Anyone with experience making molds?

General Discussion
1

I’m working on making some molds for my latest pet project, and it’s literally giving me fits.   I’ve had to do the walk-away-and-count-to-ten routine more times than I can count now.

 

My process as it stands now has been, make mold plugs from foam, cover with epoxy, fair out a bit, glazing putty a bit, fair some more, repeat, repeat.  Then I started in with the primers to get a good smooth surface.   First high-build primer, then a regular primer, then primer “sealer”, sanding finer between each successive layer.   So I got my plugs to the finish I wanted, let them sit for a day, then started to apply the Partall #2 wax to prepare to cast the molds.    The partall #2 ate up my primer sealer, had to wipe the whole mess off and sand back to regular primer.

Went through the primer sealer process again, rubbed it out to 1500, everything looking good.    This time applied turtle wax without any issue, then made a mess of trying to apply the PVA.    Washed the PVA off, re-waxed, and did about ten light coats of my newly discovered ghetto PVA (unscented Aquanet super-hold hairspray is aerosol PVA, gotta love the internet!), and everything was prepped for the tooling gelcoat.    This morning I start brushing on the gelcoat and after a couple of minutes I can see where the brush is pulling grey streaks of primer up into the gelcoat.

 

Shit.

 

Took off the gelcoat, spent a half hour with the denatured alcohol to get everything cleaned up again, and that’s where I stand right now.  The primer “sealer” (yeah right!) pretty much wiped right off the underlying primer, but there are a few places here and there where the primer came off in patches as well.   Looks like those are mainly areas where I had re-primered over the “sealer”.   I can definitely see the culprit here.

 

I need some help.   I remember seeing a spoon project here where someone did some molding, as well as a hollow kevlar board, but

I can’t for the life of me remember who did them.

 

Here’s my thoughts, and please chip in your input both positive and negative on it.   I’m going to sand back to primer layer and do some more fairing with my glazing putty, I want to improve the draft in a few areas anyway, so this will be a good opportunity for that.  Then I’m thinking an epoxy hotcoat over the entire thing.   I’m hoping that will be chemically inert enough to avoid the issues I’ve been having.   My other concern is if the gelcoat got through my PVA and wax today, will it do the same thing with an epoxy finish coat, and will that cause release issues?

 

Thanks for any help you guys can give.  This has been a real learning experience for me, seems like everything has been one step forward and two steps back.   Good experience though, I’ve covered all the different processes several times now!

2

Hi Shwaz

 

I've made enough molds in my time to know a few shortcuts. My advice for what it is worth is to apply 5 or 6 layers of your wax polish, give it time to evap between layers and rub to high gloss.. Then making sure you have no negative curves for the part or mold to mechanically hold onto the plug, I'd simply apply the gelcoat and get on with it... Found the PVA release to streak to easily and leave the mold with work to be done..Provided you've got enough wax on, that is plenty release in my experience.. I usually then apply a layer or two to the mold prior to each casting..Simple, effective and less frustrating.. Good luck.

3

  I remember seeing a spoon project here where someone did some molding, as well as a hollow kevlar board, but

I can't for the life of me remember who did them.

 

Thats my board........

 

 

You are putting way too much effort into this.  First:  is there a layer of fiberglass on your plug?  If not, that can be a problem .  You aught to just make a regular surfboard for a mold plug.  All that glazing sounds like a pain in the ass.  I will PM you a phone # because more info can be transfered that way than here on a message board

 

4

Hi shwuz,I’m not sure how large of a mold you’re making or what you’re making a mold of so this is all I have.I’ve made molds of fins using this AM 128 silicone rubber rtv.You want the item you are molding to be perfect because this stuff leaves every detail.You also want to position the item being molded so that an air bubbles in the silicone from mixing can flow to the top and you dont have bubbles on your mold by the product you are making.For example,if I were to make a mold of a fin it wold stand verticle not lay flat to keep bubbles from collecting by the fin.This stuff doesn’t stick to resins,it won’t even stick to itself,it will try to bond some to grainy woods but still tears loose.You can cut it easily with a razor blade.I’ve even had smoke coming from my molds before from mixing my poly resin too hot,and too much x55 in my RR,but nothing happened to the molds.For fins I build a small wooden box and glue the tabs to the box so the fin is suspended,then lightly line the wood box with vaseline to keep the mold rubber from sticking to the wood. Heres a link to the stuff I’m talking about,good luck.

http://www.jgreer.com/silicone-mold-making.htm

5

    Howzit shwuz, I would say to spray the gel coat instead of brushing it on, that's what we did in the boat business. Aloha,Kokua

6

I agree with Kokua. I've had a lot of sucess with sprayed gelcoats, and disasters with brush on or no gelcoat.

 

Lots of wax layers helps. Wax on.....wax off.

 

If you can't spray gelcoat resins you can use paints. Haven't used it but I'm sure even a can of clear spray would work. I had a lot of good results with spray on water based paint as a gelcoat.

7

Cool, some great info here!  

 

Yeah, I wondered about the mechanical effect of brushing the gelcoat rather than spraying it, the brush bristles rubbing through the layers of mold release.    But spraying gelcoat is way beyond my abilities at this point, and I really don’t want to get geared up to do it for just this project.    I’m wondering if I could get away with brushing it if I just started with one thin layer, basically just run the brush over it one time without going back over any of the same areas.   That seems to be when I ran into trouble this last time, I went back to touch stuff up and suddenly I was brushing out my primer coat.

 

I’ve seen some “how to’s” online and they seem to have some success brushing on gelcoat on smaller stuff.  My project isn’t big, certainly not surfboard-sized.    Really don’t want to go through the whole process again…

 

Yeah, just found this:  “If spray equipment
is not available, the gel coat can be applied with a brush.
Brush the
gel coat onto the plug with a very full brush load, brushing in
one direction
only. Keep the brush well loaded with gel coat”.   

Looks like the trick is to not mechanically abrade the release agents with the brush by going back and forth, that’s where I screwed up before.

 

Ok, going to apply epoxy hotcoat to plug today, get it back to the shape I want it.    Wish me luck!

8

Oh my f-king god…

 

Fisheyes in the hotcoat.  CRAZY fisheyes in the hotcoat.

OF COURSE there are fisheyes in the hotcoat, I only waxed the damn thing about a hundred times!  That was before I sanded it WAY back with 120, but I guess the wax penetrated really well.

So.   Right.

 

Another coat of primer to get a chemically inert surface, and THEN maybe my hotcoat again.

 

Now I understand why certain composite products cost so much!   I’m sure the molds aren’t created by clueless monkeys like myself though.

9

I’ve not experimented much with varieties of molding techniques and materials. But I have built molds and molded various items ranging from small fins to 18’ Class A catamaran hulls (between 1958 and the present) using a technique that has been reliable for me. As you’ll see, it’s similar to techniques described by the previous posters to this thread.

 

To wit:

Step 1 Make the male plug just as you would a surfboard: shape foam (or whatever), glass, hot coat and sand, then gloss coat.

Step 2 Wet sand the gloss coat until you’re satisfied with it’s smoothness. Then polish it with rubbing compound.

Step 3 Clean off any residual sanding dust and/or rubbing compound off the plug.

Step 4 Apply carnauba paste wax to the plug and rub it out. Repeat a couple more times.

Step 5 Coat the plug with PVA. I have not had very good luck with using a brush (esp. if the plug has been waxed). Instead I use a cheap “Dockin” spray gun. The PVA should be applied just like painting a car with lacquer: In short, initially apply a very light, thin coat comprised of tiny droplets of PVA. The objective of just “dusting” the surface of the plug with PVA is to avoid runs and to allow the tiny droplets of PVA to rapidly dry. Repeat this several times. Once enough of a PVA film has been applied, apply a thicker, wetter final coat of PVA. This heavier coat partially dissolves the PVA from the previous applications, causing the film to smooth out and ensure a continuous film of PVA.

Step 6  Allow the PVA to thoroughly dry.

Step 7 Coat the plug with laminating resin (that’s what I use for gel coat). I don’t have a spray gun that will spray resin (at least at a comfortable rate) so I use a brush. One should try to minimize the number of strokes used to coat any particular area  of the plug. In general, it’s better to miss a spot than to damage the PVA (1 stroke is ideal).

Step 8 Lay up layers of fiberglass and resin (or whatever) onto the plug in order to build up (“rigidize”) the mold. (Note that any “dry” spots missed during application of the “gel coat” will be wet out during this glassing process).

Step 9 Add any external reinforcements/stiffeners to the mold (if deemed necessary).

Step 10 Wait for the resin to throughly cure.

Step 10 Choose an appropriate location and part the mold from the plug. How difficult this can be is dependent on the stiffness of the plug and the mold, as well as the shape of the two parts.

Steps 11…?  Use the same treatments on the mold as was previously done on the plug (sanding, polishing, waxing, coating with PVA, laminating resin applied to the PVA as a gel coat)  to prepare it for fabrication of the molded part.

 

[An aside:

Attached is a picture of some kneeboards I built between 1968-1969. I had previously built a surfboard (reshaped twice), but these represent my first attempts at building kneeboards. So the set represents kind of an education. KB #1 is to the far left; KB #2 is the second from the left; KB #3 (far right) is board #3; and the red/orange and blue/green kneeboards are #'s 4 and 5, respectively. The red/orange board was the prototype for the blue/green board.
Approximately 50 of these molded boards were sold commercially in the
San Diego area.These boards were entirely constructed of molded parts (no foam was used at all). The white “board” to their right is the bottom mold used to construct them. In addition there is a mold for the front “yoke” and another for the center glass stringer. The yoke and stringer were fabricated first, then incorporated into the bottom mold during it’s layup. The fin box was fabricated and incorporated into the bottom during the lay-up process as well.]

 

 

 

Do this several more times

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10

If your part is small you can spray with dispasable sprayers. Prevale is the name. Most paint stores sell them. A glass jar with a airesole top. Put what ever you want to spray in and spray. I use then for gellcoat alot in my boat repair world.

11

personally ive never had to use a sealer primer.

my stuff goes.

foam

resin/fiberglass

bodyfiller

glaze

sand to 320-400

high build primer

sand to 600-1000

make flanges if compounding curves are involved

wax up to five times with partall #2

spray with PVA to get a nice film

lay up alternating colors of gel coat ( usually just two layers)

pull mold off and sand inside to 1500-2000 starting with 400-600. You can tell how wavy it is if the other color of gel coat comes through when sanding.

 

hope it helps. 

l

12

cheap way to spray gel coat:

 

Buy grizzly engine cleaning gun.  Remove cup.  Modify nozzle so it is as short as possible.  add a flow control valve.  dip the straw into a paper cup with gelcoat.  it sprays like diareha but it works great and it costs a fraction of what a regular dump gun costs. go to youtube and search for video.  like this one....

http://www.youtube.com/watch?v=nsvgaWHPMF4

 

http://grizzly.com/outlet/Engine-Cleaning-Gun/G5357

 

 

13

Don’task, I’m going to look into those sprayers, they seem cheap and should be just the thing…   The grizzly sprayers sound like a good deal too, but I don’t own a compressor and don’t really want to spring for one right now.

 

I keep taking one step forward and two steps back on this project…     I got three thin coats of epoxy hotcoat to stick finally, sanded it back to about 600 since the PVA flows out over the deep scratches anyway.    Waxed it with partall #2 and it didn’t eat the epoxy.  Stuck it down to a sheet of glass then filled the edge with some clay.  Everything looking good so far…   I sprayed with a dozen very light layers of my ghetto PVA (Aquanet) until it flowed out into a nice glossy film, only a couple little runs.   let that dry for several days, then painted on a coat of tooling gelcoat.   Let that gel for an hour, then brushed on another coat to fill the brushmarks of the first coat.

The second coat caused the first coat to bubble.

Two steps back again.

Doh.

14

Dontask, BIG thanks on the suggestion for the Preval sprayer.  That allowed me to get my REAL PVA on my mold plug, and it looks a hundred times better than it did with my hairspray PVA.  Perfectly glossy and smooth.     Really really happy about that.   I’ve got a good feeling about this step forward.   Now hopefully the gelcoat goes on as smoothly! 

15

hey     glad it worked for u.  For gelcoat you have to thin it with styrene to get it thou the preval. Way better than using a brush. Go slow and build up thin coats. It sucks when you think its just right and then it slides down into a mess.

16

Yeah, I’ve read a few things online about thinning the gelcoat to spray with the preval.  A lot of folks recommended thinning it with acetone, which is what I have.   I have just a tiny bit of styrene too, is that a better bet?   The good thing about this stage is that the surface I currently have in my PVA is what the inside of the mold is going to look like, no matter how bad the gelcoat goes on.

But I am interested in learning more about applying the coats of gelcoat.  I don’t want my additional coats to cause my first coat to bubble off, because then I’m back to square one again.   Any tips on that?   Just spray one decently thick gelcoat and call it good?  is there a certain level of curedness I want the first layer to reach before I spray additional layers?   It doesn’t have to spray pretty, as long as it sticks to the PVA and stays stuck.   I’ve also read you want to apply your first layer of glass matting over the gelcoat the very first day to keep the shape from changing as it cures.   How soon to apply that so it doesn’t cause the gelcoat to bubble?

 

I know I’ve been  a bit obtuse about what I’m up to (you guys know how I love a big reveal), but here’s a little hint…

17

acetone will work but styrene is better. To much acetone is not good for gelcoat. Spray just like paint, tack coat first then a full coat or two or three or…   If your gelcoat does not have wax in it , it will stay tacky just like lam. resin.(not tacky aganist your mold,just on the outside)  Let it cure and lay up your glass. If you want a test, wax a spot on some widow glass spray gel  layup glass do every thing you want to do and adjust from there.  Even prop the glass up so u get a idea how gravity will work against the gelcoat.    Gelcoat  1/16-1/8" thick–cure–sand off anything that will mess up glassing–Lay up glass–Let cure – reinforce mold where needed-- let cure some more-- pray-- remove from mold-- rejoyce

18

    Howzit shwuz, I worked in the boat industry for a couple of years as a gelcoat repair guy,we were the ones that checked for any flaws or air in the gelcoat and we used nothing but acetone for thinning and never saw any styrene at the 3 different places I worked. Aloha,Kokua

19

when I was a teenager I was a member of the school canoe club, so its been a while. We used existing canoes as plugs, but added polyurethane foam in places to modify their shape. The polyurethane foam was shaped then coated with mould plaster (like plaster of paris). Sanded and sealed. Over the sealer went “release agent”, painted on with a brush. Moulds were made with chopped strand mat and no woven cloth, for speed of building up the thickness and economy.

The finished moulds were painted with release agent (may have been waxed first - I can’t remember although at least some of our pop out surfboards were released with wax only). Gel coats always brushed on and never thinned. What I can remember is that sloppy brush work during gel-coating - such as accidentally catching the metal band at the top of the bristles on the mould surface could accidentally cause the release agent to peel. This doesn’t happen with wax, however wax was more difficult to release and unless a good amount of wax was put on it was hard to release. Release agent did give some brush marks whereas wax gave the better finish.

20

Good point about waxing Mr J.

In all of the aeronautical molds we made and used, the plugs and the products were only waxed. Wax on, wax off, several times, thoroughly. Yes, slightly more difficult to release, but we never had a problem, and never used pva or the like.

The only problem parts were the few we made without using gelcoat. I wouldn't recommend rolling, squeegeeing or brushing heavily on the waxed surface. That's why they say use a gelcoat laden brush, so there's no excess pressure on the wax, or the pva if you're using it. Spray gelcoat better.