Matched mold, vacuum assist, composite molding: ideas, questions, feedback

For over 30 years I've sought out and mulled over in spare moments all aspects related to producing molded boat hulls. But it only amounts to incomplete head knowledge. Recently I was given four sets of sea kayak molds (female hull and female deck molds). Come spring begins the hands on messing around with them. I'm intrigued with the matched mold set up I saw on you tube under "fibreglass vacuum moulding" (uk spelling, nine and a half minutes long), and I would like to apply this to one of my sea kayak molds. Two questions come to mind: does the 17 foot kayak mold length present additional problems, and how would you go about making a hard but flexible inner fiberglass mold? I'm interested in both industry standard and original ideas. Thanks

 

For low volume production just use normal infusion with a one time plastic bag taped to the mold. For higher volume production you can mold a reusable vacuum bag. Closed mold tooling is only necessary if you desire a perfectly smooth interior in high volume production, in wich case I suggest paying a consultant to help develop the tooling and set up the process.

 

As has been said,these days infusion with a vacuum bag is a much less expensive way to get a better hull.The matched mould process inevitable uses more resin and gives a heavier structure that isn't necessarily stronger.A point you will appreciate if you ever lift the kayak onto a roof rack.The inner mould typically doesn't have anything like the flexibility of a conventional vacuum bag and you have a host of technical challenges to overcome.Some of which are creating the seal between the moulds,ensuring that the placing of the inner mould doesn't dislodge the fibres while doing the assembling,designing the flow channels for the resin so that it isn't drawn directly to the vacuum extraction points and coping with a steady feed of resin so that no air is drawn in.The OP may be familiar with the writhing mass of plumbing that sometimes gets used on infusion setups and even then there is sometimes clamping of feed lines and squeezing of resin rich areas to push excess resin around.I suggest first of all building a conventional kayak from the moulds to see if they have any particular virtues as the previous owner thought that giving them away seemed like a good idea.If the finished boat seems OK then that may be the time to start learning about vacuum systems and infusion.It is a huge amount less difficult that producing the tooling for matched inner and outer moulds.I won't even mention the additional complication of a new inner mould should the layup specification need to be changed.

 

If I understand correctly, you are looking to do vacuum infusion in kayak molds that were not designed for infusion and you want to replace the disposable bag with something reusable.
My thoughts are that a vacuum formed plastic sheet part might give you the right stiffness and flex.
Dave Clark said he achieved some level of reuse in vacuum bagging in his UFO production. He did a video on it but he might be interested in advising you just to know how it worked out.

 

There is a difference between infusion and resin injection with matched moulds.Infusion uses an infusion mesh to transfer the resin evenly and the matched mould system is closer in concept to casting with gates and runners to distribute the resin.The system Dave Clark uses is infusion,done with a re-useable silicone bag.Such bags are some way from cheap and would certainly cost more than a kayak mould that was given away.Which would make me think that it would be quite a gamble to invest in one before determining that the moulds in question would produce a hull of some quality.I didn't see any indication of the laminating experience of the OP and he might have a way to go before turning out a quality boat if he hasn't done one or two previously.

 

I think I found the video the OP was inspired by. If it is not already clear, that is nothing like I thought.

In the video they just dump the right amount of mixed resin in the bottom of the mold and the space between the matched molds pushes in in place. To use that process the part must be designed for it and the two mold halves would need to be designed and constructed together.

 

Good luck with that..... A former business contact of mine was taken in by the promoter of that kind of "system" and was going to produce several hundred mouldings each year with nice shiny surfaces on both sides.After thirty or so pieces went in the bin he reverted to simple hand laminating and then flowcoating.The process time was a bit longer,but the failure rate was negligible.Dry zones,frothy resin failing to wet out the fibres and numerous voids all featured among the failures and the fibre/resin ratio was pretty dismal.Infusion is a much better idea and more adaptable.

 

Skyak. Thanks for the careful reading of my post. I see you found and viewed that utube. It sure helps to see the tooling, the process, and the end result in what appears to be a simple, repeatable, and predictable process and product with fairly short cycle times. You're right about the part needing to allow this process, but in my use it seems the main requirement is that the tooling be flared on sides and ends, which the female mold is; that way the male mold easily pops out of the kayak hull which in turn pops out of the female mold. My cyclic time could be much shorter though due to no gel coat being applied in mold and the use of 2 minute cure UV infusion resin. You mention the inner and outer molds being made together but this is what I've heard the process is: First, you complete the female mold. Then you install sheet wax on the entire mold surface, which mimics the thickness of the layup schedule. Then you form up the male mold inside of that. I perceive a few caveats in this process: it seems most demos shows the wax in small molds, and wax sheet is expensive, and resin shrinkage can be a real problem. Even just 1% shrinkage in some epoxies and polyesters would end up with a one inch gap on each end of a 200 inch (17') kayak. Are there any special no shrink resins on the market?

 

Sheet wax is a very useful commodity.Just what you need if your laminate is of uniform thickness all over.If there is any variation ,for example a bit more glass on the bottom beneath the paddler,you won't be producing a laminate of uniform thickness throughout and will have to work out a way to overcome the situation.For a modern design,that had been proved by a prototype to have good paddling qualities, you could use a 5 axis CNC machine to produce plugs for both moulds concurrently and thus get producing in a shorter time.Since the OP was given a collection of moulds I would still advocate building a trial boat to verify that the design has sufficient good points to justify the effort just in case the creator of the moulds had tried them and found them lacking.I wish him well but having seen mouldings for both industrial parts and boat hulls that were made this way I won't be holding my breath or volunteering to lift such a hull.

 

This isn't a process that leads to a light weight performance craft, it would be significantly heavier than required due to the limitations of the process.

I'm not sure what you meant when you said you could eliminate the gel coat, plus use a UV cure resin.

You need some type of finish on them, and gel coat is the easiest, fastest and lowest cost option for molded parts.

UV cure only works where you can expose every surface to an equivalent amount of light energy, and it's line of sight, any surface that doesn't get UV rays doesn't cure.

The molds would need to be UV transparent for the part to cure correctly. And before you go down the UV cure rabbit hole, check the cost of a UV cure set up.

 

RTM light would be a suitable process.

It uses a stitched lofty materials consisting of stitched chop / polyprolene mesh / stitched chop (Rovicore).

Low profile (low shrink) resins are available for RTM / RTM Light process. I’ve personally used AOC R049 with great success.

Fiber content is usually around 30%. Injection pressure is low thanks to the partial vacuum at the catchpot (usually 15 inhg), in which to lower the differential pressure inside the injection cavity.

MP or HP RTM is better if a higher fiber content is desired, but because of the high injection fluid pressures, the mold has to be able to support tons (literally) of pressure without flexing. Steel framed molds are utilized.