Different types of boat mold

[SilverAnchorProject]

You can use a one shot mold. Using CNC on polystyrene blocks, apply mat layers, finishing and then the ordinary processy, mold/tooling gelcoat, wax etc..
If you need more info don't hesitate to contact me

 

[Burnside Style]

That is a totally feasible approach.I would point out that correctly applying a vacuum bag over foam isn't imposing any forces other than those it experiences as a result of atmospheric pressure.What can,and sometimes does cause problems is having a badly applied vacuum bag that bridges gaps and consequently doesn't have full contact.

If you bag to a caul plate that is thin it will warp the plate. All of our steel and invar layup tables at work have massive steel structure underneath to resist warping from bagging "to the table". Bagging to a foam only mold could cause it to warp or change shape if there is no additional structure. You can certainly design foam tools that you can bag to but generally they are - high density tooling foam usually with an additional eggcrate type structure underneath.

 

[wet feet]

If you have an invar table of any size you must be working on aerospace projects.That somebody has designed a massive structure beneath indicates that they are anticipating a heavy load of,presumably,tooling block.I will repeat that a vacuum bag itself only removes the air inside the bag and the atmospheric pressure exerts the same force on the outside of the bag as it did on the surfaces of the foam/tooling block before the bag was applied.If an insufficiently trained operative has made the bag too small to follow all the contours of the stack then I would agree that distortion can occur.Those I have trained don't make that mistake.

 

[gonzo]

If you believe that, please describe how it differs from atmospheric pressure when the vacuum bag is not present.

When there is no bag the pressure on the foam is equalized throughout. When there is a bag with differential pressure, there is stress between one side of the bag and the other and between one face of the foam and the other. The stress generates tension on the bag (it wrinkles and tightens), and it also deforms the foam.

 

[wet feet]

If foam is getting deformed,it is because the bag is too small and is bridging some corners.With an adequately sized bag the removal of the air between the bag and any object pushes the bag onto the surface and re-imposes atmospheric pressure,no additional force is involved.

 

[gonzo]

The bag wraps around the edges and will create compression between them. Atmospheric pressure will compress the foam and the friction against the mold should prevent it from moving too much if well designed. In large parts without too many corners, the edge compression is not significant. Small molds may have more problems.

 

[BoatBuildLog]

What are you building the mold for and are you planning a one-off hull or something you might reuse for multiple boats ?

 

[wet feet]

I'd like not to hijack the thread with discussions about vacuum bags but I do wonder if gonzo has been influenced by witnessing the type of silicone bag that is sold for veneering panels and which stretches over the workpiece as the metal frame,with an incorporated seal,closes on a flat plate.A sufficiently large bag,made of vacuum bagging material doesn't introduce any forces to the job. It simply allows atmospheric pressure,or the amount the limit switch is set up for,to exert a force on the outside of the bag, by removing the air inside the bag.Any creases which may develop have balanced forces on each side.

 

[TANSL]

We've reached the absurd situation where anyone can fear being accused of hijacking a thread simply for telling another member they're wrong or don't really know what they're talking about. A very sad situation, isn't it? You be the judge of whether this is the case or if I'm exaggerating.

@wet feet, please continue expressing your opinion on whatever you like, even if they label you a hijacker or even delete your post without warning or explanation.

 

[Boat Design Net Moderator]

As long as a post provides new information relevant to the design or boatbuilding issue being discussed in a thread, it's great to add to a thread and very much appreciated.

(wet feet's posts above are fine - don't see an issue here; the way I read it, when he posted "I'd like not to hijack the thread with discussions about vacuum bags..." he simply said, politely, that he didn't want to go on too long or get too far away from the OP's original question/thread. If there are a few more replies on vacuum bags a few more are just fine. But if there are going to be 50 posts on a specific question of vaccum bagging back and forth that isn't necessarily in-line with the OP's thread, it might be better to start a new thread on vacuum bags to work this question out so as to not create a side-loop that derails the original thread. Don't see an issue here.)

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