Last minute advice needed - Plug making

General Composites Discussions Composites Talk
#1

I recently posted on a project that I’m working on (experimental aircraft inner landing gear doors – see: http://www.compositescentral.com/showthread.php?t=7145 ). I’m looking for some last minute advice so this is an update/follow on to the origianal post. See the original post here:

Please see the attached pictures:

http://www.flickr.com/photos/81745619@N08/7482729802/in/photostream
http://www.flickr.com/photos/81745619@N08/7482720818/in/photostream
http://www.flickr.com/photos/81745619@N08/7482730086/in/photostream
http://www.flickr.com/photos/81745619@N08/7482728148/in/photostream
http://www.flickr.com/photos/81745619@N08/7482728644/in/photostream
http://www.flickr.com/photos/81745619@N08/7482728972/in/photostream

At this point I’ve completed the following steps:

[FONT=Calibri]1. [/FONT]Created a skeleton frame using 1/8” x ½” basswood strips over the brake calipers & landing gear half forks (to streamline the areas of the brake calipers and half fork that protrude from the bottom of the wing when the landing gear is retracted). The strips were feather sanded where they contact the surface of the wing and hot glued in position.
[FONT=Calibri]2. [/FONT]Fillled the cavity with A/B mix polyurethane foam (after damming up the wheel well with balsa planks and covering the brake caliper and half fork with plastic).
[FONT=Calibri]3. [/FONT]Sculpted the foam to the desired contour (the basswood strips were my sanding guides… sanding the foam flush with the basswood will provide the required 1/8” clearance between the finished gear door and the brake calipers.
[FONT=Calibri]4. [/FONT]Filled the porous foam with a mixture of vinylester resin and micro balloons (it was shocking how much of this mixture the porous 2 part foam absorbed).
[FONT=Calibri]5. [/FONT]Sanded the resin/micro mixture to get “close” to the desired contour of the gear doors
[FONT=Calibri]6. [/FONT]Applied 3 x layers of 9oz 7781 eGlass (vinylester resin) – trimmed the laminates to the edge of the aerodynamic blister
[FONT=Calibri]7. [/FONT]Mixed up a small batch of resin/cabosil to a paste consistency and applied to the edge of the laminates to smooth the transition to the wing surface in preparation for step #8
[FONT=Calibri]8. [/FONT]Applied 2 x layers of 9oz 7781 eGlass (vinylester resin) – trimmed the laminates to 1” beyond the edge of the aerodynamic blister (in other words the last two laminates lapped 1” onto the wing surface)

This is five (5) layers of 9oz glass or approx 1/16" thick which shoudl be durable enough for a plug.

Next steps:
[FONT=Calibri]a. [/FONT]Sand, fill with bondo, sand… repeat until the shape is perfect (not worried about weight at this point because this is simply going to be the “plug” for my gear doors – 95% of the aerodynamic blisters will be cut away after I’ve complete the female mold off of these plugs) - only the small crescent shaped “door ramps” will remain as part of the wing.
[FONT=Calibri]b. [/FONT]Prime with duratec primer
[FONT=Calibri]c. [/FONT]Sand with progressively finer paper and ultimately polish with a good compound

Since I’m using the wing surface as part of the plug I can’t afford to have the mold not release from the plug… I also can’t use semi perm release on the plug as the wing surface still needs to be prmied/painted at some point in the future.

[FONT=Calibri]1. [/FONT]What kind of pot life can I expect out of duratec gray surfacing primer at 100 degree ambient temp (hot here in Phoenix this time of year).
[FONT=Calibri]2. [/FONT]Do I need to add gloss additive to the duratec primer or is it only added to the gelcoat (as in: gelcoat that I apply to the plugs before laying up reinforcement layers of fabric when building the female molds)
[FONT=Calibri]3. [/FONT]Once I have the plugs primed… what grits of sand paper should I use to progressively sand the plugs… ending with a polishing compound?
[FONT=Calibri]4. [/FONT]What polishing compound works the best?
[FONT=Calibri]5. [/FONT]I don’t want to use PVA so can I at this point apply 5-6 coats of a good release wax (Maguires?) and then proceed with laying up the female mold? As in: Brush on gel coat (I’m going to brush on because I don’t have a gel coat gun), apply reinforcement layers, apply wood reinforcement frame to support flange, let cure and hopefully the female mold will release from my plugs without a hitch.
[FONT=Calibri]6. [/FONT]Ideally the female molds won’t require much prep work before they’re ready to vac infuse my parts (gear doors – constructed of 1 layer eGlass, 2 layers CF, ¼” core, 2 layers CF, 1 layer eGlass… all with infusion epoxy)

Looooooong post I realize but I’m pretty new to all of this stuff so I’m hoping you guys can help me out.

Thx!

James
Phoenix, AZ

#2

[FONT=Calibri]1. What kind of pot life can I expect out of duratec gray surfacing primer at 100 degree ambient temp (hot here in Phoenix this time of year).
A. Being in South Florida, our ambient temp is between 85 - 90 degrees. Our pot life is about 15 min. So we are able to spray on a tack coat. Let that sit for a few min, then come back and spray on the flow coat. We let that cure till its tacky and then spray on another flow coat. We then let it cure fully, sand the surface with 220 - 400, then recoat. After that we sand with 800 - 1000.

  1. Do I need to add gloss additive to the duratec primer or is it only added to the gelcoat (as in: gelcoat that I apply to the plugs before laying up reinforcement layers of fabric when building the female molds)
    A. If you want to spray the Duratec primer through an automotive primer spray gun then yes you want to apply the duratec. Don’t add more than 50%. I’m not sure if it is ok to put Duratec HGA in the tooling gel coat, but I never have done that. I always spray tooling at 2% through a 6mm tip on a dump gun. Has not failed yet :).

  2. Once I have the plugs primed… what grits of sandpaper should I use to progressively sand the plugs… ending with a polishing compound?

A. To make sure you have full coverage you’ll want to sand the primer with 220 - 400. Apply two more coats to regain that loss of primer, then sand with 800 + till you are able to polish. A good buffing compound is Frecla 200 for gel coats. You can buff 600 grit and up. I recommend Frecla 200 XL. 200 XL won’t dry too fast in the direct sun.

Compared to other compounds, Frecla is BY FAR the best. Its a 100% water based material so it does not gum up like solvent based polishes. The best part of Frecla is when you think you need to apply more compound. Just mist a tiny bit of water over what you are buffing and it loosens the dried compound on the pad. Good tip ;-).

  1. What polishing compound works the best?

Frecla! I switched everything from 3M And Maguieres to Frecla. Its more economical, faster cutting, and easier to clean. Just throw the pads in the laundry.

  1. I don’t want to use PVA so can I at this point apply 5-6 coats of a good release wax (Maguires?) and then proceed with laying up the female mold? As in: Brush on gel coat (I’m going to brush on because I don’t have a gel coat gun), apply reinforcement layers, apply wood reinforcement frame to support flange, let cure and hopefully the female mold will release from my plugs without a hitch.
    On such a risky job as this. You will want to use PVA and then just polish the mold instead of spending your time and polishing the plug. IF you don’t want to do that. Then just polish up the plug as best as you can. Get a gel coat sealer. Apply the sealer. That sealer fills up the pores in the primer. Keep in mind primer is very porous. After that wax the mold at least 10 times, (out of the sun) with a wax that is manufactured for mold releasing.[/FONT]
#3

Thanks for the tips John! A couple of follow on questions:

  1. So then its not recommended to polish a new plug sprayed only with duratec primer to a high gloss, wax (10 coats per your tip) and then apply the gelcoat followed by the reinforcement fabric? I’m a bit confused because I’ve read a ton of posts from other folks on here that say they’ve stopped using PVA altogether. I’m concerned primarily with the mold releasing from the plug but of equal concern is having a great quality mold from which to pull multiple parts.

  2. Correct me if I’m I dont belive I should apply a sealer because part of my plug is the actual lower surface of the wing which will eventually need to be re-primed, sanded and painted (need the primer/paint to stick).

  3. If I have to use PVA I will but I’m trying to avoid the problems others have had with the mold surface when using PVA on the plug (gel coat fish eyeing, rough surface on the mold gel coat, gel coat alligatoring, etc).

Bottom line: I need to ensure a good release but at the same time I’d like a top quality mold from which to produce several parts. Oh, and I also plan on lightly scratching the outline of the gear doors on the surface of the plug so a reference line will transfer to the part (for trimming later). Wouldnt using PVA cover up my reference scratch on the plug?

Thanks again for the advice!

#4

Either spray the PVA and generate a mould surface which can be polished to a high sheen, or do not use PVA and use a paint system on the plug, and a release system that you know is going to work.

For spraying PVA, see attached.

PVA will not create fish-eyes in the gelcoat as it is a low surface energy system (or a high surface energy system, I keep forgetting). If that occurs look into the gelcoat. usually a spray gelcoat is applied from to short a distance from the mould. (based on polyester gelcoat). The acetone which is in it, should evaporate when the gelcoat flies to the mould.

Alligatoring is a reaction which occurs due to undercured gelcoat, so keep an eye on catalyst level, temperature (which should not be a problem in Phoenix), layer thickness (measure using a wet film thickness gauge).

Other systems are painting the mould with 2K PU paint, let cure 7 days, polish, clean and apply semi perm sealer, then high temp wax. Too late for a test with your materials, however.

#5

Thanks for the advice guys! It looks like I will be spraying PVA even though I was hoping to avoid doing so. A couple of quick questions:

  1. Since the plug surface is quite small (approx 25" square with a convex shaped bulge in the middle). Can I use a standard siphon detail spray gun to shoot the PVA? If so, are there any special clean up proceedures so I dont ruin the gun for future use (primer/paint)?

  2. How thick should the tooling gel coat be when I spray it in on the plug (by your comments Herman I take it that you dont recommend brushing it on) and how many coats does it typically take to get to this thickness?

  3. Lastly, on the mold (that I’m building off of the plug): Do I need to use PVA before laying up the parts or can I just polish the mold to a high sheen and then wax with a good release wax? NOTE that I dont plan on using gel coat on the actual parts. My planned layup schedule is as follows: eGlass, CF, CF, CF, 1/4" core, CF, CF, CF, eGlass (was going to lay up only four total layers of CF but I’m thinking the extra two layers of CF (9oz twill) will give some more stiffness since the parts have to be extremely rigid to prevent flexing in the (up to) 350mph airflow - the eGlass on the outter layers is to give me some material to sand before sanding into the CF).

#6

Please, please make clear whether you plan on using polyester or epoxy.

The reason I recommend PVA is that you used gelcoat or are using gelcoat to finish the plug. The outer surface of the gelcoat is still somewhat “green” which actually causes 15% or so of the polyester moulds made on them with only wax, to stick. I do not want you to belong to that 15%. That’s why PVA.

As for the gelcoat: 0,6 mm. Check the datasheet of your tooling gel. It should be in there. Spraying or rolling: Again check the datasheet. Gelcoat for spraying application is different than for rolling or brushing. Always specify which one you would like when ordering.
I have always rolled gelcoat. Find a roller which does not leave too much of orange peel (you will always encounter orange peel) and you are good to go. Ask your gelcoat supplier for a wet film thickness gauge. They should give them for free. If not, threaten them with applying only half of what is needed.
After the first 2 or 3 products from your mould the surface will not be that active anymore, and you can do without PVA.

#7

To answer your question: I plan on making the mold with Polyester resin & CSM (the plug was built up using vinylester/7781 BID eglass (5 layer laminate). I used vinylester for the plug because the aircraft is constructed with vinylester and the “ramps” in front of and behind the gear doors (currently part of the plug) will remain as a permanent part of the aircraft to streamline the airflow over the gear doors. Once the mold is pulled off of the plug the the center part of the plug (gear door area) will be cut away leaving the ramps (the ramps are the crescent shaped areas marked with black marker to the far left and far right of the newly glassed area - click the last image link above).

Tomorrow I’m going to fill the low areas w body filler (Rage Extreme) and sand the gear door profile to perfection, then prime with duratec surfacing primer.

#8

Herman: I just re-read your post and noticed something that I failed to catch before. The plug will not have gelcoat, just the duratec PE surfacing primer. Do you think I’m safe to simply polish the duratec primer, then wax 8-10 coats, then layup the mold? When laying up the mold the first step will be to brush on the tooling gelcoat (I noticed in one of your recent posts that you said that you brush on gelcoat), then layup the glass reinforement (lightweight woven cloth followed by multiple layers of CSM) using PE tooling resin. I could however, lay the first layer of lightweight cloth with VE resin if that would be produce a higher quality mold. Thoughts?

#9

Sorry for the silly question but what is exaclty is “2K PU paint”… I understand the “PU” to be polyurethane but I dont understand the “2K” part.

Also, if I were to go down the path of painting the plug to avoid using PVA (the plug is just finished in duratec PE surfacing primer at this point): Will the use of semi perm sealer make cause problems later on down the road when I get ready to prime/paint the wing (remember: a portion of the wings lower surface and the “ramps” leading up to the new gear door part of the plug will become a permantent part of the wing thus will need to be preped to accept primer/paint in the future).

The “ramps” are represented by the blue areas in the pic below, the “red” area represents the shape of the gear door: (disregard the “green” areas as that is simpy masking tape to help keep resin off of the wing when laminating the glass over the plug to give it some durability)
http://www.flickr.com/photos/81745619@N08/7500697882/in/photostream

#10

On PVA you got the question. But remember the eleventh commandment: Thou shalt do tests…

As for mould making:

-VE tooling gelcoat (brush or spray version, I like to roll the brush version)
-lightweight glass veil with VE resin
-100 gr/m2 or 150 gr/m2 CSM, fine tex (preferably 10 or 12 tex)
-idem
-Only after this you can add some more bulky layers, preferably a low shrink tooling system, UP or VE based.

After each layer inspect for air bubbles. Open them and fill with resin.

2K = 2 component

#11

Fiberglass Supply sells 3/4 oz CSM and 1.5oz CSM. Is the 3/4oz the stuff I should use as a veil and the 1.5oz for bulking my molds?

http://www.fiberglasssupply.com/Product_Catalog/Reinforcements/Woven_Roving_and_Chop_Strand/woven_roving_and_chop_strand.html

#12

I see they have no veil. In that case use the 3/4 oz CSM. use it a bit resin rich.

1.5 oz is for bulking up.