FR2549415A1 - METHOD FOR MOLDING A HOLLOW BODY SUCH AS A SAILBOARD COMPRISING A THERMOPLASTIC ENVELOPE AND A FILLING OF CELLULAR PLASTIC MATERIAL - Google Patents

Abstract

PROCESS FOR MOLDING A HOLLOW BODY SUCH AS A WINDBOARD CONTAINING A SHELL IN THERMOPLASTIC MATERIAL AND A FILLING IN CELLULAR PLASTIC MATERIAL. THE ENVELOPE IS PRODUCED BY BLOW MOLDING OF A TUBULAR PARAISON 3 WHOSE INTERNAL WALL SHOWS ASPERITES 4 HAVING THE GENERAL SHAPE OF POINTS WHICH LATER ENSURE THE MECHANICAL HANGING OF THE FILLING ON THE INTERNAL WALL OF THE ENVELOPE. THE PARAISON IS DEBITED THROUGH AN EXTRUSION HEAD WHOSE CORE 2 INCLUDES GROOVES 5, THE TEMPERATURE OF THE CORE BEING AT LEAST 20C LOWER THAN THE TEMPERATURE OF THE EXTERNAL DIE 1. THE INVENTION ALLOWS IN PARTICULAR REPRODUCTION MORE ECONOMICAL OF SAILING BOARDS.

Description

Method for molding a hollow body such as a windsurf board comprising

  The invention relates to a method for molding a hollow body, such as a windsurf board, comprising an envelope. The invention relates to a method for molding a hollow body, such as a windsurf board, comprising an envelope. thermoplastic material and a cellular plastic filling hanging on the inner wall of the envelope in which the casing is made by blow molding and, after demolding,

  the filling is injected into the molded envelope.

  The production of such hollow bodies first requires the realization of an envelope, which can be manufactured economically by blow molding from a tubular plastic parison 10 such as a polyolefin and then, to ensure the resistance mechanical and buoyancy of this envelope, the injection into it of a rigid cellular material generally based on polyurethane

  which must be hung on the inner wall of the envelope.

  Since cellular materials, in particular those based on polyurethane, do not naturally adhere to polyolefin surfaces and in particular to polyethylene surfaces, it is necessary to provide a surface treatment such as flammaga or ionization of the inner wall of the polyethylene. envelope before injection of the material

  to improve its adhesion on this wall.

  Such surface treatments prove to be laborious, time-consuming, expensive, difficult to perform correctly and often random in their result. Lack of adhesion therefore requires the use of relatively thick envelopes, which is expensive.

and not very advantageous.

  The present invention relates to a method for producing such hollow bodies which makes it possible in a simple and economical way to obtain an excellent attachment between the inner wall of the casing and the filling of cellular plastic material without having to

  resort to special surface treatments.

  To this end, the invention relates to a method for molding a hollow body such as a windsurf board, comprising a thermoplastic envelope and a cellular plastic filling hung on the inner wall of the envelope in which the casing is produced by blow molding and, after demolding, the filling is injected into the molded casing, characterized in that the casing is molded from an extruded tubular parison, the internal wall of which has roughness having the general shape of spikes spread over its entire surface, said asperities

  subsequently ensuring the mechanical attachment of the filling on the inner wall of the molded envelope.

  The asperities present on the internal part of the extruded tubular parison are constituted by ridges, projections, barbs, protuberances, etc. of tapered appearance normally directed with respect to the internal surface of the tubular parison, these asperities having a length from less than 2 mm and can reach and even exceed mm In this way, during the subsequent injection of the filling cellular material into the molded envelope, these asperities cling in this filling material and ensure a very effective mechanical anchoring between This material and the inner wall of the casing blow molded from the tubular parison It has, indeed, been found that these asperities are not modified by the subsequent operation of blow molding. The attachment of the cell filling on the inner wall of the envelope is achieved very satisfactorily when the inner wall of the parison has at least one roughness per dm 2 of surface and preferably at least 3 asperities per dm 2 of surface. To make an extruded tubular parison whose internal wall has asperities as defined above, it has, moreover, been found that it is sufficient to use an extrusion head comprising a smooth annular outer die and a core the outer surface of which is hollowed out with annular grooves forming, with the longitudinal axis of the core, an angle of between 45 and 1350, the best results being achieved when the grooves are hollowed -3 in planes substantially perpendicular to the axis of the kernel It is not essential to use a kernel whose surface finish is perfect On the contrary, it was noticed that a kernel of rough external structure could be beneficial The number of grooves dug in the kernel can be arbitrary In general Satisfactory results are obtained by providing two grooves. These grooves may have any cross-section, the rectangular profile (C-shaped) being Preferably, also, the blanks of the grooves form a sharp angle in their connection with the outer surface of the core. Finally, it has been further found that the grooves must have a cross section.

  at least 2 mm to achieve good results.

  It is assumed that the presence of the grooves dug in the core causes the appearance of attachment zones on the inner wall of the shaped tubular parison and that they cause in these places perturbed material flows which are at the origin the formation of asperities having the general shape of spikes randomly distributed on the surface of the inner wall of

the tubular parison debited.

  It was finally found that the formation of asperities is accentuated when the temperature of the extrusion head is adjusted so that the temperature of the core is kept at least 20 ° C lower than the temperature of the external die. are obtained when the temperature of the outer annular die is maintained at the usual extrusion temperature of the constituent thermoplastic material of the tubular parison to produce and when the core temperature is maintained at a lower temperature, the temperature difference being of the order

from 30 C.

  The method according to the invention is particularly suitable for producing hollow bodies of great length such as sailboards or floats but it can also be suitable for producing other types of hollow bodies such as containers

  used for packaging fragile items.

  -4 The constituent material of the tubular parison intended to form the shell of the hollow body may be arbitrary, preference being given to polyolefins, such as polyethylene

  high density, with vinyl resins and acrylic resins.

  The cellular filling material which is injected into the envelope after demolding thereof may consist of any expandable resin preferably leading to the formation of a rigid cellular structure. Preference is given to polyurethane resins leading to so-called foams. rigid. The method according to the invention is furthermore explained in more detail by the following practical example of embodiment which relates to the production of a windsurf board.

  description of this example, reference should be made to the drawing figures

  appendices in which: FIG 1 is a partial sectional view of an extrusion head used for producing the tubular parison known to form the envelope of the windsurf board FIG 2 is a partial sectional view of the windsurfing produced.

Example

  To achieve a windsurfing by the method according to the invention is discharged through an accumulating extrusion head partially shown in fig 1 a tubular parison of appropriate length

  from high density polyethylene (0.950).

  As it appears in FIG. 1, the extrusion head comprises

  an external die 1 of conventional construction and a core 2.

  The end of the core, of rough external structure, is provided with four superimposed annular grooves 5 and made in planes perpendicular to the longitudinal axis of the core. The grooves 30 of rectangular profile have a cross section of 4 mm. the tubular parison 3, the temperature of the extrusion head is adjusted so that the outer die 1 is

  maintained at 190 ° C and that the core 2 is maintained at about 160 ° C.

  It can be seen that the extruded tubular parison 3 has asperities 4 having the general shape of spikes on its inner wall.

  distributed randomly over its entire surface.

  -5 The tubular parison debited is introduced directly into a suitable blow mold, not shown, and is blow-molded so as to be transformed to a windsurfing envelope which is then demolded by an opening provided in the envelope a suitable quantity of expandable polyurethane resin is injected into the latter, leading to the formation of a

rigid cell structure.

  After 24 hours, two cuttings are made in the windsurf board obtained in planes perpendicular to its longitudinal axis, the two cuts being spaced 10 cm apart. In this way, a sample is obtained as shown in section in FIG. It can be seen that the cellular filling 6 adheres strongly to the inner wall of the casing 7 of the board and this through the asperities 4 present on the inner wall of the casing 7. In addition, if we exclude the two walls of the casing 7 with a force sufficient to destroy the structure obtained, it is found that significant portions of the cell filling remain

hung on these walls.

-6

Claims (9)

R E V E N D I C A T IO N S   Process for molding a hollow body such as a windsurf board comprising a thermoplastic envelope and a cellular plastic filling hung on the inner wall of the envelope in which the envelope is made by blow molding and, after demolding, the filling is injected into the molded casing, characterized in that the casing is molded from an extruded tubular parison whose inner wall has asperities having the general shape of spikes distributed over its entire surface, said asperities subsequently ensuring the mechanical attachment of the filling on the inner wall of the the molded envelope.   2 Method according to claim 1 characterized in that implements an extruded tubular parison whose inner wall   has asperities having a length of at least 2 mm.   3 Process according to claim I or 2 characterized in that implements an extruded tubular parison whose inner wall has at least one asperity by dm   4 Process according to any one of claims 1 to 3   characterized in that, for the production of the tubular parison, an extrusion head is used comprising a smooth annular outer die and a core, the outer surface of which is hollowed out with annular grooves forming, with the longitudinal axis of the core, a angle between 45 and 135 '.   Method according to Claim 4, characterized in that a core is used, the outer surface of which is hollowed out with annular grooves situated in planes substantially perpendicular to the axis of the nucleus.   Process according to Claim 4 or 5, characterized in that   a core is used whose outer surface is hollowed out by at least two annular grooves.   Process according to one of Claims 4 to 6   characterized in that a core whose outer surface -7 is hollowed out with annular grooves having a cross section of at least   Process according to one of Claims 4 to 7   characterized by adjusting the temperature of the extrusion head such that the core temperature is kept lower   at least 20 C at the temperature of the die.   Process according to any one of claims 1 to 8   characterized in that the polyolefin shell and the   polyurethane cellular filling.   Process according to Claim 9, characterized in that   realizes the high density polyethylene envelope.