[littlesmith]

Originally Posted by Explorer

Something just sank in for me, because sometimes I'm a little slow.

You're working only on making a negative mold, instead of making a positive and then a negative from that positive?

Given some of the issues you've already run into, would it be preferable to perfect the positive, so you can see and feel the perfect surface before then making the mold?

The short answer is yes, both a positive and a negative shape.

On the left a shape called a plug, on the right a negative derived from the plug



The long answer :

-First you make a shape of a guitar as it would normally be, only its from solid MDF (or solid spruce or something) and not a hollow soundbox like normally. At this stage you can feel it and hold it like a real guitar, you have to feel with your eyes closed as well and check for for bumps or cavities. When you are happy you should put the entire thing in clearcoat or epoxy (but not the bottom if you intend to glue it on the baseboard), then sand it super flat up to European 3000 grit.

-Then you glue it on a super flat baseboard or attach it with bolts from below (there must be no space between the plug and the baseboard where gelcoat can seep in). A guitar baseboard has a second surface in an angle where the headstock is. Add other components if needed like resin flow channels, channels for reusable vacuumbag sealing.

-Then you put it in high temp release wax and / or liquid release agents.

-Buff it.

Now you start to make the negative mold,

-First a polyester or epoxy based gelcoat with a brush, let it cure (or apply more layers). 3 millimeter is a good thickness for a gelcoat.

-Now backing material for strength and dimentional stability, most common is fiberglass, build it up to approximatly about 0.5 cm (ideally in a vacuumbag under vacuum so you don`t get airpockets between layers).

-Now you can make a frame for dimentional stability (optional), weld steel in or a hardwood frame. Cover the steel with more fiberglass.

-Seperate the guitarshape (plug), and the negative carefully by inserting wedges all around the perimiter, then it pops loose at some point if the release agent was applied properly. The part can never have an overhang or they can never ever be seperated. Parts with an overhang must have a 2 part mold.



Split mold example (not mine) :



-Wax the negative with release was and / or liquid agents.

Now you can START making a composite guitar body, hahaha.

In my case this isnt even the end of the process, because i want to make a 5mm thick reusable vacuum bag instead of the flimsy 0.2mm thick disposable plastic. I must skip the release wax stage of the negative and put in a calibrated wax sheet (very different kind of wax) that represents my exact part thickness.

Like this : http://www.easycomposites.co.uk/#!/p.../sheetwax.html

When you calculate the part thickness, you should make sample plates under vacuum and don`t forget to add the thickness of resin flowmesh and peelply to the final thickness or the complete stack won`t fit.

Put release wax on the wax sheets.

Then i apply the mixed 2 component sillicone reusable vacuumbag and let it cure. When its done, i will remove the reusable flexible vacuumbag, but leave the calibrated was sheets and make a hard uppershell as well like RTM manufactering and test which one gives a better result.

Then i can remove the wax sheet, and put release wax. I will now have a space between my mold surface and and the reusable vacuumbag that is the exact thickness of my part.

I hope my explanation of the process didn`t make it even more complicated. Don`t worry if its not 100% clear, it took me 3 years to really grasp composites. Youtube has many movies and tutorials about this. Just be prepared for the fact that composites is more expensive then a drug addiction.

First you just start with 100 bucks of epoxy and a brush and some carbon fiber. Then you want a vacuum pump to get better quality and resin to fiber ratio. then you want to increase the quality of the curing with an oven. I made my own oven with those small pizza ovens that fits half a pizza, i bypassed the times which is not safe at all. Now due to the funding i have an aerospace heating balnket with built in temperature control ($1000).



I will put the fabric in dry (without resin) and use special infusion spray glue that dissolves in the resin to place the fabric. Then i will put the reusable avacuum bag or the hard shell, put it under vacuum. Then if it passes the droptest (1 hour under vacuum and the needle must not move of the vacuum gauge). Then i will suck the resin into the mold, that`s called resin infusion.

That is the second highest quality level together with RTM (injecting resin instead of sucking it in under vacuum). The higest level of composites is called prepreg (pre impregnated), and this has to be stored in a freezer, the resin is in the fabric already in optimal ratio and wactivated with heat. I went to a company and they quoted me 150 Euro per body on location, then 3 weeks later iby email it was 1500 for just the body. I don`t want to be in the business of selling 5000 Euro guitars if nobody knows my brand, so thats why i went back to making everything myself.