The present invention relates to an apparatus and process for blow
molding a thermoplastic preform into biaxially oriented, shaped articles and,
more particularly, to annealing returnable polyethylene terephthalate bottles which
are subjected to washing and reuse.
Refillable plastic bottles reduce landfill and recycling problems
of disposable plastic beverage bottles and, more particularly, those bottles formed
from polyethylene terephthalate or PET.
A refillable plastic bottle must remain aesthetically pleasing and
functional over numerous washings and refillings as discussed by U.S. Patent Nos.
4,755,404, 4,725,464 and 5,066,528. Cracks, color changes, volume or structural
change must be minimized.
U.S. Patent No. 4,385,089, teaches how hollow biaxially oriented
shaped articles are formed from intermediate products which may be sheets or other
shapes when thermoformed or parisons or preforms when injection molded, injection
blown or extrusion blown. The preform may be prepared and immediately used hot
or may be stored and reheated later to a temperature having sufficient elasticity
to be shaped into a bottle or other form by drawing and blowing in a cooled mold
to obtain the final shape of the article. The preform is next often subjected
to a heat treatment at well above the glass transition temperature of the thermoplastic
to increase the articles strength and resistance to gas loss. Heat treatment also
prevents distortion when the bottle is reused, including distortion during a hot
For other heat treating patents, see U.S. Patent No. 4,233,022 and
patents cited therein. The '022 patent teaches use of a blow mold comprising four
sectional members each separated by insulating portions used to heat treat blown
bottles at a temperature within the range of 150°C to 220°C.
U.S. Patent No. 5,085,822, teaches it is old to blow in a mold at
130°C and cool to 100°C to prevent deformation on removal of the container from
the mold. Also taught is to retain a container in the blow mold and heat to remove
stress and thereafter transferring the deformable container to a separate cooled
mold to solidity. The "822" patent holds the molded container for a predetermined
period of time to heat set the container followed by introducing a cooling fluid
into the bottle. Also disclosed is heat setting a blown container in a separate
U.S. Patent No. 4,505,664, teaches transporting the blowing cavity
and blown article to a second station where medium is circulated through the article.
U.S. Patent No. 4,488,279, biaxially orients the article which can
then be heat set.
U.S. Patent No. 5,080,855, teaches blow molded articles which may
be heat set in a second mold. Also, see also U.S. Patent No. 4,485,134, 4,871,507
and 4,463,121 which discuss heat treating biaxially oriented bottles.
U.S. Patent No. 4,572,811, teaches heat treating a PET container
to form a spherulite, opaque texture which we have found leads to stress cracking
when bottles are recycled.
U.S. Patent No. 4,588,620, teaches preforms having a thinner bottom
wall and which permit longer or deeper stretch of the shoulder and sidewall portions.
While it is known to biaxial stretch a preform using pressure, we
have found the annealing of the blown preform to be a critical factor in producing
carbonated beverage bottles that are refillable.
From GB 2 009 029 A a process for preparing a saturated polyester
resin bottle from a tubular bottle blank made of saturated polyester resin is known,
which comprises a first step of heating the bottle blank to a temperature suitable
for biaxial orientation and setting the same in a heated mold for blow-molding,
a second step for blowing compressed gas into the bottle blank while an extruding
rod is inserted downward into the blank to effect biaxial orientation, a third
step of heating the bottle to a temperature higher than said temperature to which
the bottle blank has been heated while maintaining a gas pressure in the bottle
to effect thermal fixing after the formation of the bottle by said biaxial orientation
and a fourth step of lowering the temperature of the bottle and removing the bottle
from the mold.
From DE 23 39 019 A1 a method and apparatus for blow-molding a hollow
container from a preform is known, in which one or more tempering molds are used
to temper the preform before blow-molding the same into the final container. The
tempering mold may comprise different longitudinal sections forming a longitudinal
It is an object of the present invention to reduce stresses created
during the biaxial stretching of the preform to the bottle shape.
This object is solved by the method as defined in claim 1 and the
apparatus as defined in claim 4.
A hot preform, about ≈90°C to 110°C, is rapidly expanded against
the inner surface of the warm mold and held there by internal pressure until the
temperature of the shaped container reaches the annealing temperature of the mold
wall in the case of at least the neck-shoulder and body portions of the bottle.
The bottom heel portion which is relatively thick and amorphous is cooled as rapidly
as possible to reduce the base temperature to below the body wall annealing temperature.
Portions of the mold section have channels for passing warm water
through the mold wall to control the wall temperature and anneal the blown article
at the desired temperature.
Each portion substantially abuts and often contacts adjoining portions
so that the temperature of the mold wall near the edge of each portion exhibits
a gradual temperature profile and avoids sharp temperature differences which can
stress the bottle and result in bottle failure during reuse. The body section of
the mold is maintained at about 80°C using 80°C warm water. The neck-shoulder
area is maintained at 70°C or below normally about 60°C using warm water. These
temperatures rapidly reduce the blown thermoplastic temperature for PET from just
above the glass transition temperature to 80°C on the side wall and 60°C on the
upper wall section, thus, annealing the container and reducing stress. The bottom
heel portion of the molded article is cooled with cold water to rapidly reduce
the thicker base to below 80°C normally below 70°C.
The preform is designed with an inside and outside uniformly tapered
wall which increases the wall thickness from the neck to side or body portion of
the preform at least two fold.
- Fig. 1 shows a preform having taper wall, prior to blow-molding and annealing.
- Fig. 2 shows a blow-mold section in which, when closed with another opposing
section, the preform is rapidly expanded and illustrates the three temperature
controlled portions of the mold used for annealing.
- Fig. 3 is a block diagram illustrating the various steps of the process and
features of the apparatus.
The present invention relates to an apparatus and method for annealing
a blown molded thermoplastic article immediately after the preform is blown to
the shape of the blow-mold. Rather than using a hot mold for heat treatment, where
stickage can result, or a cold mold to rapidly cool the blown article, where stress
can be developed, the side or body portion of the mold walls, upper bottle or neck-shoulder
portion of the mold walls and the bottom and shoulder portion of the mold walls
are each regulated to reduce and control the wall temperature or anneal the bottle
wall which reduces and equalizes stress created during the biaxial stretching
of the preform to the bottle shape. The annealing temperature is cool enough to
allow the bottle to be removed from the mold without deformation.
The bottle herein described is a 1.5 liter carbonated beverage bottle
which can be further treated or allowed to cool, stored and be later filled with
product. The bottle may be cleaned using hot caustic and reused. Various size
bottles are possible by making commensurate changes in the size of the preform
and blow mold.
The preform 1 is shown in Fig. 1 where one-quarter of the
preform has been cut in a plane perpendicular to the paper shown as A-A and within
the plane of the paper exposing the quadrant marked 3 having relatively thin screw
cap area 5 which becomes the neck portion 63 of Fig. 2, a tapered portion shown
as 7 and 13 which when drawn and blown into a bottle forms the slowly tapering
bottle surface apparent in Fig. 2 at 27 of the neck-shoulder portion
22 of the mold. There is a relatively long wall 9 which are drawn and blown
into the long bottle wall contacting 28 shown in body portion 20 of the
mold in Fig. 2. The preform base 11 may initially contain less thermoplastic than
the side wall 9 but after being blow molded into a bottle is relatively thicker
than the side walls and more difficult to cool and would contact surface 29 in
the bottom and shoulder portion 24 of the mold shown in Fig. 2. The degree
of taper of the inside 13 surface and outside 7 surface of the preform of Fig.
1 is extensive and sufficient to increase wall thickness at least 2 fold from
neck to body.
For the 1.5 liter bottle the top of the neck or cap 2 of the preform
of Fig. 1 has a thickness of 2.1 mm and the neck has a length of 28 mm prior to
the beginning of the tapered portion. The thickness at the beginning of the taper
at 4 is 2.75 mm and 6.9 mm shown at 6 on the body wall thickness. The tapered portion
is 20 mm long and the constant circumference length of the body portion is 94
mm. The wall thickness at the narrowest portion of the bottom of the preform is
Referring now to Fig. 2, there is shown a mold section 21 having
four mold portions 20, 22, 24 and 60
which are in cooperative
and normally adjacent relationship to at least one of the other mold portions
and comprise one-half of a female mold which, when closed, forms the general shape
shown by the line marked 25, 27, 28 and 29 which outlines a cavity surface generally
shown as 62, 42, 26, and 52. The cavity is normally
formed by preparing a mold section in the shape of the bottle as if the bottle
were cut along its axis B-B into two equal volumes. Of course more sections could
be employed, if desired, as long as when closed they form a cavity having the shape
of the desired bottle.
Certain surface ornamentation can be added as shown in 31, 33 and
35. Small openings 37 to remove gases may or may not be noticeable in the final
blown and annealed container.
Warm water cooling channels, one of which is shown at 30, are equally
spaced about the body portion 26 of the cavity in Fig. 2. These channels
are connected to a warm water supply containing 80°C water which is circulated
throughout the metal body mold section shown as 20. Each channel may be
connected to each other in either series or parallel relationship and maintain
the surface of the mold cavity 26 at about 81°C during operation, the wall
being slightly hotter than the water supply resulting from contact with the hotter
blown preform. Hot water is conducted through 32 and up through the channel 30
and out through 34 to another channel not shown in series operation or to a manifold,
not shown for parallel operation. The size of the channels is governed by the
amount of heat to be removed and the heat transfer characteristics of the mold
and can be determined by one of ordinary skill in the art.
The neck-shoulder portion of the mold section shown at
22 including the upper wall surface 42 is maintained at about the
temperature of the warm cooling water. Warm water below 70°C and normally about
60°C is conducted throughout the cooling channels generally shown as 40 by dotted
lines. Inlet 44 and outlet 46 can be connected in parallel or series as desired.
The neck-shoulder portion 22 is normally cooled to about 60°C which allow
the bottles removal from the mold without deformation.
The bottom heel portion 24 of the mold section 21 is also
cooled by cold water passed through channels shown as 50 in a manner similar to
the other portions of the mold. Cold water is used to lower the bottle wall in
contact with surface 52 as quickly as possible to reduce the thermoplastic
wall temperature to below 80°C, preferably below 70°C.
A fourth mold section 60 is shown about the neck portion of
the preform which is not normally heated or cooled and remains cool and amorphous.
If desired, heating or cooling channels or equivalent heating or cooling means
may be provided.
Obviously, mold portions 60, 20, 22, and
24 of mold section 21 can be contained within an outer hydraulic mold system
surrounding at least a portion of the mold section 21 outer wall shown as 70. If
desired, channels for controlling mold wall temperature may be contained in the
outer mold system in addition to or alternatively to the channels in the mold portions
60, 20, 22 and 24.
The mold portions are normally affixed to each other or an outer
mold system by means well known to the art and not shown. The mold portions generally
substantially abut and often touch each other at 74, 76 and 78, without use of
insulation, which allows the metal in adjacent mold sections to reach a temperature
which gradually changes in the area of 74, 76 and 78 preventing stresses caused
by the difference in bulk temperature of sections 20, 22,
24 and 60.
While water is described as the usual heat transfer fluid, any appropriate
oil or other fluid might be used. Other appropriate heating or cooling means known
in the art can be used in place of and in conjunction with the heat transfer fluid.
Resistance heating may be employed, for example, in the body area. The cooling
channels may be of any desired shape and configuration but are generally circular
and are straight through the mold portion. If the channel is made to abut another
portion of the mold, other shapes can be easily formed like those shown as 50 in
In operation the annealing process may be employed as part of a rotary
or linear blow molding process. We prefer a linear configuration of stationary
molds because of the ease of feeding heat transfer fluid through stationary piping
and the limited number of bottles under manufacture should there be mechanical
failure or problems. However, rotary annealing configurations can be employed
if desired and provide higher output of bottles for a given factory area.
Referring now to Fig. 3, room temperature preforms 81 are conveyed
to a preform feed unit 80. The preforms are gripped by the neck 5 of Fig.
1 and placed on transport mandrels at 82. The preforms are passed through
infrared quartz heaters at 84 to bring the sidewalls and bottom 7, 9 and
11 of Fig. 1 to proper temperature for blowing usually between about 90 to 110°C.
The preforms are allowed to equilibrate at 86 so that the heat is allowed
to flow throughout the preform reducing the high surface temperature and adjusting
the preform temperature throughout its wall thickness. From there the preforms
are transferred to a blow station 88
where they are blown using high pressure
air or other gas against two closed mold one shown as 21 in Fig. 2. The axial
direction is also generally stretched by mechanical means such as push rods which
drive the closed end of the preform to the bottom of the blow mold. The blown
article is annealed in the blow mold 88
to below 95°C, preferably about 65°C
to 85°C, preferably about 80°C in the body portion 20 of Fig. 2, below 70°C
and usually controlled at 60°C at the neck-shoulder portion 22 of Fig.
2, and below 70°C in base and shoulder portion 24 of Fig. 2. Usually up
to 25 seconds, preferably up to 10 seconds is required to maintain the expanded
thermoplastic against the segmented mold portions to properly reach the desired
wall temperature of 80°C for the main wall surface 26 in portion
20 of Fig. 2, below 70°C in the upper wall surface 42, preferably
60°C in portion 22 of Fig. 2 and below 70°C in the base and heel surface
52 of portion 24
of Fig. 2. The bottles 89 in Fig. 3 exit the
blow station and go on to further treatment, for example, air cooling and storage
preparatory to filling. The mandrels 87 are returned to the loading station.
In a rotary system the preforms are fed to the loading station. At
the loading station the preforms are loaded onto the transport mandrel. A rotating
heater is equipped with a number of stations holding the transport mandrels as
they pass in front of the heating units. The preforms can be rotated on their own
axis to insure uniform heating. Infra-red quartz lamps are controlled separately
to obtain the desired temperature profile for each preform. While the bulk of the
body side wall should be at a temperature of about 90°C to 110°C for PET, adjustments
in temperature can be made to insure best preform blowing conditions.
The preform temperature is next equalized by passing the preforms
to an equalizing wheel which may have neck cooling to insure the neck area is cool
for blowing. The object of the equalization wheel is to allow time for the temperature
to become even or equilibrated across the wall thickness. From the equalizing
wheel the heated preforms are transferred and are locked into position in each
of a number of water cooled mold stations. Mold halves are pneumatically actuated
and locked into place. The preform is stretched using a stretch rod while high
pressure air at 27,6 to 41,4 bar (400 to 600 psi) is used to rapidly expand the
preform against the inner mold surfaces. The blown bottle is maintained against
the segmented mold portions shown in Fig. 2 up to 25 seconds, preferably up to
10 seconds and normally about 2 to 6 seconds to bring the bottle wall temperature
to the desired annealing temperature.
In the linear version of the process the molds are stationary and
the preforms indexed into the mold which is mechanically or hydraulically closed.
The process is particularly useful for polymers which are generally
blown from amorphous to crystalline state such as mono and copolymers of ethylene-glycol-terephthalic
acid-esters generically known as polyethylene terephthalate or PET.
Biaxial orientation of the articles, particularly bottles useful
for still or carbonated beverages, is accomplished by stretching the thermoplastic
material, such as PET, in the axial and hoop directions simultaneously as the
article is being formed. Often stretching in the axial direction is assisted by
a mechanical rod used to force the closed end of a preform to the base of a mold
as high internal pressure is applied to the preform causing stretching in both
the hoop and axial directions. The preform is forced against the outer mold surfaces
to shape the article and anneal the article at about 95°C or below which further
strengthens it and prevent stress cracking and other problems.
Biaxial orientation provides excellent properties such as strength
and resistance to gas loss to the article. However, the stretching with immediate
cooling (or stretching in a high temperature mold for heat setting by developing
further crystallization) leaves the article under high stress which causes excessive
shrinkage on reuse. Areas that have a thicker shell, such as the bottom heel and
neck have less or no orientation and crystallinity and the contrast between amorphous
and crystalline areas causes stress cracking on reuse of such bottles. This problem
is overcome by using annealing. Preforms are also often heat profiled at different
temperature to improve the wall thickness distribution of the final article and
this causes further stress in the walls of the blown article.
This invention is directed to a process and apparatus for reducing
wall stress in articles such as bottles by subjecting the blown thermoplastic article
to direct and immediate annealing by regulating and controlling the wall temperature
of the blow mold so that the body or main wall portion of the article, which has
undergone the most biaxial stretching and crystallization is lowered to 65°C to
85°C and more preferably about 80°C by maintaining the mold wall in contact with
that portion of the article at that temperature. The upper portion of the wall
near the neck, referred to herein as the neck-shoulder portion is lowered to 70°C
or below usually about 60°C while the lower or bottom portion of the bottle referred
to herein as the bottom heel portion is reduced to below 85°C and preferably below
70°C using cold water since the bottle is thickest at this point and most difficult
The thermoplastic material is normally blown at a temperature which
will vary depending on the polymer but which is generally between 90°C and 110°C
for PET. This means that usually substantial heat must be removed from the article
by cooling the mold to the desired annealing temperature. The bottle wall annealing
temperature is that temperature where the bottle can be removed from the mold
without deformation. One might refer to our process as warm blowing since the mold
surfaces are warm in the body and neck-shoulder region. Normal processes employ
a cold mold cooled as rapidly as possible.
We have found that annealing the blown bottle at about 65°C to 85°C,
depending on the area of the bottle, allows one to reuse the bottles, including
cleaning them at 60°C, without losing the strength developed during biaxial stretching
and the annealing treatment. The annealing process, in addition to reducing thermal
stress and biaxial stress differences, also strengthens the bottle, makes it more
resistant to stress cracking and improves gas barrier properties.
The period of contact between the warm mold and the hollow shaped
hot article is dependent on the thickness of the walls and the time necessary to
reduce the wall temperature to the desired 65°C to 85°C a range. Up to 25 seconds
residence time is sufficient with 1 to 10 seconds preferred. Usually from 3 to
6 seconds is sufficient to lower the base-shoulder region of the wall (the thickest
part) to 70°C or below.
Various portions of the segmented mold may be cooled, usually by
warm water with the base heel portion cooled with cold water. While other cooling
fluid could be used, as well as any known techniques for rapidly and effectively
removing heat, we have found water to work well.
The process of this invention can be employed on any thermoplastic
article where annealing is required and where reuse is likely. The process anneals
the thermoplastic article at a temperature above that likely to be employed for
washing the article; generally at least 5°C or 10°C above the highest contemplated
washing temperature. In general, for PET, the temperature of the article body
is reduced to 85°C, preferably about 80°C while the neck-shoulder portion is lowered
to about 70°C and the base and shoulder portion lowered to 70°C or below.
The process of this invention can also be employed on multilayer
articles containing thermoplastic materials especially PET.
The process of this invention maintains high transparency, relieves
stress, prevents stress cracking, and improves dimensional stability in the temperature
range used for filling or cleaning the shaped articles.