MXPA06004186A - Plant and method for thermally conditioning plastic items - Google Patents

Abstract

The method for thermally conditioning preforms in accordance with the invention comprises the following steps:- Placing the preforms on a chain conveyor (2);- Conveying the preforms through a first thermal conditioning stage (1) and subjecting them to a first heating or cooling heat exchange;- Removing the preforms from the first thermal conditioning stage and holding them outside for a predetermined amount of time in order to redistribute the temperature inside the preforms;- Subjecting the preforms to at least a second heating or cooling heat exchange in a second thermal conditioning stage (1) or in the first thermal conditioning stage. The invention also concerns the plants for heating or cooling preforms in accordance with the above method.

Description

PLANT AND METHOD FOR THE THERMAL CONDITIONING OF PLASTIC ITEMS TECHNICAL FIELD This invention relates to a plant and a method for thermal conditioning of plastic objects such as preforms used in the blow molding process used to form bottles, cans or other containers. Particularly, the invention relates to heating or cooling stations for preforms that will be blow molded using the method known as "cold cycle".

HISTORY OF THE TECHNIQUE Nowadays, the production by blow molding of containers of different shapes and sizes - for example plastic bottles, boats, or other objects - using the method known as "cold cycle", involves injection, compression, or extrusion of mold forms or preliminary shapes made of a suitable plastic -for example, PET- and then, cooling and storing the work pieces before the blow molding process.

To use the injection or any other type of press as effectively as possible, the pre-form cooling stage in the mold is kept to a minimum by transferring the pre-forms, as soon as possible, to specific external cooling stations , where the cooling is completed.

To achieve this, state-of-the-art sequential cooling stations can be used. Here, the preforms are cooled by fastening them in specific can-shape fasteners that are cooled using water (a sequential cooling station is described, for example, in Patent Application WO 01/54883 filed by the Applicant). Said fasteners are placed in groups -which usually form a square or a rectangle- in cooling plates used sequentially, for example the groups of preforms are handled by sequence: a group of preforms that leave the mold -normally, all the pre-forms made with the same blowing press - it is put on the plate's clips to cool them simultaneously; then, the plate is released from the cold pre-forms and the station repeats the operating cycle.

Nowadays, there is no known continuous cooling station - that is, those that can handle a continuous flow of pre-forms coming out of a continuous molding station, such as a rotary molding station, described, for example, in the patents EP 759 844 or US 5 071 339-. Currently it is not convenient to pair a sequential cooling station with a continuous molding unit, especially one of high production.

Without importing the molding station, it is always advisable to make a cooling station compact and small enough to occupy the smallest possible portion of the plant surface.

After cooling, as previously mentioned, the pre-forms or preliminary shapes should be heated again -normally, being held in a chain or conveyor belt and moving them towards suitable ovens- to achieve that they reach the appropriate temperature for the molding process by blowing; frequently, said heating ovens are infrared ovens.

The disadvantage of infrared furnaces that exist today is that they are very large - especially along - for plants with a high production, which have a particularly high speed of transport by chains.

OBJECTIVES AND BRIEF DESCRIPTION OF THE INVENTION It is the aim of this invention to provide a plant for the continuous thermal conditioning of plastic articles, especially pre-forms. In addition, this plant must have limited dimensions, especially in regard to minimizing the used area of the manufacturing plant.

According to the first aspect of the invention, this objective is achieved by means of a method for the thermal conditioning of plastic articles comprising the following steps: fastening at least one article in the conveyor chain or in other continuous transportation devices; transporting at least one plastic article through the first stage of thermal conditioning and making it subject to a first heat exchange of cooling or heating, which produces much more heat exchange than would result in putting at least a plastic article in an environment external to said first stage of thermal conditioning during the same period of time; - removing at least one plastic article from said first thermal conditioning stage and holding it outside said stage for a predetermined period of time to achieve redistributing the internal temperature of said plastic article with a predetermined degree of uniformity; - making subject of at least a second heat exchange to said plastic article, which produces a much greater heat exchange than would result in putting said plastic article in an environment outside said thermal conditioning stage during the same period of time.

According to a second aspect of the invention, the aforementioned objective is achieved by using a plant for the thermal conditioning of plastic articles that is blow molded to be suitable for implementing the aforementioned method, wherein said plant is composed at following: a chain or some other continuous transport device suitable for transporting at least one plastic article to be thermally conditioned; a first and a second step of thermal conditioning, wherein each step is crossed by said chain and is suitable for thermally conditioning by heating and / or cooling at least one plastic article carried on said chain; wherein said chain or any other continuous transport device follows a suitable path to feed with at least one plastic article to be thermally conditioned through said first thermal conditioning step, removing it from the first heating step for a predetermined period of time to achieve redistributing the temperature inside said plastic article with a predetermined degree of uniformity, and then feeding it to the second step of thermal conditioning.

According to a third aspect of the invention, the above object is achieved by means of a plant for the thermal conditioning of plastic articles that is blow molded suitable for the implementation of the method as claimed in claims 1 and / or 2, wherein said plant is composed of: a chain or some other continuous transport device suitable for transporting at least one plastic article to be thermally conditioned; at least one thermal conditioning step that is crossed by said chain and is suitable for thermal conditioning by heating and / or cooling of at least one plastic article carried on said chain; wherein said chain or other device for continuous transport follows a suitable path to feed at least one plastic article to be thermally conditioned through the first thermal conditioning step, removing at least one plastic article from the first step of heating for a predetermined period of time to redistribute the temperature within said plastic article with a predetermined degree of uniformity, and refilling at least one plastic article to the first thermal conditioning step.

In this way, plants can easily be built for the thermal conditioning of plastic articles in the form of heating stations - for example, infrared ovens - and cooling stations - for example, chambers with forced ventilation - starting with a structure Mechanical with many parts in common or at least similar - for example the guides and the conveyor chain, and the housing of the furnace or the heating unit - allowing the manufacturing of the plant to reach significant levels of standardization of the components in the two modes of operation (heating and cooling).

This invention makes it possible for the dimensions of the heating and cooling units of pre-forms to be compact, particularly in terms of the dimensions of the cooling / heating units and of the surface used in the plant. In addition, this invention can be used to make heating units with minimized length ovens and cooling units with ventilation tunnels of minimized length.

In addition, the thermal conditioning units-both for cooling and for heating the preforms-according to this invention, are units that operate not in the sequential mode, but in the continuous mode. Consequently, they are particularly suitable for use in continuous blow molding plants that operate at high production rates - where "particularly suitable" means that they do not form the bottleneck of the production line; therefore, it is possible to make an entire production line of continuous preforms that operates at high production rates without sacrificing efficiency.

These and other advantages of the invention will become apparent later, for experts in the field, in the detailed description of two particular versions of the invention, given as non-limiting examples and in conjunction with the drawings listed below.

BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 and 2 show a schematic perspective view of the path of the conveyor chain used to feed the preforms to the furnace to heat the preforms according to a first embodiment of this invention; Figure 3 shows a schematic sectional view of the furnace shown in Figure 1 according to a plane view perpendicular to the conveyor chain; Figure 4 shows a schematic perspective view of a detail of the two lengths of the chain inside the heating tunnel of the furnace of Figure 1; Figure 5 shows a schematic diagram of the thermal cycle to which the preforms are fastened in the furnace of Figure 1; Figure 6 shows a schematic perspective view of the path of the transportation chain used to feed the preforms to a plant for cooling the preforms according to a second embodiment of the invention; Figure 7 shows a schematic cross section of the cooling plant of Figure 6 according to a plane view perpendicular to the transportation chain; Figure 8 shows a schematic perspective view of a detail of the chain of plants in Figures 1 and 6; Figure 9 shows a schematic enlarged view of the chain of Figure 8.

DETAILED DESCRIPTION OF THE INVENTION Figures 1 to 5 show a first embodiment of a plant, according to this invention, for heating plastic articles, especially preforms made of PET for the blow molding of bottles or flasks.

This plant for heating preforms comprises a heating oven operated by infrared rays 1 and a conveyor chain 2 suitable for feeding many preforms P through said oven.

Furnace 1 may be similar, for example, to one described in Patent Application WO01 / 49075 requested by the Applicant, with appropriate adaptations resulting from this description. Said furnace, see Figure 3, comprises the following: Many infrared lamps L arranged preferably but not necessarily on both sides of the two coils -as will be explained in more detail below- of the transportation chain 2 (not shown in the Figure) 3), along one or more tunnels or ducts 6, 7 machined from the inside and through the oven in which the preforms are heated that can be fed to the transportation chain that transports them; One or more R reflection screens, positioned behind the lamps L, to reflect the infrared rays radiated by the lamps; said screens, as explained in more detail in the Patent Application WOO 1/49075, are preferably perforated to allow the blown air to flow through a V fan; A metal housing M which stores the lamps L, the reflection apparatus R, and the conduits through which the hot air flows.

Preforms or other items to be heated - are heated in the oven 1 mainly by radiation and convection.

The infrared lamps L are positioned around the conveyor chain 2, along its path, arranged in one or, in general, more vertical columns SI, S2, ... SN - See figure 4- positioned one next to the another and capable of heating a variable length of the chain 2 depending on different factors, including the feeding rate of the chain 2 and the temperature at which the preforms P must leave the oven.

The part of the furnace that heats a section of the chain 2 included between an inlet 4 and an outlet 5 of the furnace should be called a "heating step" from now on.

Figures 1-3 show a particular version of the heating plant according to this invention. In this version, the conveyor chain 2 forms a closed circuit and two spirals SP1, SP2. To be heated, a preform P enters the first heating step 8 -see Figure 3- and is transported along a first straight section RL1 of the first spiral SP1 -see figures 1, 2- and leaves the first heating step . Then, the preform continues - for example, but not necessarily - along another straight section and, later, along a first curve Cl exposed to the outside environment, which is located outside the oven and far from the infrared lamps. other heating elements. Then, the preforms reentran the homo 1 going through a second heating step 9 -see Figure 3- and passing along the second straight section RL2 of the spiral SP1.

Once again, the preform P leaves the furnace 1, travels through a second curved section C2, which is also exposed to the external environment outside the furnace and away from the infrared lamps or any other heating element, and reentra the homo for a third time to pass along the straight section RL3 of the second spiral SP2, which may be but not necessarily more internal than the first spiral SP1.

Then, the preform P passes through the first heating step 8 again, leaves the oven 1 for a third time, returns to the oven again after traveling along the curved section C3 of the second spiral SP2, travels to the along a fourth straight section RL4 through the second heating step 9, and leaves the homo 1 again. Then, the preform travels along the curved section C4 where the chain drops down from the level of the spirals SP1, SP2 to a lower level - in fact, in this particular example the spirals SP1, SP2 and the sections curve C1-C3 are essentially in a horizontal plane - and arrives at the loading / unloading station 3. Here, the preform P is loaded by the chain 2 to be heated, and the preform when heated to the appropriate temperature, is removed from the chain to proceed to the next step of blow molding, wherein the preform P is formed in its final shape bottle, boat or container of any other type.

The transport chain 2, after passing the loading / unloading station 3, travels along the curved section C5 -which can, like the fourth curved section C4, be positioned in a vertical plane- rising to the level more high to start moving another preform P along the first straight section RL1 of the outermost spiral SP1.

In order to travel along the curved sections C4, C5 that go down and up in relation to the level of the two spirals SP1, SP2 forming a path that is essentially non-flat, the chain 2 is articulated in such a way that it is capable of bending in the three spatial dimensions: an example of a string with such characteristics is described in Patent Application WO 99/62693 filed by the Applicant and is shown schematically in Figures 8 and 9.

Figures 1-4 show a heating plant in which each preform P is subjected to a thermal cycle shown schematically in Figure 5, the abscissa showing the time or the position of the preform along the path of the transportation chain 2, while the ordinates indicate the thermal power received or dispensed by the preform in the unit of time: in the straight sections RL1-RL4 inside the oven -during the fourth heating steps- the preform P receives thermal energy and is heated; unlike, in the curved sections C1-C4, the preform is outside the oven and is exposed to the outside environment or in any way to an area where more thermal energy is dispensed than received and, therefore, it is slightly cooled -this it is true particularly in the outer layers of the preform, which are hotter, that spread the heat by conduction to the innermost layers of the preform, obtaining a more uniform distribution of thermal energy over the entire width.

Therefore, the spiral path taken by the transportation chain 2 makes it possible to heat the preforms by alternating periods of heating in the oven with periods of "recovery" outside the oven, preventing the outer layers of the preform from overheating. : in fact, in infrared heating furnaces, the heat generated in the furnace usually heats by radiation starting from the outer layers of the preform that are in front of the heating elements of the homo-infrared lamps or reflectors; the preform is also heated by convection of any hot air flow generated in the oven; then, the heat propagates by convection to the innermost layers of the preform. Therefore, in such furnaces it is generally necessary to take appropriate measures to avoid excessive overheating of the outer layers of the preforms - in fact, overheating could cause unwanted burns or crystallization of parts of the material of the preform.

Said measures according to this version of the invention consist in alternating the periods of heating in the oven with periods of "recovery" outside the oven, in which the heat received by the surface of the preforms has time to be distributed by conduction to the layers. more internal of the preform, allowing the temperature of the entire preform to be more uniform.

The spiral path of the transportation chain allows a relatively high number of "recoveries" and periods of thermal homogenization to be easily carried out without increasing the dimensions of the plant and, in particular, the length of the ovens.

The numerical data of the example described below clarify this.

Figures 1-4 show a version of this invention where, advantageously, the different heating steps 8, 9 through which the chain 2 passes are contained in a single homo 1: this, compared to the case in which the different Heating steps are found in more than one oven, minimizes the thermal losses of the ovens and maximizes the thermal efficiency of the plant and the process.

Advantageously, multiple coils - two or more - are contained simultaneously and side by side in each heating step. Figure 3 shows a version in which two straight sections of the two spirals SP1 and SP2 are contained side by side in each of the two heating steps. In this preferred version of the invention, in addition, the infrared lamps R in each heating step are positioned on both sides of the two coils SP1, SP2 to make the heating of the preforms more uniform and prevent PET from crystallizing in a side of said preform despite increasing -compared with a homo where the lamps are positioned on only one side of the two spirals SP1, SP2- the power of the radiation per linear meter of the chain and reducing the thermal dispersions of the plant .

Advantageously, the straight sections RL1, RL3 and RL2, RL4 of the two spirals SP1, SP2 are positioned side by side in the different conduits or tunnels 6, 7 of the homo so that the preforms P seized thereon are arranged in essentially quincuncial fashion: This distribution allows the lamps on each side of the heating duct of the oven to radiate the preforms in both spirals that pass in front of them, preventing the different preforms from shading each other and optimizing the energy efficiency of the oven.

EXAMPLE To better illustrate this invention, an example is given with numerical data related to a plant made by the Applicant according to the version of the invention shown in Figures 1-4.

In this plant, the transportation chain 2 forms a path with the two SP! SP2 spirals entering and exiting - as in Figure 2- of a single homo 1 on four occasions. The fact that both spirals SP1, SP2 of the chain pass in two conduits 8, 9 reduces the thermal losses of the furnace, making its thermal efficiency -defined as the ratio between the thermal energy required to make a certain number of preforms pass through of an initial temperature at the desired temperature, and the energy produced by the homogen increases to approximately 21-28%, against 15-20% of the types of known infrared ovens, for example a homo of the type described in WO0149075 with a only conduit. The greater efficiency of the oven made it possible to realize the total path of the preforms in front of the IR lamps approximately 0.7 times longer than the corresponding paths in a homo of the type described in WO0149075, obtaining a shorter homo 1.

In addition, the fact that both spirals SP1, SP2 are positioned side by side in each duct, made it possible to cut the total length of the ducts in front of the infrared lamps in half: therefore, the total length of the duct in front of the infrared lamps is indicatively reduced by 0.7 X 0.5 = 0.35, in other words, it is reduced by approximately two thirds of one of a furnace of the type described in WO01 / 49075.

The above data are purely indicative: in some cases, in order not to risk a too strong exposure of the preforms to infrared rays -with the consequent crystallization of their external surfaces- it may be necessary to reduce the thermal power per unit length and, for therefore, to maintain the total power required, slightly increase the length of the oven. However, indicatively, it can be estimated that with this invention it is possible to at least halve the dimensions in length L? Ot, Figure 1, of the station for heating preforms compared to the length of an oven constructed in accordance with WO01 / 49075 instructions.

Figures 6, 7 show a second version of the plant for cooling plastic articles, especially preforms, according to this invention.

This plant comprises a ventilation cooling unit 1 'and a 2' transport chain of preforms. The ventilation cooling unit-hereinafter referred to as "ventilation unit 1" is comprised of the following: A metal housing that defines entrances for air and conduits through which fresh air is sucked by the fans 10-gripped to the housing - which is transported to the preforms P that require cooling (see Figure 7); One or more conduits 8 ', 9' through which the preform transportation chain can pass.

According to the preferred version of the invention, many cooling lugs, for example of the type described in the aforementioned Patent Application WO02 / 074518 presented by the Applicant, are attached to the chain: each lug comprises a box for the preform that is it's going to cool; The box is made to cool the preform by thermal conduction and hold it adequately to prevent or minimize thermal deformation during cooling. Each handle can be equipped with cooling fins that accelerate the exchange of heat between the handle and the flow of air produced by the ventilation unit 1 '.

In the cooling plant shown in Figures 6 and 7, the preform transportation chain 2 'follows a path with three spirals SP1, SP2, SP3 which causes each preform P to enter and exit the homo 1' six times.

The chain 2 'descends to a lower level following the curve section C6 - which is, for example, in a vertical or oblique plane - and travels along the loading / unloading area 3 'analogous to the discharge area 3 of the plant shown in Figure 2. Then, it goes up to the highest level of the three spirals SP1, SP2, SP3 following the curve section C7 ascending.

Inside the homo 2 ', the three spirals SP1, SP2, SP3 are positioned side by side in the two conduits 8', 9 ': when positioning more than one spiral of the chain 2' side by side in the same conduit, it is possible to optimize the operation, efficiency, and adjustment of fan 10 or fans 10.

In the case of a unit exclusively for cooling, the version shown in Figures 6 and 7 forms a more continuous than sequential unit, and can be connected to a continuous molding station more efficiently, in terms of the production range of the line, than a sequential cooling station. The road of the transport chain, with concentric spirals that are positioned side by side in the same cooling tunnel, makes possible the realization of the station with relatively confined dimensions and the occupation of a compact surface in the manufacturing plant.

The versions described above may be subject to various modifications and variants as long as they remain within the scope of this invention: generally, according to this invention, the chain 2, 2 'or other continuous transportation device follows a suitable path to feed at least one plastic article for heating and / or cooling - and not just a preform P - to a first conditioning step by heating or cooling where the preform exchanges more heat than would result when positioning said item (s) of plastic in an environment outside said first step of thermal conditioning during the same period of time; removing said plastic article (s) from said first thermal conditioning step for a predetermined period of time to redistribute the temperature within the plastic article with a predetermined degree of uniformity; and feeding said plastic article (s) to a second thermal conditioning step. The transport chain can follow a closed path without spirals but, for example, in the form of a simple ring and can pass through the same heating step many times or can pass through many heating steps in different furnaces than in the same oven 1. The heating ovens do not have to be infrared: they can be ovens with electrical resistors or other types of heating elements. The number of spiral sections that are positioned side by side in the same conduit 6, 7, 6 ', 7', can vary from 1 to a generic number "N" of spiral sections. The cooling unit 1 does not have to be of the convection cooling type with forced air: it can be of another type-for example, a cooling unit for spray or immersion.

This invention also applies to any version, changes and variants that fall within the scope of the claims and are of a similar type.

Claims (11)

1. A device for packaging plastic articles (P) molded for use in blow molding plants comprising: a chain or other transportation device (2) forming a spiral type path suitable for transporting at least one plastic article (P); at least a first thermal conditioning step comprising at least one conduit (6, 7) through which said at least one plastic article (P) can be transported and which is traversed by a length of said chain or other transportation device continuous (2) and is suitable for thermally conditioning by heating and / or cooling said at least one plastic article (P); wherein said chain or other continuous transportation device follows a path comprising the sections of feeding said at least one plastic article (P) through said first thermal conditioning step, removing said plastic article from said first heating step during a predetermined period of time to achieve redistributing the temperature inside said at least one plastic article (P) with a predetermined degree of uniformity, and feeding said at least one plastic article to said first thermal conditioning step, characterized in that said conduit (6, 7) is traversed in at least two sections of said chain or other continuous transportation device (2), positioned side by side, in which said sections each belong to a different spiral (SP1, SP2, SP3) ) of said chain or other continuous transportation device (2).

2. A plant according to claim 1, comprising at least one heating element suitable for heating by radiation many of said plastic articles arranged in at least two sections of said chain, each belonging to a different spiral (SP1, SP2, SP3) of said chain, when said plastic articles (P) pass side by side in at least one of said conduits (6, 7).

3. A plant according to claim 2 wherein said chain or other continuous transportation device (2) is suitable for feeding said at least one plastic article (P) to be thermally conditioned in said first and second thermal conditioning steps.

4. A plant according to one or more of claims 3 to 5 wherein said spirals (SP1, SP2, SP3) are positioned substantially one inside the other.

5. A plant according to one or more of claims 3 to 6 wherein said chain or other continuous transportation device (2) forms a closed path.

6. A plant according to claim 7 in which said chain or other continuous transport device (2) is articulated so as to be able to bend in the three spatial dimensions and essentially form a non-planar canine.

7. A plant according to one or more of claims 1 to 6 wherein said at least one plastic article represents a plurality of preforms (P) and said at least two chain sections are equipped with a plurality of suitable fixings for housing a plurality of preforms (P), and said at least two sections pass side by side through said at least one conduit wherein the preforms are arranged essentially quintally.

8. A plant according to one or more of claims 1 to 7, wherein said first and second thermal conditioning steps occur in one or more heating furnaces (1) suitable for heating said preforms (P).

9. A plant according to one or more of claims 1 to 8 wherein said first and second thermal conditioning step occur in cooling stations suitable for cooling said at least one plastic article more than would result in putting said at least one article of plastic in the environment outside said first and second conditioning steps.

10. A plant according to one or more of claims 1 to 9, comprising a heating oven in which said first and second heating steps occur in said oven.

11. A plant according to one or more of claims 1 to 10, comprising a cooling unit and wherein said first and second thermal conditioning steps occur in said cooling unit.