Apparatus and method for adjusting a mandrel and kernel...

Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion

Reexamination Certificate

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Details

C264S167000, C425S325000, C425S381000, C425S392000, C425S396000, C425S466000, C425S467000

Reexamination Certificate

active

06210617

ABSTRACT:

The present invention relates to an apparatus for producing pipes of a mouldable material, preferably plastic, comprising an extruder for extruding molten plastic from at least one annular nozzle, a substantially cylindrical mould as an extension of the extruder for shaping the outer surface of the plastic pipe, a longitudinal mandrel substantially inside the extruder, and a kernel outside the nozzle as an extension of the mandrel. The present invention also relates to a method for producing plastic pipes.
An apparatus of this kind is known from U.S. Pat. No. 4,846,660, for example. As the production speed of manufacturing apparatuses for plastic pipe increases, the kernel situated in the centre line of the apparatus should be made ever longer so that the plastic pipe could cool sufficiently before the calibrating effect of the kernel to the inner surface of the pipe ceases. The mandrel and the kernel joined thereto can be attached to other parts of the apparatus, most often to an extruder, only at the front end of the mandrel in which the attachment elements will not stop the flow of plastic. A disadvantage of this known attachment method is that the mandrel and the kernel are not coaxial with the centre line of the extruder and the mould, but are situated, the kernel in particular, below the centre line. As a result of the eccentric position of the mandrel and the kernel, the wall thickness of the plastic pipe produced in the apparatus varies in the direction of the circle which is detrimental to the durability of the pipe. The eccentricity of the wall of the pipe causes particularly many problems when pipes are produced of materials whose shrinkage is dependent on crystallinity degree. For example, polyethylene pipes will bend easily into a banana shape just because of this problem.
WO 91/06417 discloses an apparatus for manufacturing plastic pipe in which the mandrel is attached flexibly into an extruder so that the mandrel and thus also the kernel can be adjusted in the transverse direction in order to achieve coaxiality. In order to bring about flexibility, the mandrel is attached to the extruder by means of calotte-like surfaces adjustable with respect to one another.
A disadvantage of the prior art apparatus shown above is that it is almost impossible to render the unit formed by the mandrel and the kernel coaxial with the centre line of the extruder and the mould. It has been shown in practice that it is virtually impossible to get both the mandrel and the kernel into a coaxial position as their longitudinal axles are quite often at an angle with respect to one another. Therefore, if the mandrel is adjusted at its point of attachment in such a manner that it is situated coaxially inside the extruder, the kernel usually settles into a position where its tail end is lower down than the front end joined to the mandrel. It is not shown in said WO reference how the adjustment of the point of attachment of the mandrel is carried out in practice, but in prior art apparatuses with this possibility for adjusting, the apparatus has to be halted during adjustment.
The object of the present invention is to provide an apparatus for producing plastic pipes where both mandrel and kernel can be put into a coaxial position with respect to the extruder and the mould. The apparatus according to the invention is characterized in that the mandrel and the kernel are joined together in such a manner that the angle between their centre lines is adjustable at the junction.
The invention is based on the fact that the plastic pipe blank is considerably softer at the front end of the kernel than at its tail end when the apparatus is in use. When the angle between the mandrel and the kernel is altered, the kernel which is supported at its tail end to the rigid pipe blank rotates around a point of the edge of the tail end, whereby the front end of the kernel moves in the mould in the transverse direction of the mould and transports the tail end of the mandrel with it. The invention is particularly useful in apparatuses in which the combined length of the mandrel and the kernel has to be especially great because of the raw material of the pipe, for example. If means for effecting a relative movement for the joint surfaces are placed between the mandrel and the kernel, the adjusting process can be carried out during the operation of the apparatus.
One preferred embodiment of the invention is characterized in that the end surface of a conically expanding portion of the mandrel comprises at the centre line of the mandrel a protruding pin which is surrounded by the joint surface of the mandrel, and that the joint surface of the kernel is situated in an annular end piece that surrounds the pin. This structure makes the mutual adjustment of the mandrel and the kernel possible in a simple manner. In order that plastic substance would not enter between the mandrel and the kernel, it is preferable that the pin of the mandrel comprises means for pressing the joint surfaces of the mandrel and the kernel against each another. These means can be formed of a nut threaded to the pin or a spring surrounding the pin.
According to one very preferred embodiment of the invention, there is between the mandrel and the kernel an adjusting piece that changes its shape and/or size with the effect of temperature by means of which piece a relative movement of the joint surfaces of the mandrel and the kernel can be effected. By means of this adjusting piece, the angle position between the mandrel and the kernel can be changed in a simple manner by altering the temperature of the adjusting piece by means of electric current, for example. The adjusting piece provides especially the advantage that the apparatus need not be halted during adjusting. The adjusting piece may be situated between the mandrel pin mentioned above and the end piece of the kernel, for example.
The present invention further relates to a method for producing pipes of a mouldable material, preferably plastic, by means of an apparatus comprising an extruder for extruding molten plastic from at least one annular nozzle, a substantially cylindrical mould as an extension of the plastic pipe for shaping the outer surface of the plastic pipe, a longitudinal mandrel substantially inside the extruder, and a kernel outside the nozzle connected to the mandrel.
The method according to the invention is characterized in that in order to adjust the position of the mandrel with respect to the mould perpendicularly against the centre line of the mould, the angle between the centre lines of the kernel and the mandrel is adjusted until the front end of the kernel, which is supported at its tail end to the already cooled pipe blank, has transferred the tail end of the mandrel into a desired position.


REFERENCES:
patent: 4936768 (1990-06-01), Lupke
patent: 5139730 (1992-08-01), Holso et al.
patent: 5286183 (1994-02-01), Tonsi et al.
patent: 5346384 (1994-09-01), Hegler et al.
patent: 5674440 (1997-10-01), Klinedinst

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