As many of you know I’m busy to construct a mold for a car bonnet/hood. I’m trying work as cheap as possible…i know it’s wrong, but it’s a test run. I make use of traditional techniques, Gel coat, and lots of layers CSM. I try not to lay more than 1 layer a day to make sure all the heat escapes gradually. I’m on my second layer of CSM and still need to go at least 5 layers. But I want to add some sort of stiffness, with either wood or some sort of ribbing. Just to make sure there isn’t any twisting and flexing. I had this idea to make a big “X” with tubes or pipes across the mold and overlay it with more CSM. Remember the current layers on the mold are cured and rock hard. Someone in the composite shop told me today it’s going to leave an imprint on the gel coat…although the layers beneath it are cured. Can this really happen?
I’m assuming your speaking of your reinforcement ‘X-frame’ printing through on the mold right ?
If so…yes , it can occur depending on how long the mold has had time to cure. Heated post cure can aid in avoiding this.
It is always best I’ve found just to avoid placing braces in the middle of fields that will be visible if possible.
Yep. It’s definitely possible especially with higher shrink resins like polyester.
I also agree. I made a mold for a sunroof and put stiffening ribs across the backside of the mold. I did exactly what you did. I laid up 5 layers of CSM and let it cure for 24 hours. Then I put hte ribs on and another 3 layers. After demold, my mold had some pretty serious print through. Good enough for a race car part, but certainly something I won’t do again. This was with PER.
Indeed. keep stiffeners away from the part itself, if possible. If the stiffener has to touch the part, try and locate it on a hard spot, for instance a fold that runs over the middle of the bonnet. (not many cars have a fold like that).
I would weld up a frame that follows (roughly) the perimeter of the mould. Tape the frame with plastic. Now mix an amount of polyester putty, and put that on the corners of the mould. Drop your frame in the putty, so it sits nice and even. Fill the rest of the seam between mould and steel as well, and rework nicely, now it is still wet).
After cure, remove the steel, clean up, and replace the steel (minus tape) with MS Polymer sealant, approx 0,5cm thick.
Paint the rear of the mould with topcoat. Paint the steel in a contrasting colour.
Make sure you can attach the steel frame to a universal cart, which makes working in the mould easier, and helps moving the thing around.
I always add bars, wood, or tubes but only to the flanges. Never behind the actual part. If its a flimsy part I use plenty of tri-axial fiberglass to help keep the mold from distorting.