Plug questions...

Im going to be building a plug to make a composite shell which closely resembles a kayak, altho with a smaller cross section. Im planning on MDF mainly because its easily shaped/sanded and I have access to CNC to cut the formers and skins etc(the shell is mostly "boxy" in shape)

My questions are about the best/strongest way to join the 2 halves and the easiest way to make the plug so someone(maybe me or maybe not) can most easily make the mold(s)

Are there any other methods than butt joint or shoebox to fit the two halves together? Im going to assume shoe box with inner layers added is likely stronger, altho possibly harder to get just right due to thickness of the outer layup?

When making my plug is it worth the hassle to make it in two halves, split on the joint for the final peices? (this really wouldnt be "too" hard considering my shape) Should I just make two exact halves? Two parts which are both sligtly over half?(extra for trimming), or even make two "top hat" type plugs which already have a flat flange on them(I belive this is a bad idea as it would make the fibres turn an abrupt corner in the mold which wouldnt be ideal for layup purposes)

TIA

 

I read in another post that you're building an experimental or proof-of-concept hull. You may want to consider the ease with which you can disassemble the components should you decide to make a substantial change in your design during testing. For that reason I like the top hat option (or flange).

The joint need not be (nor should it be) an abrupt angle. As you surmise, it could create a weak spot. A gentle and generous radius will give you a strong part, though.

I disassembled an Aqua Lark (flange), and I acquired an Illusion 12 (shoebox). I sure hope I don't have to disassemble the Illusion 12. Much needless destruction of innocent fiberglass would result. Usually a flange can be split fairly easily, and in a worst case, can be sawn apart. (I've noticed, over my decade-plus of messing with fiberglass products of many stripes, that components will often separate at a layer of roving. If I decided to join some panels that MIGHT have to be separated at a later date, I'd throw in a couple layers of roving just to make separation easier.)

Your final version could use a shoebox joint for beauty.

 

thanks for the reply.

I had indeed figured if I was to go with a butt joint and inner layers of tape the flange would be basically useless in terms of the part itself. If I used a butt joint I only wondered if the mold should be "over half" to allow material to trim back to the exact joint line.

I expect at least some layers to be carbon and possibly Kevlar so Im aware already of issues with trimming and what will make the part easiest to cut/trim to size and then join etc

As far as being modular I wont need the two halves of this component to be as such, but the rest of the design is very much so

Im also a bit concerned that my two halves arent very large and access to make tape layers which are strong in a confined psace may be difficult. For this reason I think shoebox is the way to go.

Im still on the fence about attempting the mold myself so I want to be extra sure I do it the easiest way possible. I may also want to have someone else produce my mold to ensure the highest quality and perhaps keep the costs down by using poly. (If I make the it I wont even consider using anything except epoxy)

Parts coming out of the mold will almost for sure be epoxy save for a first few "test parts"

 

I would use a generous flange on the plug and mold, to facilitate vacuum bagging later on.

Also I would not split the plug, but if the release or layup of the part could be a problem, I would make a split mold. For a split mold, your plug does not need to be split. This way your 2 halves will have an exact fit.

 

The part will be similar to a kayak so the mold for sure needs to be split somehow which is why Im asking. The plug will be built up with ribs and formers cut on CNC so making the plug in two halfs where the joint will be isnt an issue but it is more work. If its not going to make it easier to make the mold obviously I wont bother but I have a feeling it will. I guess im leaning towards making it 2 pcs and adding a larger flat section to create a flange on the bottom of each half which will in turn create a flange on the mold but im concerned what the fibres will do during layup at this abrupt turn. I think maybe im better to actually make the parts "over half" AND with a flange and just do the layup untill the corner. I'd have to make a simple jig to then cut back to the halfway line of each side.

I just dont like the idea of messing with temporary dams and walls on a plug to make the halves if I can just take care of that in the design stage of the plug. I realize often there is no other way when you are pulling a mold off an existing part or component.

I am also still on the fence about joint style and how tricky it will be to gain internal access to reinforce the joint. I'd estimate the cross section of most of this enclosed space to be less than 2 sq' and the openings in the one part to be maybe 10" wide. Basically a box maybe ~12" x ~16" with access holes on the 16" face

 

Keeping the plug in 1 piece has the advantage that your 2 mold halves will be exact.

Even with CNC techniques you will encounter differences in shape of 0,5mm or more. This will give you a hard time joining the 2 halves in a way that is pleasing to the eye. Also getting the 2 halves to join nicely has some guesswork involved.

As for the dams: There are relatively easy, and do not need to be 100% exact. You can shape some plastic board or anything with a smooth surface, and tack it in place with modelling clay. Also fill the seam that surely will be left with clay. Prewax the board and the plug, so you do not need to mess with wax anymore. Then gelcoat, and fill the corner with some polyester with aerosil added. The light CSM you will use as a first layer will easily follow the contour now. After the first layer you can visually inspect, and if there is a small air pocket, cut it open and fill with (thickened) resin.

As for constructing the part: You can opt for vacuum bagging 2 halves. Clean up the keel line afterwards, and join the 2 mold halves. Now impregnate a piece of tubular braid of choice (glass, aramide) with epoxy with some aerosil added. With a nail on a stick you can quite easily place the tape on the seam. Then use a paintbrush on a stick to smooth it out.

On the outer side (after demolding) you can do the same if you wish. If you want to have the tape in a recess. make that recess in the mold using wax sheet (available in many calibrated thicknesses). It is self adhesive, and easy to be worked with. Make sure you get fresh material, it hardenes over time. (although some heat will help it keeping workable)