Large fiberglass mold questions

[industrialite1]

Hello,
I am involved in building a mold for a small single person wet submarine for an international contest my college engineering club has decided to enter for the 2nd year in a row.
Last year another college built out hull and it didnt make it to this year... I have decided to undertake the construction process of mold making and casting the hull out of fiberglass.
I have a few kinks to work out and I will start with the basic plan of attack in order to get feedback.

After designing the general dimensions of the hull, we plan on making foam washers of the desired diameter and skewering them onto a pipe to make the basic outline of the body. --- it will look like a function that has been estimated by a riemann sum. The surface will be sanded smooth by rotating on a rotisserie. The pipe will be removed and a layer of bondo will be applied. When sanded smooth again, it will be molded.
This is where it gets a litte grey.
What I was thinking of doing was taking two 1/4 inch sheets of plywood and cutting the shape of the plug into the center of each in oder to bisect the plug. After prepping the plug with mold release, laying layers of glass to one side, letting it dry and repeating for other side. Then splitting the two pieces of plywood in order to remove the plug. Now I sould have my two sides of the mold to cast the final product into.


This sounds like the best way of doing this to me but I have not any experience in mold making of this size.

Not to mention, Im not sure of what type of foam to use...

Any suggestions would be appreciated.

 

[BillPSU]

Instead of splitting the mold making a (2) piece hull make it a one piece hull and cut your access panels into the hull and remove the foam core using hand cutting tools. This will avoid the joining problems of putting the two halves together. This could present other problems with adding stiffeners and mounts but should be achievable. If possible you could have the hull filament wound. I don't know where you are located but I would bet there is a filament winding shop close to you who would do it for material cost or free if you put there name on a plaque on you sub as a sponsor. Read your contest rules if you are allowed to do this.

 

[industrialite1]

The beauty of making it a two section piece is to reduce sanding time from laying glass over a surface.
Underwater, smooth taperd surfaces are key to speed (forgot to mention it is a speed contest (human powered)). Not to mention that having a company constructing the hull eg. filament winding is against the rules.
Its gotta be split so we can install and secure our equipment because its gonna be a tight fit for one person as it is.

 

[BillPSU]

The surface area of a one piece shell will be smaller than the surface area of a split shell unless the mating flanges are internal to the halves. The area will then be equal.

How are you going to get the person inside the shell? Split the case of through an access hole? Emergency removal of a person from a split case system I believe would be time consuming and more dangerous.

Could you post a link to the rules of this competition?

 

[industrialite1]

There will access panels molded into the hull, one for the person in case of an emergency, and one for mechanical perposes. There is aslo a need for a viewing window. All these can be acheived easily with negative mold in a multi-stage lay up, that is another reason to do make casts.



I dug further today and found a reliable source that gave some instruction to work out the kinks I had. I currently have a firm plan of action and plan on executing it by this weekend.

 

[BillPSU]

Several months ago I had the chance to go through the Cobalt boat factory here in Kansas. The large molds were all female molds with extremely good finishes. The hulls and top sections molds were first treated with a mold release. The gelcoat was applied. Hand layup of the cloth was performed. Stiffeners were added for mounting of the parts to the hull and topsections. The stiffeners where made of wood and metal and fiberglassed in. The hulls were also glassed using chopper guns. Foam and plywood reinforcing was used in the critical area of stress along with reinforcing sheets of glass cut to special shapes. Removal of the hull and top sections was unusual as hoists were attached to the hull and the complete assembly of the hull and mold was hoisted and then seperated using wedges and pry bars.