GB2586943A

GB2586943A - Method of manufacturing a water-soluble mandrel

Info

Publication number: GB2586943A
Application number: GB1900037.1A
Authority: GB
Inventors: Lange Paul
Original assignee: Aero Consultants Uk Ltd
Current assignee: Aero Consultants Uk Ltd
Priority date: 2019-01-02
Filing date: 2019-01-02
Publication date: 2021-03-17
Anticipated expiration: 2039-01-02
Also published as: GB2586943B; GB201900037D0

Abstract

Method of manufacturing water soluble mandrel comprising providing an aqueous solution of polyvinyl alcohol (PVOH) 10, mixing the PVOH with gypsum plaster to form a gypsum plaster slurry 12, setting the slurry in a mould to form a mandrel preform 14 and baking the mandrel preform at a temperature between 130 degrees C and 200 degrees C to form the mandrel 16. The aqueous solution of PVOH may comprise at least 0.5% weight PVOH, up to 18% PVOH. The aqueous solution may be formed by preparing a saturated solution at an elevated temperature and diluting with water at room temperature. The method may comprise removing air bubbles from the plaster slurry. The air bubble may be removed by subjecting the plaster slurry to a vacuum. Also claimed is a water-soluble mandrel comprising a gypsum plaster and a binder comprising polyvinyl alcohol. The mandrel may comprise a surface sealer. The surface sealer may comprise PVOH.

Description

I

TITLE: METHOD OF MANUFACTURING A WATER-SOLUBLE MANDREL

DESCRIPTION

TECHNICAL FIELD

The present invention relates generally to a method of manufacturing a water-soluble 15 mandrel for use in the fabrication of hollow composite structures.

BACKGROUND ART

Braiding, hand lay-up and filament winding techniques are used to make complex, hollow composite structures. Typically, carbon or glass fibres with a suitable resin are laid onto a mandrel in a predetermined configuration. After the laying operation is completed and the resin is cured, the mandrel is removed to obtain the desired hollow structure. Some mandrels, particularly those used to form hollow structures with complex geometries, are made of soluble material and are "washed-out" to reveal the hollow-core once the resin has been cured. For example, US 3425982 discloses a water-soluble mandrel comprising foundry sand and a binder of polyvinyl-pyrrolidone.

US 6828373 relates to a low density, water-soluble mandrel uscd for the fabrication of composite parts. The mandrel is manufactured from a blend which includes a polymeric binder, water and one or more additives selected to modify the physical properties of the binder and enhance the characteristics of the finished product. The polymeric binder is a water-soluble thermoplastic binder such as polyvinyl-pyrrolidone. The additives may include ceramic microspheres, plaster, metal particles, and graphite. The plaster can be used to improve the castability of the blend when making the mandrel, and the types of plaster that may be used includes plaster of Paris and gypsum plaster. The blend is placed into a mould form so that it may be cast, and de-watered, e.g. by allowing water to drain from the mould, either by gravity or by application of a partial vacuum. The de-watered residue is removed from the mould and subjected to a drying operation to form the water-soluble mandrel.

The present applicant has identified a number of problems with existing water-soluble 5 compositions for mandrels, and these include: * Mandrels are expensive to produce as manufacture is labour intensive.

* While a smooth finish capable of producing an A-face carbon fibre part is possible it can often involve an extensive amount of time to apply the sealer to the required surface profile finish, particularly with the sand and ceramic grit aggregate based materials.

* Production quantities of large mandrels arc difficult to produce. This is because either the machinery to produce the mandrel would need to be prohibitively large when using pressure injected ceramic grit based material, thc mandrel itself would be prohibitively heavy when using a sand based material or the mandrel would have structural weakness due to multiple castings, when using the currently available powder based materials. Multiple castings are required for a few reasons namely rapid onset of cure and excessive foaming under de-gassing to around 2x initial mix volume. This means that only a few litres can be mixed and cast at a time. With the powder based material, cracking would sometimes appear on large mandrels due to temperature differentials during the heating/cooling cycle while undergoing processing.

* The mandrels require sealing as they are porous and the resins used in the production of composite parts will infuse into the mandrel material. Once these resins infuse into the mandrel it becomes insoluble. Sealing with self-adhesive Teflon tape is labour intensive and does not produce a good enough seal for RIM (resin transfer moulding) processes. Also the overlap of the tape can result in a surface finish that is undesirable for some applications, such as fluid piping. Spray-on sealers are available which are suitable for these two processes but in order to have the correct properties for coating very porous, water soluble surfaces they seemingly by virtue do not have optimum viscosity and flow characteristics for spraying. Also around seven coats are required and this is very time consuming, which adds to the cost of mandrel production.

* Mandrels with a high aspect ratio or those with a small diameter for use in filament winding require a high flexural strength. This is not possible to achieve with the powder or ceramic grit based materials. While it is possible with the sand based material the surface finish of this material is coarse, due to the large particle size of the sand. A course surface finish on a mandrel results in a course finish on the final carbon fibre part and this is often undesirable.

The present applicant has sought to alleviate or obviate at least some of the aforementioned problems.

sTA IEMENT OF INVENTION In accordance with a first aspect of the present invention, there is provided a method of manufacturing a water-soluble mandrel, comprising: providing an aqueous solution of polyvinyl alcohol (hereinafter "PVOH"); mixing the aqueous solution of PVOH with a gypsum plaster to form a gypsum plaster slurry; setting the gypsum plaster slurry in a mould to form a mandrel preform; and baking the mandrel preform at a temperature in the range 130°C to 200°C to form the mandrel.

Setting allows dehydrated or partially hydrated calcium sulphate present in the gypsum plaster to react with water in the aqueous solution of PV0II, forming gypsum (CaSO4.2H20). The reaction rapidly gives the mandrel preform a "green" strength, allowing it to be removed from the mould and handled, for example in order to place it in an oven for baking. Baking the mandrel preform above 130°C will drive off "free" water, allowing PVOH to solidify and bind gypsum particles together, and will also dehydrate gypsum, reforming gypsum plaster which is water soluble. The solidified PVOH will act as a binder for the reformed gypsum plaster. Baking the mandrel preform below 200°C will help prevent cracking and charring of solidified PVOH. Typically, the mandrel preform is baked at a temperature in the range 150°C to 180°C.

The aqueous solution of PVOH may comprise at least 0.5% by weight PVOH. The aqueous solution of PVOH may comprise up to 18% by weight PVOH, possibly up to 15% by weight PVOI I, and even up to 10% by weight PVOH. For example, the aqueous solution of PVOH may comprise between 0.5% and 10% by weight PVOH, possibly even between 6% and 8% by weight PVOH.

Providing the aqueous solution of PVOH may comprise dissolving PVOI I in water at a temperature above 50°C. In one form, providing the aqueous solution of PVOH may comprise dissolving at least 150g of PVOH per litre of water at the temperature above 50°C. For example, the method may comprise dissolving 180g -200g of PVOH per litre of water at about 85°C. The solution of PVOH may be substantially saturated at the elevated temperature. The PVOH may have an average molecular weight of at least 50,000, perhaps 60,000 -120,000, and possible about 72,000. In one form, providing the aqueous solution of PVOH may further comprise cooling the PVOH dissolved in water to room temperature, and diluting with water to achieve a viscosity of up to about 60eP at 25°C. A biocide (such as Didecyl Dimethyi Ammonium Chloride "DDAC") may be added if long-term storage of the solution will be required.

l'he gypsum plaster may comprise a powder with a maximum particle dimension in the range 40)tm -200 pm, and an average particle dimension in the range lp.m -40um. The aqueous solution of PVOH may be mixed with the gypsum plaster such that the gypsum plaster accounts for at least 50% by weight of the plaster slurry. For example, the gypsum plaster may account for 50% -75% by weight of the plaster slurry.

The method may further comprise removing air bubbles from the plaster slurry, preferably before setting the plaster slurry in the mould. Removing air bubbles may comprise subjecting the plaster slurry to a vacuum. For example, the solution of PVOH may be mixed with the gypsum plaster under a vacuum or partial vacuum, for example in a vacuum mixer. Alternatively or additionally, removing air bubbles may comprise adding an anti-foaming agent when mixing the aqueous solution of PV0I-1 with the gypsum plaster. The method may further comprise incorporating a retarding agent to the gypsum plaster slurry. For example, retarding agent may be added when mixing the aqueous solution of PVOH with the gypsum plaster.

Baking the mandrel preform may continue until the mandrel formed has a mass which is at least 30%, perhaps even at least 55%, less than that of the mandrel preform, due to dehydration. For example, the mass of the mandrel formed may be about 50% of that of the mandrel preform before baking.

The method may further comprise coating the mandrel with a sealer. The sealer may comprise a further aqueous solution of PVOH. The further aqueous solution of PVOH may comprising at least 10% by weight PVOH, possibly at least 15% by weight PVOH. The further aqueous solution of PVOH may be formed by dissolving PVOH in water at a temperature above 50°C, before cooling the solution to room temperature. In one form, providing the further aqueous solution of PVOH may comprise dissolving at least 150g of PVOH per litre of water at the temperature above 50°C, for example by dissolving 180g -200g of PVOH per litre of water at about 85°C. The resulting solution of PV0I-1 may be substantially saturated at the elevated temperature. As before, the PVOH may have an average molecular weight of at least 50,000, perhaps 60,000 -120,000, and possibly about 72,000.

In one form, the sealer may further comprise a diluent, such as isopropyl alcohol. The diluent may be included to improve the drying characteristics of the fiirther aqueous solution of PVOH. With the diluent added, the sealant may comprise between 3% and 8% by weight PVOH, possibly even between 3% and 7% by weight PVOH. For example, the further aqueous solution of PVOH and the diluent may be mixed in a ratio of about 1:1 by volume for a relatively thick sealer, and in a ratio of about 1:2 for a relatively thin sealer. Different sealer thicknesses or viscosities may be advantageous, with the relatively thick sealer being more appropriate for mandrels with a relatively low PVOH content, and the relatively thin sealer being more appropriate for mandrels with a relatively high PVOH content.

In one faun, coating the mandrel with the sealer may comprise dipping the mandrel in the sealer. The sealer coating may air dry, or may be dried in an oven.

In accordance with a first aspect of the present invention, there is provided a water-soluble mandrel comprising a gypsum plaster and a binder comprising polyvinyl alcohol.

The water-soluble mandrel may comprise at least 50% by weight gypsum plaster, possibly even at least 75% by weight gypsum plaster. The water-soluble mandrel may comprise at least 0.2% by weight PV0II. The water-soluble mandrel may comprise less than 10% by weight PVOH. For example, water-soluble mandrel may comprise between 2% and 5% by weight PVOH. The water-soluble mandrel may comprise a biocide, such as DDAC. The water-soluble mandrel may also comprise an anti-foaming agent.

The gypsum plaster may comprise a powder with a maximum particle dimension in the range 40um -200um, and an average particle dimension in the range lum -40um. The water-soluble mandrel may comprise a surface sealer. The surface sealer may comprise PVOH.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described with reference to the accompanying drawing in which: Figure 1 shows a flow chart illustrating steps in a method of manufacturing a water-soluble mandrel.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENT

Figure 1 shows steps in a method of manufacturing a water-soluble mandrel. An aqueous solution of polyvinyl alcohol (hereinafter "PVOH") is provided at step 10. The aqueous solution of PVOH comprises between 0.5% and 18% by weight PVOH, and possibly between 0.5% and 10% by weight PVOH. The aqueous solution of PVOH is mixed with gypsum plaster (CaSO4.0.5 H20) at step 12 to form a gypsum plaster sluny. The gypsum plaster shiny comprises at least 50% by weight gypsum plaster. Typically, a vacuum mixer is used to reduce foaming and prevent air bubbles from becoming entrained in the gypsum plaster slurry. The gypsum plaster slurry is poured into a mould at step 14, where it is allowed to set to form a mandrel preform. Setting occurs due to the reaction of water in the aqueous solution of PVOH with the gypsum plaster, resulting in the formation of gypsum (CaSO4.21-120). The mandrel preform is removed from the mould, and is then baked at step 16 at a temperature of between 130°C and 200°C. Baking the mandrel preform above 130°C will drive off -free" water, allowing PVOH to solidify and bind gypsum particles together, and will also dehydrate gypsum, reforming gypsum plaster which is water soluble. The solidified PVOH will act as a binder for the reformed gypsum plaster.

EXAMPLE 1

An aqueous solution of PVOH is prepared by dissolving 180g-200g of PVOH with a molecular weight of 72,000 per litre of water, heated to a temperature of 85°C. The solution is cooled to room temperature, whilst stirring as solution will be substantially saturated with PVOH. Once at room temperature, the solution is diluted with water until a viscosity of up to about 60c11 at 25°C is achieved. The amount of water which needs to be added to achieve this viscosity will depend upon the starting viscosity of the aqueous solution of PVOH, but typically Ito 2 litres of water will be added to each litre of the aqueous solution of PVOH. Optionally, 0.25 g of a biocide DDAC is added to the aqueous solution of PVOH, and may be desirable if the aqueous solution of PVOH is to be stored for future use.

The gypsum plaster slurry is prepared by mixing 0.8 Kg of the diluted aqueous solution of PVOH (with viscosity of 60 cP) with 1 Kg of gypsum plaster in a vacuum mixer. The gypsum plaster is a powder with a maximum particle dimension in the range 40pm -200pm, and an average particle dimension in the range lpm -40pm. An anti-foaming agent is added to the gypsum plaster slurry.

The gypsum plaster slurry is immediately poured into a mould where the gypsum plaster crystallises into gypsum and sets to form a mandrel preform. Setting time can be anywhere from 20 minutes to a few hours, depending if and how much retarder is added. Typically for 1.8 Kg gypsum plaster slurry, 0.6 grams of retarder (trisodium citrate) is added, providing a setting time of around 45 minutes at 25°C. The mandrel preform is then removed from the mould and baked in an oven at 150°C or above until there is a 45 -50% loss in mass due to evaporation of water, resulting in chemical decomposition of gypsum back to gypsum plaster which is bound together by solidified PVOH Baking time is generally between 3 to 30 hours, depending on volume and aspect ratio. For example a mandrel with a volume of approx.1 litre; diameter 32 mm x 300mm will require roughly 4 hours at 165°C. A block with a volume of 4 litres; 200mm x 200mm x 100mm will require around 30 hours.

The mandrel is ready for carbon fibre application, but may be sealed before use. Following curing of the carbon fibre, the mandrel is simply washed out with water-the solidified 20 PVOH dissolves, releasing the gypsum plaster.

EXAMPLE 2

For a mandrel to produce a carbon fibre pipc elbow with an internal diameter of 150min and a length of 200mm high bulk mechanical properties are generally not required. Also less oven-time and faster washout are desirable. This can be obtained by reducing the fraction of PVOH in the mandrel makeup: An aqueous solution of PVOII is prepared by dissolving 180g-200g of PV0II with a molecular weight of 72,000 per litre of water, heated to a temperature of 85°C. The solution is cooled to room temperature, whilst stirring as solution will be substantially saturated with PVOH. Once at room temperature, the solution is diluted with water until a viscosity of 10cP at 25°C is achieved.

The gypsum plaster slurry is prepared by mixing 3.2 Kg of the diluted aqueous solution of PVOH (with viscosity of 10 cP) with 4 Kg of gypsum plaster in a vacuum mixer. 24 grams of an anti-foaming agent and 2.4 grams of a retarding agent arc also added to the gypsum plaster slurry.

The gypsum plaster slurry is immediately poured into a mould where the gypsum plaster crystallises into gypsum and sets to form a mandrel preform after approximately 45 minutes at 5 25°C -35°C. The mandrel preform is then removed from the mould and baked in an oven at I65°C until there is a45% -50% loss in mass due to evaporation of water, resulting in chemical decomposition of gypsum back to gypsum plaster which is bound together by solidified PVOH. After sealing the mandrel is ready for carbon fibre application. Following curing of the carbon fibre, the mandrel is simply washed out with water: the solidified PVOH dissolves, 10 releasing the gypsum plaster.

EXAMPLE 3

For a mandrel having an approximate volume of 2 litres to produce a braided carbon fibre strut with steps in internal diameter from 35mm to 40mm High mechanical strength, dimensional stability and ability to withstand some moisture ingress without warping during the sealing procedure is desirable. An aqueous solution of PVOH is prepared by dissolving 180g-200g of PVOH with a molecular weight of 72,000 per litre of water, heated to a temperature of 85°C. The solution is cooled to room temperature, whilst stirring as solution will be substantially saturated with PVOH. Once at room temperature, the solution is diluted with water until a viscosity of 60cP at 25°C is achieved.

The gypsum plaster slurry is prepared by mixing 1.6 Kg of the diluted aqueous solution of PVOH (with viscosity of 60 cP) with 2 Kg of gypsum plaster in a vacuum mixer. 6 grams of an anti-foaming agent and 0.45 grams of a retarding agent are also added to the gypsum 25 plaster slurry.

The gypsum plaster slurry is immediately poured into a mould where the gypsum plaster crystallises into gypsum and sets to form a mandrel preform after approximately 35 minutes at 25°C -35°C. The mandrel preform is then removed from the mould and baked in an oven at 1 65°C until there is a 45% -50% loss in mass due to evaporation of water, resulting in chemical 30 decomposition of gypsum back to gypsum plaster which is bound together by solidified PVOH. After sealing the mandrel is ready for carbon fibre application. Following curing of the carbon fibre, the mandrel is simply washed out with water: the solidified PVOH dissolves, releasing thc gypsum plaster.

Claims

CLAIMS: 1. A method of manufacturing a water-soluble mandrel, comprising: providing an aqueous solution of polyvinyl alcohol (hereinafter "PVOH"); mixing the aqueous solution of PVOH with a gypsum plaster to form a gypsum plaster slurry; setting the gypsum plaster slurry in a mould to form a mandrel preform, and baking the mandrel preform at a temperature in the range 130°C to 200°C to form 10 the mandrel.
2. A method according to claim 1, wherein the aqueous solution of PVOH comprises at least 0.5% by weight PVOH.
3. A method according to claim 1 or claim 2, wherein the aqueous solution of PVOH comprise up to 18% by weight PVOH.
4. A method according to any one of claims 1 to 3, wherein the aqueous solution of PVOH is prepared by forming a saturated solution at elevated temperature, before diluting with water at room temperature.
5. A method according to any one of claims 1 to 4, wherein the PVOH has an average molecular weight of at least 50,000.
6. A method according to claim 5, wherein the PVOH has an average molecular weight in the range 60,000-120,000.
7. A method according to any one of claims 1 to 6, wherein, the gypsum plaster comprises a powder with a maximum particle dimension in the range 40pm -200pm, and an average particle dimension in the range Him -40pm.
8. A method according to any one of claims 1 to 7, wherein the aqueous solution of PVOH is mixed with the gypsum plaster such that the gypsum plaster accounts for at least 50% by weight of the gypsum plaster slurry.
9. A method according to any one of claims 1 to 8, further comprising removing air bubbles from the plaster slurry.
10. A method according to claim 9, wherein removing air bubbles comprises subjecting the gypsum plaster slurry to a vacuum or partial vacuum.
11. A method according to any one of claims 1 to 10, further comprising adding an anti-foaming agent when mixing the aqueous solution of PVOH with the gypsum plaster.
12. A method according to any one of claims 1 to 11, further comprising incorporating a retarding agent to the gypsum plaster slurry.
13. A method according to any one of claims 1 to 12, wherein baking the mandrel preform continues until the mandrel formed has a mass which is at least 30%, perhaps even at least 5 40%, less than that of the mandrel preform, due to dehydration.
14. A method according to any one of claims 1 to 13, further comprising coating the mandrel with a sealer.
15. A method according to claim 14, wherein the sealer comprises a further aqueous solution of PVOH.
16. A method according to claim 15, wherein the sealer further comprises a diluent, such as isopropyl alcohol.
17. A method according to any one of claims 14 to 16, wherein coating the mandrel with the sealer comprises dipping the mandrel in the sealer.
18. A water-soluble mandrel comprising a gypsum plaster and a binder comprising 15 polyvinyl alcohol.
19. A water-soluble mandrel according to claim 18, comprising at least 50% by weight gypsum plaster, possibly even at least 75% by weight gypsum plaster.
20. A water-soluble mandrel according to claim 18 or claim 19, comprising at least 0.2% by weight PVOH, and possibly between 2% and 5% by weight PVOH.
21. A water-soluble mandrel according to any one of claims 18 to 20, wherein the gypsum plaster comprises a powder with a maximum particle dimension in the range 40m -200p.m, and an average particle dimension in the range Igm -40pm.
22. A water-soluble mandrel according to any one of claims 18 to 21, further comprising a surface sealer.
23. A water-soluble mandrel according to claim 22, wherein the surface sealer comprises PVOH.