The invention relates to a hose extrusion mandrel. Such a hose extrusion mandrel is used in a hose manufacturing process, such as for example described in EP 1563985. EP

1563985 describes an extruder, which ejects a thermoplastic mass in the form of a tube. To obtain an acceptable tolerance of the inner dimensions of the extruded tube, a solid mandrel is inserted in the extruded thermoplastic mass. The extruded tube is then advanced into a braiding station, either with the mandrel still inserted or without, such that a reinforcement layer is arranged on the extruded tube. After the braiding station, the tube with the reinforcement layer is advanced through a second extruder to cover the tube and reinforcement layer with a cover layer of thermoplastic material.

When the solid mandrel should remain in the extruded tube, while advancing the tube, then a substantial length for the mandrel is required in order to manufacture substantial lengths of tube.

It is known to use flexible plastic mandrels or rubber mandrels for the manufacturing of hoses. The advantage of flexible mandrels, is that these mandrels can be wound on a reel. Such flexible mandrels are supplied from a reel to the extruder and after finishing the tube, the mandrel can be wound on a further reel, while still being inserted into the tube. The flexible mandrel is then removed out of the tube at a later stage.

However, with increasing diameter of the hoses to be manufactured, the diameter of the required mandrel also increases. As a result of the increasing diameter, the

flexibility of the mandrel is reduced, such that flexible mandrels can no longer be wound around a reel. In order to provide more flexibility to the mandrel, the mandrel could be made from rubber instead of the more common solid thermoplastic material. However, a rubber mandrel tends to wear quickly and for bigger outer diameters the tolerance on the dimension is high, which is undesired for mass production of hoses.

Solid thermoplastic, flexible mandrels are suitable for smaller hose diameters, but have the disadvantage that a residual film is left on the thermoplastic mandrel after a hose has been extruded over the mandrel. This residual film is called in the field "snake skin" and needs to be removed when a next length of hose is to be manufactured with the mandrel. It should also be prevented that the snake skin is left behind in the manufactured hose, as this could cause blockages in the hose .

Another disadvantage of thermoplastic, flexible mandrels is the chance on pinhole formation. In order to prevent such pinhole, the extrusion process needs to be carefully controlled.

Solid thermoplastic, flexible mandrels are typically too stiff for mandrel diameters starting at size -20 SAE (31.4 mm) . For said sizes and above typically a non flexible mandrel is used and the length of the hose to be manufactured is limited .

It is an object of the invention to reduce the above mentioned disadvantages.

This object is achieved according to the invention with a hose extrusion mandrel for manufacturing rubber hoses, which hose extrusion mandrel comprises:

- a solid thermoplastic, elongate core;

- a rubber compound outer layer covering the surface of the elongate core.

Because the hose extrusion mandrel has a thermoplastic core covered by a rubber compound layer, the diameter of the core and thus of the thermoplastic material can be smaller, such that the flexibility is less impacted when the outer diameter of the hose extrusion mandrel is increased. This allows for hose extrusion mandrels with outer diameters of size -20 SAE (31.4 mm) and over, which still can be wound around a reel and which allows for the production of longer lengths of hoses.

Although the solid thermoplastic, elongate core is covered by a rubber compound layer, it has been found, that the wear of the rubber compound layer is considerably used compared to a full rubber compound hose extrusion mandrel and has a smaller tolerance in the outer diameter compared to a rubber mandrel.

A further advantage of the hose extrusion mandrel according to the invention is that the rubber compound layer does not show the snake skin effect nor the problem of pinhole formation .

Thus the hose extrusion mandrel of the invention allows for larger outer diameters, while still being able to wind the mandrel on a reel, and the snake skin effect and pinhole problem is prevented.

Preferably, the ratio between the diameter of the core and the thickness of the rubber compound layer is between 8 and 11. Within this range an optimal flexibility of the mandrel and reduction of the wear of the rubber compound layer is obtained.

In a preferred embodiment of the hose extrusion mandrel according to the invention the diameter of the core is at least 24 millimeter.

Examples of the dimensions of the hose extrusion mandrel are:

a core diameter of 24.8 mm 25.4 mm with a rubber compound layer thickness of 3.0 mm - 3.3 mm to arrive at a hose extrusion mandrel of size -20 SAE (31.4 mm diameter) .

- a core diameter of 37.4 mm with a rubber compound layer of 3.0 mm to arrive at a hose extrusion mandrel of size -24 SAE (37.4 mm diameter)

In a further preferred embodiment of the hose extrusion mandrel according to the invention the solid

thermoplastic, elongate core is of polypropylene or polyamide.

Preferably, the rubber compound outer layer is of Ethylene propylene diene rubber (EPDM) or acrylonitrile butadiene rubber (NBR) .

The invention further relates to a method for manufacturing rubber hoses, which method comprises the steps of :

- providing an extruder having a die with a die opening;

- providing a hose extrusion mandrel according to the invention, which hose extrusion mandrel has an outer diameter smaller than the die opening;

- extruding a rubber mass through the die opening;

- advancing the hose extrusion mandrel together with the extruded rubber mass through the die opening, such that the hose extrusion mandrel is lined with a layer of the extruded rubber mass.

By advancing the hose extrusion mandrel together with the extruded rubber mass, the tolerance of the inner diameter of the hose is maintained while the extruded rubber mass is subjected to further process steps.

Due to the rubber compound layer of the hose

extrusion mandrel according to the invention, the extruded rubber mass will not show the snake skin effect, such that a better quality hose is provided with the manufacturing method according to the invention. A preferred embodiment of the method further comprising the steps of:

- braiding a reinforcement layer around the hose extrusion mandrel being lined with the layer of the extruded rubber mass, downstream of the die opening;

- extruding a covering layer of rubber mass around the hose extrusion mandrel being provided with the

reinforcement layer to cover the reinforcement layer.

With the hose extrusion mandrel still inserted in the extruded rubber mass, the extruded tube can easily be provided with a reinforcement layer and cover layer, while the tolerance of the inner diameter is maintained.

In a further preferred embodiment of the method according to the invention the hose extrusion mandrel is provided wound on a reel and the hose extrusion mandrel is supplied to the die opening by unwinding the hose extrusion mandrel from the reel.

Due to the increased flexibility of the hose

extrusion mandrel according to the invention, it is possible to manufacture hoses with larger inner diameters, while still having the advantage of longer lengths of hoses, as the mandrel can be wound on and unwound from a reel.

Yet a further embodiment of the method according to the invention comprises the further steps of:

- after a length of hose is manufactured, cutting the rubber mass at the die; and

- removing the hose extrusion mandrel from the manufactured length of hose.

These and other features of the invention will be elucidated in conjunction with the accompanying drawings.

Figure 1 shows a cross sectional view of an embodiment of a hose extrusion mandrel according to the invention . Figure 2 shows a schematic view of an embodiment of a method according to the invention.

Figure 1 shows a hose extrusion mandrel 1, which has a solid thermoplastic elongate core 2, preferably made out of polypropylene or polyamide. The outer surface of the core 2 is covered by an outer layer 3, which is preferably made out of Ethylene propylene diene rubber (EPDM) or acrylonitrile butadiene rubber (NBR) .

The core 2 has a diameter D _c of for example 31.4 millimeter. The thickness t of the outer layer 3 is for example 3 millimeter, such that the resulting hose extrusion mandrel 1 has an outer diameter D _m of 37.4, which corresponds to a size -24 SAE . The ratio between the diameter Dc of the core 2 and the thickness t of the rubber compound outer layer 3 is thus in this example 10.5.

Figure 2 shows schematic an embodiment of a method for manufacturing a hose according to the invention. A hose extrusion mandrel 1, as shown in figure 1, is wound on a reel 10 and supplied therefrom to an extruder 11. The extruder 11 extrudes a rubber mass 12 around the hose extrusion mandrel 1, such that a hose is shaped around the mandrel 1.

The mandrel 1 together with the hose shaped rubber mass 12 is fed through a braiding device 13, which braids a reinforcement layer 14 around the hose shaped rubber mass 12.

The mandrel 1, hose 12 and reinforcement layer 14 are fed together to a second extruder 15, which extrudes a cover layer 16 around the reinforcement layer 14.

The resulting braided hose 17 is then wound on a reel 18, while the hose extrusion mandrel 1 is still present in the hose 17. This extrusion mandrel 1 is removed from the hose 17 after the full hose extrusion mandrel 1 has passed through the extruder 11, the braiding device 13 and the second extruder 15. The embodiment shown in figure 2 is an inline process. The method according to the invention not only includes such an inline process, but also step by step processes .