Casting help

[Hammer3246]

I've never done this and was wondering what the best way to go about casting this part would be with this kit? According to the instructions the RTV is a one mix deal so I asume I can't make a 2 part mould.

The part is from the Leo for my bomber (crappy pic), trying to save money on kits.

Cheers

Ron

[modelnutz]

Ron, you should be able to do a 2 part mold with that kit.
You just need to seperate the 2 parts of the RTV so you can do 2 pours.
It should be a 10 to 1 ratio...am I correct ?
Usually, it's measured out by weight.
However, silicone is pretty forgiving about mix ratios...just be sure to use a minimum of 10 % catlyst ( or, you'll be waiting forever for your Rubber to cure )

here's how I would proceed with your part....

1) I would get some sulfer free clay ( or, go to Michael's and get some of the Laguna Clay...white works well )
2) Roll out a 1/4" layer of the clay on to a piece of Plexi-Glas ( or wood..not too important..it's just a firm work base )
3) Press your part in to the clay...1/2 the thickness of you part is good.
4) Using a flat bladed tool ( a spatula...or even a piece of wood with a flat chisel like end ) gently press the clay up against your part ( The object here is to push the clay up tight to the part...ideally you want to end up with a nice clean, crisp transition from the flat surface of the clay to the very edge of the part...any gaps will allow the rtv to flow around and possibly under your part.
5) Trim the clay to 1/4 to 1/2 inch away from your part..this surface will be your mold's mating surface
6) Using another tool ( A pen cap...A chop stick...whatever ) press in "dents" all round your part...these are the locks for your mold...they keep the 2 halves from becoming mis-alligned...they should be approx. 1/8 deep ( or deeper )
7) make a box around the clay...foam core works real well for this step...I use hot melt to hold it together...but super glue will work just as well....make sure that it's well glued down to the base.
8) mix your silicone as per instructions and pour...save half for the second side of the mold ! Note...when pouring your RTV...pour very slowly from at least 3 to 4 feet above the mold box..and pour in as thin of a stream as you can...this will helkp squeeze out most air bubbles.
9) When you RTV is cured ( don't rush it ! ) Pull the mold box/clay/part off of the work board ( A thin knife helps with this step ) and carefully remove the exposed clay. Clean you part...remove all traces of the clay. Note...don't get too upset if the mold box falls apart...just make a new one....Try very hard to NOT DISLODGE the part from the RTV.
10) Using a mix of Vaseline and Mineral Spirits, brush the mix over ALL exposed RTV ( This will keep the mold halves from bonding.)
11) Mix the remaining RTV and pour as above.
12) Remove the mold box and seperate the halves...you should have a very clean, accurate set of cavities to work with.
If all has gone well to this point...Pat yourself on the Back...the hard part is over.

The final step is to cut in a pour tube and an air vent.
This is where experiance ( read...making mistakes ) really comes in handy !
Use a sharp X-Acto and cut a channel leading to an outside corner ( your part looks to be a rectangle with a cutout area at one end...I would consider the cut out area to be the bottom of your mold when pouring. ) Then, at the opposite corner I would cut a smaller channel for air to escape the mold.
The secret to being sucsessful with this step is to picture the flow of the resin in to the mold...you want the resin to enter the mold chamber at a point below where the air is escaping...the air vent should be at the highest point of the mold so it fills last.

Now that you've created the mold...here's how I'd use it...

1) Brush a small amount of Talc ( Talc is found in most baby powders...check the ingredients ) in to the mold to help the resin flow better
2) Strap the mold between 2 flat panels ( Plexiglas works real well for this step ) using packing tape...don't strap real tight..you don't want to sqeeze the mold.
3) Using a polyproplene glue type bottle ( Found in the candy section of Michael's ) sqeeze in your resin mix till you see resin filling the air vent

Allow the resin to cure the appropriate amount of time ( or, longer for a thin part..you don't want warpage..do you ? )...stip off the tape...and remove your perfect casting.

I know this sounds like a lot of steps...but, I tried to be as concise as possible....don't be intimidated if this is your first time casting....just read through the steps a few times till you're certain that you understand the steps....then have at it !
It's really easyer than it sounds.

Any questions...don't hesitate to ask......and, good luck !

[Hammer3246]

Thanks for the steps! I did watch a few videos but this is way more detailed info, I'm looking forward to trying my first cast.

Cheers

[427 Cobra]

It'd be a lot easier with photos. I'll be doing some casting with any luck this summer, as I need some parts replaced that I had to fabricate. I'll post some tutorials, but I need to make a list to remind myself to do it, or I'll forget! Please show us some pictures when you have something you can show, even if it's a failure. This is the only way to learn. At least you'll have the forum to help if something does go wrong.

~ Cobra Chris

[Hammer3246]

The part is on top of the box in the photo (its not a great pic), once I get a chance to start on the moulding i'll take some pics before I pour anyting tho just to get some input.

[MonsieurTox]

Ok I see what part you want to mold and cast. I did mold an cast this part when I did my Bomber, using the same Alumilite set you have. Alumilite is very cool but the problem is that it's not very fluid and it cures very quickly... To avoid airbubbles with the alumilite, I used baby powder on the mold (still use it with my new resin), then I dropped tears of resin in the mold details, using a tooth pick to remove airbrubbles. Then I mixed more resin and poured it into the mold to fill it. This way no airbubble, even with this pretty thick resin !

[Hammer3246]

I haven't cast it yet but i'll keep that in mind when I start, thanks for the tips. This should go smoothly.

Cheers

[JAWS]

These guys know what their talking about. Yoiur in good hands thats for sure. Good luck. Nice tutorial Model Nut.

[dbhs]

Hey guys. The written tutorial is very good and easy to follow. I do have a question. Do you really pour the silicone in from 3 to 4 FEET above the box? Should that be inches or are you talking about large molds where the box is fairly big?

[modelnutz]

(06-08-2010, 11:45 AM)dbhs Wrote: Hey guys. The written tutorial is very good and easy to follow. I do have a question. Do you really pour the silicone in from 3 to 4 FEET above the box? Should that be inches or are you talking about large molds where the box is fairly big?

Thanks for the compliments on the tutorial guys...they tell me that I should have been a teacher ;-)

dbhs...You read right...3 to 4 feet.
Works for any size mold...I mostly use the technique on molds that will only be used for a short run of castings...they're called "waste molds"...due to the fact that your only pulling a few castings and then throwing the mold out. This is usually done when making multi-chambered production molds where you need ( say ) a dozen copies of a part for said mold.
This technique was developed to overcome the lack of a vacume chamber.....letting the silicone stretch out into a very thin stream helps to pop air bubbles that would normally be removed by de-gassing the RTV in a vacume chamber.
Don't be too concerned when doing small molds this way...if there is no air blowing the silicone stream around as you pour...it should be very controllable.

Keep in mind though...the ideal mold will be made from RTV that has been fully de-gassed in a vacume chamber....there is no other way to be completly sure that you will not get the dreaded resin beads on the surface of your castings....if you pressure cast with a mold made with the "small stream" technique, your mold may work fine...testing is recommended