GB2143465A

GB2143465A - Composite body moulding process

Info

Publication number: GB2143465A
Application number: GB08418134A
Authority: GB
Inventors: Gerard Moraine
Original assignee: Bourguignonne dApplications Plastiques SA
Current assignee: Bourguignonne dApplications Plastiques SA
Priority date: 1983-07-18
Filing date: 1984-07-17
Publication date: 1985-02-13
Also published as: GB2143465B; ES534382A0; FR2549415A1; GB8418134D0; ES8504539A1; IT1175583B; FR2549415B1; IT8421898A0; BE900157A

Abstract

A process for moulding a composite body comprising an outer thermoplastic envelope and a cellular plastic filling anchored to the inner wall of the envelope, e.g. a sailboard, is described. The envelope is produced by blow- moulding a tubular parison (3) whose inner wall has point-shaped roughnesses (4) which subsequently provide the mechanical anchoring of the cellular filling to the inner wall of the envelope. The parison is delivered through an extrusion head whose core (2) possesses grooves (5), the temperature of the core being at least 20 DEG C below the temperature of the outer die (1). The grooves (5) cause the points (4) to be formed. After the parison (3) is formed as shown in Fig. 1, it is introduced into a blow-mould (not shown) and blown, and after de- moulding the cellular plastic, e.g. expandable polyurethane, is injected into the blow-moulded envelope. <IMAGE>

Description

SPECIFICATION Hollow body moulding process The present invention relates to a process for moulding a hollow body, such as a sailboard, comprising a thermoplastic envelope and a cellular plastic filling anchored to the inner wall of the envelope, in which process the envelope is produced by blow-moulding and, after demoulding, the filling is injected into the moulded envelope.

The production of such hollow bodies requires first of all the production of an envelope, which can be manufactured economically by blow-moulding from a tubular parison of a plastic, such as polyolefine, followed, in order to ensure the mechanical strength and buoyancy of this envelope, by injection into the latter of a rigid cellular material, generally based on polyurethane, which must be anchored to the inner wall of the envelope.

Since cellular materials, especially based on polyurethane, do not naturally adhere to polyolefine surfaces and in particular to polyethylene surfaces, it is necessary to provide a surface treatment such as a flame treatment or ionisation of the inner wall of the envelope prior to injection of the cellular material, so as to improve the adhesion of the latter to the said wall.

Such surface treatments prove laborious, long, expensive, delicate to carry out correctly and frequently random in their result. The lack of adhesion hence makes it necessary to employ relatively thick envelopes, which is expensive and rather disadvantageous.

The present invention relates to a process for producing such hollow bodies, which process makes it possible to achieve, simply and economically, excellent anchoring between the inner wall of the envelopes and the plastic cellular filling, and to do so without the need to resort to special surface treatments.

To this effect, the invention relates to a process for moulding a hollow body such as a sailboard, comprising a thermoplastic envelope and a cellular plastic filling anchored to the inner wall of the envelope, in which process the envelope is produced by blowmoulding and, after demoulding, the filling is injected into the moulded envelope, characterised in that the envelope is moulded from an extruded tubular parison whose inner wall has roughness possessing the general shape of points distributed over its entire surface, the said roughnesses subsequently ensuring the mechanical anchoring of the filling to the inner wall of the moulded envelope.

The roughnesses present on the interior of the extruded tubular parison consist of ridges, projections, barbs, protuberances, etc., of tapered appearance running at right-angles to the inner surface of the tubular parison, these roughnesses having a length which is at least 2 mm and can be as much as, or even more, than 10 mm. In this way, upon subsequent injection of the cellular filling material into the moulded envelope, these roughnesses become engaged in the said filling material and provide very efficient mechanical anchoring between this material and the inner wall of the envelope which has been blow-moulded from the tubular parison. It has in fact been found that these roughnesses are in no way altered by the subsequent blow-moulding operation.

The anchoring of the cellular filling to the inner wall of the envelope is achieved very satisfactorily if the inner wall of the parison has at least one roughness per dm2 of surface area and preferably at least 3 roughnesses per dm2 of surface area.

It has inter alia been found that to produce an extruded tubular parison whose inner wall has roughnesses such as those defined above it suffices to use an extrusion head comprising a smooth annular outer die and a core whose outer surface is hollowed with annular grooves which form an angle of between 45 and 1 35' with the longitudinal axis of the core, the best results being achieved if the grooves are hollowed in planes which are substantially at right-angles to the axis of the core. It is not essential to use a core whose surface finish is perfect. On the contrary, it has been found that a core of the rough outer structure can prove beneficial. The number of grooves hollowed in the core can be whatever is desired.

In general, satisfactory results are achieved even when providing only two grooves. These grooves can have any desired cross-section, a rectangular profile (in the shape of a C) being preferred. It is also preferred that the sides of the grooves should form a sharp angle where they join the outer surface of the core. Finally, it has also been found that the grooves should have a cross-section of at least 2 mm2 in order to achieve good results.

It is assumed that the presence of the hollowed grooves in the core causes anchoring zones to be formed on the inner wall of the shaped tubular parison and that these zones produce, in these positions, disturbed material flow patterns which are responsible for the formation of roughnesses having the general shape of points distributed at random over the surface of the inner wall of the tubular parison delivered from the die.

Finally, it has been found that the formation of the roughnesses is accentuated if the temperature of the extrusion head is controlled so that the core temperature is kept at least 20 C below the temperature of the outer die. Noteworthy results are obtained when the temperature of the outer annular die is kept at the usual extrusion temperature of the thermoplastic constituting the tubular parison to be produced, while the temperature of the core is kept at a lower level, the difference in temperatures being of the order of 30 C.

The process of the invention is particularly suitable for the production of hollow bodies of large length, such as sailboards or buoys, but it can also be used for the production of other types of hollow bodies such as containers used for the packaging of fragile articles.

The tubular parison intended for moulding the envelope of the hollow body can consist of any material, preferred materials being polyolefines, such as high density polyethylene, vinyl resins and acrylic resins.

The cellular filling material which is injected into the envelope after the latter has been demoulded can consist of any expandable resin which preferably leads to the formation of a rigid cellular structure. Polyurethane resins giving so-called rigid foams are preferred.

The process according to the invention is furthermore explained in more detail by the practical embodiment given below, which relates to the production of a sailboard. In the description of this example, reference will be made to the figures in the attached drawings, in which: Figure 1 is a view, in partial section, of an extrusion head used to produce the tubular parison employed to form the envelope of the sailboard and Figure 2 is a view, in partial section, of the sailboard produced.

Example To produce a sailboard by the process according to the invention, a tubular parison of appropriate length, consisting of polyethylene of high density (0.950), is delivered through a partial-accumulation extrusion head shown in Fig. 1.

As may be seen in Fig. 1, the extrusion head comprises an outer die 1 of conventional construction and a core 2. The end of the core, which has an externally rough structure, is provided with four superposed annular grooves 5, these grooves being formed in planes at right-angles to the longitudinal axis of the core. The grooves, of rectangular profile, have a cross-section of 4 mm2. During the extrusion of the tubular parison 3, the temperature of the extrusion head is regulated so that the outer die 1 is kept at 1 90 C and the core 2 is kept at about 1 60 C. It is found that the extruded tubular parison 3 has roughnesses 4 over its inner wall, the roughnesses having the general shape of points distributed at random over its entire surface.

The tubular parison delivered is introduced directly into a suitable blow-mould, which is not shown, and is shaped by blowing so as to convert it to a sailboard envelope which is subsequently demoulded. Through an aperture provided in the envelope, a sufficient quantity of expandable polyurethane resin is injected into the envelope, causing the formation of a rigid cellular structure.

After 24 hours, two cuts are made in the sailboard obtained, along planes at rightangles to the longitudinal axis of the board, the two cuts being 10 cm apart. A sample such as that shown in cross-section in Fig. 2 is thus obtained. It is found that the cellular filling 6 adheres strongly to the inner wall of the envelope 7 of the board and does so by virtue of the roughnesses 4 present on the inner wall of the envelope 7. Moreover, if the two walls of the envelope 7 are pulled apart with sufficient force to destroy the structure obtained, it is found that substantial portions of the cellular structure filling remain anchored to these walls.

Claims

1. Process for moulding a hollow body such as a sailboard, comprising a thermoplastic envelope and a cellular plastic filling anchored to the inner wall of the envelope, in which process the envelope is produced by blow-moulding and, after demoulding, the filling is injected into the moulded envelope, characterised in that the envelope is moulded from an extruded tubular parison whose inner wall has roughnesses possessing the general shape of points distributed over its entire surface, the said roughnesses subsequently ensuring the mechanical anchoring of the filling to the inner wall of the moulded envelope.

2. Process according to Claim 1, characterised in that an extruded tubular parison whose inner wall has roughnesses possessing a length of at least 2 mm is employed.

3. Process according to Claim 1 or 2, characterised in that an extruded tubular parison whose inner wall has at least one roughness per dm2 is employed.

4. Process according to any one of Claims 1 to 3, characterised in that to produce the tubular parison an extrusion head is employed which comprises a smooth annular outer die and a core whose outer surface is hollowed with annular grooves which form an angle of between 45 and 1 35' with the longitudinal axis of the core.

5. Process according to Claim 4, characterised in that a core whose outer surface is hollowed with annular grooves located in planes substantially at right-angles to the axis of the core is used.

6. Process according to Claim 4 or 5, characterised in that a core whose outer surface is hollowed with at least two annular grooves is used.

7. Process according to any one of Claims 4 to 6, characterised in that a core whose outer surface is hollowed with annular grooves having a section of at least 2 mm2 is used.

8. Process according to any one of Claims 4 to 7, characterised in that the temperature of the extrusion head is regulated so that the core temperature is kept at least 204C below the die temperature.

9. Process according to any one of Claims 1 to 8, characterised in that the envelope is made of a polyolefine and the cellular filling is made of a polyurethane.

10. Process according to Claim 9, characterised in that the envelope is made of high density polyethylene.

11. Process according to claim 1, in which an extrusion head substantially as described with reference to Fig. 1 of the accompanying drawing is employed to produce the tubular parison.

12. Process according to claim 11, substantially as described in the foregoing Example.

13. A hollow body moulded by a process according to any of claims 1 to 12.

14. A hollow body as claimed-in claim 13 which is in the nature of a sailboard.