wjcalhoun

Hi guys

I have perused the site, found very much helpful info, and have a reasonable idea for my first fabrication: a cowl for a Sig Four Star 60. I'm convinced to use epoxy, not polyester. Found some at Tap plastics.
Weighing the pros and cons, I am planning to make a male mold from foam, and cover it with layers of glass and epoxy.

I bought 1 oz cloth so that it would mold well into compound curves.

The question really is how many layers. I'm thinking 4, with warp oriented 0, 45, -45, and 90 degrees from longitudinal axis of the cowl. Should give me about 4oz glass. After it is done, I'll plan to sand, prime, and paint.

Am I on the right track here? Is this enough structure? More layers needed? Other suggestions?


Thanks for any advice

AH1G

With a "male" mold you need to consider a number of things. First, how will you release the foam mold, "melt" it out of the part? Is it a one off part? What thickness do you want on the cowl? This will determine the the thickness of the layup and consequently the strength and weight of the part. Do you really want to do all that sanding and finishing of a very rough part? One layer of 5 oz. cloth maybe enough or even excessive. Consider a female mold wheather it be a one part mold if the angles are condusive or a two part mold; joined with tape, either will work. What use is a cowl that is relatively finished on the interior and tons of work to finish on the outside? If you are going to all that work to make a one off, why not build to the final spec; make a plaster female mold then layup a part and then after layup, break it apart and you will have1/10 the finishing!
Check out fiberglass specialties -$34. for a cowl http://www.fiberglassspecialtiesinc.com/catalog.htm

SCALECRAFT

My Feedback: (13)

I figure you know basic molding techniques and how to care for molds.

Our wing molds are made of first black pigmented surface coat, small sections of 2 layers 4oz cloth, 2 layers of 6oz, 1 layer 10oz and 1 layer 20 oz..

Cowls you can use black surface coat, 2 4oz, 2 6oz, 2 10oz. If you want to have a durable long life mold.

This is a bit over kill, but our molds are around 18yrs old.

Steve

wjcalhoun

AH1G - thanks. I guess that once the male plug is made, it is not that difficult to make a female plaster mold and lay up inside that. So again the question - would 4 crossing layers of 1 oz glass cloth be strong enoogh in a small cowl app?

Thanks

soarrich

My Feedback: (98)

I think you are dead on, for a light weight cowl. If it where me building a 4 Star I think I would throw in a couple more layers.

Your finished glass will be pretty smooth and there should not be much sanding. You seem to have done your homework. One thing you might want to try is to get a large party balloon, blow it up then push it against your wet lay-up while slowly releasing the air. it will give you a poor man's vacuum bagged cowl. I did it once on a small glider fuse using a condom, you would be shocked at how big they can get.

AH1G

Yes, I think four layers would be enough with an added 1/2 " wide layer around any openings. An option and the way I would build it, would be 1 oz followed by 2 oz followed by a second layer of 1 oz. The balloon sugestion is a good one. Make sure that you leave the part in the mold for at least four days; longer if you can stand it for a good cure. Green parts will warp.

wjcalhoun

Gentlemen
Thanks for your advice and suggestions. I know I could buy one, but I am interested in learning the technique, and it adds to the enjoyment of building the model. AH1G - thanks for the suggestion on reinforcine openings, and on curing time. I may have pulled it earlier not knowing that. I plan to wax the mold with mold release paste, and then coat with PVA prior to the lay up. I presume that once I start the layup, it is best to keep mixing small batches of epoxy, and get the whole thing done in one sitting - followed by the balloon. If I did the four or five layers at different times, the balloon would not have much effect.

soarrich

My Feedback: (98)

Using WEST Epoxy 105/205 you would have plenty of time using one batch. I'm assuming you're going to mix up about 2~3oz batches. Also if you mix the batch then spread it out in a foam plate you have much more working time. If you keep the mixed epoxy in a cup it heats up and kicks very quickly. WEST epoxy can be bought at any boat store.

AH1G

Your best part is going to come from a single layup, all layers in single session. I've been at this since 74''; I'm still learning and open to suggestions!

Sign this "tag it and bag it, Dano..."

wjcalhoun

Guys - a progress report and another question on my Four Star 60 cowl fabrication.

I carved and sanded the male styrofoam plug, and coated it with hard epoxy just to have a smoother surface. I sprayed the male mold with silicone spray, and then casted my plaster female mold from it. I carved the male mold such that the firewall end of the cowl is a little bigger than the prop end, and cast it nose down. I thought that would make it easier to pull the male plug out of the female mold. I let the plaster set up for a day - probably over kill, I know.

Last night, I worked on trying to loosen and extract the male plug from the plaster with very limited success. Obviously, I want to keep the female plaster mold undamaged, but I have had minimal success in extracting the male plug. Do you cut the mold in half, do you sacrifice the male mold, or is there some other trick to separating the two?

Thanks in advance

soarrich

My Feedback: (98)

You may have to sacifice the male plug. I would try a thin Mylar wedge to get between the male and female parts.

AH1G

I have to ask...
What happened to waxing and spraying the plug with PVA?

yel914

You might try spraying some compressed air between the plug and the plaster.

wjcalhoun

Gentlemen - thanks for the replies

I understood, perhaps incorrectly, that the waxing / PVA step was to facilitate release of the epoxy/fg part from the plaster mold. I did not wax / PVA the male plug - rather, i used silicone spray on that plug.

I'll try the mylar and compressed air, but it sounds as though I might have to sacrifice the plug.

Thanks again

soarrich

My Feedback: (98)

Silicone is a no-no if you're ever going to paint the part, it will mess up any paint you use. My painter friend said you can never get all the silicone off. Stick with wax and PVA.

wjcalhoun

Finally after 2 months, I have a cowl! (My 8-5 job gets in the way of this hobby). I appreciate all the advice you guys have given me. The only suggestion that did not work well was that of the balloon. My cowl has small radius corners (~.5"), and inflating the balloon pulled the fabric out of the corners. It went back in without much problem. Probably had the corner radii been larger, the ballon would have been fine.

In any case, I avoided at least a dozen mistakes by listening to advice. Thanks again.

lilhachet

My Feedback: (2)

Don't use PVA over wax.It will melt the wax and you will never get your part out. Use either the wax or PVA, just not both. I learned this the hard way. Good luck.

diggs_74

Not to give contradictory advise or anything but I use both religiously.. No issues to this point.. I wax the mold about 3 times with Partall wax and then apply a couple thin coats of PVA.. I know a number of guys that use this same procedure.. The only reason not to use the PVA is if you are masking for paint in the mold because the mask will pull off the PVA..

Just my .02 worth..

wjcalhoun

FWIW, the instructions i got with the PVA from Tap plastics said to wax the mold, and then to apply pva as a parting agent over the waxed mold. My cowl did release; interestingly, most of the pva stuck to the epoxy, rather than to the wax. I had to sand and scrape the pva off the finished cowl.

lilhachet

My Feedback: (2)

It must be me , I have yet to have a couling come out real easy.I use good wax and release agent, then I tried to spray about 3-4 coats of PVA and left it overnight before I glassed it????