WO1999004949A1

WO1999004949A1 - Method and die for extruding tubes of thermoplastic material, and the tubes obtained thereby

Info

Publication number: WO1999004949A1
Application number: PCT/EP1998/003522
Authority: WO
Inventors: Valdo Romanatti; Adriano Caiola
Original assignee: BDR Srl
Current assignee: BDR Srl
Priority date: 1997-07-22
Filing date: 1998-06-10
Publication date: 1999-02-04
Anticipated expiration: 2000-01-22
Also published as: AU8110798A; IT1293748B1; ITMI971728A1

Abstract

Method and die for extruding tubes of thermoplastic material, in particular PET, whereby the outer surface of a PET tube while being extruded and still in the plastic state is suddenly and rapidly cooled in correspondance with a narrow terminal region of the die such as to cause hardening of an external surface layer of the tube, the die surface in correspondence with said limited terminal region widening outwards.

Description

METHOD AND DIE FOR EXTRUDING TUBES OF THERMOPLASTIC MATERIAL, AND THE TUBES OBTAINED THEREBY

This invention relates both to a method and die for extruding tubes of pure or modified polyethyeleneterephthalate (PET), and also to the tubes obtained thereby.

It is known to produce tubes easily and economically by extrusion from various thermoplastic materials, such as polyethylene and polypropylene, such tubes possibly comprising internal ribs or fins, or continuous walls extending between opposing surfaces of the tubes to define therein a plurality of individual cavities separated one from another.

The thermoplastic material used to form a tube is melted in an extruder comprising one or more screws which urge this material to pass through profiled apertures provided in an extrusion die. As it passes through the die, the plastic mass is substantially uniformly cooled to a temperature at which the mass has a plasticity enabling it to be easily expelled through the discharge apertures in the die, while simultaneously assuming a consistency sufficient to prevent the just extruded tube collapsing or seriously deforming immediately on its exit from the die, so enabling the tube to be rapidly cooled, to solidify it.

The temperature range within which the thermoplastic material has sufficient plasticity to enable it to be easily extruded through the die, while at the same time possessing sufficient consistency to enable the just extruded tube, still in the pasty state, not to deform and not to collapse on leaving the die, is a fundamental fact of the known art.

Extrusion methods and apparatus can be very simple, but become relatively complex if it is sought to extrude thermoplastic materials in which the aforesaid temperature range is small. For example, US patent 4,216,253 describes a method and an apparatus involving rigorous and difficult temperature control of the entire mass of thermoplastic material present upstream of the extrusion plate, such as to maintain this temperature as close as possible to the solidification temperature of the plastic material as it becomes further cooled in its passage through the apertures of the extrusion plate, which have a carefully gauged and uniform width along their entire length. The extrusion plate is of considerable length to make the temperature of the entire material mass passing through the plate discharge apertures as uniform as possible. Moreover, to prevent temperature drop, the upstream surface of the extrusion plate is totally in contact with the plastic material within the extruder and hence has substantially the same temperature thereas. By means of these expedients, the entire plastic mass urged through the extrusion plate apertures has a uniform temperature which slowly decreases as this mass approaches the exit from the gauged apertures of the plate.

Such a process is effective for extruding most thermoplastic materials, but is difficult to manage or indeed is ineffective for extruding tubes of thermoplastic materials which are of very low consistency in the plastic state and which pass very rapidly from the plastic state to the crystalline state. Among the very many thermoplastic materials which according to US patent 4,216,253 can be extruded to form tubes, PET is also mentioned. However in practice it is not possible to extrude this material. In this respect, if PET is cooled within the extrusion plate to its setting temperature, as (according to this US patent) the entire material mass passing through the plate is of substantially uniform temperature, the apertures in the extrusion plate would be irreparably clogged by the tube solidified within them, and extrusion would not be able to proceed. In contrast, if the PET mass were to leave the extrusion plate still in the plastic state, the tube would collapse onto the outer surface of the plate. According to the teachings of US patent 4,216,253, it .vould be possible to produce PET tubes only if their cavity had no internal walls and if a cooling liquid, generally air, were circulated through such empty tubes.

US patent 4,597,932 and JP patent 51014973 (database WPI XP002048596) describe PET tubes with internal ribs. According to that described in US patent 4,597,932, these tubes arc extruded through a vertical extrusion head, such that the material forming the tubes leaves the die still in the plastic state and descends vertically by gravity, to be then cooled very energetically to harden and crystallize the PET. It is evident that this method can produce only very light tubes (because otherwise the material in the plastic state would separate from the extrusion plate by the effect of traction consequent on the weight of the tube) of small dimensions, as demonstrated by the fact that after their extrusion and solidification, the tubes have to be subjected to stretching (which would be absolutely impossible with PET tubes of relatively large dimensions and if continuous walls were present within the tube cavities) and to heat treatment.

French patent FR 2 128 146 describes plastic tubes provided with continuous longitudinal walls within their cavity, and cites many thermoplastic materials, including also polyesters. In fact, polyesters are mentioned only as part of a very long list of thermoplastic materials, but the patent describes no method which makes it possible to produce PET tubes of the stated type.

PET is a thermoplastic material possessing high resistance to cold and to defor abil ity at high temperature, however its workability is quite different from all the other thermoplastic materials usually extruded in the form of tubes. In this respect, if the

PET is given a temperature providing it with sufficient plasticity and flowability to enable it to be extruded in the form of a tube, this tube would not have sufficient mechanical strength to prevent it from collapsing onto the outer surface of the die (unless the tube is of very small dimensions and the extrusion head is vertical, or unless the tube is without internal ribs or walls and it is maintained "inflated" by feeding compressed air into it). In contrast, if the PET mass passing through the die is uniformly cooled within the extrusion plate to give the extruded tube the necessary consistency and mechanical strength, it becomes very difficult to expel this tube through the die apertures, and it becomes indeed impossible to perform the extrusion if the tube has continuous walls extending within its cavity.

The main object of this invention is to provide a method and die which enable in particular PET (whether virgin or salvaged, including the amorphous type known as APET, and whether pure or modified, ie homopolymer or copoly er) to be easily extruded into tubes, possibly with ribs or the like projecting into their cavity or with continuous walls which divide the cavity of each tube into a plurality of smaller separate cavities.

The invention also relates to the tubes obtained thereby,

The method for extruding tubes of thermoplastic material through the apertures in the extrusion plate of an extruder consists of maintaining said material in the plastic state immediately upstream of said plate, and is characterised by rapidly and suddenly cooling the surface of the material under extrusion while it passes through the extrusion plate, to a temperature such that the tube leaving the extrusion plate apertures is formed of material in the plastic state, the outer layer of which has been cooled to a temperature which is sufficiently low as to give mechanical consistency to the layer itself, the thus extruded tube being then cooled in known manner.

The die according to the invention comprises an extrusion plate which is characterised by being in direct contact with the remaining part of the die within a very limited region of its surface around the extrusion apertures, the extrusion apertures provided in said plate being at least partly bounded by surfaces which diverge towards the outer surface of the plate.

The extrusion plate is preferably of bronze construction.

To clarify the understanding of the structure of a die usable in an apparatus for extruding tubes with internal walls according to the invention, a description of a preferred embodiment thereof is given hereinafter by way of non-limiting example with reference to the accompanying drawings, on which:

Figure 1 is a schematic axial longitudinal section through a die, taken on a vertical plane; Figure 2 is a cross-section through the die on the line 2-2 of

Figure 1 ;

Figure 3 is a front view on an enlarged scale of the die viewed from the left in Figure 1; and

Figure 4 is a perspective view of a portion of PET tube obtained by the die of the preceding figures.

Figure 1 shows a schematic longitudinal section through a die comprising an element 1, 2 for connection to an extruder (of traditional structure, for example of single-screw type, not shown on the drawing), a first plate 3, a second plate 4 and a third plate 5.

From the first plate 3 there project three separate elongate (or male) appendices 6 which decrease in cross-section from the plate 3 to the vicinity of the free end of each appendix where its cross-section remains constant for a short length. Each appendix is traversed by a cavity 7 which also extends through the entire thickness of the plate 3 and has one of its ends open at the free end of the appendix itself and its other end (which is in free communication with atmosphere) opening at that surface of the plate 3 distant from the surface from which the appendices 6 project. The plate 4 (which acts as the die for regulating the thickness of the extruded tube) is traversed by an aperture (of substantially rectangular cross-section, the area of which gradually decreases in the direction away from the plate 3, with respect to Figure 1) through which there extend - to project beyond the plate 4 - the appendices 6, the outer surfaces of which define with the opposing surface of said aperture in the plate 4 an empty annular space clearly visible in Figure 1, and of which the thickness substantially corresponds to that of the tube to be extruded by the die. This annular space opens at one end into a chamber 8 provided in that surface of the plate 4 facing the plate 3, the chamber 8 communicating, via two feed channels 9 provided in the plate 3, via two channels 10 provided in the element 2 and a channel 11 provided in the element 1, with the discharge mouth of an extruder, not shown on the drawing. The connection elements 1, 2 and the plates 3, 4 are kept pressed securely one against the other (to form a very rigid single body) by tie bolts or screws, some of which are shown by dashed lines in Figure 1 (these not having been numbered for simplicity) and which pass through holes, some of which are visible in Figures 2 and 3 in which they are indicated by the reference numeral 12.

As in the case of dies of known type, the connector 1, 2 and plates 3, 4 are provided internally with temperature-controlled electrical resistance elements, for simplicity also not shown on the drawings as their structure, position and function are obvious to the expert of the art.

From Figure 1 it can be seen that the end plate 5 of the die rests against the plate 4 (to which it is fixed by a plurality of screws) within a region of very limited area all about the plate extrusion apertures within which there extend the free ends of the appendices 6. The apertures which pass through the plate 5 are superposed on the adjacent end of the apertures provided in the plate 4. The very limited contact region is a very important characteristic of the die described herein because in this manner the heat of the plate 4 is transmitted by conduction to the plate 5 only in correspondence with a limited region, so allowing simple control of the temperature of said plate 5 with minimum influence on it by the temperature of the plate 4. This means that the temperature of the plate 5 can be easily maintained at a value lower than that of the plate 4, even though the plate 5 is of very small thickness (this characteristic is also important). The plate 5, which is advantageously of bronze construction, also internally houses temperature-controlled electrical resistance elements housed in appropriate cavities provided in the plate (two of these are shown schematically in Figures 1 and 3, and are indicated by the reference numeral 14).

An essential characteristic of the die according to the invention claimed herein is that the cross-section of the aperture provided in the plate 5 and into which the free ends of the appendices 6 extend widens from its region closest to the plate 4 towards the free end of the said aperture, ie towards the outside, to hence define with the opposing surface of the appendices 6 an annular space 15 which branches or widens outwards, as can be seen in Figure 1, and also in Figure 3 in which the plate is shown frontally from the outside, enabling it to be seen that the surface of the aperture provided in the plate 5 is bounded by a double continuous line 15A. With the die shown schematically on the drawings it is possible to easily extrude a continuous PET tube even having continuous internal walls such as that shov/n in Figure 4, in which two continuous longitudinal walls 16 are provided extending between two mutually parallel opposing walls 17, on the outer surface of which there are provided continuous longitudinal ridges 18 obtained by virtue of the provision of a plurality of longitudinal grooves in that surface which bounds the aperture provided in the plate 5, as can be seen in Figure 3.

As already stated, the described die is used by connecting the free end of the part 1 of its connection element 1 , 2 to the discharge mouth of an extruder (for example of single-screw type) for example fed with pure amorphous PET or, very advantageously for many uses, with PET scrap or recycled or modified PET. The PET (a material which softens between +180°C and +215^°C and which melts at +238°C) is expelled from the extruder in the molten state and fed into the die channel 11 at a temperature of about 250^°C (± 8-9°C). By means of the temperature-controlled electrical resistance elements incorporated in the die, the temperature in the plates 2, 3 and 4 is rapidly lowered so that, at its exit from the chamber defined within the plate 4, the PET mass has a temperature of between about 205°C and +185°C, ie lower than the melting point of PET but higher than the temperature at which it begins to crystallize.

The PET mass which passes into the annular space provided in the plate 5 is suddenly and rapidly cooled thereat to reach a temperature very close to that at which it begins to soften, ie to about 195°C (± 9-10°C), at which the extruded PET mass begins to harden in contact with the cold surface of the plate 5, to form a sort of thin solid shell within which the mass is still in a sufficiently pasty state to be able to be pushed out of the die. Even though the outer surface of the extruded tube has already substantially hardened, the extrusion can continue without difficulty because this tube is pushed through an annular space which widens outwards and hence exerts much lesser friction on the already hardened outer surface of the extruded tube.

In this respect it is important to note that if the PET mass within the plate 5 were not to form a sufficiently hardened shell, the tube expelled from this plate would collapse immediately, to rest on the free surface of the plate. Moreover if the aperture provided in the plate 5 did not widen outwards, the extruded tubular body would form a plug, so blocking extrusion.

The temperature of the PET mass at the die exit must hence be sufficiently high to enable extrusion to continue (ie mass solidification of the extruded tubular body must not occur) while at the same time the temperature of the plate 5 must be sufficiently low to cause the shaped body (tube) still within the cavity in the die plate 5 to assume a sufficient surface consistency to enable the extruded tube not to collapse onto the surface of the aperture provided in the plate 5. The hardened thin outer surface layer of the extruded tube acts as a support for the ribs or continuous walls present within the tube cavity and still in the plastic state. The subsequent rapid cooling of the internal mass of the tube after it has been extruded is also facilitated by the air which penetrates into the tube, it being drawn through the cavities 7 in the appendices 6, hence rapidly causing solidification of the inner parts of the tube immediately after its extrusion.

Hence, to obtain these results the mass of thermoplastic material, in particular of PET, in the sufficiently pasty state leaving the plate 4, must be very quickly cooled in contact with the plate 5, but without blocking the die.

This is achievable if the points of direct contact between the plate 5 and the plate 4 are reduced to a minimum (ie if most of the surface of the plate 5 is not in contact with the plate 4) and if the annular space 15 defined between the aperture provided in the plate 5 and the surfaces of the ends of the appendices 6 widens outwards, in order to reduce to a minimum the friction between the extruded thermoplastic material mass which has assumed consistency and mechanical strength, and the surface of the plate aperture, so preventing the die from clogging while ensuring sufficient support or bearing for the tube undergoing extrusion.

With the described die, tubes of PET homopolymer or copolymer (for example amorphous PET or recycled PET, ie originating from salvaged mineral water or drink bottles, sheets and the like) of any form can be obtained, with any number of internal walls or fins (or even without any wall or fin) and with their outer surfaces smooth or with projections.

The extruded tubes can be subjected to the usual gauging operations commonly effected on extruded tubes of plastic material . The tubes obtained according to the invention have high mechanical strength both at very low temperature and at relatively high temperature, in addition to very good resistance to ageing.

The PET tubes can hence be used for conveying fluids or for forming any strong structure requiring the use of tubes, for example for forming window frames. A particularly interesting application is to use pieces of PET tube to form structures for supporting and transporting goods including heavy goods, such as pallets formed by joining such tube pieces together by PET nails or screws or by welding, and possibly interposing plugs of PET or other materials between the tube pieces joined together.

In the aforegoing embodiment described with reference to the drawings, the die is of single impression type, ie enabling a single tube to be extruded. It is however apparent that the die can be shaped with more than one impression, to allow simultaneous extrusion of two or more separate tubes simultaneously.

The invention hence relates both to the extrusion method and die, and also to the extruded tubes obtained thereby.

Claims

Cl ai ms :

1. A method for extruding tubes of thermoplastic material through the apertures in the extrusion plate of an extruder, said material being maintained in the plastic state immediately upstream of said plate, characterised by rapidly and suddenly cooling the surface of the material under extrusion while it passes through the extrusion plate, to a temperature such that the tube leaving the extrusion plate apertures is formed of material in the plastic state, the outer layer of which has been cooled to a temperature which is sufficiently low as to give mechanical consistency to the layer itself, the thus extruded tube being then cooled in known manner.

2. A die for extruding tubes of thermoplastic material comprising an extrusion plate, characterised by being in direct contact with the remaining part of the die within a very limited region of its surface around the extrusion apertures, the extrusion apertures provided in said plate being at least partly bounded by surfaces which diverge towards the outer surface of the plate.

3. A die as claimed in claim 2, characterised in that said extrusion plate is of bronze construction.

4. Thermoplastic tubes obtained by extrusion in accordance with the method of claim 1, or by using the die in accordance with claims 2 and 3.

5. Tubes as claimed in claim 4, characterised by being formed from PET.

6. Tubes as claimed in claim 5, characterised by being formed from PET and by comprising ribs or the like or continuous walls which extend longitudinally within their cavity.