Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Patent
1997-07-31
1999-06-15
Tentoni, Leo B.
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
2642094, 2642095, 425192R, 425377, 4253781, 425380, B29C 4700, B29C 4790
Patent
active
059119330
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for producing a tubular profile from thermoplastic material, in which the tubular profile is extruded during normal production using an extruder which comprises an internal mandrel, the tubular profile emerging from the extruder is cooled externally and internally, respectively, downstream of the extruder with the aid of an external cooling device and an internal cooling device which comprises an internal cooling unit mounted on the internal mandrel of the extruder, and a tensile force is exerted on the tubular profile downstream of the external cooling device by means of a pulling device which acts externally on the tubular profile.
DESCRIPTION OF RELATED ART
Such a method is disclosed for example, in NL-A-7605758. The use of internal cooling in the extrusion of tubular profiles made of thermoplastic material has the important and also generally known advantage that the necessary cooling of the tubular profile emerging from the extruder proceeds much more expediently and rapidly than if the tubular profile were to be cooled exclusively from the outside. As a result, the occurrence of an unfavourable internal stress distribution in the synthetic material of the tubular profile can be avoided, particularly in the case of thick-walled profiles.
The use of internal cooling during the production of thermoplastic tubular profiles is also disclosed in WO 94/05482. Both in the case of the method described in this publication and in the method according to NL 7605758, use is made for the internal cooling of an internal cooling unit which can be coupled to an internal mandrel of the extruder and which has outside dimensions which are less than the inside dimensions of the extruded tubular profile. A gap therefore remains free between the internal cooling unit and the tubular profile to be cooled, there being circulated through said gap cooled air which removes the heat from the tubular profile and delivers it to a cooling system incorporated in the internal cooling unit and being cooled by a liquid.
In these known methods, the presence of a gap between the tubular profile to be cooled and the internal cooling unit is not only required for establishing the air flow. The gap also provided to overcome a typical problem related to the use of internal cooling. Said problem occurs in practice in starting up the method according to the preamble.
The problem in starting up an extrusion process with internal cooling is that the tubular profile emerging from the extruder has to be passed over the internal cooling unit mounted on the extruder. WO 94/05482 describes how the extruder is initially set in operation during starting up without the internal cooling unit being mounted on the extruder and how the fitting of the internal cooling unit is delayed until the extrusion process has stabilized. The soft tubular profile is then cut through immediately downstream of the extruder head and the internal cooling unit is mounted on the extruder. The tubular profile then slides over the internal cooling unit as a result of the progress of the extrusion.
As long as the cooling is not operative, the tubular profile in the region adjoining the extruder, is only slightly cooled, particularly on the inside, and is consequently soft and has little or hardly any shape retention. As a result, there is an appreciable risk that the still soft tubular profile comes into contact with the relatively cold internal cooling unit and gets stuck or adheres thereto, possibly with the consequence that the tubular profile splits open at that point or will no longer pass over the internal cooling unit at all. After all, at the instant when the internal cooling unit has to be passed, the tubular profile has still not reached the pulling device and no force can as yet be exerted by the pulling device to help the tubular profile over the internal cooling unit.
GB 1 422 325 discloses an extrusion process wherein the extruded tube emerges vertically downward from the extruder. The extruded tu
REFERENCES:
patent: 5132062 (1992-07-01), Brambilla
Prenger Jan Hendrik
Visscher Jan
Tentoni Leo B.
Wavin B.V.
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