The special quality requirements of the hot filling process for PET bottles
The bottle's heat resistance is improved.
To overcome the effect of hot liquid (high temperature) on the bottle’s shape: high temperature softens the bottle, and high temperature and hot beverage create high pressure in the bottle.
At high temperatures (85°C90°C), keeping the volume shrinkage rate between 1% and 1.5 per cent is preferable.
Negative pressure must be able to be absorbed by the bottle.
The bottle wall shrinks inward to overcome the negative pressure created in the bottle after the liquid has cooled. The sidewall of the bottle deforms when it becomes flat or when the negative pressure shrinks and becomes an oval phenomenon.
Blowing method for hot fill PET bottles
One-step method of blow moulder:
After being crystallized in the crystallization furnace, the bottle preform is blown directly into the bottle using a high-temperature mould.
Advantages: include high output and ease of mass production.
Disadvantages: The high-temperature resistance is poor, and it decreases significantly with time, so storage time cannot be excessive.
Two-step method of PET blow moulder:
Two sets of moulds are used to complete the hot-filling blow moulding after a crystallization furnace crystallizes the produced preform on the bottle mouth.
To stretch and blow the preform into a super-large-volume bottle, first, use the first set of larger-volume moulds (low-temperature moulds); then send the bottle to the heating furnace for heat treatment (to eliminate the internal stress caused by stretching);
After heat treatment, the bottle is sent to a hot mould (a mould with the final required volume). It is heat-treated (to increase the crystallinity of the bottle body) and shaped before being blown into the desired shape and size.
Advantages: The bottle is more resistant to high temperatures and has a longer storage time.
Disadvantages: Low output makes mass production impossible.
◆ Steps of the blow moulding process:
(1) The preform supply system sorts the preforms before transporting them to the preform heating furnace.
(2) The preform heating furnace cools the bottle mouth as it rotates to ensure evenly heated while heating the preform. The preform is then blown by the furnace fan to ensure that the inner and outer walls of the preform are evenly heated.
(3) The blank conveying manipulator feeds the heated preform into the bottle blowing station.
(4) Pre-blowing occurs after entering the blowing mould, stretching the preform in a ring direction. When the stretching rod reaches the mould’s bottom, high-pressure air enters the cavity, stretching the preform even more and bringing the bottle wall closer to the mould wall.
(5) On the one hand, the high-pressure gas is kept in the mould for a specific amount of time, relieving internal stress caused by the stretching of the preform.
On the other hand, to improve the crystallinity of the bottle body plastic, make the bottle wall close to the mould wall.
(6) The exhaust begins after the high-pressure blowing is completed. High-pressure cooling gas is blown out of the hollow stretch rod at the same time to cool and shape the bottle wall.
Blowing low-pressure air from the bottom mould for demolding is a good idea.
If there isn’t any air blowing out of the bottom mould, the bottom of the bottle will protrude, and the bottle will not be removed.
(7) When the bottle blowing process of blow moulder is finished, the bottle conveying manipulator removes the bottle from the mould. It transports it to the bottle conveying line.
several important factors influence the quality of heat-resistant PET bottles
Intrinsic viscosity of 0.81cm3/g, viscosity drop of 4%, maximum storage time of 3 months
The far-infrared lamp in the oven emits far-infrared rays that radiate and heat the preform. The fan at the bottom of the range circulates heat to keep the temperature in the stove consistent.
The preform rotates as it moves forward in the oven, evenly heating the preform wall.
The oven’s heat is controlled by the number of lamps turned on and the overall temperature. The oven’s power and each section’s heating ratio are tandem.
When the stretching rod descends, begin pre-blowing to shape the preform.
Three important process factors are pre-blowing position, pressure, and blowing airflow.
The mould temperature is maintained between 120°C and 145°C to eliminate internal stress caused by preform stretching, increase the crystallinity of the bottle body plastic to withstand the high-temperature hydrothermal fluid, and prevent deformation of the bottle.
It is preferable to have a room temperature or a low temperature (air conditioning).
The causes and solutions for general heat-resistant PET bottle quality issues
The skew of the bottleneck
1. If the oil passage in the mould body is blocked, clear it.
2. The stretch rod’s exhaust hole is blocked. Clear the stretch rod’s blowing hole.
3. There is a problem with the nozzle sealing ring. Remove the nozzle sealing ring and replace it.
Deviation of the centre point
1. If the pre-blowing pressure is too high, it should be reduced.
2. If the pre-blowing flow is too high, it should be reduced.
3. If the pre-blowing position is too early, postpone it.
4. There is a bend in the stretch rod. The stretch rod should be replaced.
5. There is an excessive gap between the stretching rod and the bottom mould. Set the distance between the stretching rods to your liking.
6. The temperature of the preform is too high; reduce the set temperature of the preform
The bottom of the bottle is deformed
1. The oil temperature of the bottom mould is too high; reduce the oil temperature of the hot oil machine
2. The blow valve on the bottom of the mould is broken. Replace the blow valve on the bottom of the mould.
3. The temperature of the preform’s bottom is too high; lower the temperature of the preform’s bottom.
Wrinkles at the bottom of the bottle
1. The pre-blowing pressure is too small. Increase the pre-blowing pressure
2. The pre-blowing flow is too small. Increase the pre-blowing flow
3. Pre-blowing too late, pre-blowing early
1. Insufficient heating of the neck, increase the amount of heating of the neck
2. If the pre-blowing pressure is too high, it should be reduced.
3. If the pre-blowing flow is too high, it should be reduced.
4. Pre-blowing too soon; postpone pre-blowing
5. The heating furnace is positioned too high. Adjust the heating furnace’s position.
6. The stretching rod takes a long time to stretch. The stretching cylinder should be overhauled.
Poor parting line formation
1. The mould compensation seal has been tampered with. Replace the compensation seal if necessary.
2. Incorrect mould gap adjustment Adjust the mould gap if necessary.
Irrigation causes wall deformation.
1. The cooling blow duration is insufficient. Increase the duration of the cooling blow.
2. If the mould’s body temperature is too high, it should be lowered.
3. The stretch rod does not produce any cooling air. The stretch rod blowing system should be overhauled.
Wall deformation after irrigation
1. The temperature of the mould body is too low to increase the temperature of the hot oil of the mould body
2. The preform’s set temperature is too low. Increase the preform’s set temperature.
3. The cooling blow time is excessively long; shorten it.
4. Plastics are distributed unevenly. Adjust the blowing process to ensure that materials are distributed evenly.
5. The flow of hot oil is insufficient. Clean the oil filter and dredge the oil circuit.
1. Reduce the temperature of the mold, then raise it.
2. Increase the preform setting temperature if the preform temperature is too low.
3. The cooling and blowing time is excessively long; reduce the time for cooling and blowing.
4. If the oil passage is blocked, it must be cleared.
The diameter is either excessively large or excessively small.
1. Incorrect cooling air blowing time setting Change the duration of the cooling air blowing
2. Plastics distribution is uneven; adjust the process to make the material distribution even.
Common problems and solutions in the use of PET bottles in the hot filling line
1. Storage and transportation conditions and the bottle's storage period
Because PET absorbs moisture, placing PET in the air (including slices, preforms, and bottles) will absorb moisture in the air. It will absorb more water the longer it is left in place.
PET’s performance is directly influenced by its moisture content.
It will have an impact on the heat-resistant temperature of hot fill bottles.
The higher the moisture content, the lower the heat-resistant temperature of the bottle.
In general, the recommended placement time for hot fill bottles between the production of the preform and the filling of the beverage is:
Bottle storage period: >1L use within two weeks, <1L use within three weeks;
However, many manufacturers have begun to use lightweight bottles and connected production in recent years.
i.e., filling right after blowing, with a bottle storage period of fewer than 6 hours.
If the bottle is filled immediately after blowing, such a bottle can be filled with 95℃ hot liquid;
Bottles stored for more than 24 hours after blowing can only be filled with hot liquid at 88°C
The technical indicators of hot fill bottles are affected by bottle material and storage conditions (room temperature, relative humidity, storage time), namely: bottle production should be based on the above different materials, storage conditions, customer requirements, etc., and the blowing process and technical parameters should be adjusted accordingly.
When PET is melted and plasticized in average humidity, it undergoes a hydrolysis reaction.
High humidity levels frequently trigger an immediate reaction, resulting in molecular chain breaking, degradation, and a reduction in molecular weight (IV reduction).
The intrinsic viscosity IV of PET is related to its mechanical properties. The mechanical properties of PET deteriorate as the IV decreases. In Jiangnan and coastal areas, the annual average relative humidity is 85 per cent, and relative humidity in some areas can reach 90 per cent in the spring and summer.
PET will absorb moisture and reach maximum saturated humidity in a high humidity environment.
The lower the IV value of PET becomes as the moisture content rises.
The intrinsic viscosity of a specific type of PET is 0.73 when the water content is 0.01 per cent, and 0.63 when the water content is 0.02 per cent.
The intrinsic viscosity decreases by 0.10 at 180°C as the drying is cut in half.
The lower the moisture in the PET raw material, the longer the drying time, but over-drying can cause PET degradation.
When raw materials with a maximum initial moisture content of 0.3 per cent are heated to 180°C, the moisture content drops to 0.14 per cent. After drying for 4 hours, the moisture content drops to 0.004 per cent, which is the upper limit for controlling the moisture content of the preform.
Moisture in the bottle mouth molecules will speed up the crystallization of PET, while humidity in the bottle body molecules will affect the molecular chain arrangement.
2. Poor heat resistance.
2.1 hot fill bottles are heat-resistant in this way:
(1) Make a particular mould to withstand the bottle’s negative pressure:
a. After the liquid has cooled, a rectangular concave block (movable in and out of the mould) is used to absorb the negative pressure created in the bottle.
b. To prevent the bottle from becoming oval, use the neck and waist (concave ring) in the design.
c. To withstand stress or carbon dioxide pressure, use a bottle bottom design (usually petal-shaped) (concave bottom design is used for high-temperature sterilization bottles filled at room temperature).
(2) Increase the mould temperature (between 120°C and 145°C) with the hot oil machine’s high-temperature oil to eliminate internal stress caused by preform stretching. Increasing the bottle’s plastic’s crystallinity makes it more resistant to high-temperature hydrothermal fluid. There has been no tampering with the bottle in any way.
2.2 Measures to improve the heat resistance of bottles:
a. Select an appropriate preform and bottle design. The optimized preform shape and bottle mould design aid in enhancing the bottle’s wall thickness distribution and preventing distortion or shrinkage in various areas of the bottle body.
b. Preform injection cooling time should be controlled. Control the preform injection cooling time as closely as possible to allow the preform to be demolded as soon as possible. The bottle blow moulding cycle can be shortened, and the output increased in this way, and spherical crystallization can be induced due to the higher residual temperature. The crystal diameter of spherical crystals is extremely small (only 0.3mm0.7mm), so transparency is unaffected.
c. Maintain strict control over the injection and stretch-blowing process parameters, as well as the temperature distribution in each area, to avoid residual stress being released at the PET’s glass transition temperature (>75°C) and causing the bottle to deform.
d.For blowing moulds, temperature adjustment technology is used. The bottle blowing mould is usually heated using the hot oil circulation method. The bottle body hot oil cycle is one of three temperature adjustment cycles for the bottle blowing mold. Preheat the blowing mold to a temperature of 120°C to 145°C. As a result, the temperature difference between the preform and the blowing cavity is reduced, allowing more crystallization. Increase the bottle blowing’s pressure-holding time so that the bottle wall and cavity are in contact for a long time. There is enough time to increase the crystallinity of the bottle body to about 35% without destroying the transparency.
Because bottle body crystallization occurs above 100°C, mould temperatures below 100°C have little effect on the crystallinity of the bottle body. The bottom of the bottle has a cooling water circulation system. Keep the unstretched bottom part of the bottle at a low temperature (10°C–30°C) to avoid excessive crystallization and whitening. Temperature control in the bottleneck (optional).
After being demolded from the injection mould, the non-crystalline bottle mouth part was completely cooled. The majority of non-crystalline bottle mouths use a strengthened bottle mouth design (increasing the bottle mouth’s wall thickness), which improves sealing performance and prevents the bottle mouth from deforming during the capping process. After filling, the bottle mouth is usually kept within 0.2mm of its original ovality, and the thread’s outer diameter shrinkage rate is less than 0.6 per cent.
e. Bottle blowing moulder technology with 5 cycles. Controlling the deformation of the bottle after demoulding is critical when using hot blow moulding. Blowing in air and venting the cycle before opening the blowing moulder to cool and shape the bottle body to control the amount of deformation after demolding. The circulating cooling air enters through the same passage as the primary and secondary blowing but exits through the drawing rod through the small hole in the head of the drawing rod. The time it takes for the cycle to blow is between 0.5 and 2 seconds. As a result, the heat-resistant bottle blow moulder‘s high-pressure air consumption is significantly higher than that of a standard blow moulder.
3. Large fluctuations in capacity.
PET bottles that are biaxially stretched shrink at a specific rate, with a maximum shrinkage rate of about 2%. The following are the main factors that influence the capacity of PET bottles:
(1) Mold’s impact on the environment The size and shape of the mould have a significant effect on the PET bottle’s capacity. Each bottle mould’s size is usually predetermined.
When designing the shrinkage rate, keep in mind that the shrinkage rate of bottles of various shapes will vary. The fewer ribs on the bottle body and the thinner the bottle thickness are proportional to the bottle’s shrinkage rate.
(2) The impact of environmental variables The capacity of the bottle is more affected by the ambient temperature and humidity. The higher the ambient temperature and humidity, the greater the shrinkage of the bottle’s capacity.
(3) The impact of the manufacturing process. A higher blowing moulding machine pressure of blow molder is required for the bottle with a complex shape. If the blowing pressure is insufficient, the bottle will be poorly formed, and the capacity will be inadequate.; a higher mould temperature will also result in a smaller capacity.
(4) The bottle’s natural shrinkage
The size of the bottle mould should be adjustable because the PET bottle shrinks naturally (plus or minus gasket). As an example, consider a 1.5L PET bottle. The newly produced bottle has an average capacity of 1508ml. The bottle capacity will decrease by 5ml-6ml after 3 days of storage at room temperature; The power of the bottle will shrink as the time spent storing it increases, making it difficult to control. More and more production lines are now using blowing connecting filling or filling right after filling (capacity and heat resistance).
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