Abstract:
Hollow injection molded article made of plastic material, esp. preform for blow molding a container, with a wall ( 17 ) extending along axis (I), which is composed of a layer of variable thickness (ε), being remarkable in that longitudinal ribs are provided in said axis direction, wherein the wall has a tooth profile of periodic nature varying periodically between a minimum and a maximum threshold value (m, resp. M) which is determined by a typical injection molding length (Ls) for said plastic material and/or characteristic ratio (εmax/εmin). A method of manufacturing thereof ( 10 ) by injection molding comprising forming inner and outer preforms ( 11, 12 ) with formation of an integrated composite preform ( 10 ), and apparatus for this purpose.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to hollow plastic articles, particularly ribbed preforms intended as a container, notably in a particular application in overmoulding, as an injection molding technique. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is started herein with a method for producing plastics preforms by injection moulding, intended for getting processed into plastics containers. Primary raw material for the production of a preform is injected into a mould having a cavity side and a core side, between which the preform is formed. After this, the mould is opened into its two halves—cavity side and core side—wherein each core thereof bears a preform. 
         [0003]    For preforms, overmoulding constitutes an interesting technique to be adopted when the preform needs a very high light barrier, such as in containers for dairy products. In this case, an inner preform is made of a light-tight or opaque material, whilst a corresponding outer preform is made which is transparent or has a colour. As described in PCT/BE2007/000040, when blowing the preform into a milk bottle, it is white on the outer side, whilst on the inner side, there is a black or grey layer of plastic which produces the light barrier. An extrusion process of overmoulding could be used there, consisting in producing preforms to begin with the injection moulding of the grey inner layer, over which a 2 nd  white layer is further injected. This guarantees the light barrier which is required for ultra-high temperature (UHT) milk bottles. 
         [0004]    The overmoulding preform and the bottle blown therefrom can also be used however for other application fields, such as when gas-tight, moisture-tight or chemical barriers are required. 
         [0005]    A method exists in which the inner preform, as first injected product, is transferred to the cavity/core of the outer preform as product yet to be injected, by means of a transport and transfer system incorporated in the mould. Although this system offers an advantage as regards cycle time, it also has many drawbacks. 
         [0006]    US 2005/0249902 A1 concerns the compression of overmoulded preforms for containers, in which a compression injection moulding production method for a two-layer preform is disclosed. This technique is unrelated, however, to an overmoulding injection moulding process involving a stretching and blowing technique. 
         [0007]    In EP 1970181, a two-layer preform produced by an overmoulding technique is described. The preform comprises two layers, which each consist of a composition of polyethylene terephthalate (PET), titanium dioxide (TiO 2 ) and a dye with light-absorbing capacity in the visible light spectrum. However, neither a specific preparation method nor an elaborate overmoulding technique is proposed. 
         [0008]    US 2010/0092711 discloses a method and mould for producing a two-layer preform by overmoulding which differ considerably from the invention below, however, both in terms of mould design and of the course of the production method. Firstly, the technique and the mould described therein use the same core for injection into both the primary and secondary mould cavities, which calls for a vertical displacement of both the cavity side and the core side of the mould. This technique further uses alternate primary and secondary cavities. Although this reduces the amount of vertical movement which is necessary to effect transfer of the primary preforms, this also implies, however, a more complex hot runner system having alternate nozzles for the primary and secondary materials respectively. A drawback of this known system is that it needs investment in a new non-standard machine, a special mould using an as yet unknown technology, and a good deal of peripheral equipment. 
         [0009]    It is further known that there are currently limits on the output of the production system, notably no higher than 24-fold, which, in itself, is a limitation for the output, so that an increase in this is eagerly awaited. In case the requested number of products to be made is large, several such systems must be purchased, which entails additional costs. This system also operates with moving parts in a mould which is particularly complex, so that this is very prone to wear as a result of these moving parts. 
       PRIOR ART 
       [0010]    Document U.S. Pat. No. 3,327,907 essentially relates to a pressurised container provided with reinforcement ribs. 
         [0011]    JP-HO4-296520-A describes the manufacturing of composite materials, wherein as a first step an injection moulding part is formed in a first cavity; in a second step another material is pressed over this injection moulding part with a press, and in a third step a next injection moulding part is added thereto. In this method, a pressing technique is thus adopted, which by no means fits into the process which is referred to here. 
         [0012]    WO 94/16871-A1 only describes the use of two robot arms for the production of PET preforms, which is simply regarded here as a specific application in the production of overmoulding preforms, without any contribution being made to the overmoulding technique considered here. 
         [0013]    In DE-198 56 356-A1, a method is also described for the production of overmoulding preforms, for which, however, a mould with rotating core side is utilized, and wherein the injected product remains constantly on the same core and is placed into another cavity. However, this is completely opposite to the system developed below, wherein it is intended to make the mould components move as little as possible, especially in a rotary direction, for the sake of preventing wearing thereof. 
         [0014]    US2004/151937-A1 discloses a mould for the injection moulding of PET preforms, wherein the neck and cavity consist of a material having a different thermal conductivity in order to obtain a different cooling. This falls completely outside the application to the method which is referred to here. 
         [0015]    JP-2004 082622-A has no relation to preforms, which constitutes though the basic intermediate product whose production is specifically meant here, in particular for the production of containers, with its thereto associated specifies. 
         [0016]    Finally, JP-H04296520 of MITSUBISHI Heavy IND neither discloses a process for preforms nor does it mention the injection moulding technique therein, in contrast with the present invention wherein no other technique is presently envisaged. Moreover, according to this document, an initial injection is made and that so injected product is brought to the bottom, and only then a second injection is made, in contrast to the present intention proceeding to inject both simultaneously. Furthermore, the process is conducted from top to bottom here. There is a priori only one cavity and core visible on the drawings, wherein the whole system as described appears geared to one single cavity, resp. core. 
       Aim of the Invention 
       [0017]    The object consists of making injection molded products on which vertical ribs are provided, but also in a broader conception than just overmoulding products, and also wider than just preforms—in this case ribbed preforms—, such as so-called “upper PV&#39;s” being an upper pressure vessel or chamber at the top of a pressurised container, in short a combination of light weight, in any case, and strength and mechanical anchoring in the case of overmoulding. 
         [0018]    More specifically, it is essentially intended to provide products having vertical longitudinal ribs by injection molding with a higher wall thickness/injection length ratio than is normally limited—in the meaning that when injection molding PET products with a certain wall thickness such as 1 mm thick in a cold mold—, on the one hand, and to realize thin-walled light-weight products with an increased strength, on the other hand. This is notably based on an overmoulding method which brings a solution to the aforementioned problems, and remedies to the abovementioned disadvantages and/or shortcomings. It is thus desired to inject thinner-walled products with an increased strength, which is not known as such for preforms. PET can hardly be injected over a distance of merely 60 mm on 1 mm thick indeed, i.e. barely 6 cm if it is 1 mm thick. However, if an injection by means of said ribs is required yet with an average wall thickness of 1 mm, and all in PET, this would need at least a certain wall thickness, thereby making the final product far too heavy. So, ribs are made therein, which have a certain thickness with the required wall thickness of 1.3 mm. Between the ribs, there is merely available 0.6 mm average, thereby still yielding 1 mm as result. A non-uniform distribution is thus allowable yet, provided that the overall ratio is 1.3 mm. This should be continuous, and it must be an injection molding length. The configuration consists of a rib type, resp. tooth profile. 
         [0019]    When a given length must be observed, for which a flow path/wall thickness ratio of 60:1 needs to be satisfied, e.g. if need be 100:1 or even 120:1, in any case&gt;60:1, the product cannot be made completely with this thickness, because then it does not get fully injected. Conversely, in case the product is really made this way with a full injection, it would be much too heavy. 
         [0020]    However, one cannot reach 100:1 either. So one is faced with two states that cannot be realized separately, whereas this is only achievable with an average however, in which the only solution to satisfy both of these conditions consists of a ribbed preform according to the invention with a certain profile allowing an averaging. Thus, the profile should be smoothed rather than just a block shape. When a material is injected, it will constantly follow a path. The first phase allows these ribs to get fully injected and in a second step, the adjacent parts between the ribs starting from the said ribs are getting fully injected, because one cannot inject through completely between the ribs, in one time. Thanks to the ribbed preform, less PET material is needed or it yields a preform which is stronger with less PET material than in the case of simply having a mean equal thickness. 
         [0021]    The first advantage thus consists of providing articles that are lighter average. This is not useful for preforms however because they may generate warming up restrictions, since certain areas thereof are thicker so that they warm up more slowly, whereas other areas are thinner which heat up faster during blowing. So even though preforms can be made as such, they cannot be blown however, thereby resulting in that no bottle ever gets blown therefrom, so it is completely useless. Indeed, a preform is merely an intermediate injection molding product that is necessarily intended to be blown. 
         [0022]    However, not only preforms are meant here, but also injection molded products in general. Indeed, in addition to injection molding, a second objective consists of half-finished products which don&#39;t need to be blown any more, such as the abovementioned upper pv&#39;s constituting the top component of a pressurized container. 
         [0023]    This second aspect thus aims an additional reinforcement without increasing the thickness of the entire wall, thus actually consisting of the combination of both factors including a thin-walled light-weight product and the reinforcement, whereas said increased strength integrates light weight and ridge, as a result of the incorporation of a rib profile in the product. 
         [0024]    In a special application of the ribbed preform in overmoulding, there is an inner and outer preform with contiguous and mutually interlocking longitudinal ribs with their resp. mutually engaging profiles, being in mesh like in a gear tooth engagement: in said inner and outer preform the ribs hook into each other, whereas the final product shows no rib, with the great advantage that the inner and outer preform can be injection molded in two steps each alone all by themselves. Indeed, the inner and outer preforms are injected separately. Accordingly, the milk preform can be injected still lighter thanks to a particularly advantageous embodiment of the invention being applied thereto. Both the inner and the outer preform must meet the so-called 60:1 rule, contrarily to the article having those ribs for which this rule does not impose itself. So, more weight is saved yet. Less PET material is needed for such preform than in the case that the same preform is injected in one single time. 
         [0025]    A further feature is when the inner and the outer preform are not made both of PET, but the inner preform is injected from a different material such as polypropylene, and the outer preform in PET, which are two non-complementary materials. In that case, providing those ribs with an undercut offers extra added value thereto. Indeed, polypropylene and PET do not normally adhere to one another. When blowing, this causes delamination, which is thus prevented thanks to the invention, by a mechanical anchoring of the two layers. Generally, in accordance with a preferred embodiment of the invention, the outer side is the one from PET to maintain both visually and physically contact with a PET bottle, and polypropylene is applied on the inside, thereby creating a main barrier. A polypropylene bottle is felt completely different indeed, it is much heavier. Besides, PET is better formable. 
       SUMMARY OF THE INVENTION 
       [0026]    The present invention, wherein inner and outer preforms are carried out with consecutive vertical ribs whereas the final product remarkably does not show any ribs at last, is further set out in more detail hereafter. 
         [0027]    In addition, a particular overmoulding application is further proposed according to the invention in which the inner and outer preforms are performed in two different, non-complementary materials. In this case, the inner and outer preforms can be carried out with their contiguous vertical ribs being provided with an undercut, whereby a mechanical anchoring is generated between the inner and outer preform. 
         [0028]    In short, there is proposed according to the invention a hollow injection-molded article made of plastic material, esp. a preform for blow molding a container, comprising a neck portion, a wall portion adjacent thereto and a bottom portion extending axially along an axis l, wherein the product wall is composed of at least one layer with a variable thickness ε. Remarkably several ribs are provided in the axial direction, i.e. longitudinal ribs, thereby producing a wall having a tooth profile of a periodic nature, which varies periodically between a minimum threshold value m and a maximum peak value M which is determined by an injection-length Ls being characteristic for said plastics material and/or a characterizing ratio εmax/εmin. 
         [0029]    According to an advantageous embodiment of the invention, said hollow article has a square wave profile, esp. a square wave profile showing steep flanks, more particularly a profile with a variable width, even more particularly a profile with a periodic variability. 
         [0030]    According to a preferred embodiment of the invention, said hollow articles with vertical ribs are injection-molded to products with a higher wall thickness/injection-length ratio, thereby to achieve thin-walled light-weight products with an increased strength. 
         [0031]    According to a particular embodiment of said hollow article according to the invention, it consists of an overmoulding preform comprising two different materials, with an inner and an outer preform, wherein the inner and outer preform are carried out with contiguous vertical ribs, yet to such an extent that the final product exhibits no ribs, visible; more particularly wherein the inner and outer preform are carried out in two different, non-complementary materials; even more particularly wherein the inner and outer preform are carried out with adjoining vertical ribs provided with an undercut, by virtue whereof a mechanical anchoring between the inner and outer preform is established; still more particularly wherein the space between the ribs is not greater than twice the width of a rib. 
         [0032]    According to a specific embodiment of said hollow article according to the invention, it consists of primary plastic base material and at least one additional secondary material in at least a sub-area thereof, wherein the preform has at least two sub-areas, notably a primary and a secondary area respectively that are adjacent by pairs with their mutual contact sides and that have a mutually different coloration that is continuous as a whole, wherein it is bicolored; particularly wherein the sub-areas have an alternating profile of mutually substantially parallel separating lines which extend according to the axis, thereby generating at least one vertical strip with alternate color; more particularly wherein the sub-areas have a symmetrically alternating profile thereby forming a so-called zebra profile made from vertical strips of alternating color. 
         [0033]    According to a more specific embodiment of said hollow article according to the invention, at least one of the sub-areas is opaque and/or at least the other sub-area is light translucent; particularly wherein at least said opaque, resp. light translucent areas are colored; more particularly wherein sub-areas have a mutually strongly contrasted dye; even more particularly wherein at least the primary sub-area is transparent. 
         [0034]    According to an additional embodiment of said hollow article according to the invention, the inner and outer preform have a different colour, or only with the inner or outer preform being colored, particularly including selective openings in the form of specific designs and variations in colors, esp. 2; more particularly wherein the inner preform is colored opaque, wherein a through-recess is provided in the longitudinal axis of the preform, and a transparent outer preform, particularly comprising a transparent control window, over the entire length of the preform, resp. container making the filling level thereof observable. 
         [0035]    According to a still further embodiment of said hollow article according to the invention, it consists of an overmoulding preform, comprising two different materials, with the inner and the outer preform made of a different injection molding material comprising a barrier, esp. a gas barrier, a moisture barrier or a light barrier; more particularly wherein the outer preform is composed of standard PET, and the inner preform of a high-barrier or hotfill material. 
         [0036]    According to a yet further embodiment of said hollow article according to the invention, the inner preform is composed of a polyolefin, and the outer preform of PET, whereby it combines the mechanical and gas barrier properties of PET with the chemical barrier, moisture barrier and thermal properties of polyolefins. 
         [0037]    According to a further embodiment of said hollow article according to the invention, which is mainly intended for containers for dairy products having a very high light barrier, it comprises an inner preform made of a material that is light-tight or opaque, and a corresponding outer preform that is transparent or has a color. The corresponding milk bottle is white on the outside, while on the inside it has a black or gray plastic layer as light barrier, thereby using an overmoulding process consisting of making the preforms starting with injection molding the gray inner layer, on which a second white layer is furthermore injected, thereby establishing the light barrier which is required for ultra-high temperature (UHT) milk bottles, whereas the overmoulding preform and the bottle blown therefrom can also be used in other application areas, such as when gas, moisture- or chemical barriers are required. 
         [0038]    This invention also relates to an insert overmoulding method of the abovementioned type, notably wherein plastic preforms intended to be processed into containers are manufactured, wherein a plastic molding product is inlaid in a cavity of an injection mold mounted on a core thereof in a first stage, and wherein another product is injected on or around the first inlaid product in a second stage. According to a basic embodiment of the method of the invention, it is remarkable in that the plastics preform is made with an integrated added inner preform. The raw material for producing the composite preform and the added inner preform are injected into the mould with its cavity side and corresponding core side, all at the same time, i.e. both for a primary preform and a secondary preform, but not to be considered as intended for the same composite preform, however. 
         [0039]    In a 1 st  step, the injection mould containing the injected products is closed, whilst a gripping member on which a set of receiving members are fitted is brought into a set-aside standby position A. 
         [0040]    In a 2 nd  step, the mould is split into its cavity side and core side, wherein the cores for the composite preform on the core side being the primary cores, and the cores for the added inner preform being the secondary cores resp., each bear an injected product, namely a composite resp. secondary preform. 
         [0041]    In a 3 rd  step, the gripping member is set in motion under the drive of a custom-made drive unit, according to a preset moving direction between the set-aside standby position A, in which said gripping member is inactive and stands ready for onward movement, and an operating position B that is matched with the core side of the mould. The injected and cooled products are received in the gripping member by means of suction means, and each product is received in a receiving member corresponding thereto. 
         [0042]    In a 4 th  step, the gripping member is moved into a further operating position C and places the received added secondary inner preforms onto the primary cores, and keeps holding said preforms. After placing the secondary conjugated preforms onto these primary cores, the gripping member reverts to said set-aside standby position A in order to deliver the assembled composite preforms with an integrated added inner preform, to the outlet of the system for further treatment for the conditioning thereof. The cycle is then recommenced from the 1 st  step, whereby the mould is reclosed. 
         [0043]    In this case, only the injection moulding technique limited to plastics preforms instead of hollow articles is adopted, and no other technique. Both primary and secondary preforms are injected simultaneously, with the incorporation of a suitable hot runner system. 
         [0044]    The mould which is used in the system has a relatively simple construction, like existing moulds which are readily available, thus involving less complexity. There are no moving parts indeed, thus no displacements in the mould, thereby causing less wear. As a result, the whole system becomes cheaper, whilst machinery and peripheral equipment can be deployed for standard preforms. 
         [0045]    All these features according to the invention constitute valuable advantages thereof. 
         [0046]    It is also endeavoured according to the invention to transpose this complexity from the mould to the deployed robot cooperating therewith, bearing in mind that the robot is visible from the outside and is directly accessible—which is also more manageable in the event of maintenance or repair—in contrast to the mould. By virtue of this, the production process is better controllable, especially visually, so that this readily becomes more reliable as well, at least for the intended applications. Thanks to the use of existing (standard) machines, this yields a greater flexibility as a result of maintained standardization. 
         [0047]    According to an advantageous embodiment of the present invention, a primary core is used for injection into the primary cavity, whilst a secondary core is used for injection into the secondary cavity. Following the first injection step, the secondary preform is removed from the secondary cavity by an advantageously robotized grab arm, it is transported vertically and it is placed onto the primary core. This means that the vertical movement which is necessary to displace the secondary preform from the secondary core to the primary core takes place without movement of the cavity side or core side of the mould. This precludes any movements of the heavy and complex mould sides, which simplifies the process, prevents excessive wearing of the mould and allows the use of standard machines. 
         [0048]    Furthermore, no use is made in this invention of alternate primary and secondary cavities, which implies a more complex hot runner system though having alternate nozzles for the primary and secondary material, respectively. In the proposed invention on the contrary, all primary cavities and cores can advantageously be grouped together in a dedicated subfield of the mould, namely a secondary field, notably its bottom part, and all secondary cavities and cores in another sub-area of the mould, namely a primary field, notably its top part, or possibly vice versa. 
         [0049]    According to an alternative embodiment of the present invention, the primary and secondary sub-fields of each mould side are organized in a matrix pattern in a predefined number of rows and columns respectively containing an equal number of elements, just as for the primary and secondary sub-fields of the gripping member corresponding thereto. This subtends a higher number of cavities, in particular 32-fold, even extending up to 64 and above, being preferably an exponent of 2 starting from the power rank 5, if need be, even 48- or 96-fold. 
         [0050]    According to a further embodiment of the method according to the invention, in a sequential representation of overmoulding, the injection mould is closed in Ø 1 , in step Ø 2  the mould is opened, in step Ø 3  said grab arm is displaced between both mould halves in which said products are received, and in step Ø 4  the bottom products are placed onto top cores by this one grab arm. 
         [0051]    In an additional embodiment of the method according to the invention, said grab arm is provided with a vacuum plate as the receiving member for the take-up of injection moulded products consisting of said preforms, wherein the relevant mould plate constitutes the movable side having a predefined number of cores, intended for a set of primary resp. secondary preforms, and a corresponding number of cores for the secondary preforms alone, which each occupy virtually their dedicated half of the plate surface on the core side of the mould, more particularly on the top half and the bottom half. The mould plate constituting the fixed side has a corresponding number of cavities for the primary preforms and a further set having a still corresponding number of cavities intended for the secondary preforms, which occupy the other half of the plate surface, in particular the remaining bottom half. 
         [0052]    Thus in step 1, the one mould half serves as the movable side with the cores thereon, opposite to the other mould half, which serves as the fixed side, in which the cavities also are, wherein the finished products are injected and are located between the relevant cores and cavities. 
         [0053]    According to a further aspect of the basis embodiment of the method of the invention, in its following step 2, the mould is opened, and the movable mould half opposite to the fixed mould half is removed therefrom in parallel. The top products in the top half constitute the outer preforms. The inner preforms are received therein, with the formation of a finished product through the combination of a top product and an inner preform, wherein the bottom products present in the bottom half constitute the inner preforms. 
         [0054]    According to a yet further aspect of the embodiment of the method according to the invention, in the third step thereof, the grab arm with the vacuum plate, which was in the standby position, is moved vertically downwards into position C and the products are transferred from the cores to the vacuum plate. 
         [0055]    According to a still further aspect of the embodiment of the method according to the invention, in the fourth step of the method, the grab arm with the vacuum plate is moved vertically upward with the injection moulded products, whereupon the bottom products, being the inner preforms, are transferred onto the top cores. 
         [0056]    According to a yet further aspect still of the embodiment of the method according to the invention, the robot arm is then further moved vertically upwards, with said number of top products therein, wherein these are then expelled from the vacuum plate and are hence finally ready for packing. The procedure is conducted from bottom to top here, i.e. the other way round compared to the known procedure. 
         [0057]    According to a transitional embodiment of the method according to the invention, as soon as the robot arm disappears from between the mould, the mould is closed again as in step Ø1 and it is thus made ready to injection-mould a following series of products, being the integrated composite preforms at the top and a corresponding number of inner preforms at the bottom. 
         [0058]    According to a privileged embodiment of a method according to the invention, at least one second gripping member is used, which is displaced under the drive of a second drive unit, particularly wherein the latter is coordinated with the first drive unit, between said position B and the operating position C, wherein these gripping members are coordinated with one another for the mutual successive takeover and discharge of the produced preforms. 
         [0059]    According to a specific embodiment of a method according to the invention, the gripping members are moved according to a to-and-fro movement in the direction of their respective longitudinal axes Y1, Y2, wherein the gripping members are fitted above the mould and their respective movement with respect to each other is time-lagged, particularly wherein the time shift is adjustable, more particularly as a function of the number of rows of cores and cavities respectively. 
         [0060]    In a particular embodiment of the method according to the invention, said at least two gripping members are mounted on a support according to a mounting plane virtually perpendicular to the ground, and are moved in this mounting plane, according to a second direction, under the drive of a further drive unit, between a standby position A and the disengaged operating position B. 
         [0061]    According to a more particular embodiment of a method according to the invention, said at least two gripping members are successively moved according to two directions virtually at right angles to each other, wherein the first motional direction is chosen virtually vertical to the ground and both gripping members are moved between said position B and operating position C. 
         [0062]    According to a yet more particular embodiment of a method according to the invention, a first cooling time is set for the cooling, in the cavity side, of the injected preforms. Following expiry of the set first cooling time, the cavity side and the core side of the mould are removed from each other up to a mutual distance apart, which is sufficient to allow one of the aforementioned gripping members to be introduced into a thus formed space between the cavity side and the core side, wherein the take-up side of said one gripping member is matched with the core side, one gripping member is moved from said position B to said space, and said one gripping member is thus brought into operating position C with respect to the core side. The preforms are cooled there in the respectively corresponding receiving members for a second set cooling time and, following expiry of said cooling time, the preforms are passed from the core side facing one gripping member into respectively a receiving member corresponding to each core. After this, one gripping member is brought back into position B, both gripping members are shifted transversely until the further gripping member is brought into position B, and one gripping member is brought into said standby position A; subsequently, the movement executed by said one gripping member during the completed cycle is executed analogously by the further gripping member and thus, a further set of preforms is taken over by the latter from the core side of the mould and the further gripping member is then driven back into position B. 
         [0063]    According to a still more particular embodiment of a method according to the invention, during a first cycle said mould opens after expiry of a first cooling time, injected preforms standing on the core side. Once a space is formed between the core side and the cavity side, which space is sufficiently large to allow the first gripping member to be placed therein with a reliable transference of the preforms, said first gripping member is moved under the drive of a motor constituting said drive unit, along the longitudinal axis Y1 of said first gripping member between said core side and the cavity side until it stands in the operating position C. The first gripping member then takes over a complete first set of preforms from the core side. After the preforms are transferred, the first gripping member is led back along the longitudinal axis Y1 to position B, in which the preforms are held in respective sleeves, constituting the receiving elements, of the first gripping member during a following cycle O2 which commences from the moment when the first gripping member is brought into position B. The preforms are accommodated in their respective sleeves, where they are subjected to a suitable cooling, wherein the preforms of the first cycle O1 are meanwhile still present in the second gripping member. Shortly before the end of the following cycle O2, the second gripping member is moved from the standby position A into position B, whilst the first gripping member is moved into a standby position A′ with an analogous takeover process which is executed with the second gripping member. After said first gripping member has reached the position B, preforms are removed, and said steps are repeated for the following cycle O n  in a repeated process. 
         [0064]    In a still more particular embodiment of the method according to the invention, this movement of both gripping members runs at the same time in said second direction X, where the supporting plate is moved under the drive of a further motor constituting the further drive unit. 
         [0065]    The invention further relates to a device which is notably intended for the implementation of a method as set out above, comprising a mould for the forming of the preforms, which mould has mutually detachable cavity and core sides, on which a number of protruding cores are fitted for the holding of preforms, and to a gripping member provided with a set of receiving members, which can be directed at the cores, for the cooling and reception of the preforms. The gripping member is movable under the drive of a drive unit, between a disengaged position B and an operating position C, the gripping member being coupled with the core side. 
         [0066]    This is remarkable in that at least one second gripping member is provided, which is equipped with a further set of receiving members with which the cores of the core side of the mould can be aligned. Said at least second gripping member is movable under the drive of a further drive unit, between said disengaged position B and the operating position C, wherein this gripping member is coupled with the core side and the movement can be coordinated with the first gripping member. 
         [0067]    According to a further device embodiment according to the invention, each gripping member is formed by a grab arm, wherein the receiving elements are constituted by sleeves, in which the cavity side of the mould is located on a fixed machine platform, wherein the core side is fixed on a mobile platform of the machine, and wherein a core puller is provided, which has a detaining effect on the preforms remaining on the respective cores of the core side, by way of a locally applied clamping connection. 
         [0068]    According to a particular embodiment of a device according to the invention, two supporting plates or tables are provided, each having a gripping member with separate control. 
         [0069]    In summary, by virtue of the special set-up of standard elements according to a specific aspect of the invention, which are used according to specific method steps, it is possible to operate in a simplified, and globally less complex manner. The system according to the invention consists in dividing a standard mould, in particular 64-fold, into two fairly equal halves, i.e. 32 bottom parts for the inner preforms and 32 top parts for the outer preforms, or possibly vice versa. 
         [0070]    It must be clear that the scope of the invention is not limited to plastics preforms as semi-finished products, but also embraces hollow articles as final end products. In addition, it will also be clear that the robot arm, can also make a horizontal movement in order to place the secondary injection moulding part into the primary cavity, in addition to the vertical movement as an alternative for displacing the injection moulding parts described above. 
         [0071]    Further specifies and features of the invention are defined in further sub-claims. 
         [0072]    The method and apparatus of the invention are further illustrated by the appended drawings, wherein further details are explained in more detail in the following description of some exemplary embodiments of the invention with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0073]      FIG. 1  is a schematic linearly projected view of a main embodiment of a semi-finished product with a rib profile for an undercut resp. mechanical anchoring of a ribbed preform as an application of overmoulding for a container according to the invention. 
           [0074]      FIG. 2  represents a variant of  FIG. 1  with steep flanks. 
           [0075]      FIGS. 3 and 4  are each a mixed representation of a composite preform according to the invention as semi-finished product, notably obtained by using the process as further presented below in  FIG. 11 . 
           [0076]      FIGS. 5 to 10  show a series of different views of a further embodiment of a finished product as top portion of a pressurized container as an injection molded product according to the invention. 
           [0077]      FIG. 11  is a general sequential representation of an overmoulding cycle of a main embodiment of the method according to the invention. 
           [0078]      FIG. 12  shows step 1 of the method, wherein the mould is closed and the grab arm is in a standby position. 
           [0079]      FIG. 13  is a cross section along the line D-D of the device represented in the previous  FIG. 12 . 
           [0080]      FIG. 14  is an analogous representation to the previous  FIG. 13 , consisting of a cross-sectional view through the line A-A, though in this case with regard to the inner preform as the finished product. 
           [0081]      FIG. 15  represents the following phase or step 2, wherein the mould is opened. 
           [0082]      FIG. 16  represents a section along line E-E in  FIG. 15 , on which the topmost 32 products constitute the composite preforms, with inner preforms. 
           [0083]      FIG. 17  is an analogous representation to  FIG. 16 , but of a section along line B-B representing the 32 bottom products only representing the 32 inner preforms. 
           [0084]      FIG. 18  is a schematic representation of the finished product thus made, consisting of said composite preform in which the inner preform is accommodated. 
           [0085]      FIG. 19  further shows the third phase of the method according to the invention, wherein the grab arm with the vacuum plate, coming from the standby position, now receives all products from the cores. 
           [0086]      FIG. 20  is a sectional representation along line C-C in  FIG. 19 , showing a cross section, wherein the bottommost 32 products, being the inner preforms, are transferred into the vacuum plate. 
           [0087]      FIG. 21  schematically represents the fourth step of the method according to the invention, wherein the grab arm moves upwards with the vacuum plate with the injection moulded products. 
           [0088]      FIG. 22  further shows a cross section along line G-G in  FIG. 21 , wherein the injection moulded inner preforms are on the top cores. 
           [0089]      FIG. 23  represents a 3D perspective view of a detail of the mould components. 
           [0090]      FIG. 24  represents a front view of a detail of the mould components according to  FIG. 23 . 
           [0091]      FIG. 25  represents a detail in enlarged view from  FIGS. 11 &amp; 12  respectively. 
           [0092]      FIGS. 26-33  each represent a view of the robot components. 
           [0093]      FIGS. 34 and 35  are a mixed representation of a composite preform according to the invention as a semi-finished product, especially obtained by applying the method as represented in  FIG. 11 . 
           [0094]      FIGS. 36 and 37  show a combined application of overmoulding and coating on a preform, resp. the wall section thereof. 
           [0095]      FIG. 38  et seq. show a combined application of overmoulding and coating on a preform, thereby including zebra configurations. 
       
    
    
     DESCRIPTION 
       [0096]    Generally this invention relates to the injection molding of objects with vertical ribs  96  for injection-molding products having a higher wall thickness/injection length ratio than under normal limitation and providing thin-walled, light-weight products or semi-finished products with increased strength. 
         [0097]    More specifically, a special application is described in the form of semi-finished products, such as for example a preform  10 ;  81 ,  82  in overmoulding, wherein an inner preform and an outer preform with contiguous vertical ribs are performed, in which surprisingly enough the final product shows no ribs however. 
         [0098]    In a remarkable overmoulding application, the inner and outer preforms are executed in two different and not complementary materials. In this case, the inner and outer preform with contiguous vertical ribs can be made with an undercut, so that a mechanical anchoring is established between the inner and outer preform. 
         [0099]    Different profiles of ribs  96  as well as different profiles  95  for undercut resp. mechanical anchoring are represented among which one is shown with steep flanks in  FIG. 1 , e.g. wherein the width of the space between longitudinal profiles represents maximum 2 times the width of a rib  96 ; and further a variant thereof with oblique flanks in  FIG. 2 . 
         [0100]    Mainly products with vertical ribs  96  are injection molded for making products with a higher wall thickness/injection length ratio than under normal standards, on the one hand, and in order to achieve thin-walled and light-weight products with an increased strength, on the other hand, notably based on an overmolding process. In this way, one achieves remarkably to inject thinner-walled articles yet with an increased strength, which is not known in itself for preforms. 
         [0101]    Next to ribbed preforms  10  forming injection molded products with vertical ribs, finished products  1  are also proposed yet with various views notably in  FIGS. 5 to 10 . The range is thus broader than just overmoulding products, and wider than only preforms as well, notably “upper PV&#39;s” as a pressurised chamber  1  in a pressure container shown in a cross-sectional view in  FIG. 5 . In short, it is a combination of light weight in any case, and strength and mechanical anchoring in the case of overmoulding. 
         [0102]    More specifically, the product is injected with a higher wall thickness/injection length ratio than the one under normal limitation, by which is meant that when injecting PET, this goes along with a certain wall thickness, e.g. a product of 1 mm thick in a cold mold. PET material can typically be injected over a distance of 60 mm by 1 mm thick only, i.e. 6 cm against a thickness of 1 mm. E.g. with preforms of 3 mm thick, the longest PET preform to be achieved is thus up to 18 cm. Actually this simply consists of a ratio, wherein PET is thus characterized by an injecting/wall thickness of 60:1. In other words, it is possible to inject over a maximum distance up to 60 mm for each mm wall thickness, i.e. no more than 6 cm. Therefore, in case a little more is required such as up to 8 cm in height, this corresponds to a need of at least about 1.2 mm, in accordance with said ratio 60:1 rule, actually yielding 80:1.3 mm. However, if an injection is required with an average wall thickness of 1 mm on the basis of said ribs, thereby also injecting this entirely in PET, this would require at least such a wall thickness to an extent that it would make the final product far too heavy. Therefore, ribs are made in the preform, which need to have a certain thickness with regard to the wall thickness that is required, which is 1.3 mm. Between the ribs, merely 0.6 mm is made, whereby an average of 1 is thus yielded though. A non-uniform distribution is thus allowable, provided that the overall ratio is 1.3 mm. It needs to be continuous, and it must match said injection length. The configuration is in accordance with a rib, resp. tooth profile as shown in  FIG. 1 , resp. 2. 
         [0103]    With a certain length, a flow path/wall thickness ratio of 60:1 must be satisfied, e.g., merely up to 100:1 or 120:1, in any case more than 60:1. It is not possible to make the whole product with that wall thickness, because it doesn&#39;t get fully injected then. Conversely, in case the product is made completely like this, it would be much too heavy. 
         [0104]    For PET, the injection length/wall thickness ratio is 60:1. For having PET injected well, the corresponding ratio of L/c needs to be smaller than a certain value: the length L is fixed, e.g. 10 cm and the wall thickness ε is 1.5 mm, which yields 100:1.5=about 66.66 by 1. However 1.5 is too much. In case the requirement is to work at 1 mm, this yields a ratio of 100 to 1. So, the need here is of about 1.6 or 1.7, possibly even 2. If 2 mm is taken, this yields a ratio of 50:1, which is correct. The other parts then follow the ratio of 100:1. That is too much, since this corresponds to an average of 75:1. It is easier when taking it with 2 and 1 unit resp. So the injection length is 10 cm, the wall thickness here is 2 mm, and resp. 1 mm, thus yielding as ratio 50:1, respectively 100:1. This cannot be normally injected, since it is too heavy. This is the reason why it is done that way, thereby creating an average wall thickness of 1.5 mm, which is still in excess. Overweight means too expensive. So the aim is to work as thin as possible, but it is not possible to work below the actual limits being determined for the plastic. There is thus a need to inject products with a certain length and wall thickness, which cannot be injection molded in PET for example. PET can never reach a higher ratio than said 60:1, because PET is characterized by said 60:1 ratio, whereas polyethylene is characterized by a ratio of 300:1. The latter thus enables to inject much further. 
         [0105]    However, the ratio 100:1 is not feasible either. So there are two states that cannot be realized separately, which is only achievable with an average however, whereas the only solution proposed to meet these two conditions consists of a ribbed preform according to the invention that is provided with a specific profile allowing an averaging. 
         [0106]    For example, the profile may be smoothed instead of just a block shape. The purpose is that when injecting material, it will constantly follow a path. In the first phase, it will be affordable that these ribs get fully injected, whereas in a second step, the adjacent parts located between said ribs are getting full-injected starting from said ribs, because it is not possible to inject through completely in one time between the ribs. 
         [0107]    With the ribbed preform, less PET material is needed, whereas a stronger preform is achieved with less PET than in the case of a preform with simply an equal mean wall thickness. 
         [0108]    The first advantage is the ability of injecting lighter average, but this is not applicable for preforms, since they may cause restrictions due to warming, owing to some areas that are thicker thus heating up slower, whereas other areas are thinner thereby heating up quicker when blowing. Therefore, preforms can be made yet, but they cannot be blown, resulting in that it is not possible to get a bottle blown therefrom, so that such preforms are completely useless. Indeed, all preforms are merely intermediate injection molded products which are intended to be blown, since it is actually a semi-finished product. 
         [0109]    Thanks to a specific application of the ribbed preform according to the invention in the case of overmoulding, there is both an inner and an outer preform with contiguous and interlocking vertical or longitudinal ribs: in said inner and outer preform the ribs hook into each other, whereas the final product remarkably shows no rib. Indeed, in the finished product, there are actually no ribs any more, with the great advantage that the inner and outer preform can be double injection molded in two times, each individually on its own, in the meaning that the inner and outer preform are injected separately. 
         [0110]    By virtue thereof, the milk preform can be injected still lighter. Both the inner and the outer preform must meet the 60/1 rule, whereas there is advantageously no need therefor with those ribs. So, more weight is thus saved. Indeed, the final preform thus needs less PET material when thus double injected by separating in two times than in case the same preform would be injected in one single time. 
         [0111]    Another aspect arises when the inner and the outer preform are not made both of the same PET material, but instead, the inner and the outer preform are injected from two non-complementary materials such as polypropylene and PET resp. Then those ribs offer extra added value if they are provided with an undercut. Indeed, polypropylene and PET do not normally adhere to one another. This causes delamination when blowing, which is thus prevented thanks to the invention, owing to a mechanical anchoring of the two layers. 
         [0112]    Generally, in accordance with a preferred embodiment of the invention, in order to create a main barrier, the outside thereof is the one of PET, whereas polypropylene is provided on the inside, thereby maintaining both a visual and physical contact of a PET bottle. A polypropylene bottle has a completely different feeling indeed, is much heavier. Besides, PET is better formable as well. 
         [0113]    In addition to preforms, the invention yet relates to injection molded products in general as well, in other words, not only semi-finished products, but also finished products, which no longer need to be blown though. An example of this is shown in  FIGS. 5 to 10  with respect to injection molded products, such as so-called upper pv&#39;s that form a top component of a certain type of pressure container. In this case, a wall thickness of 1.5 mm e.g. is yet achievable even though it may provide insufficient resistance to pressure: it is injection molded but it expands too much in case a pressure of e.g. 8 bar is applied thereon. For a wall thickness of 1.5 mm, some parts can be made with a wall thickness of 2 mm and some other parts with 1 mm wall thickness, whereby the parts of 2 mm provide additional strength, whereas said added ribs provide additional strength without the need to thicken the whole product completely. 
         [0114]    This second aspect thus aims an additional reinforcement without having to increase the thickness of the entire wall, notably the combination of both for a thin-walled light-weight product, wherein said increased strength is a result of the light weight and the added rib, and in subsidiary order said latter reinforcement mode. 
         [0115]      FIG. 11  shows in partial views a to d a general sequential representation of overmoulding with, in φ 1 , an injection mould  3  which is closed; in φ 2 , the mould which is opened into its 2 mould halves,  31  as the core side and  32  as the cavity side; in φ 3 , a grab arm  4 , which arrives therebetween and receives products  11 ,  12 , and finally, in φ 4  the grab arm  4 , which places bottommost products  12  onto topmost cores  31 ′. 
         [0116]      FIG. 12  shows said one grab arm  4 , which is provided with a vacuum plate  40  for the reception of injection moulded products  11 ,  12 . Opposite thereto, the relevant mould plate  31  constituting the movable side, is represented, having e.g.  32  cores intended for the composite preforms  10  and  32  cores for the inner preforms  12 , which each occupy virtually half of the plate surface, here on the top half. 
         [0117]    The mould plate  32  constituting the fixed side, correspondingly has 32 cavities for the composite preform and a further set of 32 cavities intended for the inner preform, which occupy the other half of the plate surface, the bottom half here. 
         [0118]      FIG. 13  shows the mould  3 , closed in step 1 of the method, in the closed state, and the grab arm  4  in a standby position. 
         [0119]    In  FIG. 14 , the mould plate is represented as the movable side  31  opposite the mould plate represented as the fixed side  32 , in which the cavities for the composite preform  10  are also represented, in which the core is incorporated therefor, with there between the finished composite preform  10  with injected inner preform  12 . 
         [0120]      FIG. 15  is an analogous representation to the previous  FIG. 14 , consisting in a cross-sectional view through the line A-A, though in this case as the finished product with regard to the inner preform  12 . 
         [0121]      FIG. 16  represents the following phase  2 , wherein the mould  3  is opened, in particular with a section along the line E-E in the previous Fig., on which the topmost 32 products constitute the composite preforms  10 , with inner preforms. 
         [0122]      FIG. 17  is an analogous representation to the previous Fig., though of a section along the line B-B, with representation of the bottommost 32 products, which represent only the 32 inner preforms. 
         [0123]      FIG. 18  is a schematic representation of the so formed finished product consisting of said composite preform, in which the inner preform is accommodated. 
         [0124]      FIG. 19  further shows the third phase of the method, wherein the grab arm  4  with the vacuum plate  40 , coming from the standby position, presently receives all products from the cores  33 . 
         [0125]      FIG. 20  is a sectional representation along the line C-C in the previous Fig. showing a cross section, wherein the bottommost 32 products, being the inner preforms, are transferred into the vacuum plate  40 . 
         [0126]      FIG. 21  schematically represents the fourth step of the method according to the invention, wherein the grab arm moves upwards with the vacuum plate with the injection moulded products. The 32 bottom products being the inner preforms, are transferred herein onto the cores  33  of the 32 topmost composite preforms  10 . Next, the topmost finished products are deposited onto a discharge conveyor  100 . 
         [0127]      FIG. 22  further shows a cross section along the line G-G in the previous Fig., wherein the injection moulded inner preforms  12  stand on the topmost cores  33 . 
         [0128]    The robot arm then moves further vertically, with the 32 topmost products therein being the composite preforms, wherein these are further expelled from the vacuum plate  40  and are thus ready for packing. 
         [0129]    As soon as the robot arm  41  has disappeared from between the mould  3 , the mould can reclose, just as in step 1. It is then ready to injection mould the following products consisting of 32 integrated preforms  10  at the top and 32 inner preforms  12  at the bottom. 
         [0130]    It is to be understood that the method outlined above with the associated figures, is merely an example of embodiment, which is given only by way of non-limiting example, without that it may be considered that the present scope of protection is limited thereto. 
         [0131]    The overmoulding method can be used to produce bicolour preforms. For this purpose, the inner and outer preforms  11  are injected with a different colour, or only the inner or outer preform is coloured. As a result of selective recesses in the inner preform  12 , some specific designs and colour variations are obtainable. 
         [0132]    For example, an opaquely coloured inner preform, wherein in the longitudinal axis of the preform a through-recess is provided, and a transparent outer preform  11 . This generates a transparent window over the entire length of the preform and bottle, whereby the filling level of the bottle can be observed. 
         [0133]    As far as the addition of two different materials is concerned, the described method for producing overmoulding preforms likewise allows the inner and the outer preform  11  to be injection moulded in another material. This can have special advantages for, e.g. gas barrier, moisture barrier or hot-fill applications. The outer preform  11  can be produced from standard PET, and the inner preform  12  from a high barrier material or a hot-fill material. If so desired, this allows the use of a higher proportion of secondary material for barrier applications compared with known multilayer preforms. 
         [0134]    For hot-fill applications, wherein the complete bottle must standardly be made of expensive hot-fill material, the inner preform alone may consist in this case of secondary material. 
         [0135]    For further applications, the inner preform could consist, e.g. of a polyolefin, and the outer preform of PET. This bottle combines the mechanical and gas barrier properties of PET with the chemical barrier, moisture barrier and thermal properties of polyolefins. 
         [0136]    Even though this can call for a longer vertical movement between the primary and secondary injection step, it does ensure two completely separate hot runners for the primary and secondary material a, b. In addition to an extreme simplification of the hot runner systems, this ensures greater flexibility for the material, since both hot runners can be set at mutually independent processing temperatures. 
         [0137]    Examples of insert-overmoulding with unitized machine: 
         [0138]    Insert-overmoulding preforms were produced on a dual-cavity 2K PET injection machine. The hot runner was mounted such that the ‘a’ material can be injected individually into the top cavity and the b material can be injected individually into the bottom cavity. 
         [0139]    The cavities were mounted such that in the bottom cavity an inner preform was produced without screw thread, and in the top cavity an outer preform was produced with PCO screw thread. In the top cavity a core having a diameter of 0.6 mm less than the core in the bottom cavity has been placed. 
         [0140]    The take-off robot was programmed such that, after one cycle, the preform has been taken off the bottom core and placed on the top core, whilst the finished preform has been removed from the top core and subsequently cooled. 
         [0141]    Materials 
         [0142]    Test 1: in a first test, an overmoulding preform was produced, the inner layer was coloured blue in order to be able to visually evaluate both layers. 
         [0143]    Weight of inner preform 6.2 g; total weight 25.8 g 
         [0144]    A material (outer preform): PET, not coloured. 
         [0145]    B material (inner preform): PET, coloured blue. 
         [0146]    From the produced preforms, bottles were blown and evaluated. Both layers were present in the expected ratio and there was good adhesion between the layers. 
         [0147]    Test 2: In a second test, a milk preform having a highest possible light barrier was produced with overmoulding. 
         [0148]    Weight of inner preform 6.5 g; total weight 26.3 g 
         [0149]    A material (outer preform): coloured with 5% white dye. 
         [0150]    B material (inner preform): coloured with 1% black dye. 
         [0151]    From the produced preforms, bottles were blown and evaluated for light barrier with a spectrophotometer. The results indicated a markedly improved light barrier compared with only white coloured bottles.