GB2284173A

GB2284173A - Collapsible core tool for lost-core moulding

Info

Publication number: GB2284173A
Application number: GB9324345A
Authority: GB
Inventors: Alan Harper
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-02-06
Filing date: 1993-11-26
Publication date: 1995-05-31
Also published as: GB9324345D0

Abstract

An inner core tool for a lost core moulding process is made by providing a rigid mould 11, reproducing on its inner, working, surface 12A, 13A, the desired external form of the core tool; introducing a mould bag 17 into the rigid mould; causing the bag to be drawn into contact with the working surface of the mould; filling the bag with particles via a filling duct to establish close packing of the particles therein; closing the bag air-tightly to isolate the interior of the bag from ambient air pressure except via an evacuating duct 19; evacuating air from the bag interior via said duct to cause the mass of particles M to rigidify; closing the duct air-tightly to maintain a sub-atmospheric pressure therein; removing the rigidified mould bag from the rigid mould; for use as an inner mould tool for a lost core moulding process. The use of a subsidiary inflatable bag within a tool produced as above, to enable the tool, once incorporated in a mould, to be inflated further to enhance contact with mouldable material is also disclosed. Resin flow channels may be formed in the outer surface of the mould bag to facilitate resin flow over the tool. <IMAGE>

Description

METHOD OF, AND APPARATUS FOR, CASTING This invention relates to apparatus for, a method of making, a casting. 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.

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 rapped 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: I the core being heavy and not centralising itself within the product during injection. This leads to inaccurate wall thicknesses in the finished product; 2 the cost of the core being high for most applications and low throughput; 3 the occurrence of significant shrinkage in the process leading to tolerance problems.

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 comprises the steps of: 1 providing a rigid mould which reproduces on a working surface of the mould the external form of the inner core tool; 2 introducing a 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 a evacuating duct; 7 evacuating air from the interior of the bag by way of the evacuating duct to cause the mass of close packed particles to rigidify and thereafter closing the duct air tightly to maintain a sub-atmospheric pressure therein; 8 removing the rigidified mould bag from the rigid mould; and 9 using the rigidified mould bag as an inner mould tool for a lost core moulding process.

According to a first preferred version of the present invention there is provided a methods according to the first aspect including the further step of incorporating within the mould bag a subsidiary inflatable bag so that in the step of using the rigidified mould bag as an inner mould tool for a lost core moulding process particles within the rigidified mould bag can be subjected to pressurising by way of the subsidiary inflatable bag so as to urge the mould bag outwardly into contact with material juxtaposed with the mould bag. Typically in use a mould bag formed into a rigid tool is located in a working mould and resin is injected in the space between the rigid tool and the working mould (which will usually incorporate fibre or other inserts for incorporation in the finished moulded product. While the injected resin is still liquid the subsidiary bag is then inflated to finally define the outer surface of the inner tool and consolidate the liquid resin which thereafter polymerises.

Preferably according to the first preferred aspect of the present invention the rigidified mould bag has an outer surface incorporating at least one channel to facilitate the flow of liquid moulding material over the mould bag prior to the step of pressurising by the subsidiary inflatable bag; and the step of inflating the subsidiary inflatable bag serves to reconfigure the mould bag so as to reduce or eliminate the or each channel.

According to a second preferred version of the first aspect of the present invention 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 third 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 fourth preferred version of the first aspect of the present invention or the first, second or third 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 fifth preferred version of the first aspect of the present invention or any preceding preferred version thereof the filling step includes the introduction of nonparticulate material which is in filamentary or sheet form.

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

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

According to a third aspect of the present invention there is provided a moulding method for producing a moulded component comprising the steps of: I 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 as claimed in Claim 9; 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 and allowing the material to polymerise; 5 opening the mould; 6 providing an aperture in the mould bag of the inner core tool so as to depressurise the interior of the tool; 7 withdrawing at least particulate material from the tool by way of the aperture so as to enable the mould bag to be withdrawn through the aperture.

According to a first preferred version of the third aspect of the present invention there is provided moulding method for producing a moulded component characterised by an intermediate step, during the step of introducing into the plenum volume polymerisable material, wherein a subsidiary inflatable bag located within the mould bag 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 a preferred version there of.

According to a fifth aspect of the present invention there is provided a moulded component fabricated by way of a moulding method according to the third aspect.

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

In addition the use of a subsidiary inflatable bag within the mould bag enables the tool once incorporated in a mould for a moulding operation to be inflated further to enhance contact with mould axle material. Typically this enables resin flow channels formed in the outer surface of the mould bag to facilitate/optimise resin flow over the tool. Thereafter the subsidiary bag is inflated to cause the mould bag to expand slightly so effectively smoothing out the flow channels and to enhance the finished surface of the mould article in contact with the mould bag.

There is also provided the option of using a flexible heater element or network in the skin of the mould bag to provided for a heating stage for the mould in which a mould tool according to the present invention is being used. Such a heater can still be removed to enable the tool to function as a lost core arrangement.

A 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 having been removed from the rigid mould; Figure 6 shows the bag of Figure 5 located in a working mould for the manufacture of a hollow component; and Figure 7 shows a moulded hollow component.

Figures 1 to 4 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 12A, 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, 13A. 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, 13A, 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 12A, 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 6 The rigidified bag 17 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 7 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.

The use of a subsidiary bag for further inflation within the inner core tool to consolidate any reinforcement and liquid resin enhances the resulting moulded product.

It makes use of a subsidiary bag which can also be readily removed from the interior of the inner bag on completion of a moulding operation.

Claims

CLAIMS 1A method of fabricating an inner core tool for use in lost core moulding process comprising the steps of:

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

2 introducing a 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 a evacuating duct;

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

8 removing the rigidified mould bag from the rigid mould; and

9 using the rigidified mould bag as an inner mould tool for a lost core moulding process.

2 A method of fabricating an inner core tool as claimed in Claim 1 including the further step of incorporating within the mould bag a subsidiary inflatable bag so that in the step of using the rigidified mould bag as an inner mould tool for a lost core moulding process particles within the rigidified mould bag can be subjected to pressurising by way of the subsidiary inflatable bag so as to urge the mould bag outwardly into contact with material juxtaposed with the mould bag.

3 A method of fabricating an inner core tool as claimed in Claim 2 wherein the rigidified mould bag incorporates at least one channel to facilitate the flow of liquid moulding material over the mould bag prior to the step of pressurising by the subsidiary inflatable bag; the step of inflating the subsidiary inflatable bag serving to reconfigure the mould bag so as to reduce or eliminate the or each channel.

4 A method of fabricating an inner core tool as claimed in any preceding claim wherein 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.

5 A method of fabricating an inner core tool as claimed in any preceding claim wherein the step of filling the interior of the mould bag at least in part with particles involves particles of a substantially similar size.

6 A method of fabricating an inner core tool as claimed in Claim 1, Claim 2, Claim

3 or Claim 4 wherein 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.

7 A method of fabricating an inner core tool as claimed in any preceding claim wherein the filling step includes the introduction of non-particulate material which is in filamentary or sheet form.

8 A method of fabricating an inner core tool as hereinbefore described with reference to the accompanying drawings.

9 An inner core tool fabricated according to the method of any preceding claim.

10 An inner core tool as claimed in Claim 9 wherein the mould bag includes a flexible heater element adapted for heating so as to supply heat a preliminary step of a moulding process in which the mould bag is to be used.

11 A moulding method for producing a moulded component comprising 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 as claimed in Claim 9;

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 and allowing the material to polymerise;

5 opening the mould;

6 providing an aperture in the mould bag of the inner core tool so as to depressurise the interior of the tool;

7 withdrawing at least particulate material from the tool by way of the aperture so as to enable the mould bag to be withdrawn through the aperture.

12 A moulding method for producing a moulded component as claimed in Claim

11 characterised by an intermediate step, during the step of introducing into the plenum volume polymerisable material, wherein a subsidiary inflatable bag located within the mould bag 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.

13 A moulded component fabricated by the use of a lost core inner tool as claimed in Claim 9 or Claim 10.

14 A moulded component fabricated by way of the moulding method as claimed in Claim 11 or Claim 12.