U.S. patent application number 14/218190 was filed with the patent office on 2014-07-17 for process for injection molding of thin-walled preform.
This patent application is currently assigned to SA DES EAUX MINERALES D'EVIAN SAEME. The applicant listed for this patent is SA DES EAUX MINERALES D'EVIAN SAEME. Invention is credited to Gilles Bertheol, Jean-Paul Besson, Alain Colloud.
Application Number | 20140199520 14/218190 |
Document ID | / |
Family ID | 37177855 |
Filed Date | 2014-07-17 |
United States Patent
Application |
20140199520 |
Kind Code |
A1 |
Bertheol; Gilles ; et
al. |
July 17, 2014 |
PROCESS FOR INJECTION MOLDING OF THIN-WALLED PREFORM
Abstract
An injecting/blowing device includes a mold for injecting a
thin-walled hollow preform capable of being transformed into a more
voluminous hollow body. The mold includes a counter mold defining a
cavity, a cure located in the cavity, and a preform impression
located between the counter mold inner surface and the core which
will receive the melt, at least two main preferential flow channels
or at least two secondary preferential flow channels having a
recessed zone at the surface of the core and the inner surface of
the cavity. The main flow channels are located only at the
impression zone corresponding to the zone for transforming the
preform, and in the optional secondary flow channels are flow
channels discontinuous from the main flow channel.
Inventors: |
Bertheol; Gilles; (Publier,
FR) ; Besson; Jean-Paul; (Abondance, FR) ;
Colloud; Alain; (Reyvroz, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SA DES EAUX MINERALES D'EVIAN SAEME |
Evian-Les-Bains |
|
FR |
|
|
Assignee: |
SA DES EAUX MINERALES D'EVIAN
SAEME
Evian-Les-Bains
FR
|
Family ID: |
37177855 |
Appl. No.: |
14/218190 |
Filed: |
March 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
13332330 |
Dec 20, 2011 |
8691140 |
|
|
14218190 |
|
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|
12281761 |
Feb 6, 2009 |
8100687 |
|
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PCT/EP2007/052195 |
Mar 8, 2007 |
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13332330 |
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Current U.S.
Class: |
428/156 |
Current CPC
Class: |
B29B 2911/14344
20150501; B29B 2911/14133 20130101; B29C 45/73 20130101; B29B
2911/1404 20130101; B29C 2045/7393 20130101; Y10T 428/1379
20150115; B29B 2911/14106 20130101; B29C 49/0073 20130101; Y10T
428/24479 20150115; B29B 2911/1434 20130101; B29B 2911/1414
20130101; B29B 2911/14393 20130101; B29C 45/37 20130101; Y10T
428/1397 20150115; B29B 11/08 20130101; B29B 2911/1402 20130101;
B29K 2067/00 20130101; B29C 45/0046 20130101; B29B 2911/14086
20130101; B29C 49/06 20130101; B29B 2911/14033 20130101; B29K
2105/253 20130101; B29B 2911/14326 20130101; B29C 49/0078 20130101;
Y10T 428/1352 20150115; B29B 2911/14466 20130101; B29B 2911/14093
20130101; B29B 2911/14713 20130101; B29B 2911/1444 20130101; B29B
11/14 20130101; B29B 2911/14593 20130101; B29B 2911/14331 20150501;
B29B 2911/14113 20130101; B29B 2911/14343 20150501; B29B 2911/14026
20130101; Y10T 428/1383 20150115 |
Class at
Publication: |
428/156 |
International
Class: |
B29C 49/00 20060101
B29C049/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 8, 2006 |
FR |
06 50807 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. A thin-walled preform, comprising main variable-thickness zones
located only in its transition zone and optionally discontinuous
secondary variable-thickness zones, separate from the main
variable-thickness zones and located on at least one of the bottom,
the body, or the neck.
8. The thin-walled preform as claimed in claim 7, wherein the
variable-thickness zones are symmetrical.
9. The thin-walled preform as claimed in claim 8, wherein the
variable-thickness zones are distributed equiangularly over the
circumference of the preform.
10. The preform as claimed in claim 7, wherein an average thickness
(w.sub.av) is defined by w.sub.av.ltoreq.a+L/b, with
1.20<a<1.36 and 88<b<98, L being the total length in
question of the preform.
11. The preform as claimed in claim 7, wherein the
variable-thickness zones are located longitudinally or helically
with respect to the axis of the preform.
Description
TECHNICAL FIELD OF THE PRESENT INVENTION
[0001] The subject of the present invention is the injection
molding of thin-walled parts, especially preforms.
[0002] The invention blow molding or ISBM (Injection Stretch Blow
Molding) technique is well known and used for producing parts from
thermoplastics. These parts may take the form of food containers,
such as bottles.
[0003] The invention blow molding technique firstly comprises
injection molding consisting of the following steps: drying of the
material used if necessary before it melts; plasticization;
injection of the material into the mold comprising a core and a
countermold forming a molding cavity which corresponds to the
molding impression; cooling; demolding; and removal of the
preforms, and then blow molding comprising the following steps:
reheating of the preforms; stretch-preblowing and blowing.
[0004] The injection molding aspect is more specially of interest
within the context of the invention.
PRIOR ART
[0005] It is known that it is difficult to manufacture thin-walled
parts by injection molding. This is because such manufacture starts
with the injection molding of preforms which also have thin walls.
This injection molding operation poses a number of technical
problems. In particular, as the molten thermoplastic flows into the
molding cavity, the pressure drop and the cooling of the material
in contact with the mold require injection pressures that are
higher the thinner the walls. These pressures become too high and
exceed the technical capabilities of the materials used in the case
of thin walls.
[0006] The notion of a "thin" wall is dependent on the shape of the
part and on the material used. For example in the case of a bottle
preform under conventional molding conditions, the thickness of the
preform is limited by an apparent lack of material (incomplete
molding) occurring in the zone opposite the point where the molten
material is injected into the mold.
[0007] This is the case for all thermoplastics, with thickness
limits that vary depending on the melt viscosity of the material
and on the shape of the part.
[0008] In the case of a preform made of polyethylene terephthalate
(PET) for example, it is difficult to go below a thickness of 2.5
mm for a length of about 120 mm.
[0009] Patent Application WO-A-97/13696 describes a method and an
apparatus for producing multilayer preforms by injection blow
molding. The patent application stresses the difficulty of
obtaining thin-walled preforms using conventional injection molds.
This is because when such molds are used, the molten material
encounters a relatively high resistance to the flow and requires
the use of high injection pressures for injection into the cavity
of the mold. To remedy these drawbacks, indentations were inserted
into the cavity of the mold corresponding to the internal surface
of the countermold. These indentations are distributed
equiangularly in the cavity and extend from the injection point to
the opposite end of the mold.
[0010] Also known is Japanese Patent Application JP 11-090975 which
describes a multilayer preform, a process of manufacturing such a
preform and a multilayer container obtained from such a
preform.
[0011] The preform has a first layer made of a polyester injected
into a mold, to form the inner layer. Recycled polyester is then
injected to form the outer layer of the preform. Ribs are provided
on the first layer so as to be able to produce thin-walled
multilayer preforms. However, these ribs are provided over the
entire height of the preform, between the bottom and the neck of
the preform.
[0012] Likewise, the core has, on its surface, depression zones
produced in the zone corresponding to the entire height of the
preform. This configuration is suitable for multilayer preforms.
Furthermore, the ribs provided over the entire height of the
preform increase the weight of this preform and therefore the
weight of the thin-walled part obtained from this preform.
[0013] Also known is European Patent EP 1 180 424 which describes a
process for manufacturing objects using a step of injection molding
an outer preform from a first resin and then a step of injection
molding an internal preform on the inside of the first preform,
using a second resin. The process then includes a blowing step.
[0014] The core of the injection mold for molding the internal
preform has, on its outer periphery, a plurality of vertical
channels arranged at regular intervals, so that the thicker regions
formed inside the internal preform extend over the entire body of
the preform. Such thicker regions prevent any thinning-down of the
bottle formed in the central and upper zones.
[0015] Here again, the ribs provided over the entire height of the
preform increase the weight of this preform and therefore the
weight of the thin-walled part obtained from this preform.
[0016] Finally, Patent U.S. Pat. No. 4,649,068 describes a process
and a preform for the injection blow molding of containers closed
by a cap and intended to be filled with hot products. The preform
has, only in the neck region, longitudinal ribs uniformly
distributed over its inner perimeter. These ribs axially reinforce
the thread and prevent the neck from deforming under pressure when
the bottle is filled with a hot product.
DESCRIPTION OF THE INVENTION
[0017] The objective of the present invention is to obtain
thin-walled parts, especially preforms, and constitutes an
alternative solution to the solutions of the prior art.
[0018] More particularly, the objective of the present invention is
to obtain preforms enabling the weight of the thin-walled part
obtained, such as a bottle, to be minimized, while still
maintaining its mechanical properties.
[0019] The invention is based on technical principles for producing
in particular thin-walled preforms by the molding of
thermoplastics. These technical principles may be used separately
or in combination so as to obtain the expected result: [0020]
creation of preferential flow channels (PFCs) in the mold (FIG. 1)
enabling the cavity to be filled more rapidly and with less of a
temperature drop of the material and less of a pressure drop;
[0021] by the creation of depression zones, by machining the
surface of the core, which allow preferential flow; [0022] by the
modification of the surface finish of the core and/or of the cavity
(differential treatment between the preferential flow zones and the
other portions of the preform); [0023] use of heating molding
pieces allowing better flow of the material and less cooling during
the material flow step. These heating pieces must be very rapidly
heated and cooled, depending on the phase during the molding
step.
[0024] One of the aspects of the invention relates to an injection
molding device comprising at least one mold that can be used for
the injection molding of a thin-walled hollow preform that can be
converted into a more voluminous hollow body by blow molding, said
mold comprising: [0025] a countermold defining the internal surface
of the cavity; [0026] a core located in the cavity and spaced away
from said internal surface; [0027] a preform impression located
between the internal surface of the countermold and the core, which
impression will receive the molten material; [0028] at least two
main preferential flow channels (PFCs); and [0029] optionally at
least two secondary preferential flow channels (PFCs), each PFC
being bounded, on the one hand, by at least one depression zone on
the surface of the core and, on the other hand, by the internal
surface of the cavity, aid device being characterized in that:
[0030] the main PFCs are located only in that zone of the
impression which corresponds to the transition zone of the preform;
[0031] and in that the optional secondary PFCs are discontinuous,
separate from the main PFCs, and located in those zones of the
impression which correspond to the bottom and/or to the body and/or
to the neck of the preform.
[0032] Thus, according to the invention, the PFCs do not extend
over the entire height of the preform and, in particular, are not
necessarily present within the body of the preform.
[0033] Such a device allows a preform to be produced while
minimizing the thickness of the transition zone of the preform,
although this zone is subjected to high mechanical stresses, for
example when stacking bottles. The person skilled in the art has
therefore overcome a preconception that would have encouraged him
not to thin down the transition zone--a particularly sensitive
zone--so as not to risk reducing the resistance of the bottle to
mechanical stresses. The PFCs placed according to the invention
during injection allow the subsequent blow molding operation to be
carried out despite a small thickness in the transition zone. The
PFCs according to the invention make it possible to obtain a
lightweight thin-walled part, for example a bottle, while still
maintaining its mechanical properties.
[0034] Another subject of the present invention is a core that can
be used for the injection molding of thin-walled preforms,
characterized in that it includes, on the surface, main depression
zones produced only in the zone corresponding to the transition
zone of the preform, and optionally discontinuous secondary
depression zones, separate from the main depression zones and
located in the zone corresponding to the bottom and/or to the body
and/or to the neck of the preform.
[0035] The invention also relates to a thin-walled preform,
characterized in that it comprises main variable-thickness zones
located only in its transition zone and optionally discontinuous
secondary variable-thickness zones, separate from the main
variable-thickness zones and located on its bottom and/or its body
and/or its neck.
[0036] Thus, the invention described in the present application
makes it possible, after the preforms have been blow molded, to
produce lightweight flasks, while still maintaining limited overall
stretch ratios (overall stretch ratio: the longitudinal stretch
ratio (corresponding to the length of the final blow-molded
object/length of the preform ratio) multiplied by the diametral
stretch ratio (corresponding to the diameter of the blow-molded
final object/diameter of the preform ratio)), while lightening
existing preforms. It is possible to produce several weights of
preforms by changing only the molding's central portion (core).
This change of core also has an economic advantage since, in the
injection molding technique, the cores represent only 10 to 15% of
the cost of the mold. By providing a greater variation in the
thickness of the preforms, it is therefore possible to produce a
wider range of preform weights merely by changing the cores.
[0037] According to yet another aspect, the invention relates to a
process for the injection molding of thin-walled preforms, which
comprises the following steps: [0038] the molten material is
injected into a mold comprising a core as described above; [0039]
the preform is cooled down to a temperature at which the material
no longer changes; [0040] the preform is then demolded.
[0041] Such a process makes it possible to reduce the cycle times
for molding thin-walled preforms, while reducing the risks of
incomplete molding, and to injection mold highly viscous
thermoplastics, in molds shaped so as to produce thin-walled
preforms.
[0042] The term "mold" used within the context of the present
invention refers to a two-part device comprising a fixed stage,
having the cavities or countermolds, the movement of which is
provided by the closure system and a plate for supporting the
cores. The fixed stage also includes a plasticizing screw, where
the material to be injected passes from the solid state to the
molten state. The molten material is injected at the injection
point after the injection mold has been closed and clamped.
[0043] The term "impression" used within the context of the present
invention refers to the space lying between the internal wall of
the countermold and the core.
[0044] According to one particular embodiment of the invention, the
mold is characterized by the average thickness of the impression
being equal to or less than a+L/b, i.e.:
w av .ltoreq. a + L / b ##EQU00001## with ##EQU00001.2## 1.20 <
a < 1.4 ##EQU00001.3## and ##EQU00001.4## 88 < b < 98
##EQU00001.5## where : ##EQU00001.6## w av = 1 L .intg. .PHI. L w u
u u ##EQU00001.7##
where: L is the total length in question of the preform (cf. FIG.
6); [0045] u is the curvilinear abscissa at each point on the
preform; and [0046] w.sub.u is the local thickness at each point on
the preform.
[0047] The term "preferential flow channels or PFCs" used within
the context of the present invention refers to a volume lying
between the internal wall of the countermold and the depression
zones on the surface of the core. These channels allow greater flow
of molten material compared with that in the zones outside the
PFCs. When not specified, the term "PFC" covers both main PFCs and
secondary PFCs.
[0048] The PFCS may be obtained by modifying the surface finish of
the core or by machining the latter.
[0049] According to a preferred embodiment of the invention, the
PFCs are symmetrical.
[0050] According to an even more preferential embodiment of the
invention, the PFCs are symmetrical and localized equiangularly
around the circumference of the preform.
[0051] The term "equiangularly" used within the context of the
present invention refers to objects being distributed at identical
angles around a circumference.
[0052] According to the invention, the main PFCs are located only
in that zone of the impression which corresponds to the transition
zone of the preform.
[0053] According to one particular embodiment of the invention,
discontinuous secondary PFCs are provided, which are separate from
the main PFCs and may be located in those zones of the impression
which correspond to the bottom and/or to the body and/or to the
neck of the preform.
[0054] According to another particular embodiment of the invention,
the number of PFCs in the mold is between 2 and 12, at least two
being provided in the zone corresponding to the transition zone of
the preform.
[0055] In a most preferential aspect for implementing the
invention, the mold has 2 to 6 PFCs, at least two being provided in
the zone corresponding to the transition zone of the preform.
[0056] According to one particular embodiment of the invention, the
mold has PFCs which, when they are observed in cross section, are
of rectangular, polygonal, oblong or evolutive shape.
[0057] According to another aspect of the invention, the mold
includes PFCs of longitudinal, helical and/or evolutive
orientation.
[0058] According to another embodiment of the invention, the mold
further includes at least two depressions in the internal surface
of the cavity.
[0059] The term "preform" used within the context of the present
invention is defined by three different zones: the body, the
transition zone and the neck. The preform is obtained by filling
the impression with molten material injected into the mold.
[0060] According to the invention, the preform comprises main
variable-thickness zones located only in its transition zone, and
discontinuous secondary variable-thickness zones, separate from the
main variable-thickness zones and located on its bottom and/or its
body and/or its neck.
[0061] According to one particular embodiment of the invention, the
variable-thickness zones are symmetrical.
[0062] According to an even more preferential embodiment of the
invention, the variable-thickness zones are symmetrical and
distributed equiangularly over the circumference of the
preform.
[0063] The term "variable-thickness zone" used within the context
of the present invention refers to a larger volume of the preform,
corresponding to PFCs filled with the molten and cooled material.
When this is not specified, the term "variable-thickness zone"
covers both main variable-thickness zones and secondary
variable-thickness zones.
[0064] The preform has an average thickness (w.sub.av) defined by
w.sub.av.ltoreq.a+L/b, with 1.20<a<1.36 and 88<b<98, L
being the total length in question of the preform, w.sub.av being
calculated as indicated above.
[0065] According to another particular embodiment of the invention,
the variable-thickness zones of the preform are located
longitudinally, helically or evolutively with respect to the axis
of the preform.
[0066] Another aspect of the present invention relates to the core,
which can be used for the injection molding of thin-walled preforms
and includes, on the surface, main depression zones produced only
in the zone corresponding to the transition zone of the preform,
and optionally discontinuous secondary depression zones, separate
from the main depression zones and located in the zone
corresponding to the bottom and/or to the body and/or to the neck
of the preform.
[0067] The term "depression zone" used within the context of the
present invention relates a depressed zone in the surface of the
core, which may be created by machining the core or complete
molding of the core. When this is not specified, the term
"depression zone" covers both main depression zones and secondary
depression zones.
[0068] According to one particular embodiment of the present
invention, the depressions on the surface of the core are obtained
by machining or by surface treatment, said zones thus modified
being symmetrical.
[0069] According to a preferential embodiment of the invention, the
depression zones in the surface of the core are symmetrical and
distributed equiangularly over the circumference of the core.
[0070] According to yet another aspect of the present invention,
the core has 2 to 12 depression zones and very preferentially has 2
to 6 depression zones, at least two being provided in the zone
corresponding to the transition zone of the preform.
[0071] According to one embodiment of the invention, the machining
operations carried out on the core result in features that may be
of rectangular, polygonal, oblong or evolutive shape with respect
to the axis of the core.
[0072] Within the context of the preset invention, the machining
carried out on the surface of the core may allow the shape of the
machine feature created to evolve, so that the shape is better
adapted to the profile of the preform, in which case the machine
feature is then said to have an "evolutive" shape.
[0073] Another aspect of the present invention relates to a process
for the injection molding of thin-walled preforms, which comprises
the following steps: the molten material is injected into a mold
comprising a core as described above; the preform is cooled down to
a temperature at which the material no longer changes; and the
preform is then demolded.
[0074] In a preferential aspect of the invention, the core and/or
the cavity are/is heated during at least part of the step of
injecting the molten material into the mold.
[0075] Very preferentially, the heating is applied in specific
zones of the core and/or of the cavity.
[0076] The present invention will be described more precisely in
different implementations given by way of nonlimiting example and
illustrated by the following figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0077] FIG. 1 shows a longitudinal section along an axis not having
the PFCs of a closed mold according to the invention.
[0078] FIGS. 2 (A and B) shows another longitudinal section of a
mold according to the invention in the zone corresponding to the
transition zone of the preform.
[0079] FIG. 3 shows a partial longitudinal section of a preform
according to the invention, depicting the neck and the transition
zone.
[0080] FIG. 4 shows a longitudinal section of another embodiment of
a preform according to the invention.
[0081] FIG. 5 shows a cross section in the plane AB of the preform
according to FIG. 4.
[0082] FIG. 6 shows the various parameters involved for calculating
the average minimum thickness of the preform according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0083] FIG. 1 shows a mold according to the invention comprising
two parts: a fixed part, also called the cavity support or
countermold 4, and a moving part 5 supporting the cores 2. The
internal wall 1 of the countermold 4 and the core 2 define a cavity
3 into which the molten material will be injected via the injection
point 6.
[0084] FIG. 2 shows more precisely the internal wall 1 of the
countermold 4 and the core 2 which together define a cavity 3
corresponding to the impression 7 and the main PFCs 8.
[0085] According to the invention, the main PFCs 8 (FIGS. 2 A and
B) are bounded by a depression zone on the surface of the core 2,
said depression zone being produced in the zone corresponding to
the transition zone of the preform, and the internal surface 1 of
the countermold 4.
[0086] FIG. 3 shows a first embodiment of the invention in which
the preform has main variable-thickness zones 9 located only in its
transition zone.
[0087] FIG. 4 shows a second embodiment of the invention. The
preform has a neck 10, a transition zone 11 and a body 12.
According to the invention, the preform includes main
variable-thickness zones 9 in is transition zone 11 and
discontinuous secondary variable-thickness zones 13, separate from
the main variable-thickness zones 9 and located in the lower part
of the body 12.
[0088] The secondary variable-thickness zones 13 may be seen in
FIG. 5 in cross section in the plane AB of FIG. 4.
[0089] The following examples, given without being limiting, will
make it clearly understood how the invention can be put into
practice and will bring out its special features.
EXAMPLE 1
Conventional PET Preform (Weight 17 g; Length 90.44 mm) and Process
for Obtaining it
Device Employed
[0090] The preforms are produced on injection molding machines
equipped with a multicavity mold.
[0091] The PET material, predried so as to prevent hydrolysis as it
undergoes melting, is transferred under gravity to the
plasticization screw where it progressively passes from the solid
state to the molten state. This plasticization must be carried out
according to the rules of the art so as to achieve good homogeneity
of the melt. The molten material is transferred forward of the
screw by rotation of the screw, increase in the temperature and
increase in pressure: extrusion.
[0092] There is a build-up of molten material forward of the screw,
this material being transferred into the injection pot by the valve
passing into the transfer position, and an amount of material
corresponding to one complete mold (preform weight, in this case 17
g, multiplied by the number of cavities) is transferred. After this
material has been transferred, the valve is placed in the injection
position.
[0093] The viscosity of the molten material will depend on the
temperature, these temperatures typically being between 230.degree.
C. and 280.degree. C., more precisely between 245.degree. C. and
270.degree. C.
[0094] The injection takes place after the injection mold has been
closed and clamped, the clamping forces having to be sufficient to
prevent flash on the preforms, but not too high so as not to damage
the tools. In the case of a 48-cavity mold, the forces employed are
between 200 and 300 tonnes, ideally less than 250 tonnes.
[0095] The molten material is introduced into the cavities of the
mold after opening the shut-off valves. Proper filling will depend
on the flow, i.e. on the control of the injection speed and
pressure parameters and on the moment when the machine passes from
the injection phase to the holding phase.
[0096] The pressure range used is below 150 bar and more ideally
between 100 and 130 bar. The filling speeds are 40 mm per second
.+-.20%.
[0097] The hot runners of the mold are at a temperature of
generally between 270.degree. C. and 290.degree. C., this being
able to make the material relatively fluid, and therefore improving
the flow. However, increasing the temperature of the material above
290.degree. C. will cause not inconsiderable chemical degradation
and injection point defects.
[0098] The material sets upon contact with the cold walls of the
mold (cavity and core). The quality of the injection will depend on
the amount injected per unit of time. With the current injection
molding processes, the injection rate is 8 to 12 g per second,
depending on the type of PET resin--this injection speed allows
proper filling of the cavity, but also of the neck. To guarantee
this injection speed, there is a minimum thickness of the preform
to be respected, which will depend on its length. In the case here,
if w.sub.av<1.36+L/93.76, there is a risk of incomplete necks
appearing--a defect termed a critical defect.
[0099] After the cavity has been filled, the material is maintained
under pressure for a certain time, called the holding time, so as
to compensate for the shrinkage due to the material cooling.
[0100] The final step before demolding the preform is the cooling
of the latter. The material must have solidified sufficiently not
to undergo deformation and the center of the core must be below the
crystallization temperature.
[0101] The cooling time will depend on the thickness of the
preform, and this satisfies the following equation:
t.sub.C=(w.sup.2/.pi..sup.2a)In[(8/.pi..sup.2)(T.sub.i-T.sub.s)/(T.sub.d-
-T.sub.s)]
where t.sub.C=cooling time (in s); [0102] w=thickness (in mm);
[0103] a=thermal diffusion coefficient; [0104] T.sub.s=surface
temperature of the cavity; [0105] T.sub.i=injection temperature;
[0106] T.sub.d=average temperature of the preform at demolding.
EXAMPLE 2
Preform According to the Invention (FIG. 3)
[0107] The lightening of containers requires designing preforms of
ever thinner wall thickness, the thickness/length ratio of which
becomes less than 2. In this case, the design of the preform will
include PFCs for allowing the cavity to be filled. The
manufacturing process is the same as that presented above.
[0108] The lightening of the preform is achieved by changing the
core (limited investment cost), thereby reducing the thickness of
the preform.
[0109] Thus, for a preform of the same length as that of example 1,
namely 90.44 mm, it will be possible to produce a lightened preform
weighing 15.35 g. The average thickness of this preform will be 1.8
mm and this can be filled only in the presence of the PFCs in the
mold.
[0110] The addition of the PFCs allows the injection pressure to be
brought back down to between 100 and 130 bar, compared with a
maximum pressure of 150 bar, which maximum pressure, without the
addition of PFCs, nevertheless does not ensure filling of the
cavity for an average thickness of 1.8 mm without the risk of
having incomplete necks. In the present case, for an average
thickness of 1.8 mm and a length of 90.44 mm of the preform, it is
verified that the condition w.sub.av<a+L/b with a=1.36 and
b=93.76 is satisfied.
* * * * *