Due to the better safety and economy of PET bottles, they are widely used today when juice and tea drinks are the mainstream products of beverages.
Special quality requirements of PET bottles for hot filling process
- The heat-resistant performance of the bottle should be good. To overcome the impact of hot liquid (high temperature) on the shape of the bottle: high temperature makes the bottle soft, high temperature and hot liquid to produce high pressure inside the bottle. High temperature (85 ℃ ~ 90 ℃) volume shrinkage rate is controlled between 1% to 1.5% is better.
- The bottle should have the performance to withstand negative pressure. To overcome the negative pressure generated in the bottle after the liquid cools; bottle wall shrinkage (flattening, bottle sidewall deformation when contracting under negative pressure, oval and other phenomena).
The bottle blowing method of hot filling PET bottles
- One-step method.
Production of bottle preform by the crystallization furnace crystallization of the bottle mouth, directly with high-temperature mold blowing bottle forming.
Advantages: high yield, easy to mass production. Disadvantages: the high temperature resistance is poor, and the high temperature resistance decreases significantly with the extension of time, so the storage time cannot be too long.
- Two-step method.
After the produced bottle preform are crystallized by the crystallization furnace on the bottle mouth, two sets of molds are used to complete the blow molding of hot filling. First with the first set of larger volume of the mold (low-temperature mold), the bottle preform is stretched and blown into a very large volume of bottles; then the bottle is sent into the heating furnace for heat treatment (to eliminate the internal stress caused by stretching); heat treatment is good and sent into the hot mold (the final required volume size of the mold), the bottle further heat treatment (to improve the crystallinity of the bottle), shaping, and finally blown into the required shape and size of the bottle. Advantages: better heat resistance of bottles and long storage time. Disadvantages: low output is not suitable for mass production.
◆ Steps of hot filling PET bottle blowing process.
(1) Firstly, the bottle preform are sorted by the preform supply system and then transported to the bottle preform heater.
(2) While the preform heater heats the preform, it cools the bottle mouth while the preform rotates itself to make it evenly heated, and then the heater fan blows the preform to make the inner and outer walls of the preform evenly heated.
(3) The heated bottle preforms are fed into the blowing station by the preform transfer robot.
(4) After the preform enters the blowing mold, the pre-blowing air enters to stretch the preform in a circular direction; when the stretching bar reaches the bottom of the mold (ten position), the high-pressure air enters the mold cavity to stretch the preform further, so that the wall of the preform is close to the mold wall.
(5) The high pressure air is kept in the mold for a certain period of time to eliminate the internal stress caused by the stretching of the bottle preform on the one hand. On the other hand, it makes the wall of the bottle close to the mold wall to improve the crystallinity of the bottle plastic.
(6) After the high-pressure blowing is finished, the air is exhausted and the high-pressure cooling air is blown out from the hollow stretching bar to cool and shape the bottle wall at the same time. If no air is blown out of the bottom mold, it will cause problems such as the bottom of the bottle protruding and the bottle cannot be taken out.
(7) At the end of the whole blowing process, the bottles are taken out of the mold by the bottle transfer robot and sent to the juice and beverage hot filling line.
The production process affects the quality of heat-resistant PET bottles several major factors
1. Bottle preform: characteristic viscosity ≥ 0.81cm3/g, viscosity drop ≤ 4%, storage time can not exceed 3 months. Color pure, transparent, no impurities, no foreign colors, the length of the injection point and around the halo spot appropriate.
2. Heating: In the oven, the far infrared lamp emits far infrared radiation heating to the bottle preform, and the fan at the bottom of the oven carries out heat circulation to make the temperature in the oven uniform. The bottle preform moves forward in the oven while rotating itself, so that the wall of the bottle preform is heated evenly. The heat of the oven is set by the number of lamps turned on, the overall temperature, the power of the oven and the heating ratio of each section are jointly controlled.
3. Pre-blowing: Pre-blowing starts at the same time when the stretching rod is lowered to make the initial shape of the bottle. Pre-blowing position, pre-blowing pressure and blowing flow are three important process factors.
4. Mold temperature: The temperature of the mold is controlled at 120℃~145℃, which is used to eliminate the internal stress generated by the bottle embryo stretching and improve the crystallinity of the bottle plastic to resist the high temperature hot liquid so that the bottle will not be deformed.
5. environment: room temperature, low temperature (air conditioning) state is preferred.
Heat-resistant PET bottles in the production process of the general quality problems arising from the causes and solutions
Skewed bottle neck
1. blockage of oil circuit unclog the oil circuit of the mold body
2. blockage of stretching rod exhaust hole unclog the blowing hole of stretching rod
3. Damaged nozzle seal Replace the nozzle seal
Center point is off
1. Pre-blowing pressure is too high Reduce pre-blowing pressure
2. pre-blowing flow is too large reduce the pre-blowing flow
3. pre-blowing position is too early postpone the pre-blowing position
4. Tension rod is bent Replace the tension rod
5. Tension rod clearance from the bottom mold is too large Adjust the tension rod clearance
6. Preform temperature is too high Reduce preform set temperature
Bottle bottom deformation
1. Bottom die oil temperature is too high Lower the oil temperature of the hot oil machine
2. The bottom die blow valve is damaged Replace the bottom die blow valve
3. Bottom of the bottle is too high Lower the bottom temperature of the bottle
Bottom of the bottle is wrinkled
1. Pre-blowing pressure is too low Increase pre-blowing pressure
2. Pre-blowing flow rate is too small Increase pre-blowing flow rate
3. Pre-blowing too late Pre-blowing too early
1. Insufficient neck heating Increase the neck heating
2. Pre-blow pressure is too high Decrease pre-blow pressure
3. Pre-blow flow rate too high Reduce pre-blow flow rate
4. Pre-blowing too early Delay pre-blowing
5. heater position too high Adjust heater position
6. slow tension rod speed Repair tension cylinder
Poorly formed die line
1. Mold compensation seal is damaged Replace the compensation seal
2. Improper adjustment of mold gap Adjust the mold gap
Deformation of side wall before filling
1. Cooling and blowing time is too short Extend the cooling and blowing time
2. Mold body temperature is too high Lower the mold body temperature
3. No cooling air blowing out from the tension rod Check and repair the tension rod blowing system
Deformation of side wall after filling
1. Mold body temperature is too low Increase mold body hot oil temperature
2. Preform set temperature is too low Increase preform set temperature
3. cooling blowing time is too long reduce cooling blowing time
4. Uneven distribution of plastic Adjust the blowing process to make the material distributed evenly
5. The flow rate of hot oil is too low Unclog the oil circuit and clean the oil filter.
Shrinkage rate is large
1. low mold temperature Increase mold temperature
2. low bottle preform temperature Increase the setting temperature of bottle preform
3. Cooling and blowing time is too long Shorten the cooling and blowing time
4. Clogged oil circuit Unclog the oil circuit
Diameter is too large or too small
1. Improper setting of cooling and blowing time Adjust the cooling and blowing time
2. Uneven distribution of plastic Adjust the process to make the material evenly distributed
PET bottles in the process of hot filling line use of common problems and their solutions
1. Storage and transportation conditions and storage period of bottles.
- Since PET has moisture-absorbing properties, when PET (including chips, blanks and bottles) is placed in the air, it will absorb moisture from the air, and the longer it is placed, the more water it will absorb. And the moisture content in PET will directly affect its performance. For hot-filled bottles, it will affect the heat-resistant temperature of the hot-filled bottles. The more moisture content, the lower the heat resistance temperature of the bottle. Generally speaking for hot-fill bottles, the placement time is recommended from the production of the bottle preform to the period of filling the beverage.
- Bottle storage period: >1L used within two weeks, <1L used within three weeks; however, more and more manufacturers have recently used lightweight bottles and linked production, namely blow-and-fill, bottle storage period within 6 hours. Ready-to-blow bottles can be filled with hot liquids at 95°C. Bottles stored for more than 24 hours after blowing can only be filled with hot liquids at 88°C.
- Bottle materials, storage conditions (room temperature, relative humidity, length of storage time), will affect the technical indicators of hot-filled bottles, that is: the production of bottles according to the above different materials, storage conditions, customer requirements, etc., accordingly adjust the process of blowing bottles, technical parameters, etc.
- PET in the usual humidity conditions, melt plasticization will occur when the hydrolysis reaction. The mechanical properties of PET are related to the characteristic viscosity IV, and the lower the IV, the worse the mechanical properties of PET.
- The average relative humidity in Jiangnan and coastal areas is 85% throughout the year, and in some areas the relative humidity can be more than 90% in spring and summer, in the high humidity environment, PET will absorb moisture and reach the maximum saturation humidity.
- The higher the moisture content, the greater the decrease in the IV value of PET. The characteristic viscosity of a certain type of PET is 0.73 when the moisture content is 0.01% and 0.63 when the moisture content is 0.02%. at 180°C the characteristic viscosity decreases by 0.10 because the drying time is reduced by 3/4 hours.
- The longer the drying time, the lower the moisture content in the PET material, but excessive drying can also cause degradation of PET. When heated to 180°C, for a raw material with a maximum initial moisture content of 0.3%, the moisture drops to 0.14%; a moisture content of 0.004% can be obtained by drying for 4 hours, which is the upper limit of the controlled moisture content of the bottle preform. The moisture within the molecules of the top part of the bottle will accelerate the crystallization of PET, while the moisture within the molecules of the body part of the bottle will affect the arrangement of the molecular chains.
2. Poor heat resistance.
◆ Hot-fill bottles achieve heat resistance in this way.
(1) Special mold design is used to resist the negative pressure inside the bottle.
- The bottle has a rectangular concave block (which can be moved in and out of the mold) to absorb the negative pressure generated inside the bottle after the liquid cools.
- Bottle design with neck and waist (concave ring) to prevent the bottle from becoming oval.
- Bottle bottom design (generally petal-shaped) to resist stress or carbon dioxide pressure (normal temperature filling high-temperature sterilization class bottles using concave bottom design).
(2) Use hot oil machine high temperature oil to increase the mold temperature (mold temperature at 120℃～145℃), used to eliminate the internal stress generated by the bottle preform stretching, improve the bottle plastic crystallinity to resist high temperature hot liquid, so that the bottle will not be deformed.
◆ Measures to improve the heat resistance performance of bottles.
- Choose a reasonable bottle preform and bottle design. Optimal design of bottle preform shape and bottle mold design can help improve the wall thickness distribution of bottles and avoid distortion or shrinkage deformation in different areas of the bottle.
- Bottle preform injection cooling time control. Strictly control the injection cooling time to release the bottles from the mold as early as possible. This can shorten the molding cycle and increase the output, but also can induce spherical crystallization due to the higher residual temperature. The crystal diameter of spherical crystallization is very small (only 0.3mm to 0.7mm) and does not affect the transparency.
- Strict control of injection and drawing-blowing process parameters and temperature distribution in each region to avoid bottle deformation due to residual stress release at PET glass transition temperature (>75°C).
- The use of blowing mold temperature regulation technology. Usually use the hot oil circulation method to blowing mold heating, blowing mold temperature regulation has three cycles.
- Bottle body hot oil cycle. The blowing mold heated to 120 ℃ ~ 145 ℃. In this way, the temperature difference between the bottle preform and the blow mold cavity is reduced, promoting further crystallization. Extend the blowing pressure time, so that the bottle wall and cavity contact for a long time and have enough time to improve the crystallinity of the bottle, reaching about 35%, but without destroying the transparency. 100 ℃ below the mold temperature has very little effect on the crystallinity of the bottle, because the crystallization of the bottle occurs above 100 ℃.
- Bottle bottom cooling water circulation. The bottom of the bottle is kept at a low temperature (10°C to 30°C) to avoid excessive crystallization and whitening of the unstretched bottom part of the bottle.
- Bottle neck temperature regulation (optional). The non-crystallized bottle neck part is always in a completely cooled state after being demolded from the injection mold. Most of the non-crystalline bottle tops are designed with reinforced bottle tops (increased wall thickness of bottle tops), thus improving the sealing performance and avoiding the deformation of bottle tops during capping. Usually, the ellipticity of the bottle mouth is controlled within 0.2mm after filling, and the shrinkage of the thread outer diameter is less than 0.6%.
- Cyclic blowing technology. When using hot blowing mold, how to control the bottle deformation after demoulding is crucial. Blowing air is blown into the mold before opening and evacuating the cycle to cool and shape the bottle, thus controlling the amount of deformation after demolding. The inlet air of the circulating cooling air passes through the same channels as the initial blow and the secondary blow, but is exhausted from a small hole in the head of the drawbar through the inside of the drawbar. The circulating blowing time is about 0.5 seconds to 2 seconds. Therefore, the high pressure air consumption of heat-resistant bottle making machine is much higher than that of ordinary bottle making machine.
3. Larger capacity fluctuation.
Biaxially stretched PET bottles have a certain shrinkage rate, the maximum shrinkage rate is about 2%. The factors affecting the capacity of PET bottles are mainly as follows.
(1) The influence of mold
The capacity of PET bottle is mainly influenced by the size and shape of the mold. Each bottle shape mold size is usually fixed. The shrinkage rate will be different for different shapes of bottles, the less reinforcement on the bottle and the thinner the bottle thickness, the greater the shrinkage rate of the bottle.
(2) The influence of environmental factors
The higher the ambient temperature and the higher the humidity, the greater the shrinkage of the bottle capacity.
(3) Influence of production process
If the blowing pressure is not enough, the bottle will be poorly formed and the capacity will be small; the high temperature of the mold will also cause the capacity to be small.
(4) The natural shrinkage of the bottle
Since PET bottles will shrink naturally, the bottle mold size should be designed in an adjustable form (plus or minus shims) when designing. Take 1.5LPET bottle as an example, the average capacity of the newly produced bottle is about 1508ml, after 3 days of storage at room temperature, the capacity of the bottle will be reduced by 5ml~6ml; as the bottle is stored for a longer time, the capacity of the bottle will be reduced and difficult to control. At present, more and more production lines use in-line bottle blowing, that is, blowing and filling, to avoid the bottle (capacity and heat resistance) decay problem.
(5) Influence of filling method
The influence of different filling methods on capacity control also varies greatly. The quantitative filling method has the least impact on the capacity, and the self-weighting filling has the greatest impact on the capacity, which may differ by 20 ml to 25 ml at most for 1.5LPET bottles.
Therefore, to solve the bottle capacity problem, the mold (gasket) can be properly adjusted, the production process can be controlled, the storage conditions can be improved, and most importantly, the storage period of the bottle should be shortened as much as possible.
If you want to produce your own hot filling juice in PET bottles, please contact us to get free proposal and suggestions.