9 ml.
3. When hot filling at 75℃, due to the change in density of water during sterilization at 120℃, for:
● For a 360 ml plastic bottle, the volume of water will expand by 11.448 ml;
●For a 1500 ml plastic bottle, the volume of water will expand by 47.7 ml.
4. Due to the change in density of water at different temperatures, from 30℃ to 120℃, for:
● For a 360 ml plastic bottle, the volume change of the material is 18.864 ml;
● For a 1500 ml plastic bottle, the volume change of the material is 78.6 ml;
5. From the above calculation, it can be seen that the volume of the material in the plastic bottle will change greatly at different temperatures, and the plastic bottle is a soft material. For the sealed plastic bottle cavity, due to the volume of the content The shrinkage itself cannot withstand external pressure, so it can only adapt through its own deformation. In view of this, it is recommended to leave some head space in the bottle during filling so that the plastic bottle can adapt to changes in the head space during the entire sterilization process after filling.
6. For plastic bottles, when filling at a lower temperature, as the temperature increases, on the one hand, the volume of water expands (the environmental pressure of the water is higher than the vaporization pressure of the water), and the The volume is incompressible, so in the space inside a closed plastic bottle, if the volume of the plastic bottle remains unchanged, the head space in the bottle must be reduced, and the pressure of the air in the head space will increase (let the air Compression, making space for the expanding water to occupy), this is the back pressure required in the sterilization retort.
7. How much head space do you need to reserve for different bottle types? What is the appropriate filling temperature? How much back pressure is needed in the sterilization retort? All require specific calculations and experiments to obtain accurate data.
8. The calculation method is: first determine the filling amount, determine the reasonable headspace volume that needs to be reserved through the change in density of the material at different temperatures (that is, the change in the volume of the material), and then determine the volume of the plastic bottle. Specification. Or when the bottle type is determined, a reasonable ratio of materials and headspace can be determined. Only with a reasonable ratio of material to head space, the expansion of the material during the sterilization process can be compensated by the reduction of the head space under a certain pressure. Otherwise, it can only be accommodated by the expansion of the plastic bottle itself.
9. Test method: During the process of heating, maintaining, and cooling, observe the state changes of the plastic bottles in the test device at any time, constantly adjust the pressure in the device, so that the shape of the plastic bottle remains basically unchanged, and record different heating rates. The required pressure values at different temperatures are then input into the sterilization retort in the production workshop to determine the process parameters for sterilizing plastic bottles.
Through the above analysis of product materials, packaging materials, etc., temperature control is mainly related to the sterilization of the product, and also affects the physical and chemical indicators of the product, while pressure control is mainly related to the packaging materials of the product. In layman's terms, when formulating the sterilization process of a product, the temperature change parameters should be determined based on the physical and chemical properties of the product material and the sterilization requirements, and the pressure change parameters should be determined based on the physical properties of the packaging material. These two points must be mastered. Sterilizing products is crucial.
