NL8300621A - Thick-walled thermoplastic pipe connections - injection moulded in three phases to produce sealed skins with finer crystalline structure - Google Patents

Abstract

A moulding in thermoplastics such as polyalkenes, and a connecting piece in particular for thermoplastic tubes, is produced with a thick wall which has on each surface a skin of the same material as its inner core but with finer crystalline structure, the skin being rigid and sealed. Polymer is injection moulded with low viscosity and at high injection speed, first, further injection is carried out at low speed and pressure, and, in a third phase, while the polymer starts to cool, low viscosity material is injected at higher pressure. The equipment used as a heated injector. In the first, high-speed injection phase no polymer orientation takes place. The mould tools have thin inner liners with adjacent cooling bores for rapid action. After initial cooling, the moulding formed can be ejected e.g. together with the core tool and immersed in coolant until the core of the moulding has sufficiently cooled.

Description

........ + 82.5124 / vdV / Rdm / sme

Short indication; Plastic molding.

The Applicant mentions as inventor;

Roelof van der Vijgh in Zwolle

The invention relates to a plastic molded piece, in particular a coupling piece for pipes.

Plastic pipes are increasingly being used for the transport of liquids and gases. These 5 pipes are connected to each other with coupling pieces, in the form of T-pieces, bends, reducers, etc. For pipes with a relatively small diameter and a small wall thickness, the production of connecting pieces is no problem and these can easily be pressed into injection molds. However, when large diameter pipes of great wall thickness are to be used, the manufacture of couplings involves various difficulties. During injection molding or injection molding, deformations, "shrinkage cavities and" subsidence "then occur in the walls as a result of shrinking of the material. A material surcharge for obtaining sufficient strength must therefore be given. The uneven shrinkage causes inadmissible dimensional deviations, which necessitates post-processing, for example by turning out the socket ends, so that the chemically highly resistant surface layer is removed.

The object of the invention is to provide a molded piece of thermoplastic plastic such as polyolefin, in particular a coupling piece for pipes, which has good resistance to chemicals which occur in the environment and which do not require any post-processing and therefore do not need to have an excessive wall thickness. . According to the invention, a molding is characterized in that the whole is thick-walled, the wall has an outer and inner surface layer of the same material as the wall core, but with a finer crystalline distribution and that the surface layer is tight and completely closed, except for the sprue.

The advantage of this is that the molding has a high resistance to chemical attack and is wear resistant, while the molding causes less disturbance of the flow pattern of the medium in the tube.

In a further embodiment, the molding is such that the wall thickness is at least 6 mm and that it is constant over the whole product. Furthermore, a molding according to the invention has a wall core, surrounded by the surface layers of a finer crystalline composition, which is completely homogeneous and massive.

The advantage of this is that the molding made in an injection molding or flow molding die does not require a wall thickness surcharge relative to the extruded tube to which the molding will be connected in its final destination.

The invention will now be explained in more detail with reference to the accompanying drawing, in which: Fig. 1 shows a perspective view of a molding, in particular a T-coupling; and Fig. 2 shows a drawing of a cross-section of a wall part of a plastic molding according to the invention, comparable to a structure photo.

The thick-walled molding according to Fig. 1 manufactured in an injection-molding or flow-molding mold has the special property that the wall surfaces of this thick-walled molding, including the internal wall surfaces determined by mold core parts, have a continuous homogeneous surface layer of the same material but with finer crystalline distribution than the material between these layers. A coupling piece according to the invention can be manufactured with wall thicknesses greater than 6 mm, for instance of 8 mm and more, whereby in view of the material flowing into the mold, the pressures to be applied thereto will generally be small. It may therefore also be the case that the molding according to the invention is manufactured by flow molding or injection molding.

The cross-section according to Fig. 2, which is comparable to a structure photo, is illustrative. The cross-section of Fig. 2 shows a part of the molding near the socket end of the molding shown in Fig. 1, the outer wall 1 of which has a straight and tight course, as well as the inner wall 2 of the socket end. The finer crystalline distribution is hereby referred to by the 8300621 - 3 -

a 'H

number 3 is indicated and is continuous over the entire surface, both internally and externally. Due to the straight, straight course, for example of the walls 1 and 2, the molding according to the invention has the property that in axial cross-sectional planes of parts of the molding, which have straight descriptive lines in this sectional plane, the distance between the descriptive line. on the outside of a wall part and the descriptive line on the inside of this wall part, for instance formed by the walls 1 and 2, in this case the cross-sectional area is larger than 6 mm and is always constant, the finer crystalline distribution 3 of the material at the surface continuously follows both descriptive lines.

Even with curved shape, for example in a cross-section of a molding according to Fig. 1 or a curved surface at a bend-piece, the molding according to the invention has the property that in cross-sectional areas of curved wall parts whose radii of curvature are on the outside and the inside of these wall parts have the same center of curvature, the difference in length of these radii of curvature is always equal and more than 6 mm, the finer crystalline distribution of the material on the surface always continuously following the arcs described by both radii of curvature. This continuous monitoring of the surface layer is possible because the molding according to the invention does not have to be given any finishing, while a high dimensional accuracy is present within the normal tolerance limits. The remaining material 4 of the molding surrounded by the surface layer 3 with a finely divided crystalline structure, is, as with extruded material, for example, that of the tubes to which the molding is to be connected, homogeneous and solid. -

The quality of a molding according to the invention is therefore comparable to that of a molding for small pipelines and wall thicknesses, for instance from 2.5 to 3 mm.

8300621

Claims (6)

1. Molded part from thermoplastic plastic, such as polyolefin, in particular a coupling piece for thermoplastic pipes, characterized in that the molded part is completely thick-walled, that this wall has a surface layer of the same material both on the outside and on the outside. the wall core, but with a finer crystalline distribution and that this surface layer is completely tight and closed, except for the sprue. 2. Molding according to claim 1, characterized in that the wall thickness is at least 6 mm and that it is constant over the entire product. 3. Molding according to claims ΐ and 2, characterized in that the material surrounded by the surface layer is completely homogeneous and solid. Molding according to one or more of Claims 1 to 3, characterized in that in axial cross-sectional planes of parts of the molding, which have straight descriptive lines in this sectional plane, the distance between the descriptive line on the outside of a wall part in the descriptive line on the inside of this wall part in the same cross-sectional area is greater than 6 mm and is always constant, the finer crystalline distribution of the material on the surface following both descriptive lines without interruption. Shaped article according to one or more of Claims 1 to 4, 25, characterized in that in cross-sectional areas of curved wall parts whose radii of curvature have the same center of curvature on the outside and inside of these wall parts, the difference in length of these curvatures is always the same and is more than 6 mm, the finer crystalline distribution of the material on the surface always following uninterruptedly the arcs described by both radii of curvature. Molded article according to one or more of claims 1 to 5, characterized in that it is manufactured in an injection molding or flow molding mold. 8300621