Old school parts for old school modeler: resin casting question

[W Humble]

I've been teaching myself resin casting for duplicating my old school parts and through trial and error, I've gotten some good results -- finally.  One thing I can't figure out is how to cast complex exhaust headers/manifolds with very three-dimensional curves.  I've made a pretty fair resin version of the Hawk `1/25 1939 M-B Grand Prix racer DOHC V-12, and just by luck found a set of V-12 headers (parts box/origin unknown) that actually fit perfectly.  They are the simplistic semi-log type, 6-into-1 for each side, but have almost a 90-deg. curve from horizontal at ports to vertical at tail-pipe joint.  Any ideas?  Anybody want to make them for me for parts trade?

The Hawk M-B I'm actually using as the basis for a Harry A. Miller V-12 for a special 'phantom' roadster (based on the Revell Cad V-16 glue-bomb kit I have.  I can make some extra copies for trade...  Also, I am copying the Mono Indy Roadster kit's Offy DOHC 4.  Frankly, niether of these engines are great shakes; low detail and missing features: the M-B 12 was for mounting on a diplay that was included, and is missing any semblance of ignition or induction/exhaust, and the trans looks wrong to me.  The Offy lacks detail and even the cam covers are not separate, but just 'indicated' atop the head.  Not too hard to re-do with Plastruct tube, however!  

Any info or help out there?  WIck, age 76 3/4

[robdebie]

I use one-part molds and vacuum casting. I recently cast a very delicate part, to embed a steel wire in it. Shown on the right is the 'master' part, to be poured over with silicone rubber. I used very thin plastic card to 'fill the gaps'. Maybe you could do something similar for your headers? Your methods are probably totally different, I can't judge whether this will work for you.

Rob

Edited June 9, 2022 by robdebie

[W Humble]

Rob, thanks for the reply!  Your technique yields amazing results; don't know if I"m up to that standard!  I built a vacuum chamber (old paint cup and a vacuum pump) but haven't tried it yet.  Is that what you're referring to ?  (Pretty bad Eng. grammar for an ex-teacher!  Sloppy in retirement!)  Wick

[robdebie]

I've been hobby casting for 25 years now, and indeed this might be a bit ambitious if you're starting. Plus: my vacuum casting methods are 'European style resin casting' whereas the 'American style resin casting' is pressure casting. Therefore you will probably get very different comments from American modelers.

Here's a recent mold with some castings. Note the severe undercuts, made possible by the soft 10 Shore-A rubber. There are no knife-cuts in the rubber mold, that would lead to air leaks.

If your equipment isn't operational yet: for this method you need a pretty deep vacuum. If ambient pressure is 1013 millibar absolute, you need to go lower than 1 millibar absolute for the resin to de-air properly. You can translate these values in units of your preference. I *think* you need a rotary vane vacuum pump for that - AFAIK membrane pumps don't go that deep (maybe double stage membrane pumps do). Check the specs of your pump.

I use a 'vacuum desiccator', lab equipment, but you can also use steel pans, pieces of pipe, etc. But: be aware that an implosion is just as dangerous as an explosion: the shards will first move to the center, pass each other, and then move outwards like in an explosion. I've seen photos of sharp pieces of plastic embedded in doors after a vacuum implosion.

Rob

Edited June 9, 2022 by robdebie

[peteski]

Rob, from what I see it appears that your method is similar to pressure casting.  Basically you pour the resin into the open end of the mold, then you put it in a vacuum.  If there are bubbles or air pockets in the liquid resin, they will bubble up to the top. Correct?  I think I like that.  I wonder why it is not more popular in USA?  It likely takes some time to evacuate the air. What is the setting time for the resin you use?

You likely also vaccum the liquid RTV when making the molds?

Edited June 9, 2022 by peteski

[W Humble]

Again, impressive!  I invited some inside dope on casting in several forums, but didn't catch any volunteers.  I hate trial & error, but that's what I'm doing.  So far, a box of failures and mediocre projects, but I'm getting better.  My needs aren't high, at that, and I think if I can get halfway proficient at tw0-piece molds, I'll be okay.

I wonder if an electric fuel pump (12V, for a car) would make enough 'suck'? Thx!  Wick

[robdebie]

On 6/9/2022 at 3:21 AM, peteski said:

Rob, from what I see it appears that your method is similar to pressure casting.  Basically you pour the resin into the open end of the mold, then you put it in a vacuum.  If there are bubbles or air pockets in the liquid resin, they will bubble up to the top. Correct?  I think I like that.

Yes, corrrect! With one addition: the resin itself also de-airs during the process. Any bubbles that were added during the mixing are removed, plus moisture absorbed by the resin during the very limited open time of its containers. Plus: the mold has no cuts on the sides, I want to keep it water- and air-tight. The latter is of no importance in pressure casting it seems.

Now that I thought a bit about it, there is a theoretical difference. During the vacuum stage, most of the resin is pushed out of the mold cavity, into what I call the resin chamber, because it's foaming / boiling. Only when the air is admitted into the vacuum chamber again, the mold is actually (permanently) filled. Since the resin has to flow through a rather small gate, problems occur if you admit the air too quickly to the vacuum chamber. Air is then gulped into the mold cavity, and you'll have large air bubbles.

On 6/9/2022 at 3:21 AM, peteski said:

I wonder why it is not more popular in USA?  It likely takes some time to evacuate the air. What is the setting time for the resin you use?

I wondered about that difference too, it's fascinating. Maybe it's because resin casting is usually learned from someone else, and once there's an established method, it keeps repeating itself. Nothing wrong with that of couse. I developed my methods by studying mainly East-European resin parts. AFAIK they all use vacuum casting.

My resin has a pot life of 7 minutes. That's just enough for mixing, transfer to the vacuum chamber, 3 minutes of de-airing, and returning fairly slowly to ambient pressure. Air rushing in can blow resin out of the mold, and in extreme cases the molds can be 'sucked closed'.

On 6/9/2022 at 3:21 AM, peteski said:

You likely also vaccum the liquid RTV when making the molds?

Correct, you need a vacuum set-up anyway to degas the silicone rubber. I think that is done in both the European and American styles of resin casting.

Rob

Edited June 10, 2022 by robdebie

[robdebie]

On 6/9/2022 at 3:33 AM, W Humble said:

Again, impressive!  I invited some inside dope on casting in several forums, but didn't catch any volunteers.  I hate trial & error, but that's what I'm doing.  So far, a box of failures and mediocre projects, but I'm getting better.  My needs aren't high, at that, and I think if I can get halfway proficient at tw0-piece molds, I'll be okay.

I wonder if an electric fuel pump (12V, for a car) would make enough 'suck'? Thx!  Wick

I made this graphic representation of my method. It avoids having to type a lot of text ?

An electric fuel pump? I don't think that will give the deep vacuum you need to de-air resin. As reported above, I think you need a single-stage rotary vane vacuum pump. They are pretty cheap now, 100-150$ or so on Ebay.

I completely recognise the experimenting part, it also took me years to get where I am, lacking any help, and before YouTube etc. But I taught a few club members my methods in recent years, and they hit the ground running!

If you want to go the 'American way', check out Robert Tolone on YouTube. He uses totally different methods.

https://www.youtube.com/c/RobertTolone

Rob

Edited June 10, 2022 by robdebie

[robdebie]

12 hours ago, W Humble said:

I hate trial & error, but that's what I'm doing.

I hadn't spotted that line before. In that case I recommend getting:

1. a single-stage rotary vane vacuum pump
2. a vacuum desiccator or equivalent
3. soft ~10 Shore-A addition type (platinum) silicone rubber
4. relatively slow polyurethane resin, like Smooth-Cast 305

This will set you back some 250$. But you'll be able to cast resin parts as shown below.

Rob

Edited June 9, 2022 by robdebie

[peteski]

Hey Rob, thanks for very informative tutorial.  Next time I have to cast something I'll give it a try (since I already have vacuum pump and chamber I use for de-airing the RTV.  Vacuum method seems simpler than pressure casting, and  simpler (since  only vacuum is needed).

I wonder how it would work for things like car bodies (which need 2-part molds)?  I'm just theorizing since I have only been casting using single-piece open molds (like like what you are using).

I hope Wick doesn't mind me butting in on his thread asking my own questions.

[W Humble]

Peter; not butting in!  This turned out to have generated some very interesting and enlightening information, and a great graphic!  That should be on Utube or something.  Thanks to Rob!!  I may never get that sophisticated, but then again... I have a nephew who wants to learn from my experience, and he's very tech, and a perfectionist!  I'm going to will him all my model stuff, at least that my wife and/or kids don't want.   He's going to teach me decal making.   Wick

[Spex84]

Those are some great results! I'm intrigued by the heavy support structures holding each part in place...I haven't seen that before. Clearly it works well.

I've attempted to cast parts before and my results were extremely shoddy compared to the above examples. Thanks for the clear explanations!

[robdebie]

10 hours ago, peteski said:

I wonder how it would work for things like car bodies (which need 2-part molds)?  I'm just theorizing since I have only been casting using single-piece open molds (like like what you are using).

I haven't cast a car body yet, and I don't know how to approach that with my methods. Maybe pressure casting has the advantage there!

Rob

[robdebie]

8 hours ago, W Humble said:

Peter; not butting in!  This turned out to have generated some very interesting and enlightening information, and a great graphic!  That should be on Utube or something.  Thanks to Rob!!  I may never get that sophisticated, but then again... I have a nephew who wants to learn from my experience, and he's very tech, and a perfectionist!  I'm going to will him all my model stuff, at least that my wife and/or kids don't want.   He's going to teach me decal making.   Wick

Maybe I'll put the graphic on my website one day. I do have web pages on decal design on my website:

https://robdebie.home.xs4all.nl/models/decals.htm

https://robdebie.home.xs4all.nl/models/customdecals.htm

( Waving to Peteski because we know each other from the Alps printer mailing list on Yahoo / Groups.io )

Rob

[robdebie]

5 hours ago, Spex84 said:

Those are some great results! I'm intrigued by the heavy support structures holding each part in place...I haven't seen that before. Clearly it works well.

I've attempted to cast parts before and my results were extremely shoddy compared to the above examples. Thanks for the clear explanations!

You're welcome! The heavy gates (as I think is the correct word) are generally 1 mm plastic card. The thickness is required because otherwise air bubbles can get stuck in the part, being too large in diameter to pass through the gate.

Another factor is more difficult to explain, but I described the effect above. If the gate is very narrow, there's a real chance that the pressure difference during air admittance will cause the gate to (accidentally) close, and then remain closed since the pressure difference only increases. The mold is very flexible, so that happens easily. In that case the mold cavity isn't filled with resin, and you'll get a really bad casting.

Rob

Edited June 10, 2022 by robdebie

[peteski]

4 hours ago, robdebie said:

Waving to Peteski because we know each other from the Alps printer mailing list on Yahoo / Groups.io

True, we have been involved with Alps MicroDry printers, and decal design and printing for many years.

[robdebie]

Here's another small mold that I just photographed, with the master on the left side, two castings and a painted set. They are 1/72 scale.

The mold design is not optimal: the castings are difficult to release. I could have used a zig-zag pattern, for example.

Rob

[Rick L]

This 4 part rtv mold was formed over a RenShape master without pressure or vacuum. The resin casings (second photo and ready for primer) are cast upside down and sprued to the inside top where pressurized mixing guns shoot the airless plastic down into the bottom of the mold and push the air up and out through risers along the perimeter of the part. This resin kicks within 20 seconds after filled. 
The same technique can be done on a small scale by poring vacuumed resin into a syringe and pushed into a two part mold.