Device for continuously producing seamless plastic tubes

Details Device for continuously producing seamless plastic tubes Device for continuously producing seamless plastic tubes

2001-01-23

2003-09-09

Mackey, James P. (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Means feeding fluent stock from plural sources to common...

Extrusion shaping means

C425S190000, C425S326100, C425S336000, C425S380000, C425S461000, C425S462000

Reexamination Certificate

active

06616437

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention concerns a device for continuously producing seamless plastic tubes, in which at least one molten plastic extrusion issuing from an extruder is split up by means of a distributor plate which is arranged within the cross-section of a mold cavity of the device and which has plastic material distribution passages with flow paths of equal length for the molten plastic material, and fed to an annular gap from which the tubular plastic material flow formed in that way passes into the mold cavity formed for example by travelling corrugator mold jaws, wherein the plastic material distribution passages extend in a plane approximately perpendicular to the axis of the mold cavity and each terminate at an equal angular spacing from each other at a peripheral surface of the distributor plate.
In the production of plastic tubes, the molten material issuing from the extruder must be fed to an annular gap from which it then passes into the actual mold cavity. In the production of corrugated tubes, whether of a single-layer or multi-layer nature, the molten material may only issue at one location from the annular gap at which the corrugator mold jaws forming the mold cavity again bear firmly against each other, that is to say, the mold cavity is again completely closed. Now, the greater the diameter of the tube to be produced, the correspondingly greater become the entry radii of the mold jaws, which means at the same time that the nozzles of the injection heads, which form the annular gap or a plurality of annular gaps for the molten material, become correspondingly longer and longer. Nozzles of such a length are both very expensive to produce and also difficult to handle, while in particular adjustment and setting of the tube wall thickness gives rise to difficulties since, as will be appreciated, the nozzles may not bear against the moving mold jaws. On the other hand however the spacing between the annular gap from which the molten material issues and the inside wall of the travelling mold jaws may not be excessive, in order not to give rise to molding problems. Those difficulties occur very severely in particular when not just single-wall corrugated tubes but corrugated tubes consisting of two or more layers are to be produced, preferably with different materials, which means that two successive annular gaps have to be fed with material from different extruders, for which purpose the molten plastic material must then be guided into the mold cavity in separate annular passages arranged concentrically one within the other, in a forward direction to the point of injection of the individual molten material layers. If the assembly has such long and very narrow flow paths, enormously high back pressures necessarily build up, which in turn gives rise to certain problems. For example, DE 24 03 618 A1, DE 28 03 808 C3, DE 29 11 833 C2, EP 0 208 055 A1, EP 0 230 055 A2 and U.S. Pat. No 3,677,676 disclose embodiments which are used in a practical context, for corrugator injection heads.
In the known corrugator injection heads, the material is divided up to form a tube by means of a torpedo, a central divider spindle or a spiral distributor. If for example a composite tube is produced with just one extruder, that is to say the inner and the outer skins are made from the same plastic material, then in most cases, after the first step of dividing up the material to form a tube, a second division operation is effected by means of a cutting ring disposed at a downstream location. Transportation of the plastic material into the corrugator or mold cavity has hitherto always been effected in long annular passages arranged concentrically relative to each other. In that case, at least the first material division operation to form a tube is always effected in the proximity of the extruder, in which respect the structure involved means that the outside diameters produced are far above the inside diameter of the tube which is to be subsequently produced. That naturally means that a corresponding amount of space must be provided for the corrugator injection head. When the material is divided up to form a tube by means of a so-called ‘torpedo’, it is observed moreover when dealing with given plastic materials that the radial arms which serve to fix the torpedo in position in the molten material passage produce marks in the plastic tube, and such marks possibly even result in corresponding weakening and thus reduced quality of the finished tube. To eliminate marks of that kind or to achieve an adequate level of quality, it is however at least necessary to operate with a considerable back pressure in the flow passage, and that overall considerably increases the level of complication and expenditure.
A further problem in the production of a composite tube but under some circumstances also in the production of single-wall plastic tubes, is that calibration or cooling bars or mandrels have to be fitted subsequently to the annular nozzles within the mold cavity in order to smooth and cool the tube from the interior. Those components mean that the length of the injection head is again considerably increased, in which respect the particular problem is that the injection head can no longer be supported or braced over its entire length between the entry into the mold cavity and the end of the cooling or calibration mandrel, whereby major stability and vibration problems occur, which makes itself extremely disturbingly perceptible in terms of production and the end product.
A device of the general kind set forth, having an annular gap tool for the extrusion of thermoplastic material, is known from DE 27 52 932 A1. There, the molten plastic material is fed by way of an annular passage to a spiral pre-distributor. Disposed between the spiral pre-distributor and an annular gap is a distributor plate having a plurality of substantially radially extending plastic material distribution passages which are distributed uniformly over the periphery of the distributor plate. That design configuration of the tool admittedly makes it possible to reduce the axial structural length involved. A disadvantage with the known tool however is that the feed of molten plastic material to the distributor plate must be by way of an annular gap, with the consequence that the above-discussed structural problems of the known devices still occur. In particular, provision must be suitably made for fixing the parts which define the respective annular gaps, and this can be effected for example by way of suitable flow dividers (so-called torpedoes).
DE 42 18 095 discloses a head for extruding preforms of thermoplastic material, of a hose or tube form, wherein two distributor plates are arranged virtually axially in succession. In that arrangement, in the first distributor plate in the direction of flow, the molten plastic material which is fed to the mold radially from the exterior is fed to axial feed openings in the second distributor plate, by way of passages formed by a branching configuration. Then, those axial feed passages are adjoined in the second distributor plate, which is disposed at a downstream location in the flow direction, by passage portions which are substantially in the form of parts of a circle and which feed the molten material from the exterior to the exit gap which is nearer to the center of the mold tool. A disadvantage of this known arrangement is that the distributor plates are of a comparatively large diameter. In addition, as it passes through the various distributor passages, the molten plastic material experiences many very sharp changes in direction, which can result in deposits of the plastic material in the flow path and can thus result in the quality of the finished produced being adversely affected.
Therefore, the object of the present invention is to propose a device of the general kind set forth, for continuously producing seamless plastic tubes, which makes it possible to eliminate the above-discussed difficulties in the state of the art, while in particular the invention seek

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