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• Large and thick complex shapes achievable
• Integration of inserts in the preform
• Short cycle times, less labour cost
• Low pressure operation (usually under 100 psi)
• Range of resin systems available
• Mechanical properties comparable to autoclaved parts (low void content <1%)
• Process can be automated, giving higher production rates and less scrap
• Bagging materials not required
• Repeatable process
• Refrigeration of resin only
• Fewer trimming and finishing operations
• Minimal handling of resins
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RTM Limitations
• Expensive, matched-mould tooling
• Not all resins have low viscosity suitable for RTM
• Can be difficult to completely wet out fibre bundle
• Not cost effective for low series production due to tooling expenses
• Improper sealing can cause messy resin overflow
• Current resins not usually as tough as prepreg matrices:
• Toughness can be achieved via secondary approach
• Additional means (e.g. binder, stitching) required for preform consolidation/stability
Hexcel’s RTM materials package
• HexFlow®RTM 6 monocomponent epoxy system, operating at service temperatures from
-60° to 180°C (-76 to 356°F)
• HexFlow®RTM 651 monocomponent BMI system, operating at service temperatures up 232°C (450°F)
Standard aerospace qualified Injectex fabrics used with HexFlow® RTM6 are:
• G0926 (5HS, 6K, 370 gsm)
• G0986 (Twill 2 x 2, 6K, 285 gsm)
• G1157 (UD, 6K, 270 gsm)
• G1151 (formable weave, 6K, 600 gsm).
All are supplied with epoxy binder on one or both sides. The binder represents approximately 5% of the
total fabric weight.
LIQUID RESIN INFUSION AND VACUUM ASSISTED RTM
Vacuum assisted RTM (VaRTM) is a variant of the traditional RTM process described above. It is a vacuum
only process for the manufacture of composite parts, typically using single-sided moulds in combination
with a specific vacuum bag set-up, providing both resin distribution and consolidation functions. The
process has advantages over conventional RTM by eliminating the need for costly matched tooling and
allowing impregnation and cure to be conducted at low, e.g. atmospheric, pressure. Consequently, simple
ovens can be used for processing rather than expensive press or autoclave equipment.
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ADVANCED FIBRE-REINFORCED MATRIX PRODUCTS FOR DIRECT PROCESSES
VaRTM is typically a three-step process:
1. Lay-up of a fibre preform
2. Impregnation of the preform with resin
3. Curing the impregnated preform.
The reinforcement is placed onto a rigid tool surface. Once the reinforcement lay-up is complete it is covered
with resin distribution media and finally with the formable vacuum bag, as shown in the diagram below.
The resin is introduced through single or multiple inlet ports depending upon part size and shape. There
may be a network of resin distribution channels integrated into the vacuum bag set-up, governing the resin
flow through the bag and the preform. A vacuum port allows the fibre preform to be air-evacuated prior to
impregnation and provides the mechanism for the transfer of resin into the part. In addition to the pressure
gradient caused by the vacuum pressure, gravity and capillary flow effects must also be considered. Good
resin flow is crucial to adequately wet out the reinforcement and ensure there are no dry areas in the cured
laminate. The viscosity depends on the chemistry of the resin and the injection temperature. The
component manufacturer typically needs to have significant processing experience to obtain high quality
components consistently.
VaRTM materials and processing techniques can achieve the 60% fibre volume necessary for high quality
composite parts.
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Advantages of VaRTM over RTM
• Larger parts possible, particularly large surfaced components, such as wind blades and fuselage panels
• Out of autoclave cure: simple oven cure in conjunction with vacuum
• VaRTM uses simple, single-sided moulds, compared to relatively costly matched-mould tooling for RTM
• Lower cost infrastructure and tooling
RESIN FILM INFUSION (RFI) AND SEMI-PREG
In Resin Film Infusion (RFI) the fibre-reinforcements and the matrix come together at the moulding stage.
Sheets of resin film are stacked and infused through a dry fabric preform, or alternatively interleaved
between individual fabric layers. The resin is cured either by vacuum assisted infusion plus oven cure, or
by using vacuum only. An autoclave can be used if required.
RFI is suitable for large, relatively flat aerospace parts. Very fast deposition rates are achieved and when
　
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