New to Casting and need Help... Please

[theaftermath06]

Hello, I have just recently started casting propellers for working hobby airplanes. I have already cast a few 2 part molds out of MoldStar and have made a few cast of the propellers out of Task 4 Liquid Plastic. I am not getting a finished product by any means. They are not as strong as I would like but was assured they should be with the product I'm using. The propellers are only 9cm X 3/4 cm X 3/4 cm at its deepest although the actual wings of them are as thick as 5-7 sheets of paper. I am getting lots of small bubbles and not even close to a finished product. I have read through many many posts here in these forums and have learned some... confused some... and generally looking for more help as I am brand new to all of this. My questions are:

1. Any advice or suggestions as for the product I'm casting with? The props do flex without breaking but these seem to break easy at a certain point. They need to be impact resistant as much as possible but retain its shape and form.

2. I am thinking about buying a pressure pot to put the mold into after I've filled it with the Task 4 material but not sure after reading these forums if I need to use pressure pot or vacuum chamber? Also will using either of these make the product denser? I would think with pressure they would be denser and more impact resistant since there would be product and not air in spots?

3. If pressure pots are the answer then would I have to remold props with the pressure pot? Would these pressure pots that I see for paint work?

I am new to this again and hopefully posted this in the correct place and in the correct manner. If I have missed any needed info or if anyone would like to see photos of the props or talk more in detail on phone, I am open to anything as I've already invested a nice amount of money and time into this first project. I thought I was prepared better then I am but I am in need of good advice. Thank You to anyone that can give me good advice.

--- Pat
theaftermath06@yahoo.com
just email for phone number

 

[JFB64]

I am not familiar with the materials you are using so all I can say is follow the directions carefully. You may or may not need to vacuum de-gas it. The materials I use do not need degassing. The small bubbles would probably be eliminated by using a pressure pot. A 2 ½ gallon paint pot is relatively inexpensive and a good choice if you are just starting out. It will need to be modified to hold the air in and fitted with a valve at the air inlet. It should also have a pressure gauge. I charge my pot to 40 psi. I would remake the mold in the pot just to be safe. As far as making your parts denser, I don't think pressure casting as will help with that. The type of resin you are using will affect part density. Here is a picture of my set up. I would set the pot on it's bottom like the 5 gal is. I was putting the lid on the 2 ½ once and the whole pot rotated while I was tightening the clamps so my RTV ended up out of level. Cut a plywood disc to act as a floor. I hope this helps.

Joe

 

[theaftermath06]

Ok so a paint pot will work as long as its modified... very cool. I will have to try different products as the task 4 is not as impact resistant as I need. JFB Thank you very much for all the advice and suggestions. If anyone else wants to leave input please by all means do. Thanks again! Pat

 

[JFB64]

I'm glad I could help, Pat. If you decide to go with a pressure pot and change brand names of materials, be sure to get something with a longer pot life, you will need the extra working time to get the molds into the tank and lid secured. For what it's worth, I use Smooth On OOMOO RTV and Smooth Cast 310 resin. They are both 1 to 1 mix ratios and very easy to work with, I've never had a problem with them. If you have any more question, please ask, I've done quite a bit of casting over the years and would be happy to help if I can.

Joe

 

[modelnutz]

Many of the Task family of resins need to be post cured to get their ultimate strength.
Most urethanes really require 6 to 7 days at room temp to fully cure.

I might suggest that you contact Smooth-On's tech line and ask about the post cure requirements of Task 4.

FYI... I've used a number of the Task materials..and they were all brittle without post curing.

Another tip ( if you did this step...please ignore the following ) High performance resins like Task need to be pre-mixed really well.

 

[Wesman]

Just so you understand the physics here...A pressure pot takes the air bubbles and compresses them to make them tiny. A vacuum chamber makes tiny bubbles huge and they can then "boil" out.

 

[cosmic_x]

so guys, I am also experimenting with this at the moment to replicate some small parts which I would liek to use as greeblies but although my mold is very good (no cracks) the resin mix has many tiny bubbles. This means I should use a pressure pot to get rid of them?

(I am using Smooth Cast 300)

 

[modelnutz]

Get thee to the local drug store and get some baby powder.
Check the ingredients and make sure that it's TALC...not cornstarch.

Lightly powder your mold cavity with the Talc....blow it out ( you want just a light dusting ) and then pour your resin.

Do this step every time you pour and you should get much better castings.

The talc acts as a sufectant(sp?)...reducing the surface tention of the mold and allows the resin to flow over the surface much easier ( as well as flow into recesses/details)

OOOps...just re-read your post...are you having troubles with bubbles all through the castings...or are you having troubles with bubbles at the mold/resin surface ?
If the latter...some folks vibrate the table that the molds are sitting on to make the bubbles rise to the surface...then, you can easly pop them with a tool of some sort...I use twist ties with some of the paper removed....the wire sticking out of the twist tie is patted over the surface, popping the bubbles.

Hope this all helps.

 

[cosmic_x]

Ed you are the man! thanks, will test this!

By the small parts the bubbles are mostly in the lower areas, between small details, will the TALC method help me here?

 

[JFB64]

There are two kinds of bubbles that form when casting with resin. Those caused by trapped air when the resin is poured into the mold, which can be eliminated by properly venting your mold. The other kind are caused when the resin cures. These will be eliminated by the pressure pot. Mind you, I have been pressure casting for years without problems. I have never tried these fast cure RTVs or resins so I don't know how well they work.

 

[modelnutz]

Cosmic...thanks.
Yea, the Talc should help with the surface bubbles....just remember to pour slowly so the resin wets out the mold. Dumping the resin into the mold can trap air. If the bubbles keep happening in the same area of the mold...you can pour a small amount in to the troublesome area and work out any bubbles with a wire or some similar tool ( try to avoid using wood tools though...they can hold moisture...adding to the problem )...then you can finish filling the mold.

Also, there is one more cause for bubbles in resin.... moisture.
Water is a foaming agent for urethanes....if you leave your containers open for a long time on a very humid day...you could be asking for trouble.

 

[JFB64]

In all my years of casting urethane, I never knew that moisture was a problem. I don't think that I ever had a problem with it however. I've never tried talcum powder but I always use a spray mold release, which may also help displace moisture, I don't know. I agree that it's a good idea not to keep the resin containers open too long.

I just noticed where you are located modelnutz, I'm in the “big city” a few miles west of you.

Joe

 

[modelnutz]

Joe...so you live in B'flo ? Coolness...what sort of casting do you do ?
Me, I'm a freelance Modelmaker as well as a GK Producer.
I do all sorts of castings myself...figures, spacecraft, Industrial..small, large, clear, Roto-Mold..you name it...I'll cast it :lol

As for moisture...yea, it's a big problem with urethane.
I once tried flour as a filler with SC-300...got some of the nicest, most even bubbled foam I've ever seen.
I figure ( and Smooth-on's techs agree ) the foam was carrying a small amount of moisture from the air....it mixed in real well so the foaming was very even throughout the block of plastic.

Fun trick ;-)

Also, don't confuse the use of Talc as a replacement for mold release....they each do a different task.
Mold release is a barrier coat between the resin and mold surface.
The talc just reduces the "stickyness" of the mold surface..allowing the resin to flow better.
Critical if there are many small details to capture.

 

[cosmic_x]

Thank you for your valuable Input.
One more question(excuse me Pat if Im hijacking your Thread)

I read over at the Website from Smooth-on, that for clear resin you always need a pressure pot

is there no way around this?

 

[JFB64]

Modelnutz..... I started making RTV molds and casting with urethane when I worked as a model maker for Coleco Toys back in the late 80's. The rubber would have to be measured out on a triple beam balance and then you would have to calculate the correct number of drops of catalyst to add. I can't remember how we measured out the urethane. The 1 to 1 formulations of today are so much easier to use. Everything was put into a pressure pot.

I started casting in my home shop about eight years ago when I was making a studio scale Eagle. At first I didn't have a pot, but I realized soon that I needed one. I bought a 2 ½ gal pot and the bubble problem went away.

Now I have a small room in the basement that I use just for casting. I got a five gal pot from a friend of mine which is the one I mainly use now. I've gotten really good at making two part molds. I also use a squeeze bottle to “inject” the urethane into the mold.

Here are a series a pictures that show the process.

The first pour of RTV
The second pour of RTV
The molds ready for urethane
The final parts

Joe

 

[modelnutz]

Thank you for your valuable Input.
One more question(excuse me Pat if Im hijacking your Thread)

I read over at the Website from Smooth-on, that for clear resin you always need a pressure pot

is there no way around this?

I have cast their clear without a pot....but, it's not very easy.
I had to heat the resin to 180 deg. so the viscosity would drop to a water like thickness. Then, I had to work real quick as the resin wanted to cure real fast.
I wouldn't even think about trying it with a 2 part mold...only open face molds so I can pick out any bubbles that come to the surface.

In conclusion.... you need an air compressor and pressure pot..sorry

 

[cosmic_x]

thanks Ed, in other words, I wont bother testing clear resin

 

[modelnutz]

Cosmic X...have you tried post curing the Task 4 yet ?
Pressure cast a prop and once it's cured ( fully set ) put the mold ( with the part still in it ) in the oven at 180 degrees for about 1 hour. Remove from oven and let cool. Take the part out...let it sit till the next day and then test it for strength. You might find a big difference in the properties of the plastic.

 

[cosmic_x]

Cant try that Ed, the only oven I have in the house is the kitchen, my wife will kill me though if she see's me doing anything there

 

[Stormrider]

The clear resin needs a pressure pot because youll see the bubbles. It has no more bubbles than regular resin, you just see them.

While most people use the 300 series from smoothon, the smooth cast roto, actually is more flexible. It is a lot less brittle than the other resins. Its a little thicker however.

Some tips to making your parts, (including some that others have mentioned follow.

1. Shake your resin in the bottles, and then let it sit for 15 minutes. Youll have micro bubbles in it after shaking.

2. Create your mold, build the right type of mold. Some parts turn out a lot better with squish molds. Some with spouts. If I were doing a prop mold, id probably do a squish mold. By over filling it, and wetting the part your squishing in, your less likely to get bubbles.

3. Mix the above with both mold release, and talc. Even baby powder will work.

4. On a side note, if you use something like flour instead of talc, your resin will foam, and turn to a 'bone' material. It doesnt always skin perfect, but its very light and very strong.

I no longer use smoothon plastics. I prefer alumilites casting resins and silicones. Ive gotten poor customer service, and acusatory responses too many times from them when asking questions (IE: Question: Im having inhibition with your plat silicone, what type of primer can I seal my parts with. Answer: It must be something that you are doing on your end that is causing the inhibition. We can not guarantee our parts to work with the type of primer your using. .... But never an answer.

Hope some of that helps.

m2