U.S. patent application number 14/574798 was filed with the patent office on 2015-08-27 for process for making a stretch-blow moulded container having an integrally moulded handle.
The applicant listed for this patent is The Procter & Gamble Company. Invention is credited to William John Cleveland CONNOLLY, Patrick Jean-Francois ETESSE, Christian Gerhard Friedrich GERLACH, Christopher LAMB.
Application Number | 20150239169 14/574798 |
Document ID | / |
Family ID | 39708844 |
Filed Date | 2015-08-27 |
United States Patent
Application |
20150239169 |
Kind Code |
A1 |
CONNOLLY; William John Cleveland ;
et al. |
August 27, 2015 |
PROCESS FOR MAKING A STRETCH-BLOW MOULDED CONTAINER HAVING AN
INTEGRALLY MOULDED HANDLE
Abstract
The present invention relates to a process for making a
container having an integral handle, comprising the steps of: a)
providing a preform (6) in a mould cavity (1); b) stretch-blow
moulding the preform (6) to form an intermediate container (8)
which comprises at least one, preferably two, convex bubble(s) (9);
c) deforming the or each convex bubble (9) by means of an inwardly
moving plug (5) to form one or more concave gripping region(s),
whilst maintaining the pressure within the intermediate container
(8) above 1 bar and whilst the temperature of the material in the
gripping region of the intermediate container is maintained at a
temperature between the glass transition temperature, T.sub.g, and
the melt temperature, T.sub.m; d) releasing excess pressure within
the container, preferably prior to withdrawing the plug (5) from
within the container; and e) ejecting the finished container from
the mould cavity (1, 3).
Inventors: |
CONNOLLY; William John
Cleveland; (Windlesham, GB) ; ETESSE; Patrick
Jean-Francois; (Brussels (Etterbeek), BE) ; GERLACH;
Christian Gerhard Friedrich; (St. Gilles (Brussels), BE)
; LAMB; Christopher; (Ohain, BE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Procter & Gamble Company |
Cincinnati |
OH |
US |
|
|
Family ID: |
39708844 |
Appl. No.: |
14/574798 |
Filed: |
December 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12406523 |
Mar 18, 2009 |
9050750 |
|
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14574798 |
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Current U.S.
Class: |
264/534 |
Current CPC
Class: |
B29C 49/06 20130101;
B29L 2031/463 20130101; B29K 2025/00 20130101; B29C 49/4802
20130101; B29K 2027/06 20130101; B29K 2023/12 20130101; B29L
2031/7158 20130101; B29K 2023/06 20130101; B29K 2067/00 20130101;
B29K 2067/046 20130101; B29C 49/6481 20130101; B29C 2049/4807
20130101; B29C 49/12 20130101; B29D 22/003 20130101; B29C 2049/4812
20130101; B29C 49/4273 20130101 |
International
Class: |
B29C 49/42 20060101
B29C049/42 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 2008 |
EP |
08152903.4 |
Claims
1. A process for making a container having an integral handle,
comprising the steps of: a) providing a preform (6) in a mould
cavity (1); b) stretch-blow moulding the preform (6) to form an
intermediate container (8) which comprises at least one convex
bubble (9) extending outwardly from the intermediate container, the
convex bubble (9) having a surface area; c) deforming the convex
bubble (9) by means of an inwardly moving plug (5) to form a
concave gripping region having an area, whilst maintaining a
pressure within the intermediate container (8) above 1 bar, wherein
the surface area of the convex bubble (9) is less than the area of
the concave gripping region; d) releasing the pressure within the
container; and e) ejecting the container from the mould cavity (1,
3).
2. A process according to claim 1 wherein step b) comprises
stretch-blow moulding the preform (6) to form an intermediate
container (8) that comprises two convex bubbles (9).
3. The process according to claim 1 wherein step d) comprises the
pressure within the container prior to withdrawing the plug (5)
from within the container.
4. (canceled)
5. (canceled)
6. (canceled)
7. The process according to claim 1 wherein step b) and step c) are
carried out in separate cavities.
8. The process according to claim 7 wherein the intermediate
container (8) is reheated between step b) and step c).
9. (canceled)
10. The process according to claim 1 wherein the plug (5) has an
outer profile that corresponds to an interior profile of the
concave gripping region.
11. The process according to claim 1 wherein the finished container
is ejected from the mould cavity at step e) at a temperature below
the glass transition temperature, T.sub.g.
12. (canceled)
13. The process according to claim 1 wherein the gripping region of
the finished container comprises a deep grip palm rest, x, (52); a
deep grip finger rest, y, (54); and a deep grip recess depth, z,
(56).
14. The process according to claim 13 wherein the sum of x+2y+2z is
greater than 107 mm.
15. The process according to claim 13 wherein the deep grip recess
depth, z, (56) is greater than 20 mm.
Description
[0001] The present invention relates to a process for making a
stretch-blow moulded container having an integrally moulded handle,
preferably having a handle with good ergonomics.
[0002] The formation of integral handles via the stretch-blow
moulding process is highly desirable and the art contains a number
of attempts to solve the inherent problems which have not proved
successful. An integrally moulded handle is generally less
expensive than a separate, e.g. clip-on, handle. Previously
disclosed approaches to provide an integral handle typically
require the formation of a pair of opposing depressions or cavities
in the body of the bottle that form the structural basis of the
handle. These depressions can either then be welded together and
the central section, encompassed by the weld, can be removed such
as to form a completely open space through which the fingers and/or
thumb can be inserted (a `through` handle), or, alternatively, left
to simply form a grip. If the grip is formed to be sufficiently
wide and deep so that a hand can close on the grip without having
the tip of the fingers touch the bottom of the recess, then
ergonomic studies have shown the resulting grip functionality to be
as good as that of a through handle.
[0003] One of the problems associated with the prior art is that
the distribution of material in these depressions is uneven. Due to
the need for a significant difference in stretch from the original
preform to different sections of these wells, irregular wall
thicknesses are formed, resulting in adverse mechanical and
aesthetic properties.
[0004] One approach, as detailed in EP-A-0 346 518, published on
Dec. 20, 1989, requires the use of moving sections of the mould to
compress the expanding preform and form these deep wells during the
blowing process. This process, however, results in two problems.
Firstly, this process requires significant levels of stretching of
the material after contact with the mould. This results in highly
irregular wall thickness in the handle area and regular failure of
the material under stress. In addition, the complexity that is
required to move mould sections against pressure in excess of 20
bars required to blow a bottle results in moulds which are
expensive and difficult to maintain.
[0005] An alternative approach as detailed in WO-A-2006/113428,
published on Oct. 26, 2006, is to produce an intermediate container
with a convex section which can be mechanically deformed inwardly
to form the concave grip section. This process enables the designer
to produce more even stretch ratios and hence more even wall
thicknesses. However, the inversion of this section results in
significant deformation of the area around the handle, creating
aesthetic defects. These defects are moreover very difficult to
control, as the exact nature of the deformation will be highly
dependant on very small variations in wall thickness.
WO-A-2006/113428 describes multiple articulation zones in order to
minimize the issue to allow a clean inversion of the grip geometry
(from convex to concave). This results in a very restrictive design
geometry and cannot eliminate residual stresses in the plastic
sheets forming the grip resulting in undesirable wrinkles and poor
ergonomics.
[0006] It is an object of the present invention to provide a
process to create a deep, concave grip which provides an ergonomic
handle.
SUMMARY OF THE INVENTION
[0007] The present invention relates to a process for making a
container having an integral handle, comprising the steps of:
[0008] a) providing a preform (6) in a mould cavity (1); [0009] b)
stretch-blow moulding the preform (6) to form an intermediate
container (8) which comprises at least one, preferably two, convex
bubble(s) (9); [0010] c) deforming the or each convex bubble (9) by
means of an inwardly moving plug (5) to form one or more concave
gripping region(s), whilst maintaining the pressure within the
intermediate container (8) above 1 bar and whilst the temperature
of the material in the gripping region of the intermediate
container is maintained at a temperature between the glass
transition temperature, T.sub.g, and the melt temperature, T.sub.m;
[0011] d) releasing excess pressure within the container,
preferably prior to withdrawing the plug (5) from within the
container; and [0012] e) ejecting the finished container from the
mould cavity (1, 3).
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows a vertical cross-section through a
thermoforming apparatus useful for carrying out the process of the
present invention.
[0014] FIG. 2 shows a horizontal cross-section through the
thermoforming apparatus shown in FIG. 1, along the centre line
2-2.
[0015] FIG. 3A illustrates providing a perform.
[0016] FIG. 3B illustrates stretch-blow moulding the perform.
[0017] FIG. 3C illustrates deforming the or each convex bubble.
[0018] FIG. 3D illustrates the finished container.
[0019] FIG. 4 shows the grip diameter measured in accordance with
DIN33402.
[0020] FIG. 5 shows a perspective view of a bottle with an integral
handle, made according to the present invention.
[0021] FIG. 6 is a graph showing the development of topload with
time of the bottle shown in FIG. 5 made from polypropylene.
DETAILED DESCRIPTION OF THE INVENTION
[0022] By "stretch-blow moulding", what is meant herein is the
process in which preforms are heated above their glass transition
temperature, and then blown in moulds using high pressure air to
form hollow bodies, such as containers, bottles. Usually the
preform is stretched with a core rod as part of the process.
[0023] By "preform" what is meant herein is a moulded form which is
produced prior to expansion to form the finished object. A preform
is necessarily somewhat smaller than the finished object. A preform
is generally produced by, for example injection moulding, at an
elevated temperature in excess of the melt temperature.
[0024] The term "thermoforming" is used herein in its broad sense
of deformation of a material at temperature above its glass
transition/brittle temperature. Thermoforming is described in "The
Wiley Encyclopedia of Packaging Technology", Edited by Brody, A. L.
and Marsh, K. S., John Wiley & Sons, Inc., 2.sup.nd Edition,
1997, pages 914 to 921, which is incorporated herein by
reference.
[0025] Thermoforming is one of many manufacturing processes that
converts plastic resin into usable everyday products. Speed and
cost efficiency are the highlighted qualities that thermoforming
offers which lead the way for the process becoming so important in
industry today. The basic concept of thermoforming is quite simple.
A pre-manufactured thermoplastic sheet is heated until it becomes
soft and pliable. It is then forced against the contours of a mould
until it cools to its original state. Once it has cooled it is
removed from the mould while still maintaining the shape of the
mould. Thermoforming is a broad term; there are many different
types of thermoforming processes. For deep thermoforming, bubble
plug-assist forming is an important process. The advantage of this
forming technique is that it improves material distribution because
of its pre-stretching procedure. By this process, it is possible to
control the thickness of the formed article as the sheet is
stretched to guarantee an even thickness of walls. Once the sheet
has been placed in the frame and heated, controlled air pressure
creates a bubble. This bubble stretches the material to a
predetermined level. The male plug assist is then lowered, forcing
the stretched stock down into the cavity. The male plug is normally
heated to avoid chilling the plastics prematurely. The plug is made
as large as possible so the plastic is stretched close to the final
shape of the finished product. The female mould must be vented to
allow trapped air to escape from between the plastics and the
mould.
[0026] Thermoforming can take place in two dimensions, whereby a
surface is deformed, or in three dimensions where, in addition to
two dimensional deformation, a change of the thickness also
occurs.
[0027] The present invention uses the bubble plug-assist
thermoforming technique to address the problems of forming an
integrally moulded handle on a stretch-blow moulded container.
[0028] The term "deep grip" is used herein to denote a blind
handle, that is to say a gripping feature which permits the user's
thumb and fingers to wrap around a handle, but which does not allow
the fingers to pass completely behind and through the handle. A
"through" type of handle can be achieved by cutting away part or
all of the web of material which is formed between the handle and
the body of the container. The deep grip handle and the through
handle from which the web has been cut-away are both within the
scope of the present invention, although the deep grip is
preferred.
[0029] Preferred plastic resin materials for use in the present
invention are thermoplastic materials, and most commonly polyesters
such as polyethylene terephthalate (PET). Other materials suitable
for use in the present invention are polypropylene (PP),
polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC) and
polylactic acid (PLA). Particularly preferred resin material is
heat-set polyethylene terephthalate.
[0030] As in all polymer processes the temperature history plays an
important factor in the deformation behaviour. For amorphous
polymers, such as polystyrene, the forming temperature is generally
above the polymer's glass transition temperature (T.sub.g). The
glass transition temperature is defined as the temperature below
which the polymer behaves like a brittle, glassy solid and above
which the polymer behaves like a rubber and is easily deformable.
For semi-crystalline polymers, the amorphous region responds to
temperature just like an amorphous polymer described above, but the
crystalline region that is composed of crystallite structure
requires more heat energy to unravel/shear the crystallite
molecules and enable large deformation as needed for the formation
of the deep grip. The percentage of crystallinity and the rate of
crystallization depend strongly on the crystallization temperature,
the time at that temperature, and the degree of molecular
orientation during crystallization. Typically, semi-crystalline
materials are deformed close to the melt temperature T.sub.m. The
melt temperature is the temperature where all crystallites are
melted and the polymer is behaving as a fluid. Another
characteristic temperature for semi-crystalline polymers is the
re-crystallization temperature T.sub.c. The re-crystallisation
temperature is defined as the temperature at which an un-oriented
polymer when cooled down from the melt is showing significant
crystal growth within a specific period of time, typically a few
minutes. As crystal growth is slower at lower temperatures, if the
specified time period is extended, the T.sub.c effectively
drops.
[0031] Glass transition temperature, re-crystallisation temperature
and melt temperature may be conveniently measured with reference to
ASTM D3418.
[0032] As illustrative example of above, an isotactic random
co-polymer polypropylene grade typically used for reheat
stretch-blow moulding of containers shows a 1-3 minutes
crystallization inhibition time before crystal growth initiates at
60-80.degree. C. Hence, if the time period for crystallisation is
set at 1 minute, 60-80.degree. C. is effectively below T.sub.c even
when the polymer was stretched biaxially at temperatures close to
the melt temperature. PET is known to show stress-induced
crystallization during stretch-blow moulding up to a crystallinity
of 20-25%. If the polymer is then cooled down rapidly to
temperatures below T.sub.g no additional crystallinity will
develop. But if kept at temperatures above 90.degree. C. (T.sub.c)
the polymer will crystallize up to 35% within minutes. At
temperatures between T.sub.g and T.sub.c there is little or no
further crystal growth within the first 1-2 minutes which is of
advantage for the processing described within.
TABLE-US-00001 Typical Temperatures PET PP PS Glass transition
81.degree. C. -10.degree. C. 82.degree. C. temperature, T.sub.g
Recrystallisation 90.degree. C. 110.degree. C. Not applicable
(fully temperature, T.sub.c (for amorphous) time <1 min) Melt
temperature, T.sub.m 265.degree. C. 170.degree. C. 240.degree.
C.
[0033] In a preferred embodiment of the present invention a heated
preform is inserted into a blow mould cavity. This blow mould
comprises two recesses in the outer walls of the cavity to create
the pre-stretched bubble that will later be thermoformed by the
plug. The container is then stretch-blown in such a way that the
pre-stretched bubble is cooled as little as possible. This ensures
that minimal or no additional heat is needed to be applied to the
bubble in the second thermoforming process step.
[0034] The invention will now be illustrated in more detail with
reference to the drawings.
[0035] FIG. 1 is a vertical cross section of a bubble plug-assist
thermoforming cavity (3) used in the invention.
[0036] FIG. 2 is a horizontal cross section of the bubble
plug-assist thermoforming cavity (3) used in the invention. The
plug (5) is driven by pneumatic pistons (4). Not shown in the
figure are cooling/heating circuits integrated into the cavity and
separate cooling/heating circuits integrated into the plugs. This
cavity can either be integral to the blow mould or be a separate
cavity.
[0037] The process of the present invention can be well understood
with reference to FIGS. 3A-D showing in diagrammatic form, the
steps of: [0038] a) providing a preform (6), in a mould cavity (1),
FIG. 3A; [0039] b) stretch-blow moulding the preform (6) to form an
intermediate container (8) which comprises at least one, preferably
two, convex bubble(s) (9), FIG. 3B; optionally transferring the
intermediate container into a separate, thermoforming cavity (3),
and optionally reheating the intermediate container, FIG. 3B;
[0040] c) deforming the or each convex bubble (9) by means of an
inwardly moving plug (5) to form one or more concave gripping
region(s), whilst maintaining the pressure within the intermediate
container (8) above 1 bar and whilst the temperature of the
material in the gripping region of the intermediate container is
maintained at a temperature between the glass transition
temperature, T.sub.g, and the melt temperature, T.sub.m, FIG. 3C;
[0041] d) releasing excess pressure within the container,
preferably prior to withdrawing the plug (5) from within the
container; and [0042] e) ejecting the finished container from the
mould cavity (1, 3), FIG. 3D.
[0043] The step shown in FIG. 3A can be done via injection
stretch-blow moulding or reheat stretch-blow moulding where in the
latter injection and stretch-blow moulding is done on two separate
machines.
[0044] The step shown in FIG. 3B shows the option of providing the
thermoform cavity separately from the blow cavity. The container
may be re-heated during this transfer in the case the pre-stretched
bubble has cooled down too much during the blowing step and/or the
transfer to the thermoforming cavity.
[0045] In a variation, the step shown in FIG. 3C can be eliminated
if steps in FIG. 3A and FIG. 3C happen in the same cavity. This has
the advantage that the time between blow moulding and thermoforming
is minimized and that the tolerances on the container specifically
around the deep grip are tighter as there is no relative movement
of the container versus the plug. The integration in one cavity
complicates the construction of the blow mould as the blow mould
needs to have moveable plug(s), the blow moulding machine needs to
be adapted to control the thermoforming step and the total cycle
time increases as the movement of the plug adds to the blowing
cycle.
[0046] In the step shown in FIG. 3C, the bottle is pressurized to
enable a positive location of the bottle in the cavity, and a
plug-assist piston is forced into the pre-stretched bubble to
thermoform the desired deep grip recess. As the male plug fully
engages, an over pressure of from about 1 to about 5 bar is applied
inside the container in order to effectively act as the female
mould portion of the conventional pressure-bubble/plug-assist
thermoforming process. This also ensures that the non deep grip
portion of the container is not deformed as the male plug
thermoforms the deep grip portion. Once the male plug is fully
engaged, plastic is conformed to the plug and the final deep grip
geometry is achieved.
[0047] In the step shown in FIG. 3D, the pressure is preferably
first released, and then the plug-assist piston retracted, and the
bottle ejected.
[0048] Preferred characteristics of the deep grip forming process
shown in FIG. 3B to 3D are as follows: [0049] 1. The deep grip is
formed before thermal crystal growth has started. Ideally the
material is in an amorphous state where it is easily deformable;
[0050] 2. The surface area of the pre-stretched bubble is no larger
than the surface area of the deep grip; [0051] 3. The plug that
forms the deep grip is an identical male shape of the final deep
grip; and/or [0052] 4. The container is ejected at a temperature
where mechanical deformation is hindered (typically below T.sub.g)
and material crystallizes in the final desired shape.
[0053] In point 2), it is not essential that the deep grip is a
mirror shape of the pre-stretched bubble. In fact, it can be of
advantage if the surface area of the pre-stretch bubble is somewhat
smaller than the surface area of the deep grip (1-50%). In case the
surface area of the pre-stretched bubble is smaller than the
surface area of the deep grip there is a three dimensional
deformation of the pre-stretch bubble to form the deep grip. The
resulting wall thickness reduction of the pre-stretch bubble to
form the deep grip can be of advantage to give a good calibration
of the deep grip detail. It has been found that the surface area of
the pre-stretched bubble should never be larger than the area of
the deep grip, as the "excessive" surface will form folds and
wrinkles that have negative impact on aesthetics and
performance.
[0054] In point 3) it is has been found that plugs that are not a
male shape of the final deep grip cause undesired wrinkles and
folds in the deep grip that have a negative impact on aesthetics
and performance.
[0055] The following table captures particularly preferred process
settings in terms of temperature, desired microstructure in desired
bubble/deep grip geometry
TABLE-US-00002 Desired Pre-stretch bubble/ Step Temperature history
microstructure deep grip geometry a) Injection T.sub.m
Amorphous/semi- Not applicable crystalline b) (Reheat) stretch-
Above T.sub.c for the Amorphous Pre-stretched bubble of blow mould
timescale, below Tm same or less surface and typically within area
less than 1 minute c) Thermoforming Above T.sub.g and below No
significant crystal Deep grip formed by deep grip T.sub.c, and
within few growth - ideally plug with desired final minutes,
typically amorphous or only shape less than 2 minutes stress
crystalisation. e) Eject container Significantly below
Semi-crystalline, No further shape T.sub.c importantly crystal
change desired growth happening in the final shape
[0056] It can be advantageous to include venting holes in the
thermoforming plug that are designed similar to venting holes
conventionally used in blow moulds. When the pre-stretched bubble
gets thermoformed to form the concave gripping region, the surface
of the material will conform closely to the outer profile of the
thermoforming plug, and the air between the bubble and the plug can
escape through the venting holes. Furthermore, when the
thermoforming plug is withdrawn just prior to the container
ejection, the venting holes prevent a vacuum from building up
between the concave gripping region and the plug that could lead to
a distortion of the deep grip.
[0057] The deep grip should be ergonomically shaped to help the
consumer to hold the container and pour product from the container.
Anthropometric studies indicate that the minimum grip diameter of
the circle formed by the thumb and forefinger when enclosing a cone
(in accordance with DIN33402) for women aged 20-59 is 34 mm, which
corresponds to an inside perimeter of 107 mm. Accordingly, the deep
grip should provide at least this amount of grippable developed
length to ensure ergonomic functionality equal to that of a through
handle.
[0058] FIG. 4 shows the grip diameter, d, defined as the minimum
diameter of the circle made by the thumb and finger when enclosing
a cone (in accordance with DIN 33402).
[0059] FIG. 5 is a drawing of the finished container with key deep
grip functional dimensions: deep grip palm rest, x, 52; deep grip
finger rest, y, 54; deep grip recess depth, z, 56.
[0060] The deep grip should preferably be as deep as possible, the
principle limitation being the footprint of the bottle, but
preferably no less than required to generate a developed length of
107 mm. In the case of a symmetrical deep grip design as shown in
FIG. 5, then x+2y+2z should preferably be no less than 107 mm. The
depth of each deep grip recess (z) should preferably be greater
than 20 mm (i e, z>20 mm). In a preferred embodiment the deep
grip is formed by two opposing concave gripping regions, the
respective bases of which are in mutual contact.
[0061] The palm rest of the deep grip (x) should preferably be
sufficiently large so that the bottle can rest intuitively against
the palm of the hand and opens the hand sufficiently wide that the
fingers do not touch in the bottom of the deep grip recess. It has
been found by ergonomic studies that a palm rest width of greater
than 30 mm (i.e. x>30 mm) is the minimum required to provide
comfortable handling at parity with that of a through handle.
[0062] Opposing deep grip halves do not need to be symmetrical in
terms of depth and shape as the asymmetry can improve the ergonomic
performance of the container.
[0063] When the respective bases of the concave gripping regions
are in mutual contact it can also be advantageous to provide means
for interlocking the opposing concave gripping regions against each
other such as to substantially eliminate any relative movement when
gripped. An example of one such means is a "peg and pin" where both
concave gripping regions meet. The peg and pin are aligned and
interlocked during the thermoforming step. In one embodiment, the
peg and pin are formed concavely in the pre-stretched bubble and
then formed into their final convex shape during thermoforming the
deep grip. In another embodiment, the peg and pin are thermoformed
out of the pre-stretch bubble wall thickness during the deep grip
thermoforming step. This avoids the need to weld opposing concave
gripping regions. Such welds are undesirable because they provide
weak spots which easily fail, especially under drop load.
[0064] The advantages of this invention over previous art are that
formation of the final deep grip area is produced with material
that is already substantially stretched to its correct ratios. This
prevents the requirement of significant stretching against a cool
mould wall as in EP-A-0 346 518. The plugs also only need to move
against a relatively low pressure (typically less than 5 bars),
greatly simplifying the mould construction. In addition, as a deep
grip forming is not occurring in an intermediate container that has
had significant intimate contact with cool mould walls as in the
process outlined in WO-A2006/113428, the material will be much less
stressed on deep grip forming, resulting in lower deformation at
the deep grip area.
EXAMPLES
Example 1
[0065] Using polypropylene excellent results are obtained by
stretch-blow moulding the container at 125.degree. C. The container
is transferred from the blow cavity to the thermoforming cavity
within two minutes without additionally pre-heating the container
whilst the pre-stretched bubble is at a temperature of between 60
and 80.degree. C. The container is pressurized in the thermoforming
cavity to two bar and then the deep grip is thermoformed by use of
pneumatic cylinders. Optimal deep grip quality is obtained with a
plug matching the final deep grip shape. The container is vented
and then ejected at a temperature of below 60.degree. C.
[0066] In this example the polypropylene resin selected is Basell
Stretchene.RTM. 1903. The conditions for the transfer of container
from blow cavity to thermoforming cavity are chosen so that the
material is essentially still amorphous before and during the deep
grip forming step. In the case that significant crystal growth is
initiated prior to thermoforming significantly higher forces are
required to create the deep grip and the risk of aesthetic defects
is higher.
[0067] FIG. 6 shows the development of topload with time after
stretch-blow moulding the polypropylene container. The increase of
topload correlates directly with the development of crystalinity in
polypropylene containers as can be found in literature. It can be
seen that the topload is extremely low in the first 2 minutes
because the crystal growth has not started for the specific
Ziegler-Natter isotactic random copolymer used here (Basell
Stretchene.RTM. 1903).
[0068] It can be advantageous to heat the thermoforming plug to
reduce the crystal nucleation rate and keep the material warm
enough to enable a material deformation through the thickness of
the pre-stretched bubble without inducing stresses. In the latter,
there is true three dimensional deformation of the pre-stretched
bubble into the deep grip and ensure best calibration of the deep
grip shape by the plug.
Example 2
[0069] Using PET excellent results are obtained by stretch-blow
moulding the container at 90-95.degree. C. The container is rapidly
transferred from blow cavity to the thermoforming cavity within
less than 60 seconds (preferably less than 30 seconds) while the
pre-stretched bubble is still at a temperature between 80 and
90.degree. C. The container is pressurized in the thermoforming
cavity to 1-5 bar and then the deep grip is thermoformed by use of
pneumatic cylinders. Optimal quality is obtained with a plug
matching the final deep grip shape. The container is vented and
then ejected at a temperature of below 80.degree. C.
[0070] In this example the PET resin selected is Equipolymer.RTM.
C93. The conditions for the transfer of container from blow cavity
to thermoforming cavity are chosen so that the material is
essentially still amorphous before and during the deep grip forming
step. As PET is stress-induced crystallising during the
stretch-blow moulding one cannot avoid an initial crystalinity up
to 25%, but the temperature/time profile was carefully chosen to
eliminate any additional thermal crystal growth (i.e., at
temperatures below T.sub.c as undrawn PET crystallizes at
temperatures above T.sub.c) and to be warm enough deforming
rubber-like (i.e., at temperatures above T.sub.g).
[0071] For PET it can be of advantage to heat the blow cavity up to
140.degree. C. to achieve the desired temperature of the blown
container between 80-90.degree. C. at the thermoforming step.
Preferably, only the mould section that forms the pre-stretch
bubble is being heated so that this section stays at 80-90.degree.
C. while the rest of the container mould is cooled and therefore
resists better any undesired deformation during the deep grip
formation. It has been shown that unmoulding a PET container from a
cold 10-20.degree. C. blow cavity leads to an undesired container
temperature of 45-55.degree. C. prior to thermoforming which then
requires higher pressures inside the container to avoid undesired
deformation outside the deep grip area, higher forces on the plug
to thermoform the pre-stretched bubble, and leads to undesired
folds and wrinkles in the final article as internal stresses are
build up during thermoforming.
[0072] It can be of a further advantage, especially for PET, to
combine blow cavity and thermoform cavity in order to eliminate any
delay between blowing and thermoforming the deep grip and keep the
temperature of the pre-stretched bubble as close to the desired
80-90.degree. C. such that thermoforming happens before significant
crystal growth has occurred.
[0073] In comparison to polypropylene, PET will show less through
thickness deformation during deep grip forming and therefore the
benefit of a heated plug is considered of less significance.
Instead, it can be advantageous, especially for PET, to cool the
thermoforming plug to freeze the final deep grip shape ensuring
good calibration in the deep grip forming step and to minimize
post-mould shrinkage in the article.
[0074] The dimensions and values disclosed herein are not to be
understood as being strictly limited to the exact numerical values
recited. Instead, unless otherwise specified, each such dimension
is intended to mean both the recited value and a functionally
equivalent range surrounding that value. For example, a dimension
disclosed as "40 mm" is intended to mean "about 40 mm".
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