JohnW

My Feedback: (6)

On my second month of learning how to make composite parts from plugs/molds. I was curious as to what material others use to attach plugs to parting plates when making a mold. I have used modeling clay. The clay seems to work pretty well, but it starts to flow if epoxy cure heat isn't carefully regulated. I also read somewhere that one can use Bondo(TM), which I tried last night. Even with 5 layers of part-all wax on the plug, the Bondo stuck too well to my plug and did moderate damage to the plug. Any pointers appreciated. Thanks in advance. John.

Mike James

I prop up the parting board, (or build a support box for it) and also provide supports under the plug, to support it. (so nothing can shift) Then, like you, I use a non-drying modeling clay to seal any gaps between the plug and parting board, followed by several coats of wax, and finally PVA.

Are you using polyester resin? Epoxy shouldn't generate enough heat to make the clay flow, unless it's something particular to that brand of clay. (I've used several different brands, with the same results... no flow)

Anyway, Bondo is a horrible idea. Whatever seals the gap between the plug and parting board only needs to be rigid enough to keep epoxy out, and should be completely removable at the end, to avoid damage to both your plug and parting board.

aparchment

I used bondo to fill the gap in a parting board around a fuselage because the board was slightly warped downward toward the nose of the fuse, and I needed to build the parting board up. In addition I wanted a nice straight line as the part line, and I didn't think I could get it with modeling clay. It definitely takes more work than clay since you have to sand the bondo down or build it up to the right level, but I didn't have to worry about having a nice straight parting line.

FYI, if you use enough wax and let it dry between applications nothing should stick to your plug. If you are really concerned you can put plastic 3M tape along the plug where the bondo will make contact. Of course you should wax the tape as well.

Antony

davidfee

I used modeling clay to fill the gap on the first fuselage mold I ever made. I don't remember the brand name, but I bought it at Aaron Brothers. It softened and flowed under the heat of the 45-min potlife epoxy surface coat I used. I could have popped the plug out, sanded the part line flush, re-inserted the plug.. etc. But I didn't and now that mold will foreve have a wavy part line and that has always bugged me. If you use clay, it just has to move a hair to mess up your part line.

I was told about the Bondo technique and have used it to great effect ever since. It guarantees a straight and razor sharp part line, every time. My website: http://members.cox.net/davidfee/Fuselage_Molds.htm describes how I do it.

If you have enough wax on your plug, it should not stick. If you use PVA on top of your wax, then you are basically guaranteed success.

I am sorry that your plug stuck.

-David

KC-135

My Feedback: (3)

I agree with Mike.When making the parting board take your time and try to get as close as you can with the parting edge.This will also allow less clay.I try to get less than a 1/16" groove around the plug.Then lay the clay in and work it .Scrap the remainder off.You should have a thin clay line and a sharpe edge.Use a slower curing resin or surface coat for the first couple layers to minumize heat . Wax the plug and if you think you have enough coats put a couple more on. PVA is good but can leave imperfection in the mold which then you'll have to buff out which ain't to bad a job.

Mike James

Never too late too learn...

I misunderstood the earlier "Bondo" post... thinking that he used Bondo before wax and PVA. That's why I said it was a horrible idea.

After looking at David's page (above) I agree that if done right, it would create a very nice fit. Personally, I'd still use some tape or clay under the plug/parting board assembly, to be sure that no resin leaked through any hairline gaps, but it does seem like a good idea, now that I understand what you mean.

Live and learn.

tschmidt

My Feedback: (1)

John, What are you building man!!!

I use "Klean Klay" which is a modeling clay sold by fibreglast.com. and have never had a problem. Also I have used plumbers puddy with great results. Works just as good as the modeling clay in my opinion.

Todd Schmidt

JohnW

My Feedback: (6)

Hey Todd! Was about ready to email you on this one. On my third mold. Started small, cowl for a 40 sized plane, some blades for my Piccolo, etc. The plug in question is for new wheel pants for my Excellence. I was wondering about plumbers putty... what I am using is some non-harding clay I purchased at Hobby Lobby. I think If I'm careful I can get it to work, but just wanted to be sure I wasn't pulling a stupid. The bondo idea looked really slick, but it is making me a bit nervous as I already damaged the plug once. I will try the bondo method after I get a clean mold using clay first. Been meaning to ask you, what density of Polyurethane foam do you use for casting plugs? Is 2lb dense enough? Thanks. Be seeing you when the snow melts.

David, thanks for the web site. Very helpful.

Mike James

John,

Your question wasn't directed at me, but I'll jump in anyway...

I've been using 2-part expandable polyurethane (the 2 lb. density) from Fibreglast for many large scale plugs, and it's plenty strong enough after glassing, using a method suggested by DarrinC...

When the plug is properly shaped, but dimensionally slightly smaller than the final desired size, apply a thick mixture of epoxy and microballoons. Sand with something really rough. (I use 50-grit paper from a heavy duty sanding belt) Then glass and finish normally. I used to use a very wet glassing method directly on these things, skipping the "sealing" step with the microballoons, and it was not nearly as nice. I had hoped that the excess resin would seal the pores and voids during the glassing process, but it didn't. Now, sealing first and glassing second is giving me much better results, and using less resin.

Here's the cowling plug for my 1/6th scale King Air B200. After glassing, the surface was nice, with very little additional filling to do.

tschmidt

My Feedback: (1)

John, I use the same foam Mike is using from fibreglast. It works really well. Another thing I do before making the mold is spray the plug with Duratec surface primer (also sold by fibreglast). This allows you to really level things as it is high build but sands well. The nice thing is it is hard enough to polish to a class A finish that helps release the plug from the mold. Its nasty stuff to spray as its hardened with MEK but worth it in the long run. When making the mold I use a surface coat epoxy first (usually tint it green) then back it up with cloth and mat. Its a thickened epoxy past with a hardener you have to mix by weight and sets in about 20 minutes. Using this will give you a hard ceramic shiny mold surface and takes all the stress out of making very nice molds. You can also get it at Fibreglast.

Mike, nice looking plug!!

Good Luck
Todd

boater

My Feedback: (2)

I wax my plug well, then run duct tape on one side of the center line leaving about an 1/8' gap to the centerline. Cut my parting board a little loose and five minute epoxy the back side of the parting board to the duct tape. Fill the gap to the plug with bondo, leaving it a little low. Now wax my plug and parting board, PVA, and lay up my first half of mold. After curing, pop off the first half and block out the previously low (now high) spot of the mold parting line. Reinsert the plug, rewax it all and PVA, Lay up the other half. Recommend 4 1/4-20 stove round head bolts in the parting board for alignment purposes.

davidfee

Any reason you leave the Bondo low, requiring the removal of the plug for sanding?

If you sand the Bondo flush, then the plug can stay in the first half of the mold... so there is less risk of messing up the alignment and fit. At least, that's my understanding. I have my plugs rest on a balsa cradle, with Bondo making a perfect hand-in-glove fit. See the pics at my website (link above).

-David

JohnW

My Feedback: (6)

Todd, thanks for the tips. I've been using PPG automotive primer on the plugs and then sanding back from 220grit to 1000grit. Seems to work well for producing a smooth plug as you indicated.

Mike, thanks on the Poly answer.

Well, set the plug in clay tonight and just cast the first coat of epoxy. Set the cure box at 85F and ran away. Wish me luck! Thanks for all the help guys. There is definitely a pronounced learning curve to making plug and molds... I think I need more practice.

Mike James

John and everybody,

That's the redeeming value of these online forums. We not only get to share information, including photos, but can get almost real-time results to compare. I have "my" method, but am always open to a better one... Why not? Let us know how your parts come out.

Next time I make a small mold, I'll try using something hard, instead of clay. (maybe not Bondo in particular, but same technique) If it works well for me, I might also do it on a larger, more "serious" mold.

JohnW

My Feedback: (6)

I agree that online forums such as RCU are an invaluable resource. It is very nice to be able to pick the minds of experienced modelers.

I still had some minor problems with the clay. It softened enough to create some issues in my cure box. I think the problem may be the clay I am using. I may try some Klean Clay at a later date and I plan on experimenting with the bondo method as that looks like the cat's meow as long as I can keep it from sticking to my plug. However, I am pleased to say that after some hoop jumping and monkey business I was able to get a nice sharp part edge using the clay on the first mold half. It's all downhill from there. Thanks all for the help!

FLYBOY

My Feedback: (11)

You guys will probably shoot me for this one, What I do on my splitter plates is to use the clay on the underside of the board to hold the plug, then heat parafin wax and fill the gap that remains with it. Then use a straight edge to get the wax leveled exactly where the splitter plate it. You get a perfect 90 degree corner and it is clean and easy. Then the epoxy won't stick, or run, or move the clay or anything like that. Done a few this way and have not been dis pleased with it yet.

aparchment

Neat idea on the paraffin wax, but where do you get it? I will have to give that a try some time.

Antony

FLYBOY

My Feedback: (11)

I comes in boxes from places like walmart, craft stores, I think maybe even the grocery store.