mold repair

[Brian Blazer]

I made a mold for a cowl that has a couple of small defects where the fiberglass wasn't sitting right on the plug. If I fill these holes with bondo and cover it with epoxy, will the fix last a long time, or will the bondo just pull out? Any ideas on what I can do?

thanks, Brian

 

[jydracing]

hi brian first what is the mold made of epoxy or poliester. also how deep is the defect. also how thick is the layup of the mold and what type of reliese are you going to be using. jimmy

 

[MikeP]

If it was mine I would fix it with the same thing you made it with. Epoxy and silica or some kind of thickening added to make a putty.

Be sure to clean and sand all the mold release so it will stick.

You might be able to fill in the voids with putty and stick the plug back in to save some sanding work on the mold.(be sure to wax it again)

MikeP

 

[Brian Blazer]

there are about 4 areas where there are bubbles, about 3/8" diameter. The mold is made out of epoxy, and is 3 layers thick. I may try microballoons.

thanks, Brian

 

[mark poole]

If the mold is made of epoxy then it is best repaired with epoxy. Is there a particular reason you used epoxy to build the mold?..and what material did you use as a first layer (skin) when you made the mold over the plug. The first layer is the most critical but if the those are the only air voids then it should not be a problem filling them and still having a mold that makes good parts.

Mark

 

[Brian Blazer]

I use epoxy b/c there is no smell. I do all of my work in my basement. The glass I used is pretty thick. I am not sure ow thick it is though. I am going to order some 3oz and 6oz for the next one.

thanks, Brian

 

[mark poole]

I understand that. Just be sure to have adequate ventilation, even with epoxy. Ok...try to use lightweight mateial for the first couple of layers. It will go down better and you will be able to get any air out better before the resin sets. Also, for a really smooth surface on the mold put down a layer of resin (gel coat is used with polyester resins for this), then lay up the material. If you are using a thin resin that wets out the glass nicely then I would use lightweight (.75 oz.)

chop strand mat, not woven cloth. The cloth can cause "print" on the mold suface and it can delam from the mold skin after multiple pulls. If you are wanting to build a lot of these cowls then make the mold 8 to 10 layers thick. A heavy mold will work better in the long run. The set molds for the boat that I am curently building are 15 layers of chop mat. Good luck.

Mark

 

[Brian Blazer]

ok so lets start from scratch...

I have the plug complete...

Started with foam, then taped it all up and applied bondo over the whole surface. After I got the shape I wanted, I primed it and then painted it with enamel. The surface is very smooth.

To make the mold, you would recommend:

1) gel coat the plug

2) after gel coat dries (I assume), apply 10-12 layers of .75 chop strand mat. polyester resin

To make the cowls:

1) wax and pva the mold

2) use a 2oz layer with epoxy (I am using west system), then a 6oz layer.

Do you lay your cloth on the mold, then pour epoxy on it, or do you saturate the cloth first, then put it in the mold?

thanks so much for your help as I am a virgin fiberglass guy.

Brian

 

[GTR]

Hi Guy's

I recommend that after you gelcoat the plug use surfacing tissue, then light chopped strand mat, changing to heavier mat as you add more layers. Don't worry about using too much resin as the more you add the stiffer it will become. You want it to be fairly solid.

I have a few plugs/molds under my belt these days and it works for me.

GT

 

[MikeP]

Try this site!

http://groups.yahoo.com/group/moldmaking/messages/1

 

[mark poole]

Yes, wax the plug, gel coat the plug, let dry then lay up material. After you have finished the mold let it stay on the plug several (3-4) days. Why? All resins, especially polyester will dry to the touch pretty quick but will not shrink down, harden and cure for several days....sometimes longer....depends on the grade of resin and the conditions.

I have found it much better to brush down resin then lay the glass in and satuarate it. If it has to much resin in it simply use several dry brushes to sponge out the excess with a dabbing motion. A little resin rich on the plug is ok. Resin rich in the part is not ok. Excess resin in a thin part is just extra weight and actually makes the laminate weaker because the resin can fracture "around" the laminate if stressed.

Want to know a few secrets? The older you get the less you try and keep secret...after all whats the point?

Since this is a cowl mold keep it simple. Go to the hardware store and buy fiberglass resin....yes the kind with the wax blended in it, commonly known as finishing resin. Brush a nice thick coat over the plug but don't let it make deep puddles anywhere. Let it dry..couple hours. Using the same resin wet out a single layer of the lightweight mat...not woven cloth...and keep the air bubbles out. After this layer gets dry, do another layer, let dry, then another. After you get past 2 or 3 layers, put on more than one at a time if you want.

Now, a lot of old fiberglass salts will swear up and down that you can't do this with finishing resin. Yes you can. If you do all of the laminating in one day the layers WILL adhere to each other because they are getting a chemical bond between them and no you do not have to sand between coats. You would not want to do one layer one day and then one the next because the chemical adhesion "window" of the resin closes after a day. PVA is a good thing to use on the plug since it reduces the "slip factor" and lessens the chance of the skin coat buckling up.

Build the parts the same way if you want. They will be transparent and you will have to paint them. Don't get tempted to remove a part from the mold too soon. The longer it stays in the mold the better it will retain its shape.....and....it will release from the mold easier. Hope you have as much fun with the stuff as I have!!

Mark

 

[GTR]

Hi Guy's

I mostly agree with Mark - especially about the PVA as it makes it real easy to separate part and mold. You must wash it carefully clean before you can do anything with it though. Only obvious tip I can aslo add is to use some pigment in the resin - strong colours are best - it really helps show which parts are properly wetted out and which are dry. Not all finishing resin has wax added and lot's of laminating resin has wax added so the wax is not actually an issue. Yes leave parts in the mold for up to 3 days to get them to cure well, green parts will lose shape if not careful. I have a really stiff overweight cowl that I use to sit my lightweight ones on to cure out of the mold so that I can lay-up another one. This works for me. With practice you can lay-up the surfacing tissue (highly recomended) and two layers of 5 or 6Oz cloth in a single run - light stiff parts. Takes a little practice but gives good quick results. Forgot to mention if you are using mat use a grooved roller to get the mat properly blended and evenly deposited, as well as work out the air. Lot's more info out there.

GT B)

 

[Brian Blazer]

Thanks a lot guys...you have been a huge help! I will let you know how it comes out.

Brian

 

[MikeP]

I make molds by mixing up a thick putty of epoxy, silica graphite, powder. It looks like black mayonnaise. I brush it on nice and smooth about a 1/4" thick. This becomes the mold, no weave imprints, bubbles or voids. After it sets up a little but still sticky I start covering it with whatever fiberglass I have laying around to support the mold. This process makes it easy for beginners to get perfect results.

MikeP

 

[mark poole]

One more post for Brian.

I was so intent on explaining the resin and glass layup that I overlooked something. After getting the mold off the plug, wet sand the mold surface lightly with 1000 to 1200 grit sandpaper. Then use a super fine polishing compound to shine up the mold. I have used 3M Finese-it with excellent results. This type of compound is liquid and smooth....not gritty and will shine the heck out of a mold. After polishing, wash the mold out with mild soap and water, dry it out then do your waxing process. Personally I do not use PVA in the molds because I like for my parts to come out ready to go. PVA will leave a slight grainy look on a part. I use the standard multiple coat wax method. Go for it!

Mike P.

Does your black mayonnaise gel mixture resist cracking even at 1/4 inch thickness? Sounds like you get good coverage with that stuff.

 

[MikeP]

Hey Mark,

I haven't had any problems with cracking on thick gel coats, it might be because I use West Systems epoxy for everything.

Mike