Master mold making and heat proof fiberglass?

[BsSmith]

Hello,

I'm just starting to gather materials for a HPR scale Ares I (3" booster, 4.5" upper stage). The rocket will mostlybe flying on J motors. All of the transitions on the rocket will be made out of molded fiberglass, including the two nozzle pieces at the end of the booster. I have never made a mold before. I found a very nice thread on here about how to make the mold, but I haven't found anything about the master, so I have a few questions.

The masters I ordered will be made out of balsa. I know that the master has to be smooth so I can pull it out of the mold, but how hard does the master have to be? Will filled, primed, and painted balsa be enough or should I dip the transitions in laquer or some other material that will harden the balsa?

Once I have the molds made, I will be making the nozzle piece that sits under the motor. As a result of it being under a white lightning J motor, it will get very, very hot. Is there some way to heat proof the nozzle with something I can mix into the epoxy? Will JB Weld mixed with a carbon powder (to make it less heat conductive) work? The rocket will also be landing on the nozzle, so it has to be decently strong too.

 

[Pat_B]

It depends on a number of factors. The balsa only has to stand up to allowing the rubber mold material to flow around it. You want the surface to be smooth enough so that the rubber mold material doesn't get caught up on any fuzzies on your balsa and end up with some air bubbles.

Once the mold is dry then you need to remove your original balsa part. Without knowing the exact shape of what you intend to do I can only say that you might end up breaking the balsa part when you remove it from the mold. If that doesn't matter then the only issue is whether or not any balsa particles end up being embeded in your rubber mold.

If you have a more complex mold for which it would be harder to remove your original then you can consider buying some more flexible mold material. They sell a more flexible rubber mold material for those molds in which you have to do a lot of stretching.

Also realize that mold material nowadays can pick up very subtle grain patterns from balsa, so the surface quality you get out of your mold will be nearly identical to how well finished the balsa was to begin with.

Realize too that in order to get a really nice surface finish that most original parts are even waxed prior to being used to make a mold. Obviously, balsa isn't too ideal for that. It all depends on how slick of a surface you end up needing.

 

[troj]

What Pat provided will work, if you're making rubber molds for casting resin. However, it sounds like you plan to make your nozzles out of fiberglass, so the process is different.

If you're indeed planning on making fiberglass molds & nozzles, then I wouldn't suggest balsa as a material -- it's not strong enough, and you'll likely have problems removing the master from the mold.

Aluminum is the ideal material for fiberglass molds, but wood can be used -- the master for our Delta III nosecones is made from oak that has been sealed with epoxy. You just need to use a wood that's solid enough to stand up through the process of being pulled from the mold. Even when a release is used, the master sticks a bit.

As far as your materials go, if your nozzles are exposed to the heat from the exhaust, they're going to get cooked a bit. I'd suggest making sure the bottom of the motor is at, or very close to, the bottom of the molded nozzle.

For the landing question, if the motor is moved to where it's close to the bottom, then you'll have a tube and centering rings to support the nozzle, so it should be fine, with a safe landing speed.

-Kevin

 

[rocketsaway]

Hello,
Once I have the molds made, I will be making the nozzle piece that sits under the motor. As a result of it being under a white lightning J motor, it will get very, very hot. Is there some way to heat proof the nozzle with something I can mix into the epoxy? Will JB Weld mixed with a carbon powder (to make it less heat conductive) work? The rocket will also be landing on the nozzle, so it has to be decently strong too.

Many yrs ago, for my SAM NARAM scale event, I needed to protect the launcher's blast shield from heat. The launcher was used as part of the event and made of styrene from a model kit.
I coated it w/Parker High temp Furnace and Retort cement. Its a black grainy paste that adhered well. Good to 3000F. Granted, the motor was only a 1/2A.

 

[Pat_B]

Gosh, I missed the fact that you're using fiberglass. What Troj said is best. I do a lot of layups for FAI rockets and my molds (or 'plugs') are all aluminum. Some folks have gotten by with wood molds so long as they're coated with a finish such as polyurethane AND heavily waxed. But aluminum is indeed the best.

 

[BsSmith]

Maybe I'll do it in two parts.

First, I'll finish the balsa transitions and use them as masters for a mold to make a set of masters for the fiberglass mold. What easily pourable products are there to make the 1st mold (that the balsa will go in) and the 2nd master mold? I don't think I can get machined aluminum.

The transitions are just simple cones, so they should be pretty easy to pull out of the mold.

 

[troj]

First, I'll finish the balsa transitions and use them as masters for a mold to make a set of masters for the fiberglass mold. What easily pourable products are there to make the 1st mold (that the balsa will go in) and the 2nd master mold? I don't think I can get machined aluminum.

You could try resin. I'll defer to Pat, Micro, or someone else for how strong that stuff is.

One thing to note is that you need to have the balsa master perfectly smooth, as every imperfection will show on your molded parts. This is true regardless of what kind of material you're using, and is why aluminum is so nice for masters.

The transitions are just simple cones, so they should be pretty easy to pull out of the mold.

You'd be surprised.

I made some 7.5" nosecones for the Delta III; straightforward shape, nothing fancy. Getting them out of the mold was sometimes "fun".

-Kevin