Something to mold

[n3tjm]

I had an interesting idea to make some injection molds to cast some motor parts. Making the molds would not be a probem... but what I need to figure out is what to put in the molds. I need a material that will not melt, and have good tensil strenght. Also, affordability is an issue to . I want it to be cheaper then buying parts from RCS

Any ideas of what to use?

 

[teflonrocketry1]

How about Acrylic casting resin such as:

https://www.alumilite.com/

https://www.hobbyengineering.com/SectionBM.html

Bruce S. Levison, NAR #69055

 

[Stymye]

an injection mold... ? tell us more

 

[n3tjm]

Sorry.... this is an experiment I want to try out first . I want to see how doable this is, and how economical it is on the small scale. If it is pratical, I may decided to sell the casings I make to the EX enthusiests who want to make smaller motors .

My first trial will be with a 29mm motor.
Then if that works out, 24, 18, maybe even 13mm

Now I know what some of you are thinking... why make small motors....

Dual Black Smoke motors in a stock Dueces Wild

10 Second Burn G motor for monocopter and other rockets

a F14 clone for my Initiator....

Besides... everybody is making BIG motors... which is cool. But I wanna explore an area largley ignored by the ex community

 

[BlueNinja]

I dunno... At first I was thinking RTV but that's rubber... Maybe look at www.shopmaninc.com

?

B N150

 

[DynaSoar]

Originally posted by n3tjm
Now I know what some of you are thinking... why make small motors....

Not at all. Three words: frontal cross section. It's the primary factor in drag. Apogee makes the case for smaller diameter birds/motors very well in their literature and on their web site.

As to what to use, how about high thermal resistance (NOT high thermal conductivity) epoxy, such as Alchemie's EP45-5/H83 or EP 4601 mixed with a high temp matrix like graphite or carbon fiber powder. More expensive but worth looking at is using a ceramic injection molding material like zirconia for the matrix, using the epoxy instead of sintering. Either way, messy, requiring plenty of solvent for clean up, but off-the-shelf. If you could find paper tube made from very thin layers for liners, it would ablate some rather than just burn through. Maybe find some paper tube with glassine coating inside?

 

[n3tjm]

I had an interesting thought... and I am going to try it...

.25" x 1"

ring a bell?

Micromax size composite motor.

think it could be done? I am going to find out .

 

[teflonrocketry1]

The fireworks manufacturers are now using polypropylene motor casings! The problem is the casings tend to bulge during firing, that's O.K. in a firework where you don't have to extract the casing from a cardboard motor mount tube afterwards.

Bruce S. Levison, NAR #69055

 

[DynaSoar]

Originally posted by teflonrocketry1
The fireworks manufacturers are now using polypropylene motor casings! The problem is the casings tend to bulge during firing, that's O.K. in a firework where you don't have to extract the casing from a cardboard motor mount tube afterwards.

Bruce S. Levison, NAR #69055

A slightly concave cylinder to compensate? Parabolic, for maximum strength?

 

[Ryan S.]

Originally posted by n3tjm
I had an interesting thought... and I am going to try it...

.25" x 1"

ring a bell?

Micromax size composite motor.

think it could be done? I am going to find out .

how are you going to ignite it, you are going to need a super small ignitor. Also, how are you going to mix a batch that small. sounds like way more work then it is worth.

 

[teflonrocketry1]

Originally posted by DynaSoar
A slightly concave cylinder to compensate? Parabolic, for maximum strength?

Sounds like this would work but it might be a challenge to mold and fill the casing correctly.

Bruce S. Levison, NAR #69055

 

[n3tjm]

Originally posted by Ryan S.
how are you going to ignite it, you are going to need a super small ignitor. Also, how are you going to mix a batch that small. sounds like way more work then it is worth.

IF I make a motor that small, the igniter will be built into the motor. I have about three ideas how to do that.

It is worth it to see IF it could be done

 

[DynaSoar]

Originally posted by teflonrocketry1
Sounds like this would work but it might be a challenge to mold and fill the casing correctly.

Bruce S. Levison, NAR #69055

True. You'd then have to injection mold the grain into the casing, unless you were pouring a liquified grain.

 

[Hospital_Rocket]

Originally posted by n3tjm
IAny ideas of what to use?

I don't want to talk down to you, but have you ever done any work in injection molding? You are talking moderately high temperatures (around 300 degrees) and very high pressure (Tons per square inch).

Also, mold design is the work of serious engineers. If you are talking about a motor casing, make sure you design in a taper to allow the part to eject.

I presume your statement that the mold is no problem imples you have access to a machine shop. I did a quick calculation and a single cavity mold out of aluminum would cost around $600 to have machined.

As for material, I'd consider a thermosetting compound like Diallyl Phthalate. A glass or boron substrate might help.

Another possibility would be bakelite.

A

 

[n3tjm]

I do have access to a machine shop.... right next to my bedroom . Now the molds I think about makeing are not typical molds. I am not sure how it will work out, but I will find out. The only thing I need now is the money to buy what I am going to try, and the time to work on it (plus don't want to get in the way of my Dad's steam engine project)

 

[Stymye]

Doug, thats why I asked about it further up the thread

Generally speaking,
you need access to equipment to run an injection mold and the mold has to readily adapt to the machine..
mabey you could check with injection molding companys in your area and see what they would charge to run xxx quantity on your mold
..It would be a major job to build something yourself to run the mold

 

[Hospital_Rocket]

I just talked to an old friend who runs an injection molding company.

His 30 ton press cost $1500 to set up and based on the guestimate size of your piece, a single run would be about 800 pieces. that was based on a 29mm x 125mm part with a 24mm internal bore.


Now you would need to price out the resin.

I'm not reying to squash your idea...

It's just that injection molding is a high volume production process.

A

 

[n3tjm]

I am (thinking about) machining the molds from tubes and bar stock. I will make an injector that fastens to the bottom of the mold, and the solution is injecetd through the bottom, works its way up,, and exits the air holes on top. This may or may not work, and there are issues of air bubbles, and bonding issues.

I am hoping I can us that liquid stuff for this... although they say it can deform. I need to experiment with it first, but I will do that on a much smaller scale. I have some ideas how to do that too... This is pretty much a trial and error project.

This will work in a similar fashion to the injection mold device we had in metal shop in high school

 

[Hospital_Rocket]

This may sound dumb, but are you talking about injection molding or extrusion?

Injection molding is inserting a material into a closed cavity under heat and pressure then letting it cool.

Exrtusion is pushing a semi solid material though a die (think toothpaste) to obtain a required geometry.