Mold Making 101 or 201? - RC Groups
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Oct 23, 2006, 05:44 PM
Registered User

Mold Making 101 or 201?

I just watched a mold making DVD that used a gypsum plaster (Hydorcal 30). After casting in on the foam plug, you have to do 3 coats of epoxy and then 3 coats of PVA. A lot or work time and expense.

Has anyone tried putting on a layer of silicone molding material first and then Hydrocal 30 on top of that for a rigid backing? The silicone material needs no futher prep and fiberglass will not stick to it. It is plyable and allows the fuse to come out quite easly.

I though that I would ask before I try something that does not work.

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Oct 23, 2006, 06:01 PM
Speedy Tree Surgeon
Hi Tom,
I'm not sure, but i think epoxy does stick to silicone.
If i'm wrong i'm sure someone will chime in and put me right.

Oct 23, 2006, 06:45 PM
Nucking Futs
I'm Rick James's Avatar
While I don't have any experience in this I would imagine that the silicone wouldn't stick to the plaster you would want to back it with. I'm guessing it would peel right out of the plaster mold.

Oct 23, 2006, 07:58 PM
Registered User

Connecting silicone to plaster

I would use some type of mechinical fastener between the silicone and the plaster. I am thinking that the silicone mold would be about 1/8 to 1/4 inch thick, with some wood screws embeded into the silicone, head first so the theads would be into the Hydorcal 30 gypsum cement. I have to be sure that the heads do not make contact with the plug that I am covering with the silicone mold stuff.

Sounds like I need to do a small scale experiment before I do the real deal.

Last edited by TomFlyer; Oct 23, 2006 at 07:58 PM. Reason: spelling
Oct 23, 2006, 08:25 PM
Registered User
motorhead's Avatar
I have heard of this being done but I doubt you could seam it in the plug. You will probably have to maks seperate parts and then join them together. The other problem is trimming the parts along the parting line. With a hard mold you can trim against the mold. If you are not going to make many parts just use the plaster for the mold. I made one a year ago and it worked fine. After the plaster cured well I popped the plug out and coated the mold with epoxy to seal it. Then wax it and make a part.
Oct 23, 2006, 08:36 PM
Registered User
Check the price of the silicone. THe epoxy sealing of the Hyrdocal is a lot less expensive. Anyway, the big work item is making the plug, not making the mold.
Oct 23, 2006, 08:53 PM
find me @samotage
samotage's Avatar
Originally Posted by TomFlyer
... A lot or work time and expense.
Hey Tom,

This is the essence of mold making Don't start unless you have adequate supplies of both! If not, just buy a moldie - it's a far cheaper route.

That said, for me sanding, filling, epoxying, filling, sanding, sealing, polishing, waxing, and molding is an enjoyable way of passing time that would otherwise be spent doing "other activites"

Oct 24, 2006, 12:16 AM
BillO's Avatar
My experience with making a plaster fuse mold is that it does take some time -- first you have to wait a few days for the plaster to cure, then you have to surface the mold with 3 coats of epoxy as you mention.

But after that, you don't need 3 coats of PVA. I did three coats of wax (Partall #2) and then a single coat of PVA, wiped in with a soft cotton cloth until all the bubbles and streaks are gone.

You don't have to spray it, and you don't need a thick coat. Just a very thin layer will do. After using the mold the first time, I've been doing only 2 coats of wax + 1 of PVA. Releases have been great.

Oct 24, 2006, 01:26 AM
Registered User

Great hints and tips

Thanks guys for the great hints and tips. I will stay tuned.

Oct 24, 2006, 01:32 AM
Registered User
TFLG's Avatar
A buddy of mine and Dutchies made a silicone mold for a FW-190 years ago. I ended up with it and made a few fuses. While it did work it was a ROYAL pain in the arse to work with. Just laying in the cloth is nearly impossible as the silicone grabs the cloth and does not allow it to slide so working the cloth down into the mold is very difficult. Silicone molds are used for casting solid parts and it works great for that. If you want a mold for manifolds or a pilot bust or a fake engine it's fantastic. But for laying up fuses, forget about it!

Oct 24, 2006, 09:44 AM
Registered User

Thanks, I needed that

Brian: You just saved me from alot of frustration: Thanks.
Oct 24, 2006, 11:14 AM
Detail Freak
target's Avatar
Hi, Tom-

I've found that it is far better to polish the wheel, than to try to re-invent it...
So, just take the proven techniques, and refine them in your own way slightly, and you should have smooth sailing.
Here's an example:
I asked Phil Barnes why he didn't cut his core blanks just a little longer than the TE of the chord, since in the video, he mentions a possibility of the mylars "hooking" under vacuum.
His simple anwser was, "I never thought of it". So, I usually now cut the blanks about 1/8-1/4" wider at the TE to accomplish this when bagging, or doing a fully sheeted core.
But, I'll bet that Phil still uses the same method that works for him, not mine!

I made one set of small plaster molds to make bulged covers for the flaps on my Trinity.
It was not a big deal to heat and seal them...
but they fit in the oven in the house. After that they worked fine.

Good luck on your project. I wish I had your energy, I'm jealous!

Oct 24, 2006, 11:27 AM
Registered User
F3X's Avatar

Not too long I hope!

TomFlyer brought up the topic and I see it a lot so let me run down a few ideas for future mold makers.

I read all the time about guys wanting to cut corners and save time on making a mold. Well after 25 (oh my God ) years in aircraft composites (UAV, Models, full size) it comes down to the best way not to waste money, time or materials is to do it right the first time. Even a small mold will take material and time but what you end up with is now worth something, cutting corners or going cheap will get you a useless mold or mold of low quality. Either one you don't really want in the long run. Sure you can skimp but lets skimp once you know how the process works first.

(Note: Everyone does it differently and this is just my way of doing things learned by doing it the hard way years ago)

Making a master is the most labor intensive part, why take all the time making a nice master just to slap a cheap mold over the top, sure plaster molds can work but if time is your concern than a glass/epoxy mold is faster and better! (Counting cure time and final mold quality.) You can lay up a mold 1/2 one day and the other side the next. Demold the 3rd day and start making parts. Casting molds in Urethane or filled casting epoxy works also but heat and shrinkage and COST is higher than a glass epoxy mold. They are not saving time and if using casting Epoxy or Urethanes you are not saving money. Not to mention your plug will need to be heat set or heat resistant to not get deformed from the first pour.

Short cut on the master? Why mold a Lump? It takes time and materials to make a mold so why waste time to mold something your not happy with as a final part? Spend a little time and get the master (or Plug) right and all you parts will be better EVERYTIME you make one. I don't see the reasons body working every part that comes out of the mold. Look at the Euro moldies, parts are painted in the mold and the final parts look great. You may want to at least prime the part in the mold; it takes less time and weight and saves on materials.

Glass cloth can be had cheap from a few suppliers. Epoxy is still not too expensive, I get 6 liquid gallons from a local supplier for 250.00 or so. Gallon kits are much less. Good for molds and PSS fuses. I use MGS for lighter thinner parts.

A quick run down of what I would use for a fun slope project. (See note  )
Make from sheets of 1/8 blasa, Lightly spray Krylon paint one side of the sheet black and laminate the sheets together like plywood. I usually paint the center sheet red so I know where the middle is at a glance. Use laminating epoxy and clamp together. Use just a thin layer or epoxy by scraping all epoxy off or use a weenie roller and apply a thin layer.

Shape the master as you like, use the black lamination lines to help keep the left side the same as the right, using the lines will get you very close from one side to the other in a short time. The lines work like guides or rings on a tree and if you ever sanded model aircraft ply you will see the way the lines or plys show you the contour.

I then glass the plug with a layer of 3oz. Then fill with catalyzed polyester primer, sorry but nothing in a spray can will work as well or as well for making a plug. Catalyzed polyester primer will build nice and thick filling small lows and any cloth weave and when cured will form a hard shell that you can wax over for the plug that wonít come off in the mold. One of the best is Dura tec. Available in pints and can be brushed on or rolled on but spraying the final coat is best.

If I really want this baby nice I will sand to 600 wet and look at the reflection in a florescent light and make sure itís optically perfect by filling the low areas with a coat of primer (Same as used above) this is where you can cut some time if youíre not that anal. Getting the sanding rings correct from one side to the other on the plug will get you VERY VERY close, Glassing and priming and block sanding by using a firm foam block will keep ripples out of the plug. At this point the optically perfect plug is just extra time.

Splitter or parting plane. I do not cut or separate my plugs made by hand. Its 2x the work and the molds never align up perfectly. I use a splitter plate cut to shape and packed with clay, I know there are many posts showing this. Here is one:

I always key my molds so I can wet join my fuses in the mold. This saves time and is much stronger seam line by joining ďwet on wet.Ē (See Note )

Mold: I use an epoxy surface coat and epoxy glass for the mold, the mold is usually .150 to .250 thick depending on the shape or size, larger molds get a back up structure small ones like PSS size fuses donít. You can save a little time and materials here as well but really the last layers are the easiest and add life span to the mold so why not get to .150 at least. If you use 7500 glass for the molds that is something like 10 plys if your resin rich as most new guys are or 12 plys if you are better at wetting out. 7500 is 10oz plane weave cloth found almost everywhere.

Time for a small 2m F3F type fuse:
I can shape a master in about a hour that I am happy with. Glass, prime, fill & paint in about 4 hours not including cure time but this would be a VERY smooth and symmetrical plug. Production quality. For a simple F3F type fuse. Not a scale model.
Splitter: In about 2 hours total. Plug in ready to wax and mold.
Mold Ĺ: All up including surface coat gel is about 4 hours. Actual time over the mold is about 1.5~2.0 hours total. Same for 2nd half.

Thatís it. The question I ask myself is: Am I committed to the project enough to spend the $$ and time or do or do I just want a fuse that I can buy for 75.00~150.00. If I just want a fuse I will buy it unless I canít get what I want. For me itís not about making a mold as a hobby (I do that for pay) but making molds as part of your hobby is fun and as enjoyable as flying for a lot of guys.
Oct 24, 2006, 01:05 PM
I always key my molds so I can wet join my fuses in the mold. This saves time and is much stronger seam line by joining ďwet on wet.Ē (See Note )

Thanks! Great information. Do you have an illustration or link to the process you use for keying and joining your parts?

Oct 24, 2006, 01:47 PM
Registered User
F3X's Avatar
Originally Posted by F3X
I always key my molds so I can wet join my fuses in the mold. This saves time and is much stronger seam line by joining ďwet on wet.Ē (See Note )

Thanks! Great information. Do you have an illustration or link to the process you use for keying and joining your parts?

On this mold I set .500 dia steel ball as in the flange. The mold is un-trimmed in this shot just pulled from the master. You can use large rivet heads or shapes in the flange of the first 1/2 that will transfer to the 2nd mold 1/2 and then the molds will mate. Also using lots of 1/4 holes in the flange works well if done before the molds are split. But I don't use screws as pins unless they are shoulder bolts. Use pins and pins and screws as screws to get better alignment.

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