Lower Cowl Mold, times two

The lower cowl geometry doesn’t lend itself well to making it a single piece mold. The centerline scoop/tunnel/boat tail has geometry that would “capture” the plug, and subsequent parts, in the mold if it was laid up as a single piece. You can still do a single piece mold, but there are some draw backs.

If I did a single piece mold, I would have to break apart the plug to get it out and will no longer have it useable if we wanted to redo the mold or modify our shape. Also, you don’t know how the mold surface turned out until the plug is out. So you are destroying the plug and committing to making repairs on the mold surface if there are problems with the mold. Additionally, you are relying upon the final part that is laid up in the mold to be flexible enough that you can get it out of the mold without having to cut it into pieces or destroy the mold. For these reasons, it is a safer approach to make the lower cowl mold in two pieces.

In order to accomplish this, the only thing I needed to do was add a tooling flange down the centerline of the part. Of course, that means jigging the flange to our fully prepared surface. In order to protect the finish, I laid down a layer of vinyl tape first, then bondoed the tool flange on. You could also use hot glue for this.

From here, the rest of the molding process is mostly the same as what you’ve already seen me describe in previous posts. Tooling flanges, wax, PVA, and layup. Once you have one half of the mold done and cured, you can remove the centerline tooling flange backer board form. The completed mold half is left in place, and the centerline flange on the existing mold becomes the tooling flange form for the second side. This way they exactly match.

On the second half of the mold, I want a joggle or channel to facilitate sealing the centerline of the mold when assembled. This channel will allow me to put a rubber weather strip in between the halves and hopefully get an air tight seal. I wanted a 1/8″ deep channel. Turns out that my tacky tape was the perfect easy thing on hand to make the form for the sealing channel. I adhered tacky tape to the centerline flange, then I put vinyl tape over the top of the tacky tape as a releasable surface. This should give me a 1/8″ channel or depression in the second side of the mold to facilitate sealing the molds to each other. Again, the mold making process was the same as for the first half. Tooling flange forms, waxing, PVA and layup.

One thing you will notice is that one of the mold halves appears white, the other black. A few weeks ago, Dave Anders told me about a mold making technique he learned from Dave Ronnenburg, the designer of the Berkut. Dave uses essentially an “aircraft structure” approach to making molds where he has laminate-core-laminate. Dave uses micro for his core. This makes for very light, inexpensive mold structure. So on one side I tried that approach (white micro visible), the other side I did the woven cloth and chopped strand mat (see through the glass to black primer). I did this to see if I like micro core better and check if it saves time.

It turns out that both mold halves took me within 15 minutes of each other. The micro core mold took a large amount of micro-balloons, but was much less epoxy than the chopped strand mat required. Because of my temperatures, I didn’t dare mix up a large batch of micro, so I did it in 5 or 6 smaller batches. This certainly added some time to the process. I also have learned some better technique for chopped strand mat and how much epoxy it requires.

On these mold halves, I don’t see a clear winner between the two methods. However, if I could do a single mix (or maybe 2) of micro to get what I need to for the whole mold, I think it would be significantly faster and lighter to do a micro core. I can’t wait to get them off the plug and see if one is stiffer than the other.

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