WO1999058313A1

WO1999058313A1 - Improvement in a two-stage process and apparatus for producing preforms of thermoplastic material

Info

Publication number: WO1999058313A1
Application number: PCT/EP1999/002594
Authority: WO
Inventors: Ireneo De Nardi; Gino Campo
Original assignee: Sipa S.P.A.
Priority date: 1998-05-13
Filing date: 1999-04-17
Publication date: 1999-11-18
Also published as: IT1305265B1; AU4031399A; ITPN980033A1

Abstract

Two-stage process for the production of preforms (2) of thermoplastic material, comprising the stages of extrusion and/or injection of a flow of molten resin into a mould provided with a plurality of individual cavities adapted to produce respective preforms (2), and removal of said preforms (2) from said cavities, followed by a stage in which a plurality of male elements (1) having outer dimensions that are substantially equal to the dimensions of the inner volume of the preform (2), are inserted in the respective preform (2) which is then allowed to cool down to such an extent as to cause it to shrink and tighten around the respective male element (1) owing to the effect of the resulting contraction. Such a cool-down is allowed to go on until the preform (2) becomes stable; the same preform (2) is then separated from the respective male element (1). Said preform (2) are not placed in any external cool-down or forming apparatus during this process.

Description

IMPROVEMENT IN A TWO-STAGE PROCESS AND APPARATUS FOR PRODUCING PREFORMS OF THERMOPLASTIC MATERIAL

DESCRIPTION

The present invention refeis to an improved process and apparatus for the pioduction on a large-scale basis of containeis made of thermoplastic material, in particular polyethylene terephtalate (PET), polyethylene naphthalate (PEN), copolymers (PET+PEN) and polypropylene (PP), intended for use m applications implying them bemg filled with liquids, which may be even be at an elevated temperatme, le hot, and/or caibonated, le containing C0₂ (carbon dioxide) gas

For reasons of greatei simplicity, reference will be made also m the following description to a type of pieform obtained with a single extrusion/injection phase It shall howevei be appreciated that the present invention applies also, and m the same manner to multi-layei preform, including those that are obtained with co-mjection, oveπnjection and similar methods

In the field embracing the technology and the machines for manufacturing such containers there are a number of developments and impiovements aimed at obtaining, on the one side containers with increasingly enhanced properties and robustness that are capable of bemg used to hold bot hot-filled liquids and/or carbonated liquids, and, on the othei side pioduction piocesses and related apparatuses that are capable of producing said containeis m an increasingly reliable, cost-effective, versatile manner, to an increasing level of quality, m a highly competitive industrial context of very large-scale production

These production processes are generally known to be capable of being schematically giouped into two basic typologies, le. smgle-stage and two-stage processes.

In a two-phase process, a previously produced preform or parison, which is m a substantially amorphous state, is heated up again to its preferred molecular orientation, at which it is then blow-moulded into the desired shape. As used m this context, the term "two-stage process", or "double-stage process", shall be understood to cover any process that produces a preform or parison which must then be heated up from ambient temperature to the related blow-moulding temperature

In contrast therewith, single-stage processes are so defined in that they are capable of forming the so-called preform, or parison, and transferring said preform from the injection mould or extrusion die (upon it having been allowed to cool down to some appropriate temperature) to a conditionmg station, where it is allowed to evenly level at a temperature of preferred molecular orientation. Said preform or parison is then transferred to a blow-moulding mould, in which it is finally moulded into its desired form

Inherent to any smgle-stage process is the fact that an uneven heat distnbutiontakes place across the cross-section area of the wall thickness of the preform when the latter is transfened from the injection mould or extrusion die. It is basically due to this reason that almost the whole lot of patents dealing with single-stage processes actually disclose methods to mould and form preforms, or parisons, of some thermoplastic material which are m a number of various manners carried along thiough a conditioning station at a wall temperature which is uniform across the entire thickness of the same wall, wherein such a temperature corresponds to the temperature of preferred molecular orientation of the thermoplastic lesm involved In two-stage processes, which therefore imply a separate phase that is devoted solely to the production of the preforms, all above cited problems and constraints typically encountered during such a conditionmg phase are simply ignored, since the sole need arises here for a snes of preforms to be produced which must simply be cooled down to ambient temperature

Under these circumstances, therefore, a factor of paramount importance turns out to be, further to the productivity of the manufacturing plant, the actual quality of the preforms themselves

As a mattei of fact, by allowing the preforms to undergo uncontrolled cool-down upon them having been removed from the injection mould a number of drawbacks may show up, concerning m particular a possible deformation or warpmg of the same preforms during the cool-down phase

It is obviously possible for the preforms to be allowed to directly cool-down within the respective cavities of the injection mould, and be then removed when they are sufficiently cool to exclude the possibility for them to undergo any possible deformation of a thermal nature However, as all those skilled in the art are able to readily appreciate, such an operating principle visibly clashes with a basic requirement that the manufacturing plant has to comply with, le an acceptably high productivity In fact, the longer the preform is allowed to stay in the injection mould, the less will the same mould be available for use to produce further preforms, so that the productivity of the whole pioduction plant and process is lowered accordingly

Various types of precesses and apparatuses aie actually known in the art to have been developed m view of implementing conditions aimed at establishing some trade-off enabling said problem arising from such mutually clashing quality and productivity requirements to be overcome Such processes and apparatuses can ideally be grouped into a few distinct categories that aie characterized m that they feature substantially similar solutions a med at solving the above cited problem

A first such category, which is for instance best represented by the disclosure m the US patent no 4,382,905 to Valyi, is charactenzed in that the preforms are removed prematurely from the respective cavities in the injection mould, le when they are still very hot, and inserted m adequate respective cup-like receptacles that are so shaped as to enable the walls of the preform to adhere to the inner walls of the respective cup-like receptacle The mutual contact between said walls is effective m causing the preform to cool down lapidly, whereby the same preform is m fact able to be removed at an earlier point than usual from the respective cavity m the injection mould and the same cavity is conclusively made immediately available for a new injection moulding cycle

Such a type of solution, however, has a major drawback m that the rapid cool-down effect, which the preform undergoes when so inserted m the cup-like receptacle, causes the preform to shnnk, under resulting deformation or warping of the same preform, so that serious quality-related problems may reasonably be expected to arise when the preform is then blow moulded in view of obtaining the final product, le the container

A second category of solutions amed at solving the afore mentioned problems, which is best exemplified by the disclosure in US 5,501,593, is based on the circumstance of the pieform being cooled down in a forced manner while shll dwelling m the injection mould, and m a subsequent cooling-down station to which the preforms are bi ought by having male-type transfer elements inserted therem However, these male-type transfer elements do not ensure any possibility for a contmuous and large-area contact to be established with the surface of the related preform, so that they me by no means able to cool it down m a controlled manner

Cooling down pieforms in such a mannei causes practically a number of problems to arise as f i as both the actual produαion of the preforms, which are forced into cooling down m an anomalous and uneven manner, and the complexity of the production plant and the operation thereof are concerned

A further category of solutions proposed to the afore cited purpose, which is best exemplified by the disclosure m the US patent no 4,313,905 to Robert X

Hafele, provides for the preforms to be cooled down by the same male moulding elements (core pms) that are used also to carry the preforms to the subsequent conditioning station

Such a solution, although quite effective from a qualitative point of view as far as the manner m which the preforms are actually cooled down and handled is concerned, clashes with the need for said male mould elements, le core pins, to be used also foi transfeirmg the pieforms, so that it practically becomes impossible foi the injection moulds themselves to be used to carry out the next moulding cycle as the preforms are bemg so transferred This of course causes the productivity of the whole production plant to suffer significant losses

No one among the afore cited patents therefore discloses any type of solution which is really capable to effectively and contemporaneously solve the afore described problems connected with both the quality of the preforms and the productivity of a two-stage pieform producing plant

Based on these considerations, it therefore is a main purpose of the present invention to provide a two-stage process and apparatus for the production on a large industrial scale of pieforms made of thermally stable thermoplastic material, which are capable of doing away with the afore cited drawbacks, are reliable and can be easily implemented with the aid of leadily available techniques and means

This and furthei puiposes and aims of the present invention will become clear and apparent to those skilled m the art reading and understanding the description that is given below to illustrate a prefened embodiment m a detailed manner with leference to the accompanying drawings, m which - Figure 1 is a schematical view of the basic architecture of an apparatus according to the piesent invention, as shown m a first operational phase,

- Figure 2 is a view of the apparatus illustrated m Figure 1 , as shown m a second operational phase,

- Figure 3 is an outside view of a male-type form element according to the present invention, as shown before being inserted m the respective preform,

- Figure 4 is an axial longitudmal-section view of a preform removed from the mould and befoie being fitted on the respective male-type form element,

- Figure 5 is an axial longitudmal-section view of the preform illustrated m Figure 4 and the element illustrated m Figure 3, as shown m the state m which they are fitted togethei ,

- Figure 6 is a schematical perspective cutaway view of a male-type form element accoidmg to the present invention,

- Figure 7 is a view of a preform holding and transferring part according to the present invention,

- Figures 8, 9 10 and 11 are symbolical views of some phases of the preform transfer geometry according to the piesent invention,

- Figures 12 13 and 14 are schematical views, referred to a more complex embodiment, of the conditions, arrangements and cool-down phases of the preforms coi responding to Figures 9, 10 and 11 ,

- Figuie 15 is a diagrammatical view of the typical temperature profile in the body of a pieform immediately upon removal fiom the mould In order to be able to bettei understand the actual woikmg mechanism of the present invention, le how the lattei actually works, the need anses for a couple of basic circumstances and peculiarities to be stressed and duly lemembered In the first place the preforms, after having been moulded and formed in the respective injection moulds, are allowed to partially cool down m the same moulds before being then removed therefrom and allowed to further cool-down They are finally transferred to the suitably provided subsequent collection and storage phases (it should be noticed that a two-stage plant for the sole production of preforms is being considered here)

Upon its removal from the mould, the preform usually exhibits an uneven temperature profile, m the sense that the temperature is at a peak inside the wall thickness of the preform body, while it giadually decreases towards the inner and outei surfaces thereof, in particular, the outer surface is at a lowei temperature than the inner surface (TV T_e > 10 - 20°C)

Figure 15 shows a typical profile t of the distribution of the temperature across the wall thickness of the preform body, and in correspondence of the inner and outer surfaces thereof, immediately upon removal of the preform from the injection mould By plotting on the abscissa the distance at which the body temperature is measured with respect to the inner surface of the preform and the respective temperature on the ordmate it can be observed that, at the point 0 on the inner surface, the temperatuie has a value T, of approx 120°C, and then increases up to a peak value T₀ of approx 140 - 150°C at the core of the wall thickness of the preform body It finally decreases again to a value T_e of approx 105°C in correspondence of the outer surface

Aftei a certain time has elapsed to allow for said tempeiatuies to stabilize and even out, they show a profile as lepresented by the dashed curve h, as anyone skilled m the ail is well aware of, so that no further explanation and illustration needs to be set forth heie m this connection After removal from the mould and during the subsequent transfer phases, the temperature of the preform tends to level out throughout its body, including the outer and inner surfaces thereof, which therefore heat up

It has been observed that such a heating-up of the outer and inner surfaces brings about a dilatation of the preform itself with an increase m both the inside and the outside diameter thereof Since such a heatmg-up or leveϋmg-out of the temperature is just a transient process, when the preform then unavoidably cools down it tends to take its ultimate dimensions and it is exactly in this phase that the afoie mentioned diawbacks of pieform deformation, or uneven thickenmg of the walls thereof, tend to occur

It has also been observed, and can really be explained logically, that the larger the wall thickness of the preform, the greater is the temperature difference between the core portion of the wall and the outer surfaces thereof, and therefore the greater is the difficulty encountered m trying to control the deformation of the preform and the dilatation of the respective inside and outside diameters, and conclusively the greater is the extent of the deformation which the preform will ultimately undergo

In order to eliminate all such drawbacks, while still allowing for the preforms to be removed prematuiely from the respective moulds in view of assuring the desired productivity of the production plant, it would therefore be desirable to be able to rely on a piocess oi an apparatus adapted to prevent the preform from defoi ming upon its I emoval from the mould

The present invention a ms at reaching just such an aim with particular reference to Figures 3, 4 and 5, it can be noticed that the basic peculiarity of the piesent invention lies in pioviding the production apparatus with a plurality of male-type cool-down elements 1 that are inserted automatically in the preforms 2, so as to fill up the whole mnei volume thereof, when the inside diameter of the same pieforms 2 has increased to reach its approx maximum value owing to the afore mentioned tempei ature leveUing-out effect This enables male elements to be used with a diameter 3 which is substantially equal to the mside diameter 4 of the preform as the lattei is just removed from the mould, since said elements can m fact be easily inserted in said preforms without any risk of possible interferences

A basic feature m this connection is the fact that the possible different diameters 3, 3 a, 3b, etc of the male elements can be appropriately selected to exactly reproduce the respective ultimate inside diameters 4, 4a, 4b, etc that the preforms should desirably exhibit

This enables a first important lesult to be obtained in that the possibility is in this way eliminated for the ultimate diameter of the preform to possibly stabilize at a value which is smaller than and/or different from a pre-set value

The male elements that are inserted at the due moment m the respective preforms are normally cooled by means of an inner circuit through which a cooling medium is circulated m a forced manner

As illustrated m Figure 6, the male element 1 is preferably cooled by means of two coaxial conduits 11 and 12, of which one is used for the delivery of the cooling medium all the way up to almost the opposite extremity of the element 1 , while the othei one is used foi the letmn of the same medium

As soon as the male elements are so introduced m the respective preforms, these tend to cool down due to both a natural effect and the action induced by the immediate contiguity of said male elements

In the initial phase of such a cooling-down process, the preform shrinks, thereby tightening with most of its innei surface against the respective male element, so that the lesultmg dnect contact therewith boosts the coolmg-down effect and theiefoie the furthei conti action of the piefoim and, immediately thereafter, the ultimate setting of the prefoim fitting tightly on the lespective male element Considering that the temperature of the preforms upon their removal from the mould is at a value ranging from approx. 110°C to 120°C, it has been observed that the cooling-down action becomes particularly effective, without any nsk of thermal stresses being placed thereupon, when the male element is at a temperature comprised between 24°C and 40 °C

At this point the preform is practically completed and exhibits, as compared with pnor-art preforms, following improvements - absence of any defoi mation,

- accuracy of the inside diameters and the inner contour of the preform, coaxiality of the inside and outside diameters,

- constancy of the wall thickness of the preform, since the whole inner surface of the preform has established a tight-fitting contact with the male element and has therefore been able to cool down and, as a consequence, shrink to a similar extent.

Finally, the release and the ejection of the preforms from the mould is carried out by means of appropriate release/ejection devices of vanous type that are lergely known m the art and do not require any further explanation here.

With reference to Figures 8, 9, 10 and 11 , an advantageous improvement of the invention is now illustrated

Usually, the regular production procedure consists in these cases in organizing m a senes arrangement the vanous phases of a production process that can be broken down into subsequent phases.

In this particular case, the simplest and most immediate solution consists in arranging a pluiality m of cavities m a same injection mould 31 , as well as a corresponding plurality n of male elements in another part of the production plant, wherein n = m

10 With such an arrangement, n preforms are moulded at the same time, which are then removed at the same time from the mould and transferred m a synchronous manner so as to cause them to fit onto the respective male elements

Most obviously, the moulding phase and the cool-down phase, inclusive of the respective ejection, handling and similar stages, must be appropnately synchronized m order to ensure constancy of the production flow

However, if a phase is longer of the other one, le takes a lonfer time to be completed, appropriate "wait-only" phases must be clearly introduced m the process during which a part of the production plant, le the one involving the shorter or quicker operation, will therefore remain inactive

In the case being considered here, the shortened moulding phase lasts approx 14 seconds while the cool-down phase on said male elements typically lasts 42 seconds, le three times as much as the moulding phase

This brings about a loss in the production efficiency of the plant m general owing to the impossibility for all of the parts thereof to be duly satin ated, as anyone skilled m the art is able to understand

In order to avoid such a drawback, the part of the plant performing the slower phase, le the cool-down phase in the case considered here, is replicated by said multiple factoi 3 and is supplied cyclically with the preforms being produced by the quicker part of the plant, le the moulding stage

As it can be observed in the Figures, the male elements 1 are grouped into three distinct groups a, b, c Each one of these groups is supplied cyclically with pieforms arriving from a single row so that, as those skilled in the art are fully able to understand the injection mould can opeiate on a continuative basis since, as soon as they are produced and removed from the lespective cavities in the injection mould, the preforms, indicated symbolically at X, are initially fitted onto the male elements of the group a subsequently a new set of preforms is fitted onto

11 the male elements of the group b, while the preforms on the elements of the group a continue to undergo cool-down Then, as illustrated in Figure 10, a new set of preforms are again fitted onto the male elements of the group c

In the subsequent phase, which is illustrated in Figure 11 , the preforms associated to the male elements of the group a will have completed their cool-down period and are theiefore lemoved therefrom, while the subsequent shot of preforms, generally indicated at Y and coming from the same mould where they have been moulded immediately thereupon, without any interruption, are again transferred onto said male elements of the group a which had m the meanwhile completed their cool-down cycle and had been cleared of the related pieforms within an overall period of time corresponding approximately to the time required by the injection mould to produce the preforms for the group b, for the group c and for the same group a which is now again supplied with respective preforms at the beginning of their cool-down cycle

Those skilled in the art will now be fully able to understand the logic behind the mutual concatenation of the vanous preform production, transfer and cool-down operations

With reference to Figures 12, 13 and 14, it can be noticed how the present invention can actually be used also m view of improving the overall production organization The illustrations m said Figures show m fact how the possibility is actually created for a first lot of preforms to be produced at the same time, which can then be subdivided into two distmct sets H and K of preforms that are fitted at the same time onto two respective groups of male elements

The process takes of course place and goes on m accordance with the above described logic, in the sense that even the subsequent lots of preforms, as again duly subdivided into two respective distinct sets will similarly be fitted onto respective groups of male elements according to the sequences and the cycle times of the afore described elementary process

12 As anyone skilled in the art is able to understand, the production of more distmct rows of preforms produced at the same time and transferred onto respective groups of male elements can in fact be ideally considered as simply the production of a single row of preforms that is then subdivided into a plurality of preform subgroups that are contemporaneously associated to respective subgroups of male elements.

A further advantageous improvement lies m providing particular cup-shaped receptacles 15 m which the preforms are placed upon having been ejected from the respective moulding cavities

Said cup-shaped receptacles, with the preforms so placed thereinto, are transferred concurrently with said groups of male-type cool-down elements In order to prevent them from prematurely cooling down, and therefore shrinking with the risk of making it difficult or even impossible for the respective male element to be then inserted therem, during such a transfer, said cup-shaped receptacles are closed at an extremity thereof and are furthermore made of a heat-insulating matenal so as to be able to act as thermal niches for the preforms which they must transfer

With particular reference to Figures 1 and 2, these can be seen to illustrate a preferred embodiment of the present invention In Figure 1 it can be observed that the preform transfei apparatus 30, which is provided with a plurality of cup-shaped receptacles 15 foi conveying the preforms, is arranged near the mould 31 in which the preforms are being moulded, while the plurality of preforms 32, which had previously been fitted onto the respective male elements situated in the working station 34, are removed from the same elements with the help of commonly known means (not shown) and are collected by gravity into a chute-like container 33 In a subsequent phase, which is best illustrated in Figure 2, the preform transfer apparatus can be noticed to have already conveyed a new lot of preforms to the same station 34 where the same preforms have alieady been fitted onto the respective male elements, while the mould 31 has already started moulding a subsequent lot of pieforms subdivided into two similai lows 35 and 36

13 It will be appreciated that the present invention can also be implemented and carried out with processes and apparatuses that may differ from the afore described ones, provided that they are within the scope of the invention as substantially recited in the appended claims.

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Claims

1. Two-stage process for producing preforms of thermoplastic material, in particular preforms intended for subsequent moulding into finished containers, comprising the consecutive phases of extrusion and/or injection moulding of a flow of molten resin into a mould provided with a plurality of individual cavities adapted to produce respective preforms, a cool-down of said preforms in said cavities, the orderly removal of said preforms from the mould with the help of appropriate handling and transfer means, characterized in that, upon such removal of the preforms from the mould, - a plurality of male elements (1) having outer dimensions that are substantially similar to the ultimate inner volume of the preforms, are inserted in the respective preforms;

- said preforms are then allowed to cool down to such an extent as to cause them to shrink and closely fit around the respective male element owing to the resulting contraction;

- said cool-down effect is continued until the preforms becomes thermally stable;

- upon reaching said state of thermal stability, the preforms are separated from the respective male element.

2. Process according to claim 1 , characterized in that before, during and after the insertion of said male elements in said preforms, the latter are not inserted in

15 any other device that, provided externally to the same preforms, may be adapted to either cool them down or form them.

3. Process according to claim 1 or 2, characterized in that said male elements are cooled through an inner circulation of cooling medium.

4. Process according to claim 3, characterized in that the surface of said male elements is kept at a temperature comprised between 5┬░C and 20┬░C throughout the time during which they are inserted in the preform.

5. Process according to any of the preceding claims, characterized in that:

- said male elements are subdivided into a plurality of distinct groups (a, b, c) formed as many male elements as the number of individual cavities provided in said mould, - the sets of preforms being produced at the same time in said plurality of individual cavities are directed in an orderly sequence towards said distinct groups of male elements until they a saturation thereof occurs, so that each set of preforms is fitted on a respective distinct group of male elements,

- upon reaching such a saturation, the sets of preforms that are produced subsequently are directed cyclically towards the same distinct groups of male elements as they are in the meanwhile cleared from the respective preforms in an orderly sequence.

6. Process according to any of the preceding claims, characterized in that, as they travel from the injection moulds to said male elements, said preforms are at least partially accomodated m respective thermally insulating receptacles (15) in which they are so transferred.

7. Two-stage apparatus for producing preforms of thermoplastic material, in particular preforms intended for subsequent moulding into finished containers, comprising at least an injection mould (31) for moulding a plurality of preforms (2), characterized in that it is provided with a plurality of male elements (1) adapted to

16 be inserted in respective preforms after the latter are removed from said injection mould (31)

8. Apparatus according to claim 7, charactenzed m that said male elements have outer dimensions and shape. that are similar to the inner dimensions and shape of the respective preforms when the latter are cooled down to thermal stability

9. Apparatus according to claim 7, characterized in that there are provided a pluiality of thermally insulating receptacles (15) adapted to accomodate said plurality of preforms aftei they are lemoved from the injection mould and before they are fitted on the respective male elements

10. Apparatus according to claim 8 or 9, characterized in that to a definite plurality of cavities adapted to receive a flow of molten resm for moulding respective preforms there is associated a corresponding plurality of male elements, in which said plurality of male elements amount to a number of male elements which is equal to a number (X) that is substantially a whole multiple of the number of cavities comprised in said definite plurality of cavities

11. Apparatus according to claim 10, characterized m that said whole multiple number (X) is substantially similar to the ratio of the time needed to complete the whole preform extmsion, injection moulding, cool-down and ejection cycle to the time needed to complete the cycle during which said male elements are inserted in said preforms, are allowed to dwell therein and are finally removed therefrom

12. Apparatus according to claim 10 or 11 , characterized m that there are provided means adapted to transfer a first plurality of preforms to a first plurality of male elements and then, m a sequence, subsequent pluralities of preforms to respective pluralities of male elements, and so on until all of the available pluralities of male elements are so used up said means being then adapted to start transferring the next pluiality of preforms again to said fiist plurality of male elements

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