Building a foam plug

I'm reading carefully @wet feet all the posts, I just don't have anything to add yet!

 

Things to be aware of when dealing with a machined plug.You may have some steps in the surface if there has had to be a change of tool in the CNC,this is most likely if the machine is not equipped with a tool length sensor and the operator has to use a setting device.I would hope this type of machine is fading from use,but there are some around.It leaves you with the dilemma of whether to build up the low surface or to rub down the higher,or a bit of both.You have to use judgement to decide which of the surfaces is closest to the intended shape and it is extremely easy to abrade the very soft foam when using filler to rectify the situation.

If the machining has been done on a 3 axis machine it will have required the use of a ball end tool and there will be small cusps between the passes of the tool.Some machining strategies will make these less bad than others.For instance the Mastercam Flowline toolpath.Any machining strategy that involves parallel passes along offset planes will have regions where the cusps are quite large because the distance between passes is amplified the angle of the surface relative to the plane the tool is operating in,for instance cutting parallel to waterlines on a nearly flat surface.An experienced machinist may use boundaries defined in the CAM program to avoid machining this type of zone and should be able to define cutting directions in his software to minimise the cusps.It is possible to re-machine such zones if the situation hasn't been anticipated,but it takes a bit more machining time and obviously that costs more.

The other problematic area with a kayak is likely to be the line that defines the meeting of the hull surface and the mould joining flange.This needs to be a sharp corner,so that the two halves of the hull can meet smoothly but the hull surface will almost certainly have been machined with a ball end tool.Either the hull will have to be built up in stages-as described earlier-or there may be a need to sharpen the radius that will have been left.It is very easy to cause yourself problems at this stage.With some foams,and with a very sharp chisel,it may be possible to carefully remove the fillet radius that has been created.It is also possible to tear out some foam and to need to apply filler,but such filler needs to be compatible with the foam and it needs to be flatted with extreme care.The difference in hardness between foam and filler goes away once a surface coating is applied,but this particular corner is the one to be very careful with.It may be less difficult to apply a sealer coat over any tearout and then to apply some filler,you are likely to be dealing with other small blemishes at this time in any case.

 

@ondarvr after the primer and some sanding if I Put a layer of glass (~150gr) and sand it again a little bit, do you spray again primer over it and then after sanding whatever color I need?

 

Typically you use primer over the glass to get a better surface. But how good that surface needs to be is totally up to you and your desired result.

 

Also reading with interest. I've been lurking for years for info I can apply to a teardrop camper trailer and soon a strip kayak and your write ups are very informative.

 

If any particular questions come to mind,I will do my best to provide answers.Cutting formers for a strip kayak is a simple job,made much easier if a good 3d surface model exists and it isn't an enormous task to create such a thing.

 

An update. I print a small part (near the stern) which has a few difficult shapes to make a test.



It needs a cloth as it is very fragile and nearly impossible to sand close to the fittings without damaging the foam so I added a layer of 290gr of twill.



After sanding I've added some epoxy putty on some places that needed. Do I need ideally to put in the whole part to me more even?



After sanding I've added two coats of primer (with a brush) to allow me to sand and make it as smooth as possible. Most likely I will have to add a little bit extra after sanding.
This is how it looks now before start sanding.

 

That looks like a good choice for a test piece as there are several features of different types for you to practice with.You will have found that power sanders are of little use and will need to find or make several different sanding blocks.A few assorted pieces of rubber hose will be useful and for the large circular recess,a plywood semicircle with a piece of perhaps 120 grit paper will probably help with the flat areas.In fact,120 grit all over would be a good start as you are almost certain to find some pinholes or areas of weave that require filling and next time round you will be able to use finer grades of abrasive all the way through.

The big challenge will be to get the sharp corner between the moulded part and the horizontal flange,truly sharp and with a fair curve in plan view.It can be done and it may require a flexible sanding pad perhaps made from thin plastic maybe 3mm to 5mm thick to follow the curve and with the ends of the same pad tucked into the angle when using the pad on the flange surface.The best approach is to work to the standard you are happy with and to ignore the clock until you have achieved the result you want.Perhaps before going to he next paint stage it might be useful to verify that the hardware that is intended for the various recesses will actually fit now that glass has been applied.If the answer to this comes back negative then a bit of adjustment to the CAD model might be required.It is much better to determine this now than to make production moulds that have a built in flaw and even better to learn that there is no need to change anything as it will all work well as designed.Good luck.

 

When I finish with it, I will test with the deckfittings and then we will adjust the CAD. That is the easy part....the difficult is the sanding.....

 

Nobody has ever believed me when I tell them that sanding well is one of the most difficult things to become proficient at. Which normally changes after they try to do it. Along the way, we normally have the conversation about power sanders and at some point I will point out that simply pressing a switch isn't even the beginning of being competent, which always gets the comment "yes,but I can do it". It takes a close encounter with a few bumps and recesses to bring about enlightenment.

 

I don't know about other constructions but for sea kayaks the power sanders are very little used....nearly everything soon or later has to be done by hand.
Of course I agree with you @wet feet

One more question please. For the foam plug what is the recommended colth that I can put in once to avoid shrink problems etc?
I was thinking to put 1 x 150 gr as the first and then 1 x 300 gr same time.
Then wait a couple of hours (5-6) and put another 300 gr and then next day the rest (4 x 450gr)

 

Is the glass intended to stabilise the plug surface or to form the eventual mould?For the mould,there are dedicated tooling resins that shrink less than basic laminating resin,just as there are tooling gelcoats that are both more stable and able to take a higher gloss than the production component would achieve when polished.I would plan for two gelcoats and then glass tissue in the areas with sharp corners and then a light mat.Once the first mat is cured you may add the laminate to reach the desired layup and I greatly prefer not adding more that 1800 gsm in any 24 hour period with a little less being preferable.If you can leave it at this stage to cure over a weekend it will help maintain the stability and then you can add such framing and bracing as you find necessary.Good practice is to put a piece of foam or coremat beneath any structural pieces to resist the tendency for the shrinkage to leave a slight bulge on the main surfaces.

 

Latest update, I've build the sample mold but it seems impossible to release it without destroying the foam plug.
I manage to get a small gap and release a little bit of it but the rest doesn't seem that will popup easy.
I used the same waxing technique that I used in 8 molds without a problem so is not a waxing issue.

Anyway, any tips how to release it without damaging the mold?
Of course the foam will be destroyed but doesn't really matters.

 

Obvious point;did you incorporate sufficient draft?Insufficient draft can cause problems if a sanding block leads to a minor groove on the near vertical sides.For future projects it might be worth incorporating either a blow hole or a jacking plate over areas such as the hatch recess.If losing the plug you have is something you have accepted then you could either chip the foam out,piece by piece, or try drilling a 5mm hole over the hatch recess and applying compressed air but don't exceed about 3.5bar at first.It might still lead to total destruction of the foam but you will at least have tried and I don't see any other easy way to apply force.This is a general problem with foam and you are doing quite well to make a second mould from foam,which is why I mentioned the possible need to make a heavy layup from the mould to use as a plug for future moulds.What absolutely has to be determined is why the mould didn't release easily and a good picture of the surface may be helpful.

 

After thinking for a few minutes I felt that my previous post was only really partially complete.The physics of releasing any mould are the same,how much force will it take and how might it be applied?With a small piece of foam and a mould section that has no framing,the options are very limited,usually it will be wedges around the edges and the limitation is how resistant to crushing the foam might be.If the wedge applies a force that exceeds the crush resistance of the foam the inevitable result is damage.So how might more force be applied to the existing case?One possible solution would be to add some plywood or MDF framing and to bond the bottom face of the foam block to a heavy bench or to the workshop floor.You now have the possibility of applying more force by attaching cross pieces to the framing and you can use jacks or a hoist to pull the entire item away from the foam.This distributes the force and gives a better chance of success and you may also use wedges around the periphery at the same time.Sometimes-but very rarely-you can leave the framing subjected to a force and go away to get coffee and when you return it will have separated.I have had occasions when it took a combination of cranes,jacking points,air blowing and wedges to get a successful release and the relief was huge.