TECHNICAL FIELD

This invention relates to a method of fabricating an inner core tool for casting, an inner core tool fabricated by the method, a moulding method using such an inner core tool and the products resulting from such use. It is particularly concerned with the casting of hollow items in plastics material such as a glass reinforced polymer.

Currently when a moulder wishes to make a hollow moulding two mould halves are prepared in each of which the laminate is laid up. The loaded moulds are then brought together and the laid up material joined with resin glue. Once hardened the two mould halves are split and removed to leave a hollow moulding. Such a technique is used for manufacturing car spoiler mouldings.

To make use of resin transfer moulding a moulding is made in one piece using a foam core which is slightly smaller than the required outside dimension of the finished component. This foam core is wrapped with glass fibre and placed in a two piece mould. The mould is closed and resin injected to flow between the foam core and the inner mould faces so impregnating the glass fibre. Once the resin has cured the two piece mould is then split apart and the hollow moulding removed. A problem with this process is the cost of manufacturing the foam core and the weight of the core which adds to the weight of the finished product.

BACKGROUND ART

Attempts have been made to provide a process similar to that described above in which the foam core is replaced by a pressurisible bag. The pressurised bag is wrapped with the fibre reinforcement and located in a two part mould. The resin is injected and when cured the mould is split apart to allow the hollow product to be removed. The pressure bag is then deflated and removed through an aperture in the moulding. A problem found with using a pressurisible bag was that it was not practicable to readily ensure accurate working tolerances whilst the bag was being wrapped with glass fibre.

Furthermore once the partially inflated bag wrapped with glass fibre is placed in the mould and finally inflated the inflation step can result in fibre being displaced and misaligned before the resin injection occurs.

Lost core casting developed in metal foundry work have been adopted for resin transfer moulding processes. A core of wax or low melt point alloy is wrapped in glass fibre and located in a mould. Once an injected moulding has hardened the core is melted and runs out through a small hole in the moulded part. Disadvantages of this process result from: the core being heavy and not centralising itself within the product during injection; this leads to inaccurate wall thicknesses in the finished product; the cost of the core being high for most applications and the throughput low; the occurrence of significant shrinkage in the process leading to tolerance problems.

DISCLOSURE OF INVENTION

According to a first aspect of the present invention there is provided a method of fabricating an inner core tool for use in lost core moulding process characterised by the steps of:

1 providing a rigid mould which reproduces on a working surface of the mould the final external form of the inner core tool;

2 introducing a flexible mould bag into the rigid mould;

3 providing a filling duct passage to the interior of the mould bag from outside the mould;

4 causing the mould bag to be drawn into contact with the working surface of the mould;

5 filling the interior of the mould bag at least in part with particles by way of the filling duct so as to provide for close packing of the particles within the mould bag so as to cause the bag at least in part, to conform closely to the working surface;

6 closing the mould bag gas tightly so as to isolate the interior of the bag from ambient air pressure except for an evacuating duct;

7 evacuating air from the interior of the bag by way of the evacuating duct to cause the mould bag and its mass of close packed particles to rigidify and thereafter closing the duct air tightly to maintain a sub-atmospheric pressure within the mould bag; and

8 removing the rigidified mould bag from the rigid mould.

According to a first preferred version of the first aspect of the present invention the method of fabricating an inner core tool is characterised by the further step of incorporating within the mould bag an expanding device so that in use as an inner mould tool for a lost core moulding process the rigidified inner mould tool can be subjected to internal pressurisation by way of the expansion device so as to urge the mould bag outwardly into contact with material juxtaposed with the mould bag. Typically the expansion device comprises a subsidiary inflatable bag; or a capsule containing a pressurised gas, the capsule having a closure adapted to vent the capsule within the mould bag in a controlled manner.

According to a second preferred version of the first aspect of the present invention or the first preferred version thereof the step of causing the mould bag to be drawn into contact with the working surface the or each raised region provides for the creation of a channel in an outer surface of the mould bag by incorporating at least one raised region on the working surface.

According to a third preferred version of the first aspect of the present invention there or any preceding preferred version thereof the step of causing the bag to be drawn into contact with the working surface is achieved by the application of a sub-atmospheric pressure to the working surface by way of ducts through the walls of the mould.

According to a fourth preferred version of the first aspect of the present invention or any preceding preferred version thereof the step of filling the interior of the mould bag is undertaken with the interior at substantially atmospheric pressure.

According to a fifth preferred version of the first aspect of the present invention or any preceding preferred version thereof the step of filling the interior of the mould bag at

least in part with particles involves particles of a substantially similar size.

According to a sixth preferred version of the first aspect of the present invention or the first to the fourth preferred versions thereof the step of filling the interior of the mould bag at least in part with particles involves the use of particles which vary in size so that the largest are at least twice the size of the smallest.

According to a seventh preferred version of the first aspect of the present invention or any preceding preferred version thereof the filling step includes the introduction of non-particulate material which is in filamentary or sheet form.

According to a second aspect of the present invention there is provided an inner core tool characterised in that it is fabricated according to the method of the first aspect or any preceding preferred version thereof.

According to a first preferred version of the second aspect of the present invention the inner core tool includes a flexible heater element adapted for heating so as to supply heat as preliminary step of a moulding process in which the inner core tool is to be used.

According to a third aspect of the present invention there is provided a moulding method for producing a moulded component characterised by the steps of:

1 providing a rigid mould with a moulding surface reproducing the form of the component to be moulded;

2 introducing into and locating in the rigid mould an inner core tool according to the second aspect of the present invention or the first preferred version thereof;

3 closing the mould to establish a pressurisible plenum volume conforming to the form of the finished mould component;

4 introducing into the plenum volume polymerisible material,

5 allowing the material to polymerise in the form of the finished component;

6 opening the mould;

7 removing the polymerised component;

8 providing an aperture in the mould bag of the inner core tool within the

polymerised component so as to depressurise the interior of the tool; 9 withdrawing at least particulate material from the tool from the component by way of the aperture so as to enable the mould bag to be substantially emptied of particulate matter and thereafter withdrawing the mould bag through the aperture.

According to a first preferred version of the third aspect of the present invention the moulding method for producing a moulded component is characterised by an intermediate step, during or following the step of introducing into the plenum volume polymerisable material, wherein the inner core tool is caused to inflate to enhance contact pressure between particles and the inner side of the mould bag and so between the exterior of the mould bag and material located in the plenum volume.

According to a fourth aspect of the present invention there is provided a moulded component fabricated by the use of a lost core inner tool according to the second aspect or the first preferred version thereof.

According to a first preferred version of the fourth aspect of the present invention there is provided a moulded component fabricated by way of the moulding method of the first aspect.

The inner core tool of the present invention is accurately located in a mould. The bag and its filling are separately removed from a cured moulded component through a small aperture provided in the hollow component: the particulate material, being very free flowing, can be poured out and the bag, which can be of thin film, is also readily removed through the aperture.

The proposed system provides a number of advantages over existing systems. There is no sacrificial component in the method (except possibly the mould bag which in any event is inherently cheap); no heating is required to remove the core; accurate reproduction of the finished product is facilitated as no significant shrinkage takes place during the evacuating step; an evacuated core is readily obtained at room temperature using low cost tooling; there is no limitation on the size of the evacuated core that can

be fabricated and used; and due to the low density of the particulate materials available (such a micro balloons) the weight of the evacuated core is significantly lower than that of the previously known wax or low melt point metal.

BRIEF DESCRIPTION OF DRAWINGS

An exemplary embodiment of the invention will now be described with reference to the accompanying drawing of a moulding process of which:

Figures 1 to 4 is a rigid mould being used to form a rigidified mould bag; and

Figure 5 shows a completed rigidified mould bag located in a working mould for the manufacture of a hollow component; and

Figure 6 shows a moulded hollow component resulting from the working mould of Figure 5.

Figures 1 to 4 variously show the preparation and use of a rigidified mould to produce a hollow moulded component. Split case mould 11 is made up of sections 12, 13 hinged along one side to enable the sections 12, 13 to be hinged apart to give access to interior 14 of the mould 11. Port 15 provides for access to the interior 14 of the mould. Seal 16 bounds moulding surfaces 12 A, 13A once the sections 12, 13 are hinged shut as shown in Figure 1. The moulding surfaces 12A. 13A serve to provide an accurate reproduction of an external surface of a hollow component to be manufactured by way of the mould 11. A number of vacuum ports 16 are provided through the walls of the sections 12, 13 to enable a sub-atmospheric pressure to be applied to the interior 14.

Figure 1

Mould 11 is shown closed with a thin walled mould bag 17 hanging in interior 14 of the mould. Interior 18 of the bag 17 communicates with the outside of the mould 13 by way of filling neck 19.

Figure 2

Ports 16 in walls of the mould 11 provide for a sub-atmospheric pressure to be applied to cause the outer surfaces of mould bag 17 to be drawn into contact with, and so conform closely to, the moulding surfaces 12A, 13 A. If necessary this effect can be enhanced by provision of a slight overpressure to the interior 18 by way of the neck 19.

Figure 3

The mould bag 17, whilst maintained in intimate contact with moulding surfaces 12A, 13 A, is filled with hollow micro spheres of plastic material by way of a filling neck 19. The micro spheres form a mass M which during the filling operation behaves in the manner of a liquid.

Figure 4

The mass M is shown filling the interior 18 of the bag 17 to the extent that the bag 17 is fully supported in the vicinity of the mould surface 12 A, 13. A vacuum is applied to the interior 18 of the bag 17 to withdraw air to a sufficient extent to cause the micro balloons forming the mass M to agglomerate to cause the mass M, and so the mould bag 17, to cohere into a rigid component. The filling neck 19 in then sealed off as shown in Figure 4 to preserve low pressure in the interior of the bag 17.

Figure 5

The rigidified bag 17 obtained as described in connection with Figure 4 is removed from the mould 11 and has pre-formed glass fibre reinforcement 60 wrapped around it and is then located in the interior 61 of a working mould 62 and aligned to provide a casting plenum 63 between the outside of the bag 17 and inside moulding surface 64. Resin is then injected into the casting plenum 63 to immerse the reinforcement 60 and thereafter having cured form the required hollow fabrication F.

Figure 6

The fabrication F is removed from the working mould 62 and aperture 65 is prepared at one end. The bag 17 is cut to vent the interior of the mould bag 17 to atmosphere by way of the atmosphere. This serves to allow the micro balloons making up the mass M to separate slightly from one another and revert to a liquid like behaviour. Consequently the small micro-balloons are readily poured out of the bag 17 by way of aperture 65. Once the micro-balloons have been removed the bag 17 can be withdrawn also. As a result the fabrication F is left hollow. If necessary a filling cap or other closure can be provided for the aperture 65.

The exemplary embodiment represents a cheap and readily undertaken method of

manufacturing hollow fabrications. The method is readily undertaken without a need for highly trained manufacturing staff and lends itself to production line use and readily provides for both small and large scale production requirements.

The exemplary embodiment makes use of a filling of particles of substantially uniform size. However depending on the type of finished product to be produced and so the degree of precision or conformity required on the outer surface of the inner tool the filling can comprise other elements. Thus it can be of particulate material of different sizes. Typically smaller particles can be used for the outer layer of the tool to provided for particularly close conformity of the inner tool surface with the surface to be moulded from when manufacturing the inner core tool and so providing for a high definition of surface for the inner tool in that region. The inner part of the filling is then taken up by particles of larger or much larger size. Additionally the filling can include other forms of material such as filamentary or sheetlike material. The only requirement is that whatever filling material is used it must be capable of being withdrawn form the inner core tool through the aperture.