WO2008059256A2

WO2008059256A2 - Method and apparatus for making a container with a pressure accomodating base

Info

Publication number: WO2008059256A2
Application number: PCT/GB2007/004365
Authority: WO
Inventors: Robert James Blakeborough
Original assignee: The Plastic Can Company Limited
Priority date: 2006-11-15
Filing date: 2007-11-15
Publication date: 2008-05-22
Also published as: WO2008059256A3; GB2443807A; WO2008059256A9; GB0622813D0

Abstract

A container (1) having a body and a rim (3) defining an opening for the container, the container being made of a material such as PET or PEN, and being manufactured in a multi-step process. The first step comprises injection moulding a preform (15) which has a rim (3) formed at its mouth and a continuous body-forming portion (120) extending across the preform from the inner periphery (21) of the rim. In the final step, the hot preform is placed in a moulding cavity (61) of a blow moulding apparatus (60) with the preform being located by means of the rim (3). The body- forming portion (120) expands in the cavity to form the side walls (20) and bottom wall (2) of the container. The combination of the shape and thickness of the preform (15), the temperature profile of the preform after heating, and the shape of the moulding cavity (61) of the blow moulding apparatus cause the bottom wall (2) of the container to have a central rigid substantially-flat portion (4) and an annular relatively flexible portion (5) circumscribing the central rigid portion, the two portions being directly adjacent and integrally connected. The flexible portion (5) is arranged such that the central rigid portion (4) can be urged into any one of a continuum of positions between a first lowered position and a second raised position due to flexing of the flexible portion. Thus, the sealed container can accommodate changes in pressure within the container relative to atmospheric pressure.

Description

METHOD AND APPARATUS FOR MAKING A CONTAINER WITH A PRESSURE

ACCOMODATING BASE

Background of the Invention

This invention relates to a method of and apparatus for making a container with a pressure accommodating base, and to a container made thereby.

The invention is particularly suitable for the manufacture of wide-mouth containers in the nature of cans or pots, preferably with lids, from polyethylene terephthalate (PET) , though it is not limited to such manufacture. The term "wide-mouth" is well understood by the skilled person; in the industry it is understood to relate to a container with a mouth greater than approximately 35mm. "Narrow-mouth" on the other hand is understood to relate to a container with a mouth less than approximately 35mm.

In such containers, the sidewall of the container can collapse or buckle inwards some time after the containers have been filled and sealed. This is unacceptable because labels are often applied to the sidewall and deformation of the wall can render the label illegible or the container aesthetically unappealing.

The buckling is due to a decrease in pressure (relative to atmospheric pressure) or partial vacuum inside the container. We have identified several reasons for the decrease in pressure:

(1) if the container is filled with warm contents and sealed, the contents subsequently cool, causing the solvent fumes or water vapour in the headspace to condense;

(2) if oil is present in the contents, it may absorb some of the oxygen from the headspace. Both mechanisms (1) and (2) lead to a decrease in the amount of gas in the headspace, while the volume initially remains the same; thus a drop in pressure is observed;

(3) due to a decrease in temperature, as in mechanism 1, the temperature of the gas in the headspace decreases, causing a decrease in pressure (as the volume initially remains constant) . Collapse of the sidewall can be prevented by increasing the thickness of the sidewall, but this adds unacceptable cost to the container because of the increase in material required. Alternatively, strengthening ribs could be incorporated into the sidewall, but these would be visible under the label, and may therefore be unacceptable for aesthetic reasons. We have appreciated that it would be desirable to produce PET containers which use a minimum of plastics material, and yet are sufficiently strong for normal use, which occupy a minimum space when empty for transporting, and yet which are easy to handle and use individually, and finally which incorporate a means to prevent buckling or deformation of the sidewall after filling and sealing. When fitted with a lid the containers should stack readily and be easy to open. Such containers should desirably be capable of being manufactured in a variety of sizes particularly in the range 250ml to 5 litres or *s pint to 1 gallon.

International Patent Application WO97/19801 describes a method of making a container using a two-step process. In the first step an embryo container is formed by injection moulding. At this stage the rim of the container is essentially fully formed, but the body is not yet formed, and instead the embryo container walling takes the form of a central cone extending inwardly from the inner periphery of the rim. In the second step the embryo container is transferred to a blow moulding machine and the walling is expanded outwardly to form the container body. International Patent Applications WO00/46118 and WO2004/71745 describe containers with more complex rim constructions .

International Patent Application No. WO2004/106175 describes a narrow-mouth PET container in the form of a bottle with a base structure which is responsive to vacuum related forces. Figures 12a and 12b of the present application (based on Figures 5 and 6 of WO2004/106175) show side sectional views of the base of a container 300 of WO2004/106175, first in the configuration as moulded (Fig. 12a) and secondly in a configuration after responding to vacuum related forces (Fig. 12b) . The figures are described in full detail in WO2004/106175. The structure has an inversion ring 302 and further requires a central push-up 301 generally in the shape of a truncated cone. As is seen, the inversion ring forms a relatively large part of the base, whilst the central push-up 301 forms a relatively small part of the base. The base of containers that can respond to vacuum related forces must have a specific distribution of material that provides flexibility in the correct places. In the case of narrow mouth containers, as described in WO2004/106175, a central push-up 301 is required to obtain this distribution of material. The pushup 301 is required due to the nature of blow moulding narrow mouth containers . To blow mould a typical narrow mouth container (e.g. a bottle) a tubular, or test-tube-like, preform is used, which is heated prior to insertion into the mould. Initially, the preform expands vertically down the mould (i.e. without significant lateral expansion) as high- pressure gas is introduced (with optional use of a stretch rod) . When the tip of the expanding preform touches the bottom of the cooled mould, it rapidly cools ("freezes") and will no longer stretch or expand. This typically leaves the base thicker than the walls of the container. Thus, to achieve the required material distribution, a central push- up 301 is included in the base of the mould. This causes some material to continue to stretch after the tip of the preform has frozen to central push-up; thus causing the inversion ring of the base to be sufficiently thin enough to be flexible. Use of a central push-up has the significant disadvantage of raw material wastage: the central push-up cones of such containers can account for up to 10% of the total material in a container (due to its thickness being somewhat greater than, for example, the walls of the container) . Clearly this wastage of material significantly increases the cost of producing each container. Further, the push-up results in a reduction in the capacity of the container.

US Patent No. 4,880,129 describes a thermally processable food container. This container is thermoformed: a completely different technique to that used in the present invention; essentially it is stamped out of a hot sheet. In this container the bottom wall is fully flexible to accommodate the short-term increase in internal pressure over external pressure that arises during thermal processing of food containers. The bottom wall is able to distort into a bulged condition as the temperature and thus the internal pressure rises, and then revert to its normal concave condition at the conclusion of thermal processing when the internal pressure falls as the container is cooled once more to room temperature. While the base profile described in US 4,880,129 is superficially similar to that of the present invention, its purpose (accommodating a short term rise in pressure) , and its mode of operation (temporary bulging of the bottom wall to increase the volume of the container) , are completely opposite to the purpose (offsetting the effect of a long term fall in internal pressure) , and mode of operation (inward movement of a substantial part of the base in order to reduce the volume of the container) of the present invention. Further, the base of this container construction is liable to suffer from sagging when a substantial volume of liquid is placed within the container. This is unacceptable because it inhibits efficient stacking of the containers and leaves the base vulnerable to rupture if dropped.

SUMMARY OF INVENTION

The invention in its various aspects is defined in the independent claims below, to which reference may now be made. Advantageous features are set forth in the appendant claims .

Preferred embodiments of the invention are described in more detail below with reference to the drawings. The preferred embodiments take the form of a wide-mouth container (i.e. as defined previously with reference to the understanding within the blow-moulding industry) having a body of similar or wider diameter and a rim defining an opening for the container, the container being made of a material which is susceptible to deformation when heated. The container can be manufactured in a multi-step process, the first step comprising injection moulding in an injection-moulding apparatus a preform which has a rim formed at its mouth and a continuous body-forming portion extending across the preform from the inner periphery of the rim.

In the second step, the preform is heated such that it has a specified temperature profile. Preferably the heating is carried out using infra-red heaters. The step of heating can also be combined with a step of cooling certain portions of the preform, preferably using a stream of gas.

Alternatively the preform may be extracted from the injection mould while it is still hot from the injection moulding process and transferred either directly to the final step, or via an intervening conditioning step in which the hot preform may optionally be exposed to preferential heating (or cooling) .

In the final step, the preform is placed in a moulding cavity of a blow moulding apparatus with the preform being located by means of the rim. The body-forming portion expands in the cavity to form the side and bottom walls of the container.

It should be understood that the steps in the container forming process described above might be completed in one all-encompassing machine conventionally referred to within the industry as a one-stage process, or in separate injection moulding and reheat blow moulding machines, in which case it is described as a two-stage process.

The combination of the shape and thickness of the body-forming portion of the preform, the temperature profile of the preform, and the shape of the moulding cavity of the blow moulding apparatus cause the bottom wall of the container to have a central rigid substantially-flat portion and an annular relatively flexible portion circumscribing the central rigid portion, the two portions being directly adjacent and integrally connected. By "directly adjacent" it is meant that there are no intervening portions between the central rigid substantially-flat portion and the annular relatively flexible portion. The term directly adjacent is meant to encompass the case where there is a gradual taper in thickness between the rigid substantially-flat portion and the annular relatively flexible portion. Preferably the ratio of the thickness of the central rigid portion to the thickness of the annular relatively flexible portion is at least 2.3:1. More preferably this ratio is between 3.3:1 and 4.3:1. Preferably the ratio of the diameter of (or dimension across) the central rigid portion to the ratio of the diameter of (or dimension across) the base of the container is at least 0.5:1, and the ratio of the radial extent of (or dimension across) the annular flexible portion to the total diameter of (or dimension across) the base of the container is at most 0.2:1. The flexible portion is arranged such that the central rigid portion can be urged into any position (i.e. any one of a continuum or plurality of positions) between a first lowered position and a second raised position due to flexing of the flexible portion. Thus according to one aspect of the present invention the base of the container is not bi-stable (i.e. it is not stable in only two configurations). Thus, by altering its volume, the sealed container can accommodate changes in pressure within the container relative to atmospheric pressure.

The container is preferably provided with a lid which conforms with the upper surface of the rim. The lid sealingly engages the rim of the container, preferably comprising two sealing elements, one on the top and the other at the side of the portion of the lid engaging the rim. A flap may be provided at the side of the lid to assist in its removal.

It will be understood by the skilled person that in the methods of the present invention it would be possible to move the base of the stretch-blow mould during the moulding process, for example as described in US 6,277,321B, which is incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example, with reference to the drawings, in which:

Figure 1 is a side sectional view through a container embodying the present invention;

Figure 2 is a sectional detail through the rim of the container of Figure 1;

Figure 3 is a side sectional detail through the base of the container of Figure 1, with the base in its lowered position as moulded;

Figure 4 is a side sectional detail through the base of the container of Figure 1, with the base in its raised position;

Figure 5 is a side sectional detail through the base of the container of Figure 1, showing thickness, in mm, of the base and sidewall;

Figure 6 is a side sectional view through an injection moulded preform used in the manufacture of the container of Figure 1; Figure 7 is a side section view through an injection station of a one-stage machine illustrating the manufacture of the preform;

Figure 8 is a diagrammatic side section view of the heating station of a reheat blow moulding machine used to change the temperature profile of the preform;

Figure 9 is a perspective view of the heating station of a reheat blow moulding machine used to change the temperature profile of the preform;

Figure 10 is a sectional view through a blow mould illustrating the beginning of the formation of the container from the preform;

Figure 11 is a side sectional view through a second container embodying the present invention; and Figure 12a and Figure 12b are side sectional views of the base of the container of WO2004/106175, included for comparative purposes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the invention will now be described with reference to the drawings. Figure 1 is a side sectional view through a finished wide-mouth container 1 embodying the invention. The container may be of generally cylindrical shape as shown here, or may be square or other shape according to market demand with sides 20 and a base 2. The base has a central substantially flat (or substantially- planar) rigid disc 4, which is circumscribed by an annular relatively flexible region 5. The annular flexible region 5 is thinner than the thick central rigid disc 4, which causes the difference in flexibility. The thick central rigid disc 4 is supported by the annular flexible region. A rim 3 defines the mouth of the container. By substantially flat, it is meant that the portion is substantially planar (i.e. having a substantially two dimensional characteristic - the third dimension being substantially smaller than the other two) . However, the term substantially flat as used in the present invention encompasses a central rigid portion having some slight doming (as shown) , which provides additional strength.

The base construction is such that the central rigid disc can move to one of a plurality of positions between a lowered position (as moulded) and a raised position. This allows the volume of the container to decrease in response to a decrease in pressure in a sealed container. Thus, if the pressure decreases (e.g. because of the reasons outlined previously) , the base can move to the raised position, accommodating the pressure in the container and effectively equalising it with atmospheric pressure and thus preventing the sidewalls from collapsing. This is illustrated in Figure 3, which shows the lowered base position (as moulded) , and in Figure 4, which shows the raised base position in response to a decrease in pressure. The base of the container may take one of a plurality of positions (not 5. shown) in response to different pressure changes.

The shape of the flexible region 5 is such as to allow flexing in response to changing pressure differences between the inside and outside of the container and thereby prevent vacuum-induced collapse of the container sidewall 20. As0 moulded and shown in Figure 3, proceeding inwardly from the side 20, the base comprises a substantially flat portion 6 which forms the peripheral contact ring which rests on the surface upon which the container stands. This is followed by a concave portion 7 as seen from outside the container5 and this in turn by a convex portion 8 which merges into the central substantially-flat relatively rigid disc 4. The portions 4, 7 and 8 provide a somewhat concave region when seen from below. Flexure is accommodated by the disc 4 moving upwards so that the portion 8 merges into the concave0 portion 7 and itself becomes concave, as shown in Figure 4. Thus the portions 7,8 operate as an inversion ring, or in a bellows-like manner.

It is important that the base of the container has the correct distribution of material, i.e. the correct 5 thickness. The annular flexible region must be thin enough to flex at a pressure which is below that at which the sidewalls collapse. On the other hand, the annular flexible region cannot be too thin, as this would lead to sagging of the base when it is filled. Further, the central disc must0 not move so easily that it could come into contact with the ground if the container was accidentally dropped.

Figure 5 shows the desired side and base thickness (in millimetres) of a 1 US Gallon (3785ml) paint can embodying the present invention. It has a diameter of 168mm. The5 skilled person will appreciate that the desired base thickness can be adjusted for containers with different diameters; for example, for larger containers, the central disc and annular flexible regions may be thicker to support a larger mass of contents. In general, the best dimensions for any given application (container size and type of contents) can be determined empirically. However, the present inventors have found that a base with the thickness of Figure 5 performs well in containers of this diameter having volumes from 2500ml to 5000ml. The central rigid disc region preferably has a thickness of between 1.5 and 3 mm. The lesser figure relates to containers with diameters between 100 and 120 mm, while the greater relates to containers with diameters between 150 and 180 mm, with other sizes pro rata. The annular flexible region preferably has a thickness of between 0.3 and 0.7 mm. Preferably, the ratio of the thickness of the central rigid portion to the thickness of the annular relatively flexible region in a particular container is at least 2.3:1, more preferably between 3.3:1 and 4.3:1. It will be understood that the transition from the central rigid disc to the annular flexible portion can be a continuous transition; the thickness of the base can taper smoothly from the thicker central rigid disc to the relatively thinner annular flexible portion. In this case, the boundary of the central rigid disc and the annular flexible portion may be defined as the point at which the thickness of the base is half-way between the thickness of the central rigid disc and the thickness of the annular flexible portion. Thus, as defined in this manner the central substantially-flat relatively rigid disc is directly adjacent and integrally connected to the annular relatively flexible portion.

The ratio of the diameter of (or dimension across) the central rigid disc to the diameter of (or dimension across) the base of the container is preferably at least 0.5:1. In the embodiment shown in Figure 1 the ratio is about 0.52:1. Thus, the central rigid disc forms a major part of the diameter of the base.

The ratio of the radial extent of the annular relatively flexible portion to the total diameter of the base of the container is preferably at most 0.2:1. Thus, the annular relatively flexible portion constitutes a relatively small part of the diameter of the base.

In the embodiment described, there is a small variation in thickness across the profile both of the central rigid disc and of the annular relatively flexible region, although the skilled person will appreciate that the central rigid portion and the annular relatively flexible portion could each be substantially uniform in thickness. However, it is preferable that there are no abrupt changes in thickness, as these could lead to a weak point in the base. Thus, the central rigid portion tapers gradually into the annular relatively flexible region.

The rim of the container will now be described. Figure 2 is a sectional detail through the rim 3 of the container 1. In cross section, the rim 3 has, from the inside, an upstanding flange 21 defining the inside of a U-shaped channel 22 whose radially outer side 23 is lower than the flange 21. A horizontal flange 24 projects outwardly from the side 23 from the latter' s smoothly rounded top 25 and a skirt 26 depends from its outer edge. Approximately half-way down the skirt there may be a small annular outwardly- extending rib 27 as shown in the figure.

At the outer top corner of the flange 24 there is an outset and upstanding further flange 28 whose rounded top is just higher than the top of the flange 21.

The flange 21 has a slight inward taper towards the top, and the channel 22 is slightly flared.

At regular intervals around the rim 3 there are slim ribs 29 in planes radial to the axis of the container. They are shown in cross hatching to distinguish them from the rest of the rim with which they are integrally moulded. These ribs enhance the strength of the rim, and typically between eight and twelve ribs 29 (depending upon the diameter of the rim 3) may be included. For clarity the ribs 29 are not shown in the other figures.

In the finished container, the sides 20 contact the bottom of the skirt 26 to form a box section 30, which provides further strength to the rim. The lid can be based on the lid described in International Patent Application No. WO00/46118.

The method of manufacturing the container of Figure 1 will now be described. The manufacturing method is based on that described in International Patent Application No. WO97/19801. The shape of the rim is a modification of the rims described in International Patent Application No. WO00/46118. Reference may be made to both these applications for further details and variations concerning the design.

The first stage of the method is to form a preform 15 as shown in Figure 6. The preform includes a fully formed rim 3 as described above. However, the wall 20 of the body of the container is not formed at this time, but instead the preform includes a continuous dished or bowl-shaped body- forming member 120 which extends across the preform from the inner periphery 21 of the rim, but which is of reduced size compared with its final form. Thus, the body-forming member 120 of the preform does not contact the bottom of the skirt 26 as in the finished container, and therefore the box section 30 referred to in Figure 2 has yet to be formed. The shape of the body-forming member 120 can be anything from approaching a flat membrane to a substantially conical shape. The thickness of the body-forming member 120 is important, as it will at least partially determine the thickness of the final sidewalls and bottom wall. Thus, the body-forming member 120 should preferably be of thickness of between 2 and 4 mm. Preferably the region which will form the sidewall should be of substantially uniform thickness, and preferably the body-forming member should be without abrupt changes in thickness. A computer running conventional design software is normally used to calculate the dimensions of the preform based on the dimensions of the finished container, the stretching characteristics of the specific material to be used, and the desired material distribution. The preform is formed by injection moulding on an injection moulding machine. Injection moulding apparatus are very well known and thus a detailed description of such apparatus need not be given here. It should be noted however, that the rim of the preform is fully formed in this operation and adopts the shape required in the final container. The use of injection moulding allows the rim and body-forming portion to be formed with high accuracy, which is necessary respectively for good sealing of the lid and for achieving the optimal distribution of material in the bottom wall of the finished container.

The method can also comprise providing a lid which conforms with the upper surface of the rim. The lid can also be injection moulded and can be based, for example, on the lid described in International Patent Application No. WOOO/46118.

It should be noted that in this description the preform and eventual container are assumed to be in conventional orientation, that is with the mouth at the top and the base at the bottom. The terms "up", "down", "raised" and "lowered" and their derivatives should therefore be construed in this sense although in actual fact the orientation of the preform or container may be different from this.

At this point, the manufacturing process can proceed in either a one-stage or two-stage process. In the one-stage process, preforms are injection moulded, removed from the injection mould while they are still hot from the injection moulding process, if necessary the temperature profile can be adjusted at a conditioning station, and the hot performs are then blown into finished containers in a single continuous process on one machine. A section through the injection station of a typical one-stage machi-ne is shown in Figure 7. The injection mould has two main sections, namely a lower section formed by a cavity insert 42 and an upper section formed by a two-part inner preform core pin 44 and an outer preform core pin section 46. Neck splits 48 are also provided, carried by neck split carrier plates 50, on the machine carousel 52. Plastics material is injected through an injection orifice 54 in the cavity insert 42 at the bottom centre of the preform 15.

The preform is designed in such a manner that a straightforward vertical movement of the central mould core (pin 44 and 46) will allow removal of the preform from the mould. There are no undercuts or slides in the construction of either the core or the cavity.

In the two-stage process, the preforms may be cooled to room temperature after discharge from the injection moulding machine and if desired may be stored or shipped to a completely different location for reheating and blowing to give the finished container.

The shape of the preform enables them to internest or stack with other like preforms, which allows for more efficient storage and transportation. However, the preforms are easy to separate, because individual preforms can be grasped easily.

When the preforms are to be moulded to form the finished containers, they must first be reheated, as illustrated in Figures 8 and 9, so they have the correct temperature profile. Reheating is achieved using an array of infra-red lamps 201-208. These lamps allow the preform 15 to be heated in specific places in order to achieve the correct distribution of material in the finished container.

The preform is held in front of the infrared lamps by a clamp 209, which engages with the preform 15 on its rim 3. The clamp 209 revolves around a central axis 210. Thus, the preform rotates about its longitudinal axis, evenly exposing the body-forming portion 120 of the preform to the heaters.

The preform is held in front of the heaters for approximately 72 seconds, although a person skilled in the art of reheat blow moulding would appreciate that this time can be adjusted in order to achieve the correct distribution of material in the finished container.

In order to achieve the correct heating profile, the intensity of the lamps must be carefully adjusted. The present inventors have found that the intensities displayed in Table 1 below have been found to give a good distribution of material in the finished product. However, the skilled person would again appreciate that the lamps' intensities can be adjusted in order to achieve the correct distribution of material in the finished container. The skilled person would be able to alter the intensities accurately based on the appearance of the finished container.

Table 1

After heating has taken place, the preform should have a temperature of approximately 102⁰C (measured halfway down the preform sidewall immediately after the preform has been heated) . This temperature is approximate and could be adjusted by the skilled person.

The required temperature profile can be defined with reference to the heating pattern as described in Table 1, with further adjustments understood to be possible by the skilled person. Broadly, more heat is applied to the upper part of the body-forming portion (i.e. near the rim) and towards the very bottom of the body-forming portion. Thus, these areas have a higher temperature after heating, will deform more readily during the blow moulding, and will be thinner as a result. The temperature profile of the preform achieved by the aforementioned heating is important in determining the distribution of material in the finished container .

Figure 9 illustrates the automated heating process in a perspective view. The preform 15 is seen to be held by the revolving clamp 209, engaging with the preform rim 3. This clamp moves along the length of the infra-red lamps 201-208 such that the preform is exposed for approximately 72 seconds and is heated to a temperature of approximately 102⁰C.

In addition to heating the preform as described above, it is also possible to cool certain portions of the preform in order to achieve the correct temperature profile. The cooling can be achieved by passing a directed gas current at certain portions of the heated preform during the heating process. Preferably the directed gas current is a jet of pressurized air. The gas can be directed through openings 212 to cool the body-forming portion 120 of the preform, and longitudinal slit 211 to cool the rim 3 shown in Figure 9. Once the preform has been heated to the correct temperature profile, it is moved to a blow moulding machine. A blow moulding machine is one type of pressure moulding machine; a less preferred alternative would be to employ a machine that uses suction rather than blowing. Blow moulding machines, and methods of blow moulding are well known in the art and are describes, for example, in International Patent Applications WO97/19801, WO00/46118 and WO2004/71745.

The hot and pliable preform is then transferred to the blow moulding station 60, shown in Figure 10. This comprises a cavity 61 defined at the sides by two moveable mould halves 62 and 63 with preform retention and locating pieces 71 and 72 at the top and at the bottom by a moveable base or punt 64. The cavity 61 has a shape which conforms with the shape of the finished container. Thus, the cavity 61 has a base 64 which has a central disc region which is substantially planar which will cause the central rigid disc portion 4 to be formed, and also an annular ridge 68 circumscribing the central disc, which will cause the formation of the annular flexible portion 5 as described previously. The top of the cavity is defined by a clamp 209 which has a circular opening through which an axially vertically moveable central stretch rod 66 can be lowered into the open top of the preform 15 such that it engages the body-forming portion 120 of the preform 15. As it descends it causes mechanical downward stretching of the body-forming section. Note that the part of clamp 20-9 which engages with the preform rim 3, may be flat as shown in this simplified view, or preferably will be profiled to closely match the shape of U-shaped channel 22.

Thus, the sequence is that first the two blow mould halves 62, 63 with preform retention and locating pieces 71 and 72 close around the pliable preform 15 supported by the preform carrier 209. Then the stretch rod 66 is lowered to force the material 120, which is to constitute the container walls, towards the base 64 of the mould cavity 61. Simultaneously, compressed air is introduced through the middle of the preform carrier 209 into the preform. This causes the plastics material at the top of the preform to move outwardly until it contacts the side of the blow mould cavity, contacting the bottom of the skirt 26 as it does so, thus forming a box section in the rim.

As the compressed air continues to blow, an aneurysm forms that rapidly inflates until a proto-container, slightly smaller than the mould cavity, has been formed. At this point, higher pressure compressed air is introduced to form the finished container by compressing the still-soft plastics material of the preform hard against the metal sides of the blow mould. This continues to be assisted by the mechanical stretching caused by the stretch rod 66. The stretch rod 66 is also used to centre the gate (the central or bottom point) of the preform in the bottom of the container. The amount of mechanical stretching imparted by the stretch rod 66 is actually small compared to the stretching caused by the compressed air.

As noted above, the shape of the mould cavity 61 is such as to impart the desired shape to the container side walls 20 and bottom wall 2.

Once the material has cooled and solidified, the high pressure air is exhausted and the blow mould opens and the container is then lifted out by means of the rim 3. The metal sides of the blow mould may optionally be chilled to help cooling of the plastics material.

Figure 11 is a side sectional view through a finished wide-mouth container which is a second embodiment of the invention. The container may be of generally cylindrical shape as shown here, or may be square or other shape with sides 20 and a base 2. The base has a central rigid substantially-flat disc 4, which is circumscribed by an annular relatively flexible region 5. The annular flexible region 5 is thinner than the thick central rigid disc 4, which causes the difference in flexibility. The thick central rigid disc 4, is supported by the annular flexible region 5. A rim 3 defines the mouth of the container. As described previously the central rigid disc can move to any one of a plurality of positions between a lowered position (as moulded) and a raised position, allowing for accommodation of changes in pressure. This movement is similar to that illustrated in Figures 3 and 4. Optionally, it is possible for the central rigid portion to be provided with some slight doming, which provides additional strength; this can still be regarded as substantially flat.

The shape of the flexible region 5 is such as to allow flexing in response to changing pressure differences between the inside and outside of the container and thereby prevent vacuum-indexed collapse of the container sidewall 20. As moulded, proceeding inwardly from the side 20, the base comprises a substantially flat portion 106 which forms the peripheral contact ring which rests on the surface upon which the container stands. This is followed by a concave portion 107 and this in turn by a convex portion 108, followed by a further concave portion 109 which merges into the central relatively rigid disc 4. Flexure is accommodated by the disc 4 moving upwards so that the portion 108 merges into concave portions 107 and 109, itself becoming concave. Thus the portions 107, 108 and 109 operate in a bellows-like manner.

The shape of the flexible region is designed to achieve the correct distribution of material in the finished container to achieve the required amount of flexing whilst retaining sufficient strength. This design is particularly good at achieving the required flexibility in region 5 in smaller diameter containers (up to 120 mm) , but can also be used for larger diameter containers (greater than 120 mm) . The containers are preferably made of a plasties material, by which it is meant any of various organic compounds produced by polymerization, capable of being moulded, extruded, cast into various shapes and films, or drawn into filaments . Containers are more preferably made from polyethylene terephthalate (PET) , for which the process is particularly suitable. Other thermoplastic resins may however be used, including polyethylene naphthalate (PEN) , also PET and PEN blends and other blow mouldable plastics including polyvinyl chloride (PVC) , polycarbonate, and polypropylene (PP) . In principle, any material suitable to deformation when heated may be used, but the method is particularly advantageous with PET etc.

PET resins are characterised by their intrinsic viscosity (IV) , which is a number related to the molecular chain length (the longer the molecule, the higher the IV and the better the mechanical properties) . Resins having intrinsic viscosities in the range of 0.76-0.84 have been found to be most preferable for production of containers embodying the present invention.

The containers can be formed as pails, buckets, or cans and are particularly suitable for enclosing solvent- based contents for storage for long period without solvent loss. They may, in particular, be used as cans, especially lever-lid cans, for paint and other coating materials. The container itself is made as a unitary element without the need for welds, which are costly and can be unreliable. The containers can be formed accurately of PET, which requires blow moulding to give the biaxial stretching necessary for optimum strength. The base of the container is designed to equalize a pressure differential between the interior of the container (when sealed with a lid) and the atmosphere by decreasing the volume of the container. Thus, the base prevents the sidewalls from collapsing, which would be unacceptable for aesthetic reasons and would prevent a label on the container from being intelligible. The pressure equalising base means stiffening ribs are not required in the container body, which would otherwise be visible beneath a label. Further the design of the base allows for controlled movement of the central rigid portion; the central rigid portion will not flop about (i.e. move to multiple positions in multiple directions) , but is substantially confined to movement in one vertical direction between a raised or lowered position. Yet due to the size of the central rigid portion - the central rigid portion forming a large part of the diameter of or dimension across the base - the central rigid portion will readily move up when the external pressure exceeds the internal pressure before the container sidewall 20 begins to collapse.

The base embodying the present invention is sufficiently strong that it will not sag or deform when the container is filled.

The base design illustrated not only provides the above advantageous features, but also does so with the minimum excess material. By using the methods embodying the present invention, no central push-up is required in the base. Such a push-up would lead to effectively wasted material; thus, costs are minimised. The design of the base is made possible by the method embodying the present invention. By a combination of the design of the preform, the temperature profile of the preform after reheating, and the shape of the blow moulding cavity, the present method can achieve the requisite distribution of material with sufficient strength and flexibility without the need for a push-up portion.

Although preferred embodiments of the invention have been described and illustrated, by way of example, many modifications may be made to the method, apparatus and containers described.

Claims

1. A method of manufacturing a container with a pressure accommodating base and having a body and a rim defining an opening for the. container, the container being made of a material which is susceptible to deformation when heated, the method comprising the steps of: injection moulding in an injection-moulding apparatus a preform which has a rim formed at its mouth and a continuous body-forming portion extending across the preform from the inner periphery of the rim; providing the preform with a predetermined temperature profile; placing the preform in a moulding cavity of a pressure moulding apparatus with the preform being located by means of the rim; and moving the body-forming portion in the pressure moulding apparatus downwardly and outwardly whereby the body-forming portion expands in the cavity to form the container side and bottom walls, the body-forming portion contacting the sides of the cavity to form a shape defined by the interior shape of the cavity; wherein the shape of the preform formed by injection moulding, the temperature profile of the heated preform, and the shape of the interior of the moulding cavity of the pressure moulding apparatus are selected such that the distribution of material in the finished container provides the bottom wall with a central rigid substantially-flat portion and an annular relatively flexible portion circumscribing the central rigid portion, the said two portions being directly adjacent and integrally connected, with the flexible portion being arranged such that the central rigid portion can be urged into any one of a continuum of positions between a first lowered position and a second raised position to accommodate pressure variations in the container.

2. A method according to claim 1 wherein the steps of injection moulding the preform, providing the preform with a predetermined temperature profile and pressure moulding the finished container are carried out in a one-stage continuous process .

3. A method according to claim 1 wherein the step of providing the preform with a predetermined temperature profile comprises heating the preform.

4. A method according to claim 3 wherein the steps of injection moulding the preform, heating the preform and pressure moulding the finished container are carried out in a two-stage process.

5. A method of manufacturing a container with a pressure accommodating base and having a body and a rim defining an opening for the container, the container being made of a material which is susceptible to deformation when heated, the method comprising the steps of: providing a preform which has a rim formed at its mouth and a continuous body-forming portion extending across the preform from the inner periphery of the rim; heating the preform, the heating being such as to give the preform a predetermined temperature profile; placing the preform in a moulding cavity of a pressure moulding apparatus with the preform being located by means of the rim; and moving the body-forming portion in the pressure moulding apparatus downwardly and outwardly whereby the body-forming portion expands in the cavity to form the container side and bottom walls, the body-forming portion contacting the sides of the cavity to form a shape defined by the interior shape of the cavity; wherein the shape of the preform, the temperature profile of the heated preform, and the shape of the interior of the moulding cavity of the pressure moulding apparatus are selected such that the distribution of material in the finished container provides the bottom wall with a central rigid substantially-flat portion and an annular relatively flexible portion circumscribing the central rigid portion, the said two portions being directly adjacent and integrally connected, with the flexible portion being arranged such that the central rigid portion can be urged into any one of a continuum of positions between a first lowered position and a second raised position to accommodate pressure variations in the container.

6. A method according to any of claims 3 to 5 in which the step of heating further comprises the step of cooling specified regions of the preform in order to produce a desired preform temperature profile.

7. A method according to any preceding claim wherein the container is a wide-mouth container.

8. A method according to any preceding claim in which the container material comprises polyethylene terephthalate (PET) , polyethylene naphthalate (PEN) , a PET and PEN blend, or polypropylene (PP) .

9. A method according to any preceding claim in which the interior of the moulding cavity of the pressure moulding apparatus is shaped so that the annular relatively flexible portion is an inversion ring which is moveable between a first lowered position and a second raised position.

10. A method according to any preceding claim in which the pressure moulding apparatus is a blow moulding apparatus .

11. A method according to any preceding claim in which the step of moving the body-forming portion is assisted by mechanical stretching of the body-forming portion in the pressure moulding apparatus.

12. A method according to any preceding claim, further comprising the step of providing a lid for the container, the lid conforming with the upper surface of the rim.

13. An apparatus for manufacturing a container with a pressure accommodating base and having a body and a rim defining an opening for the container, the container being made of a material which is susceptible to deformation when heated, the apparatus comprising: an injection-moulding apparatus adapted to injection mould a preform which has a rim formed at its mouth and a continuous body-forming portion extending across the preform from the inner periphery of the rim; means for heating the preform to a predetermined temperature profile; a pressure moulding apparatus having a moulding cavity; means for placing the preform in the moulding cavity of the pressure moulding apparatus with the preform being located by means of the rim; and means in the pressure moulding apparatus for moving the body-forming portion in the pressure moulding apparatus downwardly and outwardly whereby the body-forming portion expands in the cavity to form the container side and bottom walls, the body-forming portion contacting the sides of the cavity to form a shape defined by the interior shape of the cavity; wherein the injection moulding apparatus provides the preform with a shape, the means for heating gives the heated preform a temperature profile, and the interior of the moulding cavity in the pressure moulding apparatus is shaped such that the bottom wall comprises a central rigid substantially-flat portion and an annular relatively flexible portion circumscribing the central rigid portion, the two portions being directly adjacent and integrally connected, and such that the flexible portion is arranged such that the central rigid portion can be urged into any one of a continuum of positions between a first lowered position and a second raised position.

14. An apparatus for manufacturing a container with a pressure accommodating base and having a body and a rim defining an opening for the container, the container being made of a material which is susceptible to deformation when heated, the apparatus comprising: means for heating a preform, which has a rim formed at its mouth and a continuous body-forming portion extending across the preform from the inner periphery of the rim, to a predetermined temperature profile; a pressure moulding apparatus having a moulding cavity; means for placing the preform in the moulding cavity of the pressure moulding apparatus with the preform being located by means of the rim; and means in the pressure moulding apparatus for moving the body-forming portion in the pressure moulding apparatus downwardly and outwardly whereby the body-forming portion expands in the cavity to form the container side and bottom walls, the body-forming portion contacting the sides of the cavity to form a shape defined by the interior shape of the cavity; wherein the means for heating gives the heated preform a temperature profile, and the interior of the moulding cavity in the pressure moulding apparatus is shaped such that the bottom wall comprises a central rigid substantially-flat portion and an annular relatively flexible portion circumscribing the central rigid portion, the two portions being directly adjacent and integrally connected, and such that the flexible portion is arranged such that the central rigid portion can be urged into any one of a continuum of positions between a first lowered position and a second raised position.

15. Apparatus according to claim 13 or 14 wherein the pressure moulding apparatus is a blow moulding apparatus.

16. Apparatus according to any of claims 13 to 15 wherein the pressure moulding apparatus includes means for mechanical stretching of the body-forming portion.

17. Apparatus according to any of claims 13 to 16 wherein means for heating is an array of infrared heaters .

18. Apparatus according to any of claims 13 to 17, further comprising a means for cooling the preform in specified regions.

19. Apparatus according to claim 18 wherein the means for cooling the preform is a directed gas current.

20. Apparatus according to claim 19 wherein the directed gas current is a jet of pressurized air.

21. Apparatus according to any of claims 13 to 16 wherein the means for heating the preform is incorporated into the pressure moulding apparatus.

22. Apparatus according to any of claims 13 to 21 further comprising means for forming a lid for the container, the lid conforming to the upper surface of the rim.

23. A container with a pressure accommodating base and having a body and a rim defining an opening for the container, the container being made of a material which is susceptible to deformation when heated, the container comprising: a rim formed at the mouth of the container; and a container side wall and bottom wall; wherein the bottom wall comprises a central rigid substantially-flat portion and an annular relatively flexible portion circumscribing the central rigid portion, the two portions being directly adjacent and integrally connected; and wherein the flexible portion is arranged such that the central rigid portion can be urged into any one of a continuum of positions between a first lowered position and a second raised position in response to a change in pressure within the container.

24. A container according to claim 23, wherein the container is a wide-mouth container.

25. A container according to claim 23 or 24, wherein the container material comprises polyethylene terephthalate

(PET), polyethylene naphthalate (PEN), a PET and PEN blend, or polypropylene (PP) .

26. A container according to any of claims 23 to 25, wherein the annular relatively flexible portion is an inversion ring which is moveable to any one of a continuum of configurations between a first lowered position and a second raised position.

27. A container according to any of claims 23 to 26 wherein the thickness of the central rigid portion is between 1.5mm and 3mm; and wherein the thickness of the annular relatively flexible portion is between 0.3 and 0.7 mm.

28. A container according to claim 27 wherein the thickness of the central rigid portion is substantially uniform; and wherein the thickness of the annular relatively flexible portion is substantially uniform.

29. A container according to any of claims 23 to 26 wherein the ratio of the thickness of the central rigid portion to the thickness of the annular relatively flexible portion is at least 2.3:1.

30. A container according to any of claims 23 to 26 wherein the ratio of the thickness of the central rigid portion to the thickness of the annular relatively flexible portion is between 3.3:1 and 4.3:1.

31. A container according to any of claims 23 to 30 wherein the central rigid substantially-flat portion is substantially disc-shaped.

32. A container according to any of claims 23 to 31 wherein the ratio of the diameter of the central rigid portion to the diameter of the base of the container is at least 0.5:1.

33. A container according to any of claims 23 to 32 wherein the ratio of the radial extent of the annular flexible portion to the diameter of the base of the container is at most 0.2:1.

34. A method according to any of claims 1 to 12 wherein the shape of the preform, the temperature profile of the heated preform, and the shape of the interior of the moulding cavity of the pressure moulding apparatus are selected such that the distribution of material in the finished container provides the ratio of the diameter of the central rigid portion to the diameter of the base of the container being at least 0.5:1 and the ratio of the radial extent of the annular flexible portion to the diameter of the base of the container being at most 0.2:1.

35. An apparatus according to any of claims 12 to 22 wherein the perform is provided with a shape, the means for heating gives the heated preform a temperature profile, and the interior of the moulding cavity in the pressure moulding apparatus is shaped such that the ratio of the diameter of the central rigid portion to the diameter of the base of the container being at least 0.5:1 and the ratio of the radial extent of the annular flexible portion to the diameter of the base of the container being at most 0.2:1.

36. A method substantially as hereinbefore described with reference to the figures.

37. An apparatus substantially as hereinbefore described with reference to the figures.

38. A container substantially as hereinbefore described with reference to the figures.