For a number of years now, prepregs have become the preferred material technology for the production of high performance race boat structures, as evident in the America’s Cup.
As normally happens when a new technology is adopted on high end products, this form of construction finds its way into the production boat market. Manufacturers in this market are keen to exploit higher performance materials and are finding that the reduced labour content, improved process control and health and safety now make prepreg a viable option. With the advent of truly compatible Gelcoat systems and core technology, high quality, cost effective prepreg components can be released directly from the mould.
Processes such as vacuum infusion are also finding favour, but resin formulation constraints and potential process/property variations mean these processes cannot give the consistently high property/process control envelope prepregs offer.
Master
The master, which could be used either as a male tool or plug for a female mould, is manufactured using marine ply headers. These headers are skinned with MDF or plywood, which is then coated with several layers of epoxy/glass, applied as a wet lay-up. Finally, the wet lay-up is then covered with a primer/filler, followed by a topcoat. Typically, epoxy coatings are used, but products such as Durabuild VE are now gaining favour.
It is good practice to confirm the stability of the master at curing temperature by subjecting it to a reduced heating cycle prior to final finishing. The surface of the master should be worked to a suitably smooth condition and the vacuum integrity of the surface checked prior to the application of the release agent.
Tool Manufacture
ACG has developed a specific range of vacuum processing tooling materials specifically for the marine industry. Please refer to the Tooling Product Selector Guide for full details
A typical mould construction is as follows:
• LTF318
surface ply.
• VTS263
syntactic ply (optional print blocking ply).
• VTM264
or ZPREG264 bulking layers.
• VTS263
syntactic (optional, but must be used if ply is used on the face side
of tool).
• VTM264
lightweight ply to balance surface ply.
Cure cycle is typically 16 hours @ 65°C (149°F), followed by a post-cure of up to 100°C (212°F).
Note: This construction, after post cure, can be used up to 80°C (176°F). If higher temperature stability, e.g. 120°C (248°F), is required then it is recommended that LTM/S317 be substituted for the VTM products.
Refer to: Recommendations for Manufacturing Large Scale Marine Mould Tools from LTF/VTM260 Series Materials (PDF)
Part fabrication is carried out in three stages:
The inner/outer skin (dependent on whether the master/tool being made is male or female) is laid-up. GC260® Gelcoat and tie layer, available from Scott Bader Company Limited, can be applied to the mould first if desired.
If a core is to be used (foam, honeycomb, etc.) it is bonded using a film adhesive. Alternatively, a VTF261/Soric ply can be used.
The final skin is laminated and bonded to the core using film adhesive.
Curing can be carried out in two stages (after application of the core, then after the second skin) or three stages (after the first skin, core and then final skin). In some cases it is possible to carry out a single ‘one-shot’ cure, but extreme care must be taken to avoid the risk of laminate/core movement. As a consequence, this procedure is generally not recommended.
Key process advantages of VTM Prepregs
• Rigorous control of resin content, weight and laminate properties.
• Excellent handling characteristics (reducing labour time).
• Improved Health and Safety.
• Ability to process in a similar manner to conventional vacuum assisted wet lay-up/infusion.
