360/430 Spider Hardtop Mold Complete!

Inner mold polished! Took me 30 hours of hand wet sanding, mostly with several blocks to get the shape pretty smooth.

Here's a reflection from the middle. Not perfect but so much better than it was. To make it perfect, I'd need to spray it with a good two part clear coat, then wet sand it again, another clear and polish. I'll probably do that before I make the first production unit. For now, this is fine for the next step.
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30 hours to sand just the inner mold! That doesn't count the thousands of hours of sanding I've done on this project. Sometimes, I think I've died and been sent to sanding hell, where no matter the effort, the surface is never perfect, and I'm compelled to keep trying.

 

Inner mold is ready for the first casting. To prepare the mold, it's waxed with a mold release wax, which is like an old fashioned car wax, then sprayed with PVA, or Poly Vinyl Alcohol, a nifty substance that sticks to everything when wet and nothing when dry. It's resistant to epoxy but dissolves easily in water. Once properly sprayed, the entire coating can be peeled away intact. It's like spray on shrink wrap film that melts in water.

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Tomorrow I start smearing it with epoxy. It will take a number of clear epoxy layers followed by several layers of cloth, followed by heavy duty stitched material. It should be about an eighth of an inch thick when complete.

This inner casting is sort of a two dimensional transparent hardtop. All the mountings and interfaces with the car are located in this casting. I'll use this part to position the latches and trim pieces, make the cutouts, and then transfer those back to the mold so the next copy comes out with those things cast in the right position.

 

First casting complete! I used the vacuum bagging method instead of just a straight wet layup. Using a vacuum bag is both easier and more complex. It's easier in that the fabric layers can just be gobbed up with resin rather than painstakingly fitted around all the corners and curves. Additionally, the method uses a layer called a "peal ply" and another called a "breather" that together drain out the excess resin and any air bubbles in the layup so that the finished article is as light and strong as it can be. The strength in a fiberglass composite isn't the resin. It's the glass. Squeezing out the excess with air pressure reduces the resin to just what's necessary.

Here's the vacuum bag stage
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This is my first vacuum bagging experience. It went remarkably well and the screwups I made were easy to correct. I managed to hold the vacuum at 28" with my measly pump. I certainly learned a lot of things not to do when contemplating vacuum bagging with epoxy.

In all the previous layups I've done, I do one layer at a time, cure it out and then add another layer. This takes days to do with all the waiting for the layers to cure. I can get excellent results this way but it's very labor intensive and time critical. With the vacuum bagging method, the layers don't have to be perfect. They just have to be saturated with resin and have sufficient material to cover, and not cause a "bridge" (gap between the mold and a curve or inside edge). Some bridging is ok as the resin will seep into that area and fill it, a sort of automatic filet.

One thing that's bad about the vacuum bagging method is that it's extremely time critical. All work has to stop at the two hour mark from first mixing and the bagging system installed by the end of the third hour. Going past this time limit would mean the epoxy has started to gel and things could go south pretty quickly after that. I drafted my teenage daughter to help and she really did a great job. I hardly had to give her any instructions. She just went to work. I got everything done within the two hours and by the end of the third hour, I had a vacuum pulled on the casting. She did one thing bad. She proved she's a valuable resource.

Here's the finished casting right out of the mold.
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All the edges are sharp and the curves smooth. The finish is excellent and the few blemishes are easily fixed. I'm quite impressed with the quality of the part. Next thing is to trim off the flanges and round everything up so I can handle it without gloves. Right now, it's a porcupine of glass quills.

 

Thanks! I did a wet layup with vacuum bagging as a clamping method. If I ever get far enough along, I'll go the extra mile with vacuum infusion. That would be pretty awesome.