The practical application of the vacuum injection
technique in boat building.
The injection of the hull of the Contest 55.
A r l e n Koebergen
Centre of Lightweight Structures TUD-TNO P.O. Box 5058
2600 GB Delft, The Netherlands
Abstract
Closed mould injection techniques (Resin Transfer Moulding, RTM) have been successfully applied for small products in large series. This has proven the advantages of a closed mould injection technique. The application of a closed mould injection technique for large products in small series (i.e. boats) is hardly practised, although the technology exists! The risk of failure is often considered too high.
An introduction into the resin injection technology is given. It will be shown that a large product needs to be injected using vacuum (with a resulting maximum pressure difference of 1 bar). The following main options to optimise the injection fill time are identified:
1. Optimisation of reinforcement permeability: 2. Choice of injection strategy.
The general procedure for the injection of a large product is given, focusing on one of the most critical aspects: proper positioning and securing of the dry reinforcement and cores. Finally, the injection method for the hull of the Contest 55 is described in detail as well as the main production steps.
Although the RTM process and the vacuum injection technique already exists for quite a while, transfer of the technology is well possible but most of the times not simple and straightforward. However, this paper shows that a structured approach will lead to the successful vacuum injection of any product. To aid dissemination of the technology a process control tool to ensure the successful injection of a large product needs to be established.
1. Introduction
Closed mould injection techniques (Resin Transfer Moulding, RTM) have been successfully applied for small products in large series [ 1 , 2, 3 ] . This has proven the advantages of a closed mould injection technique (better working conditions, less styrene emission from polyester or vinylester resins, better quality products and less labour costs !). The application of a closed mould injection technique for large
products in small series (i.e. boats) is hardly practised, although the technology exists! Boat builders and other composite manufacturers often want to switch to a closed mould (i.e. vacuum injecdon) technique. However, the risk of failure is often
considered too high. This is mainly due to a lack of knowledge of the technology and a lack of process control tools. This paper will show that a stnjctured approach will lead to the successful vacuum injecdon of any product. Before going into details about the practical application and the injection of a hull of the Contest 55 an introduction into the resin injection technology is given. The main variations in the technology and the most important processing parameters ai-e discussed. The general procedure for the injection of a large product is given, focusing on one of the most critical aspects: proper positioning and securing of the dry reinforcement and cores. Finally, the injection method for the hull of the Contest 55 is described in detail as well as the main production steps.
2. The resin injection technology
RTM, VARI, SCRIMP, Infusion, Vacuum bagging [4, 5, 6, 7, 8]. There are many words and many abbreviations which try to describe a process which is basically very simple:
Dry reinforcement is placed in a mould, the mould a closed and resin flows into the mould and impregnates the reinforcement. The driving force for the flow of the resin is a pressure difference.
Why do people use all these names and abbreviations? Because people 'invent' variations on the basic principle and want to give it a new and unique name. To fully appreciate the resin injection technology one needs to understand the basic principle and the possible variations.
The filling time of the injection of a rectangular strip can be calculated [ 9 ] using equation (1).
fill - time = -T—~- ( 1 )
2 K * A P ^ ^
With 9 porosity of the reinforcement
K permeability of the reinforcement
T| viscosity of the resin
1 flow distance (length of the strip)
Equation (1) clearly shows how different parameters influence the fill time and hence the cycle time of a product. The pai'ameters can be split in:
• Material properties: • resin viscosity
• reinforcement porosity and permeability • Product properties:
• size, volume and shape (influencing flow distance) • Process properties
• Pressure difference
• Injection strategy (influencing flow distance)
2.1 Pressure and mould type
The pressure difference applied and the size of the product also determine the type of mould (besides other aspects like product series and surface quality required). This gives us a first classification of resin injection techniques as shown in table 1. Resin transfer moulding and vacuum assisted resin injection require süff moulds to prevent deformation of the mould due to the injection pressure. Vacuum injection however, can be practised using one flexible or semi flexible mould halve (either single-use foil, re-usable foil or (silicon) rubber pads or thin fibre reinforced plastic shells). Large products can only be manufactured cost effectively using a vacuum injecdon technique, since the use of pressure would require very stiff and very costly moulds. This however, implies that a maximum pressure difference of approximately
1 bar can be achieved.
Table 1. Classification of injection techniques based on applied pressure difference.
Injection technique Applied pressure difference
Injection technique
Inlet port Oudet port
Resin transfer moulding -1- 1
Vacuum assisted resin injecdon + 0
Vacuum injecdon 1 0
+ : indicates a pressure higher than atmospheric pressure 1 : indicates atmospheric pressure
0 : indicates a pressure below atmospheric pressure (vacuum)
2.2 Material properties
The driving force in vacuum injection is limited to 1 bar. In order to achieve a mould fining process (and cycle time) that is fast enough it is necessary to optimise some material properties. These properties are the viscosity of the resin and the porosity and permeability of the reinforcement. However, viscosity of the resin is usually limited to 100 to 500 mPas. The fining time is directly related to the viscosity (see equation 1). Consequentiy there is not much to be gained in trying to optimise resin viscosity. The porosity of most reinforcements used is in between 0.5 and 0.85. The permeabihty of reinforcements however, can differ greatly. In addition core materials such as Balsa contourkore can have channels which not only fill with resin but can also have a
relative high permeability. Accurate measurement of permeability is difficult [10]. Approximate values of porosity and permeability of materials are shown in table 2. Table 2. Permeabilitiy and porosity of (reinforcement) materials.
Material porosity
[-]
peiineability [e-lOm^]
Saertex 935 0/90/random lay-up 0.63 0.6
Unifilo816 450 random mat 0.77 5
Rovicore 450/D3/450 random mat + 'core' 0.83 50
Balsa 1" core 0.11 33
Feeder material 0/90 grid 0.95 . 100
Feeder material braiding 0.95 500
A product is usually not made up of only one type of reinforcement. Different
materials are combined to get the required lay-up. The permeability of such lay-up can be roughly calculated from the permeability of the constituent materials using a simple rule of mixtures. However, this rule of mixtures is not very accurate, especially when the permeability of the constituent materials differ greatly. This is due to
3-dimensional flow that occurs, which results in impregnation of the poorly permeable layer through resin flow from a highly permeable layer. Knowing this effect, we can use highly permeable layers as resin feeder material. This can be applied either within the laminate (using Balsa, Rovicore, Multimat or sometimes even a continuous random glassmat) and creating a resin rich core, or outside the laminate (using any feeder material). The feeder materials can be separated from the actual laminate using a peel ply. In this way this feeder layer can be removed from the laminate after cure. Cores applied in the laminate can be very useful in creating permeable channels allowing a quick injection of the laminate.
2.4 Iiyection strategy
The pressure difference is limited to 1 bar and material properties are only to be varied within limits. Therefore, we need to optimise the injection strategy in order to achieve an acceptable filling time for a lai"ge product. There are only thi-ee basically different injection strategies (shown in figure 1). A l l other injection strategies are always a combination of. the following three:
1. Edge injection 2. Point injection 3. Peripheral injection
The filling time is greatly influenced by the injection strategy. This shown by 8 examples of possible injection strategies for a flat rectangular plate of 1 x 2 metre and 10 mm thickness (see table 3 and figure 2). The fastest injection strategy is about 25 times faster than the slowest injection strategy!
I . Edge injection • ] J 2. Point injection
_^ 1
i 1t
i
-tt
3. Periplieral injectionFigure 1. The 3 basic injection strategies.
From these simple examples we can generally conclude that injection time is governed by:
• Injecdon distance.
• Ratio of length of the injection channel and length of the flowfront. Table 3. Influence of injection strategy on fill time.
Injection strategy Fill time
1 Edge injecdon short side 400
2 Point injection (1 injection point) 200
3 Peripheral injection 20
4 Edge injecdon long side 100
5 Point injection (2 injection points) 100
6 Edge injection with additional injection channels 20
7 Sequential edge injection with additional injection channels 30
8 'Edge' injection with a grid of injection channels 15
2.5 Conclusions
When we want to inject a lai-ge product we need to use vacuum injection (with a maximum pressure difference of 1 bar). We can identify the following options to optimise the injection fill time.
1. Optimisation of reinforcement permeability:
• Use permeable reinforcement (random mat type materials) • Use permeable core materials (channels)
• Use peiTneable feeder materials 2. Choice of injection strategy.
3. How to inject a large product ?
For any product that we want make we can identify the following general procedure: 1. Define product requirements
2, Define processing requirements 3. Identify processing method
4, Describe and elaborate production steps
In the next chapter these four items will be discussed based on the hull of the Contest 55. Especially the processing method and the injection strategy will be discussed in detail in chapter 4. Before going into detail about this injection the overaU production steps will be indicated. One aspect, which is of crucial importance, will be treated thoroughly: the positioning and securing of the dry reinforcement. Another aspect, the selection and formulation of the resin, is also crucial but will not be treated in detail here. However, one should beai- in mind that resin properties like viscosity, geltime and peakexotherm can be critical and should be thoroughly discussed with the resin supplier.
3.1 Production steps
Based on product and process requirements and tlie cliosen injection strategy the production steps can be elaborated and fully described. Generally these steps are:
1. Mould preparation 2. Apply gelcoat
3. Apply first laminate (hand lay-up or spray-up) 4. Position and secure dry reinforcement and core
5. Position and secure injection channels and outlet ports and channels 6. Seal mould with foil
7. Test for leakage and final check 8. Inject resin
9. Cure resin
10. Demould product
Most of the above indicated steps are quite straightforward and easy but require dedicated personnel and accuracy. One of the most critical steps is the positioning and securing of the dry reinforcement. Initially this seems rather surprising. However, poor securing can cause the diy layers to move or even fall down. Poor positioning can lead to highly peimeable channels. Because of its importance this aspect is treated in detail below,
3.2 Positioning and securing of reinforcement and cores
In the traditional hand lay-up or spray-up method the sticky resin also ensures that the reinforcement remains in place. Dry reinforcement layers on vertical mould parts however, can move, slide and even fall down. In addition the dry reinforcement layers are bulky and resilient. On applying the vacuum in the mould this can lead to folds and wrinkles, mainly occurring around edges. Depending on the size and the
compression these folds and wrinkles can lead to highly permeable channels causing resin raceti-acking and the occurrence of dry spots. Also cores must be positioned and secured properly for the same reason (i,e, occurrence of channels in between different sheets of balsa core).
There are 3 different ways of securing: 1. Temporary mechanical fasteners 2. Permanent mechanical fasteners
3. Adhesives
Clearly it is a requirement that these methods do not influence the laminate properties. The options given above are listed in order of priority. Temporary mechanical
fasteners (such as clips and clamps) ai-e always preferred when possible. Otherwise permanent mechanical fasteners can be used (such as staples) which can be very convenient for securing reinforcement layers on cores.
Adhesives are veiy convenient to use as well however, they need to be used with caution because only little is known about the short or long term effect on laminate properties. Therefore we have investigated several adhesives on both ease of application and the influence on laminate properties.
3,2.1 Experimental set up
After a first evaluation on ease of application and bonding sti-ength 4 adhesives were selected out of 12. These 4 systems ai-e:
A. muld-purpose adhesive spray in an organic solvent (cyclohexane) B. polyethylene hot melt
C. polyurethane hot melt D. cyano-aciylate adhesive
Firstly, for each adhesive the required amount (g/ffl^) to achieve a proper bond strength was determined. Secondly, test laminates were manufactured using the respective determined adhesive amounts. The build up of the test laminates was chosen to resemble the build up to be used for the manufacture of the deck. This test laminate build up contains:
• gelcoat
• hand lay-up layer (225 g/m^) • adhesive • Saertex 935 • adhesive • Rovicore 450/D3/450 • adhesive • Rovicore 450/D3/450 • adhesive • Saertex 935
A reference laminate (without adhesive) was manufactured to be able to compai'e the results. In addition laminates were aged using rather severe cyclic temperature changes (6 hrs, -20°C and 6 hrs 80°C).
Subsequently the following material properties were determined: • Inter laminar shear strength (ILSS)
• 3-point-bending strength • 3-point-bending modulus
These material properties were detennined at 23°C and 50%RH for laminates aged 0. 50 and 100 cycles, and in addition at 80°C for the laminates which were not aged.
3.2,2 Results
The test results are shown in figures 3 to 5. They show the influence of ageing (0, 50 and 100 cycles) and temperature (80°C) for the laminates with different adhesives on respectively the ILSS, bending strength and bending modulus. Adhesive C and D showed clear- decreases in interlaminar shear strength and bending strength, especially at higher temperahires and after ageing, compared to the reference laminate. Adhesive A also showed a decrease, but less severe than C and D, Adhesive B showed hai'dly any difference in material properties compared to the reference laminate. This adhesive was therefore selected for further testing to investigate the sensitivity for the amount of adhesive used. The results are shown in figure 6, From these results it is clear that more adhesive results in lower mechanical properties, although bending modulus is hardly affected, the ILSS and bending strength drops to about half the value of the reference laminate. 25 g/m^ is sufficient for achieving proper bonding
strength. Twice or even three times this amount appeal's to be acceptable without a too large influence on the material properties. This encourages the use of this adhesive since accidentally spraying too much will occur occasionally and this will not result in a dramatic decrease of properties.
Raftiwwi A B C
Lamlnalt
Figure 3. T h i hlTuence of adhisiws and ageing on Lurinatc ILSS
Umlnstt
Figure 5. Innuence of adhesi^-cs and ageing on turmalo bcndhg mxlulis
Figure 4. Influence of adhesives and ageing on hmmaic bcndiig sirengili
NocfingnnUui birKTngitrtogth
m•c^anlo«tprop•^ty
Figure 6. T h : Wtucncc ofadheshü atTBunt on mechancfil propcnics
3.2.3 Conclusion
It is rather surprising that a polyethylene hot melt is hardly influencing the material properties, compared to the other adhesives. However, this can be explained by taking into account that the hot melt was sprayed on the laminate in small droplets. This will result also in small droplets of PE in the cured laminate. These small droplets can be compared with voids, since there will be no interaction between the PE droplets and the resin, Because the droplets are always sprayed very evenly on the reinforcement the mechanical properties are not influenced severely. The clear advantage of the hot melt against an adhesive spray is the absence of organic solvents. This not only is much healthier for the employees, it is also rather contradictoiy to use a closed mould injection technique preventing styrene emission and simultaneously using an adhesive spray consisting primarily of an organic solvent.
4. The injection of the hull of the Contest 55
4.1 Product requirements and description
The product is the hull of a Contest 55. Table 4. Contest 55 hull data
A new yacht of the Conyplex shipyard. Length hull 16.40 m
Outer dimensions were fixed (see table Width hull 4.50 m
4). Conyplex decided to try to realise a Height hull 2.50 m relatively simple mould: a female mould Length waterline 13.55 m made from laminated wood WITHOUT
the help of a male plug (i.e. a female plug was made directly, which would act as the female mould directly). Consequently the hull would need filling, sanding and polishing to achieve the required surface finish.
Designer Dick Zaal was responsible for the laminate lay-up. The proposed hand lay-up / spray-up laminate was slightly adjusted to facilitate the injecdon. The main laminates build up of the hull (laminate codes A to E) are shown in table 5, A description of the different materials used can be found in table 6, The first hull would be made as a Balsa sandwich shell consti-uction (without sdffeners, skeg or stringer integrated). Integration of stiffeners, skeg and stringer were to be realised at a second injection.
Table 5, Main laminates build up.
material L a m A L a m B L a m C L a m D L a m E outside M450 450 450 450 450 450 Saertex 935 935 935 935 935 935 Saertex 935 935 935 935 935 935 Saertex 935 935 935 935 Saertex 935 935 Saertex 935 935 Saertex 935 935 Saertex 935 935 Saertex 1128 1128 Saertex 1128 1128 Saertex 1128 1128 M450 450 450 450 450 M450 450 450 450 450 450
Balsa 24 mm Balsa Balsa Balsa Balsa Balsa
M450 450 450 450 450 450 M450 450 450 450 450 Saertex 935 935 935 935 935 935 Saertex 935 935 935 935 935 Saertex 935 935 Saertex 935 935 Saertex 1128 1128 1128 Saertex 1128 1128 1128 Saertex 1128 1128 1128 M450 450 450 450 inside Total glass (g) 5540 6440 7375 15919 12758
Table 6. Materials used in the hull.
Material Type and build-up Supplier
M450 Random glass mat 450 g/m
UnifiloU816
Vetrotex
International S.A. Saertex 935 Bi-directional glass fabric
0° - 335 g W 1200 tex 90° - 288 g/m^ 600 tex CSM - 300 gin? 2400 tex
Saertex Wagener GmbH & Co KG
Saertex 1128 Bi-directional glass fabric 0° - 480 g/m^ 1200 tex 90° - 336 g/m^ 600 tex CSM - 300 gin? 2400 tex
Saertex Wagener GmbH & Co KG
Balsa 24 mm Balsa contourkore
thickness 24 mm
Baltek Corporation
4.2 Process requirements
Since the Contest 55 hull would be the first to be made with vacuum injecdon, it is clear that Conyplex was unfamiliar with the vacuum injection technique. Therefore, one of the main process requirements was to keep the process as simple as possible and easy to control. Furthermore, the process should also be feasible for all other Contest yachts.
4.3 Injection method
4.3.1 Overall injection strategy
The Contest 55 hull is both a long (16,4 m), wide (4.5 m) and high (2.5 m) product to inject in one shot. The height results a hydrostatic pressure difference which can results in a lower driving force for the injecdon. There are three basic variations possible on the edge injection strategy to distinguish:
1. Edge injection downward 2. Edge injecdon upward 3. Edge injecdon sideways
The injecdon downward is not preferred for two reasons. Firstly, bubbles in the resin and laminate will be entrapped more easily and secondly, there is higher risk on the occurence of dry spots due to racetracking of the resin thi-ough highly permeable mnner channels.
The injecdon upwai-d can only be can-led out with sequential injection channels (otherwise injection time will be too long). This however, would result in about 10 sequential injection channels each of which need to be controlled and opened at the right time,
Based on the requirement to have a simple process there was a clear preference for an injection with a grid of injection channels in which the majority of the channels would allow for a sideways injection. This would allow for one single resin inlet port (and consequentiy also one resin tank level to control). This grid would consist of a main
injection channel running from the stern to.the bow (via the keel) and branches of channels from the main channel in the keel to the flange (with the deck). The overall injecdon channel pattern is shown in figure 7.
Figure 7. The injection strategy and position of the injection channels.
4.3.2 A local injection problem
Based on the overall chosen injection strategy we must verify that there are no local details which can cause the formation of dry spots or else disturb the injection. The Balsa core was sufficiendy peiTneable. Therefore there was no additional feeder
material required on all lanunates containing Balsa. However, the laminate used in the ' keel would have a low overall pei-meability. This could cause dry spots on the keel which were clearly shown using flowfront simulations. Figures 8 and 9 show the simulations done without (figure 8) and with (figure 9) additional feeder material on top of the reinforcement. A small scale experiment was caitied out to validate this
simulation result. A small part of the hull laminate was injected on a flat plate. This part consisted of the keel laminate.(pardy with and partiy without additional feeder material) and the balsa sandwich laminate shown in figure 10. It was injected using a main
injection channel at the edge of the keel laminate and an injection channel branch in the middle of the laminate. The carefully registered flowfront.proved the occurrence of a diy spot on the part without the feeder material and the absence of a diy spot on the part with the additional feeder material.
Figure 8. Part of the hull with dry spot Figure 9. Part of the hull without dry formation at the location of the keel. Spot formation.
branch channel side view
Balsalrgmirtate V/ithoutfeeder With-fe9der \ flow front -Main injection channel
4.3.3 Final injection simulation
Finally the injection of the complete hull was simulated using the simulation program Pi7 (developed by TNO; future simuladons are carried out using RTMworx of
Polyworx, the successor of Pi7). The hull and all laminate properties were modelled, Simulations were carried out to optimise injection channel position and length. The final injection simulation is shown in figure 11.
• 7 i n n 1.17 HTM • i i n u i j l . u n IK \ P I 7 _ n i H fnM\HnDFtS\iLi.i|iiJ,<t,i.ilil,.ri | i i 7 | B O D
Fib Ed* W E W ShUjiiijn Qt^idli C J o i t tl^W
r i u i H G r i H C - i o '
Figure 11. Flow front simulation of the injection of the hull of the Contest 55,
4.4 Production
For the production of the hull of the Contest 55 the following steps were undertaken. Mould preparation • ' .
The female mould was checked on leakage and a release agént was applied. Apply gelcoat •
A transparent ISO-NPG gelcoat was applied Apply first laminate
A first laminate (450 g/m^ random mat) was applied with hand lay-up. This layer is applied for two reasons; Firstly it protects the gelcoat when the dry reinforcement is positioned, secondly it provides a high quality layer óf iso-phtalic polyester resin, while the injection resin is an otho-phtalic polyester.
Position and secure dry reinforcement and core
All layers of dry reinforcement and the balsa core are posidoned and secured. Main methods of securing are clamping on the flange (see picture 12) and stapling. Only when these methods were not sufficient the PE hot-melt was used for securing the reinforcement locally.
Posidon and secure injection channels and oudet ports and channels
The injection channels were positioned and secured with the PE hot-melt. The resin inlet tube was fixed to the main injection channel at the stern. Oudet ports (permeable strips) were positioned at the flange, in between injection channel branches. These outlet ports were 'connected' to a vacuum channel which was located on both flange sides from stern to bow. This vacuum channel was connected to the vacuum pump (with an overflow vessel).
Seal mould with foil
A double line of tacky tape was put on the mould flanges. The nylon foil was positioned in the mould and sealed on the double line of tacky tape (see picture 13).
Test for leakage and final check
First vacuum is applied between the double line of tacky tape. This allows a proper sealing of the foil on the mould. Secondly, a vacuum is applied in the mould. This is done very gently and slowly to allow the reinforcement to be compressed properly and the foil to be repositioned where necessary. The sealings, all connections (outlet and inlet) and the foil are checked for leakage very thoroughly using an ultrasonic leak detector. A leakage is sealed using tacky tape. Finally a final check is carried out to ensure the correct position of injection channels, etceteras.
Inject resin
The required amount of Reichhold ortho-phtalic polyester resin (1200 kg) is mixed and the injection valve is opened. Resin flows into the injection channels and stai'ts to impregnate the reinforcement (see picture 14 and 15). During the injection the product is carefully watched to detect a possible leakage. If a leakage occurs in the foil this can be sealed with tacky tape. The flowfront is watched as well and compared with the predicted flowfront. After IVi hours the hull is fully injected (see picture 16), The resin inlet tube is closed.
Cure resin
The resin gelation started after 4 to 5 hours. The peak exotherm was measured at different spots in the hull and limited to approximately 80°C,
Demould product
After the resin was cured the vacuum pumps were switched off, the foil and injection channels were removed. After installation of several bulkheads the hull was demoulded (see picture 17),
5. Conclusions
In this paper the stepwise approach to realise the vacuum injection of a large product (the Contest 55 hull) is presented. Not all aspects have been covered, but only some major ones have been described. Although the RTM process and the vacuum injecdon technique already exists for quite a while and transfer of the technology is well possible, most of the times this is not simple and straightforward. The main reasons are:
1. There is (sdll) a lot of labour and craftsmanship involved. 2. Consequently process control is difficult to establish.
Therefore, in order to be able to apply the vacuum injection technology it is essential that the manufacturer bears in mind the following:
• Understand the basic principles of the technology.
• K J I O W the properties and requirements of the materials you are going to use. • Never JUST do something: KNOW why you are doing it and WHAT the
consequences are on the process and the final product properties. Critical aspects to detenuine before starting are:
• Which materials to use? • How to inject the resin?
• How to secure the reinforcement?
• What can go wrong during the injection? (AND: can you solve it?)
This paper has shown that a structured approach will lead to the successful vacuum injection of any product. To aid dissemination of the technology the main research area which must get attention is the realisation of a proper process control tool. This tool must be able to ensure the successful injection of a large product.
Acknowledgements
The work presented was funded through Senter (SMO95103) of the Dutch Ministiy of Economical Affairs. Much of the practical work has been carried out by dedicated employees of Conyplex under the supervision of Han van Aalst and Cor van Zantvliet. This work was not possible without the close co-operation with the Dutch shipyards Friesland Polyester and Standfast Yachts, resin supplier Reichhold and CVK.
References
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Picture 12. Securing the reinforcement and Balsa in the bow by means of clamping and stapling.
Picture 14. The resin starts flowing through the injection channels and impregnates the reinforcement.
Picture 15. At the stern large parts are impregnated, while at the bow the impregnadon-just starts.
Picture 16. The hull has been fully inected.
Picture 17. Demoulding of the hull. 19
