Due to the safety and economy of
PET bottles, they have been widely
used today when juice drinks and tea
drinks have become mainstream
beverage products.
1.
The special quality requirements
of hot filling process for PET
bottles
1. The heat resistance of the bottle is
better. To overcome the influence of hot
liquid (high temperature) on the shape
of the bottle: high temperature makes
the bottle soft, and high temperature
and hot liquid cause high pressure in
the bottle. It is better to control the
volume shrinkage rate between 1% and
1.5% at high temperature
(85℃～90℃).
2. The bottle must be able to withstand
negative pressure. It is necessary to
overcome the negative pressure generated
in the bottle after the liquid is
cooled; the bottle wall shrinks (the
side wall of the bottle deforms when the
negative pressure shrinks, and becomes
elliptical).
Second,
the blowing method of hot-filled
PET
bottles
1. One-step method: After the produced
bottle preform is crystallized on the
bottle mouth by the crystallization
furnace, it is directly blown into the
bottle with a high temperature mold.
Advantages: high output, convenient for
mass production. Disadvantages: The high
temperature resistance is poor, and the
high temperature resistance decreases
significantly with time, and the storage
time cannot be too long.
2. Two-step method: After the produced
preform is crystallized on the bottle
mouth by a crystallization furnace, two
sets of molds are used to complete the
hot-filling blow molding. First use the
first set of larger-volume molds
(low-temperature molds) to stretch and
blow the preform into super-large-volume
bottles; then send the bottles to the
heating furnace for heat treatment
(eliminate the internal stress caused by
stretching); After it is finished, it is
sent to a hot mold (a mold with the
final required volume), and the bottle
is further heat-treated (to increase the
crystallinity of the bottle body),
shaped, and finally blown into a bottle
of the required shape and size.
Advantages: the bottle has better high
temperature resistance and long storage
time. Disadvantages: low output is not
suitable for mass production.
◆ Steps of blowing
process:
(1) The preforms are sorted by the
preform supply system and then
transported to the preform heating
furnace.
(2) While heating the preform, the
preform heating furnace cools the bottle
mouth while it rotates to make it evenly
heated, and then the furnace fan blows
the preform to make the inner and outer
walls of the preform evenly heated
.
(3) The heated preform is fed into the
bottle blowing station by the blank
conveying manipulator.
(4) After the preform enters the blowing
mold, the pre-blowing air enters to
stretch the preform in a circular
direction; when the stretch rod reaches
the bottom of the mold (ten position),
high-pressure air enters the mold cavity
to further stretch the preform, so that
The bottle wall is close to the mold
wall.
(5) The high-pressure gas is kept in the
mold for a certain time, on the one
hand, it eliminates the internal stress
caused by the stretching of the preform.
On the other hand, make the bottle wall
close to the mold wall to improve the
crystallinity of the bottle
plastic.
(6) After the high-pressure blowing is
finished, the exhaust starts, and at the
same time, high-pressure cooling gas is
blown out from the hollow stretch rod to
cool and shape the bottle wall. While
demolding, blow low-pressure air from
the bottom mold for demolding. If there
is no air blowing from the bottom mold,
it will cause problems such as the
bottom of the bottle protruding and the
bottle not being taken out.
(7) After the entire bottle blowing
process is over, the bottle conveying
manipulator takes the bottle out of the
mold and sends it to the bottle
conveying line.
3.
Several main factors that affect
the quality of heat-resistant
PET bottles during the
production
process
1. Preform: Intrinsic viscosity
≥0.81cm3/g, viscosity drop ≤4%, storage
time cannot exceed 3 months. The color
is pure, transparent, free of
impurities, no different colors, and the
spot length and surrounding spots are
suitable. 2. Heating: In the oven, the
far-infrared light tube emits
far-infrared rays to radiate and heat
the preform, and the fan at the bottom
of the oven performs heat circulation to
make the temperature in the oven
uniform. The preform rotates while
moving forward in the oven, so that the
preform wall is evenly heated. The heat
of the oven is set by the number of
lamps turned on and the overall
temperature. The power of the oven and
the heating ratio of each section are
jointly controlled.
3. Pre-blowing: Start pre-blowing while
the stretching rod is lowered to make
the preform take shape. Pre-blowing
position, pre-blowing pressure, and
blowing air flow are three important
process factors.
4. Mold temperature: The temperature of
the mold is controlled at 120℃～145℃ to
eliminate the internal stress caused by
the stretching of the preform, increase
the crystallinity of the bottle body
plastic to withstand the high
temperature hydrothermal fluid, and make
the bottle not deformed. 5. Environment:
room temperature and low temperature
(air conditioning) are better.
4.
Reasons and solutions for
general quality problems of
heat-resistant PET bottles in
the production
process
Bottleneck
skew
1. The oil passage is blocked, clear the
oil passage of the mold body
2. Exhaust hole of the stretch rod is
blocked. Clear the blowing hole of the
stretch rod
3. The nozzle seal is damaged. Replace
the nozzle seal
Center point
deviation
1. Pre-blowing pressure is too high,
reduce pre-blowing pressure
2. The pre-blowing flow is too large,
reduce the pre-blowing flow
3. Pre-blowing position is too early,
postpone pre-blowing position
4. The stretch rod is bent Replace the
stretch rod
5. The gap between the stretch rod and
the bottom mold is too large. Adjust the
gap between the stretch rods
6. The preform temperature is too high,
reduce the preform setting
temperature
The bottom of the bottle is
deformed
1. The bottom mold oil temperature is
too high, reduce the hot oil engine oil
temperature
2. The bottom mold blow valve is
damaged. Replace the bottom mold blow
valve
3. The bottom temperature of the preform
is too high, reduce the bottom
temperature of the preform
Folds 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 is too late, pre-blowing
early
Stiff neck
1. Insufficient neck heating Increase
the amount of neck heating
2. The pre-blowing pressure is too
large, reduce the pre-blowing
pressure
3. The pre-blowing flow is too large,
reduce the pre-blowing flow
4. Pre-blowing too early, delay
pre-blowing
5. The position of the heating furnace
is too high. Adjust the position of the
heating furnace
6. The stretching rod is slow. Overhaul
the stretching cylinder
Poor forming of parting
line
1. The mold compensation seal is
damaged. Replace the compensation
seal
2. Improper adjustment of the mold gap
Adjust the mold gap
Wall deformation before
irrigation
1. The cooling blow time is too short.
Extend the cooling blow time
2. The mold body temperature is too
high, reduce the mold body
temperature
3. There is no cooling air blowing out
of the stretch rod. Overhaul the stretch
rod blowing system
Wall deformation after
irrigation
1. The mold body temperature is too low
to increase the temperature of the mold
body hot oil
2. The set temperature of the preform is
too low. Increase the set temperature of
the preform
3. The cooling blow time is too long,
reduce the cooling blow time
4. Uneven distribution of plastics
Adjust the blowing process to make the
distribution of materials even
5. The flow of hot oil is too small,
clear the oil circuit and clean the oil
filter
Large shrinkage
1. Low mold temperature Increase mold
temperature
2. The preform temperature is low,
increase the preform setting
temperature
3. The cooling blow time is too long,
shorten the cooling blow time
4. The oil passage is blocked, clear the
oil passage
The diameter is too large or too
small
1. Improper setting of cooling blow time
Adjust the cooling blow time
2. Uneven distribution of plastics
Adjust the process to make the
distribution of materials even
5.
Common problems and solutions in
the use of PET bottles in hot
filling
lines
1. Storage and transportation
conditions and the storage period of
the bottle.
Due to the hygroscopic properties of
PET, placing PET (including slices,
preforms and bottles) in the air will
absorb moisture in the air. The longer
it is placed, the more water it will
absorb. The moisture content in PET will
directly affect its performance. For
hot-filled bottles, it will affect the
heat-resistant temperature of hot-filled
bottles. The more water content, the
lower the heat-resistant temperature of
the bottle. Generally speaking, for
hot-filled bottles, during the period
from the production of the preform to
the filling of the beverage, the
recommended placement time:
Bottle storage period: >1L within two
weeks, <1L within three weeks; but
recently, more and more manufacturers
use lightweight bottles and connected
production, that is, blow and fill, and
the bottle storage period is within 6
hours. Blow-and-fill bottles can be
filled with hot liquid at 95°C. Bottles
stored for more than 24 hours after
blowing can only be filled with hot
liquid at 88°C.
Bottle materials and 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 should be based on the above
different materials, storage conditions,
customer requirements, etc., accordingly
Adjust the blowing process and technical
parameters. PET will undergo a
hydrolysis reaction when it is melted
and plasticized under normal humidity.
High humidity content often leads to
immediate reaction, resulting in
molecular chain breaking, degradation,
and molecular weight reduction (that is,
lower IV). The mechanical properties of
PET are related to the intrinsic
viscosity IV. The lower the IV, the
worse the mechanical properties of
PET.
The annual average relative humidity in
Jiangnan and coastal areas is 85%. In
some areas, the relative humidity can be
as high as 90% in spring and summer. In
a high humidity environment, PET will
absorb moisture and reach the maximum
saturation humidity.
The higher the moisture content, the
greater the drop in the IV value of PET.
When the water content of a certain type
of PET is 0.01%, its intrinsic viscosity
is 0.73, and when the water content is
0.02%, its intrinsic viscosity becomes
0.63. At 180°C, the intrinsic viscosity
decreases by 0.10 due to the reduction
of the drying time by 3/4 hour.
The longer the drying time, the lower
the moisture in the PET raw material,
but excessive drying can also cause PET
degradation. When heated to 180°C, for
raw materials with a maximum initial
moisture content of 0.3%, the moisture
drops to 0.14%; drying for 4 hours can
obtain a moisture content of 0.004%,
which is the upper limit for controlling
the moisture content of the preform. The
moisture in the molecules of the bottle
mouth will accelerate the
crystallization of PET, and the moisture
in the molecules of the bottle body will
affect the arrangement of molecular
chains.
2. Poor heat
resistance.
◆ Hot-filled bottles are heat-resistant
in this way:
(1) Use a special mold design to
withstand the negative pressure in the
bottle:
① The bottle body has a rectangular
concave block (which can be moved in and
out on the mold) to absorb the negative
pressure generated in the bottle after
the liquid is cooled.
② Bottle design, use neck and waist
(concave ring) to prevent the bottle
from becoming oval.
③ Use bottle bottom design (usually
petal-shaped) to withstand stress or
carbon dioxide pressure (concave bottom
design is used for high temperature
sterilization bottles at room
temperature).
(2) Use the high temperature oil of the
hot oil machine to increase the mold
temperature (the mold temperature is
between 120°C and 145°C) to eliminate
the internal stress caused by the
stretching of the preform, increase the
crystallinity of the bottle body
plastic, and resist the high temperature
hydrothermal fluid. The bottle is not
deformed.
◆ Measures to improve the heat
resistance of the bottle:
① Choose a reasonable preform and bottle
design. The optimized preform shape
design and bottle mold design help to
improve the wall thickness distribution
of the bottle and avoid distortion or
shrinkage in different areas of the
bottle body;
② Control the cooling time of preform
injection. Strictly control the cooling
time of the preform injection to allow
the preform to be demolded as soon as
possible. In this way, the molding cycle
can be shortened and the output can be
increased, and the spherical
crystallization can be induced due to
the higher residual temperature. The
crystal diameter of the spherical
crystal is very small (only
0.3mm～0.7mm), which does not affect the
transparency;
③ Strictly control the injection and
stretch-blow molding process parameters
and the temperature distribution in each
area to avoid the release of residual
stress at the glass transition
temperature of PET (>75°C) and lead
to bottle deformation.
④The application of temperature
adjustment technology for blowing molds.
The hot oil circulation method is
usually used to heat the bottle blowing
mold. There are three kinds of cycles
for the temperature adjustment of the
bottle blowing mold: The bottle body hot
oil cycle. Heat the blowing mold to
120°C to 145°C. In this way, the
temperature difference between the
preform and the blowing cavity is
reduced, which promotes further
crystallization. Extend the
pressure-holding time of the bottle
blowing, make the bottle wall and the
cavity contact for a long time and have
enough time to increase the
crystallinity of the bottle body,
reaching about 35%, but without
destroying the transparency. The mold
temperature below 100°C has little
effect on the crystallinity of the
bottle body, because the bottle body
crystallization occurs above 100°C.
Cooling water circulation at the bottom
of the bottle. Keep the bottom of the
bottle at a low temperature (10℃～30℃) to
avoid excessive crystallization and
whitening of the unstretched bottom
part. Bottleneck temperature adjustment
(optional). The non-crystalline bottle
mouth part has been completely cooled
after being demolded from the injection
mold. Most of the non-crystalline bottle
mouth adopts a strengthened bottle mouth
design (increasing the wall thickness of
the bottle mouth) to improve the sealing
performance and avoid deformation of the
bottle mouth during the capping process.
Generally, the ovality of the bottle
mouth after filling is controlled within
0.2mm, and the shrinkage rate of the
outer diameter of the thread is less
than 0.6%.
⑤ Cycle blowing technology. When using
hot blow molding, how to control the
deformation of the bottle after
demolding is very important. Before
opening the mold, air is blown into the
blowing mold and the cycle is exhausted
to cool and shape the bottle body to
control the deformation after demolding.
The air intake of the circulating
cooling air passes through the same
passage as the primary blowing and
secondary blowing, but exhausts from the
small hole in the head of the drawing
rod through the drawing rod. The cycle
blowing time is about 0.5 seconds to 2
seconds. Therefore, the high-pressure
air consumption of the heat-resistant
bottle making machine is much higher
than that of the ordinary bottle making
machine.
3. Large fluctuations in capacity.
Biaxially stretched PET bottles have a
certain shrinkage rate, and the maximum
shrinkage rate is about 2%. The main
factors affecting the capacity of PET
bottles are as follows:
(1) Influence of mold The capacity of
PET bottles is mainly affected by mold
size and shape. The size of each bottle
mold is usually fixed. The shrinkage
rate of bottles of different shapes will
be different when designing the
shrinkage rate. The less the ribs on the
bottle body and the thinner the bottle
thickness, the greater the shrinkage
rate of the bottle.
(2) The influence of environmental
factors The environmental temperature
and humidity have a greater influence on
the capacity of the bottle. The higher
the ambient temperature and the greater
the humidity, the greater the shrinkage
of the bottle’s capacity.
(3) The influence of the production
process. When blowing bottles with
complex shapes, a higher blowing
pressure is required. If the blowing
pressure is insufficient, the bottle
will be poorly formed and the capacity
will be small; higher mold temperature
will also cause the capacity to be
small.
(4) Natural shrinkage of the bottle As
the PET bottle shrinks naturally, the
size of the bottle mold should be
designed in an adjustable form (plus or
minus gasket). Take a 1.5L PET bottle as
an example. The average capacity of the
newly produced bottle is about 1508ml.
After 3 days of storage at room
temperature, the bottle capacity will
decrease by 5ml~6ml; as the bottle
storage time increases, the bottle
capacity will shrink and become
difficult to control. At present, more
and more production lines use in-line
blowing, that is, blowing and filling,
to avoid the attenuation of bottles
(capacity and heat resistance).
(5) The influence of filling methods
Different filling methods have different
influences on volume control. The
quantitative filling method has the
least influence on the capacity, and the
self-weight filling has the greatest
influence on the capacity. For 1.5L PET
bottles, the difference may be up to 20
ml to 25 ml. Therefore, to solve the
bottle capacity problem, the mold
(gasket) can be appropriately adjusted,
the production process can be
controlled, and the storage conditions
should be improved. The most important
thing is to shorten the bottle’s storage
period as much as possible.
