GB2410458A

GB2410458A - A moulded fibre reinforced composite product with a core

Info

Publication number: GB2410458A
Application number: GB0401689A
Authority: GB
Inventors: David Irving
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-01-27
Filing date: 2004-01-27
Publication date: 2005-08-03
Anticipated expiration: 2024-01-27
Also published as: GB0401689D0; GB2410458B

Abstract

A production system for fibre reinforced composite products with cores comprises making the core 2 and the fibre reinforced skin 1 simultaneously by resin injection. A mould has the fibre reinforced skin 5 and 6 laid into the top and bottom halves 3 and 4 of the mould, which is then clamped shut. A mix of resin and filler particles (9 figure 6) comprising the core is pumped into the closed mould, forcing the reinforcement against the mould walls at which point the injection pressure increases and surplus resin within the mix is squeezed into the fibre reinforcement. A vacuum may be used to assist the resin in impregnating the reinforcement. The core density may be adjusted by means of careful placement of the vacuum ports and inlet sprues (8 figure 4). Alternatively the core may be made lighter by using pieces of foam in the core after placement of the reinforcement. The composite products may be turbine blades, aircraft propellers, yacht rudders or dagger boards. Later embodiments relate to a production system wherein a fibre pack and a core of filler particles are injected with resin simultaneously, a production system for making syntactic foam and a production system for fibre reinforced products with cores where a styrene or other resin constituent has its boiling controlled.

Description

1 241 0458 Improvements to moulded composite products with cores This

invention relates to the simultaneous moulding of cores and the fibre reinforced skin in moulded composite products.

Many fibre reinforced composite products comprise a shaped core and a fibre reinforced skin such as: Small wind turbine blades Aircraft propellers Helicopter blades Yacht rudders and dagger boards Spoilers Large Fan blades The finished moulded shape is typically an aerofoil section, sometimes tapered and sometimes with twist. It is advantageous for the product to be made in one piece and this can be achieved by resin transfer moulding (RTM).

In this case the core must first be moulded to the exact shape of the finished part less the thickness of the fibre reinforced skin. The core can be made from 2 part polyurethane foam or a thermoses resin mixed with a filler such as glass bubbles (syntactic foam). The dry reinforcement is wrapped around the core and the whole 'pack' is placed in a mould containing seals around the flange which is clamped shut very tightly.

Resin is injected into the reinforcement cavity and cured to form the finished product, moulded in one piece, with the core inside and continuous reinforcement around the outside.

Moulding the core involves a separate operation and a separate mould. The moulded shape must be very accurate and the reinforcement must be placed exactly or resin will inject too easily where the cavity is not packed tightly enough with glass, and not at all where it is packed too tightly. The surface of the core must be abraded so the injected resin will bond to it.

Some of these problems are addressed in the Dowty patent (UK Pat no 2105633). In this case the core (in 2 part pu foam) is moulded in the main mould with the dry reinforcement pack in place so that the resulting core is exactly the correct shape.

In the 'Smartcore' process (UK pat no 2284173A) a removable core is made in a separate mould from filler particles inside a vacuum bag. The vacuumed bag will maintain its shape while the reinforcement is wrapped around and it is placed in the mould. After the resin has been injected and cured the vacuum is released and the core removed by pouring out the particles.

In all the above processes the core must be made in a separate operation.

According to the present invention both the core and the fibre reinforced skin are made in one operation in one mould. The dry reinforcement is paced in the top and bottom halves of the mould and the mould clamped shut leaving the space to be occupied by the core empty. Catalysed resin is then mixed with filler particles (glass bubbles) to form a 'runny' mix which contains a high enough proportion of resin to flow and be pumped. This mix is then pumped into the core cavity though a sprue in the mould. The mix will eventually fill the whole cavity and force the dry reinforcement against the mould. The pump injecting the resin / filler mix will see a sudden rise in injection pressure when the whole cavity is filled. The injection pressure is maintained and resin is squeezed out of the filler mix into the reinforcement pack. This can be assisted by applying a vacuum at the exit ports (witness holes). The vacuum can be turned off once the whole reinforcement pack is infused and resin appears at the witness holes. Alternatively the vacuum can be maintained and more resin is sucked out of the filler mix to produce a lighter weight core.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig 1 Shows a cross section through a typical part (in this case a small wind turbine blade) with a fibre reinforced skin on the outside 1 enclosing a core 2 inside Fig 2 Shows a cross section through a typical mould to make this part. The mould comprises a top 3 and bottom 4. The mould is opened up to load reinforcement.

Fig 3 Shows a cross section of the mould closed up, with reinforcement in place, ready for injection.

Fig 4 shows the mould during injection of the runny mix, about two thirds full Fig 5 shows a plan view of the mould and flange showing the position of inlet sprue 8 and vacuum ports 11 Fig 6 shows a cross section of a mould packed with reinforcement and dry filler particles ready for injection.

Fig 7 shows a 2 dimensional representation of the 3 dimensional packing of spheres of 2 different sizes To produce the part shown in Fig 1 dry reinforcement 5 (Fig 2) is loaded into the top mould 3 and bottom mould 4. The reinforcement can be held in place by touching it to tacky gell coat or with a spray adhesive. An overlap 6 is left on the bottom reinforcement. The mould is closed (Fig 3) and the runny mix 7 is injected into the space inside the reinforcement via a sprue 8. As the runny mix fills the core cavity it forces the overlap 6 against the top reinforcement 5.

When the core cavity is full of runny mix all the reinforcement will be forced against the sides of the mould and the pump injecting the mix will see a sudden rise in injection pressure. This pressure is maintained and surplus resin is squeezed out of the mix and into the reinforcement. A vacuum can be applied to assist this By careful placement of the vacuum ports and the inlet sprue the density of the core can be varied. If the vacuum port on the mould of, say, a rudder or small wind turbine blade were placed near the tip then more resin would be sucked out here giving a lighter core. The part of the core in the root would be much further away from the vacuum port and less resin would be sucked out giving a heavier, stronger core.

Also, if it was required to make the core even lighter then pieces of foam or other light weight material could be placed in the core cavity after placement of the reinforcement. On injection the runny mix will fill the space around the pieces of foam and then infuse resin into the reinforcement pack in the manner described above.

The advantages of this production system are that the resin only has to travel a very short distance to infuse the fibres compared to normal resin transfer moulding in which the resin has to travel through the fibre stack from the inlet port to the vent ports. Also the core will be exactly the correct shape to give an even volume fraction of reinforcement wherever the glass (or other reinforcement) is placed. The bond between the reinforcement and the core will be exceptionally good as they are both moulded simultaneously. The process is quicker and more robust. Many potentials for operator error are eliminated, such as misplacement of the glass and failure to abrade the core.

In a further version of the invention the filler particles 9 (Fig 6) are pumped, or poured into the core cavity in the mould dry. Vibration could be applied to pack the filler particles in tighter They force the dry reinforcement against the sides of the mould in the same way as the runny mix. When the core cavity is full the filler particles are held under pressure by closing the inlet sprue.. Resin is injected from a line gate along the leading edge 10 and it infuses the fibre reinforcement and the dry filler particles at the same time. The minimum size of the filler particles is selected so that the resin infuses the particles at the same rate as the fibre reinforcement. This must be found by experiment for each type of reinforcement.

If the filler particles are spheres of the same size and they are packed tightly together then approximately 70% of the volume will be occupied by the spheres leaving 30% for the resin.

If a true density of 125 kgs / cubic metre is assumed for the filler particles, and 1100 kgs / cubic metre is the density of the resin then the density of the mix will be: (0.7 X 125 + 0.3 X 1 100) = 418 kgs / cubic metre This compares with the density of PU foam at around 30 kgs / cubic metre and PVC foam at 80 kgs / cubic metre.

If it was necessary to produce a lighter core then smaller spheres could be introduced in such a proportion that they occupied the interstices between the larger spheres in a proportion of 70% spheres and 30% resin. In this case the volume occupied by the resin per cubic metre would be: 1 - (0.7 + 0.3)< 0.7) = 0.09 cubic metres And the density would be (0.7 X 125 + 0.21 X 125 + 0.09 1100) = 213 kgs / cubic metre The diameter of the smaller spheres would need to be small enough to fit easily into the interstices between the larger spheres, typically less than 5% of the diameter of the larger spheres. They would be mixed in a proportion of 1. 0 parts of larger spheres to 0.3 parts of smaller spheres by nominal volume (nominal volume means the volume of a bucket full of spheres, that is the volume of the spheres and the air between them) If it was necessary to reduce the density of the core even further then the interstices between these smaller spheres could be filled with a mix of 70% even smaller spheres and 30% resin. In this case the volume occupied by the resin 1 - [0.7 + 0.21 + (0.09/ 0.7)1 = 0.027 cubic metres And the density of the mix would be: (0.7 X 125 + 0.21 X 125 + 0.063 X 125 + 0.027 X 1100) = 152 kgs / cubic metre The diameter of the smallest spheres would need to be less than around 5% of the diameter of the middle sized spheres, and all 3 sizes of spheres would be mixed in the proportions by nominal volume: 1.0 parts large spheres, 0.3 parts medium spheres, 0.09 parts small spheres.

In practice it would be impossible to achieve perfect packing and the volume of resin would be higher. The minimum density achievable would be higher.

Vibration could be used to pack the spheres tightly together.

The smallest commercially available glass bubbles are approximately 20 microns in diameter. If these were used in the type of mix described above then the next size larger bubbles would need to be around 20 times larger or 0.4 mm diameter.

If a third size of sphere was required (for an even lighter core) then these would need be around 8mm diameter. Glass bubbles are not available in this size and would be too delicate. Expanded Polystyrene beads are available in 6 - 8mm diameter. They could be used with epoxy resin, but not polyester, to produce a lighter weight but lower strength core.

As with any resin / filler mix there will be a tendency for the lightweight filler particles to float to the top and the heavier resin to sink to the bottom. This can be used to advantage by standing the mould on end with the tip at the top and the root at the bottom while the resin is curing. This would produce a lighter weight core in the tip and a heavier, stronger, core in the root. If it was necessary to prevent this separation then the filler particles would have to be packed in tight under pressure and vibration so that the larger spheres were jammed together close to their tightest packing arrangement. Alternatively the mould could be rotated while curing.

Another way of reducing the density of the core would be to allow cantoned boiling of the styrene within the Polyester resin in the core to produce small gas bubbles within the core. Normally in vacuum infusion of Polyester or Vinylester resin it is desirable to avoid boiling the Styrene and for this reason it is normal to keep the level of vacuum to below 60% of a full vacuum (ie above 400 mbar pressure absolute) at room temperature. If the level of vacuum and / or the temperature is increased then boiling of the styrene is possible. The level of heat build up would naturally be higher in the middle of the core where it is harder for the heat to be conducted away. This effect could be used to encourage boiling within the core but not in the reinforcement where it would cause weakness. The level of vacuum and the temperature would be carefully adjusted to produce the desired level of boiling. The vent ports would be left open to allow surplus resin to be forced out by the expanding volume of gas produced by the boiling. When a sufficient volume of resin had been bled out then boiling could be stopped quickly by closing the vent ports. This would cause the pressure to rise which, in turn, would raise the boiling point. Also accelerated curing of the resin would occur where there was heat build up and this would provide a natural stop to the process. The presence of filler particles packed closely together within the core would encourage the formation of many small bubbles rather than larger ones, which would cause weakness. It is necessary for some styrene to remain in liquid form to react with polyester molecules and provide the cross-linking necessary for curing the resin, so the boiling must not be allowed to progress too far.

Alternatively extra styrene, or other reactive diluent could be added to the resin before injection.

Potential problems of making the core and skin simultaneously are exotherm and shrinkage.

Exotherm can be alleviated by making sure the core has a high volume fraction of filler particles in the thicker sections (up to 65 %). This dilutes the resin content.

Also a slow reacting resin can be used and cooling pipes can be placed in the mould. These pipes can revert to being heating pipes once the resin has gelled.

Shrinkage can be alleviated by using epoxy resin or a low shrink polyester resin.

Also holding the core under pressure while the resin gene will help to reduce shrinkage as the filler particles will be forced to butt up against each other forming a rigid structure, which is harder for the resin to distort on curing.

Claims

Claims 1. A production system for fibre reinforced composite products with

cores wherein the core and fibre reinforced skin are manufactured simultaneously and resin is infused into the fibre stack by squeezing surplus resin out of the core.
2. A production system as claimed in claim 1 wherein the core is made from a mixture of a thermosetting resin and filler particles.
3. A production system as claimed in claim 2 wherein a mixture of resin and filler particles is injected into a closed mould in which dry reinforcing fibres are placed adjacent to the mould surface
4. A production system as claimed in claim 3 wherein the injection of resin and filler particles into the space inside the reinforcement forces the reinforcement against the surface of the mould and the maintenance of pressure on the resin / filler mix forces resin out of the mix and into the fibre reinforcement.
5. A production system as claimed in any preceding claim wherein vacuum is applied to the reinforcement cavity to assist the passage of resin from the filler mix into the reinforcement fibres.
6. A production system as claimed in claim 5 wherein continued application of vacuum causes more resin to be sucked out of the resin / filler mix to produce a lighter weight core.
7. A production system as claimed in claim 6 wherein vacuum is applied at selected points to produce differential bleeding of resin from the core and hence a variable density core.
8. A production system for fibre reinforced composite products with cores wherein the fibre pack and dry filler particles comprising the core are injected with resin simultaneously.
9. A production system as claimed in claim 8 wherein dry filler particles are pumped into a closed mould in which dry reinforcement fibres are placed adjacent to the mould surface.
10. A production system as claimed in claim 9 wherein the pumping of dry filler particles into the space inside the reinforcement forces the reinforcement against the surface of the mould.
1 1. A production system as claimed in claim 10 wherein the same resin is injected into the reinforcement and into the dry filler particles comprising the core at the same time
12. A production system as claimed in claims 8 - 11 wherein the minimum size of the filler particles is chosen so that resin will inject through the filler particles at a rate compatible with that through the reinforcement.
13. A production system as claimed in any preceding claim wherein the filler particles are glass bubbles.
14. A production system for making syntactic foam wherein the filler particles are two or more distinct different sizes and they are combined in specific proporhons to produce the most efficient packing.
15. A production system as claimed in any preceding claim wherein foam pieces or other light weight pieces are placed in the cavity occupied by the core to reduce its weight even further.
16. A production system for fibre reinforced products with cores in which controlled boiling of styrene, or other resin constituent, occurs in order to reduce the density of the core.
17.A production system as claimed in claim 16 wherein temperature and pressure are adjusted to produce the desired amount of boiling.
18. A production system for moulded composite products with cores substantially as described herein with reference to Figures 1 - 7 of the accompanying drawings.