Torpedo tube casting for 1/72 PT Boat

The new ones are coming out better thanks to Aimee's advise.  The only piece from the kit is the front end of the tube cover.  The rest is a combination of scratch building and parts from WEM's photo etch set.  I'll get some photos organized eventually.

Dave

Looks like your castings turned out very well! Are these scratchbuilt or based on the Revell kit?

I've also recently started learning to cast model parts - I'm using white metal rather than resin as it's not quite so expensive (mistakes and badly cast parts can be melted down and recycled - and early on I made a LOT of mistakes). I'll probably be using this technique a lot in future for making fittings for scratchbuilt models.

Well, I followed Umi's advise on making and preparing molds and I now have an excellent mold and the first castings have gone extremely well.  I should have all four tubes poured before the weekend.  Thanks so much for the advise. 

A friend of mine came up with I think is a great idea for pulling air bubbles out of liquid rubber and resin.  Use a FoodSaver (a.k.a. seal-a-meal) system.  They are low cost and serve a purpose beyond modeling (like we really need one).    I'd be interested to hear if anyone else has used this device for this purpose..... pulling air from liquid resin and rubber.

Thanks

Dave

I use these polypropelene cups, and nothing sticks to them. Well maybe epoxy sticks in them, but a thin layer of casting resin can be peeled out of the cup, or popped out if the bottom has a thick layer.

16 oz cup is what I typically use, the larger ones are not as durable.

Everything else is scraped clean, or protected with disposable cardboard overlay.

The Easyflo-60 is what Bare Metal Foil is selling.  I think I'll go back to that product after I'm done with the material I'm using.

What do you use to clean up your cups after use?  I can't find any instructions on cleaning up this material.

Thanks once again.

Dave

I really like the 6 hour cure, as it moves projects or small part molds right along. However, I feel like the quick cure makes the molds more brittle or subject to tearing later in the lifespan. The 24 hour cure is more flexable, I often support each side with some thin plywood, or fome-core board. If you are doing large molds the 24 hour cure is going to give you the best working time. There is nothing more frustating in mold making or casting then having the material "kick" during a pour.

I used to get all my supplies exclusively from Tap Plastics. Their cups are reusable and come in several sizes, They also have paper cups for small pours, and I also use them for epoxy mixing.  Tap has some local franchise stores in town. Anyone can pick up all they need for moldmaking or fiberglassing or acrylic work just by walking into the store. Resin is all personal preferance, and availability. I loved the Tap quick cast till they changed to a thicker formula(beige) I switched to a Polytek brand, Easyflo-60, or Easyflo-90. Both are white resin, but each has different working time and viscosity. After curing, they both can become very flexible under hot water or "in the car temperatures". I had a hydrofoil wing sag after sitting in the car for a quick drive across town. I ended up using a BJB brand polyurethane for those parts. My choices are based on curing time and viscosity, and then stiffness.

Again, Resin choice seems to be left mostly to availability for most people. I have tried many, but never run across, or had the need to try any of the bare metal foil, or micro mark brands.

Thanks once again... okay... more questions....

Have you used the 24 hour RTV and if so have you had better luck with it?  The 24 hour RTV seems more flexible which might be good or bad.  I'd appreciate your opinion.

also...

Do you reuse your mixing cups?  If so what do you use to clean them up?  I bought some from MicroMark which ended up being rigid plastic.  They crack with a single use.  I ordered some of the flexible cups from another company.  I bought some of these from Bare Metal Foil before but they only seem to sell them in their packs with other materials. 

finally....

Do you have a preferred source for resin?  I've bought it from Bare Metal Foil (beige when cured) and now Micro Mark (white when cured).  The Bare Metal Foil resin seems more rigid than the Micro Mark product.  I think I prefer the Bare Metal Foil resin but am interested in your take.

Thanks once again

Dave

RTV can be de-aired in a vacuum till the material cures.... The few times that I have seen RTV stop bubbling in a vacuum, was when it was a very small pour, or it was heavily thinned. The thinned RTV is subject to shrinkage. I typically don't de-air for more than seven minutes.

A concern I have with mold making under pressure, is that if there is an air bubble near the part surface it will be compressed to it's smallest possible size at, say, 45 psi.  The air bubble is not gone it is compressed. So when you release the chamber pressure, you now have an air bubble sealed in the rubber that will want to expand. It does this each time it is put under pressure and brought back to normal atmospheric pressure.  This will tend to be a weak spot in the mold. It may also be why you had a tear so early in the mold life.

I use a spray mold release, Pol-Ease® 2300 Mold Release, available from Tap Plastics.

Tap Plastics

I used to use their polyurethane "Quick Cast", but they recently began selling a different fromula, and the viscoscity is thicker than I prefer to work with.

On some wider flat surfaces, the mold release can often be seen on the surface of the casting. Because the mold release is supposed to create a "barrier" to help release the part,  it often prevents the casting material from actually contacting the mold. This is in micro millimeter distances, but it is evidant by micro bubbles on the cast part's surface, or what sometimes looks like "water stain". The talcum powder wicks the casting material right up to the mold surface and prevents the micro bubbles or "stain" from occuring.

A simple way to help remove bubbles from your casting material is to draw it up in a seringe. The bubbles will all be up at the top of the seringe, and you "inject" the casting material and not the air. Air that is trapped in the mold while pouring, is problematic, and is best dealt with by properly designing the mold to not trap air in the first place. Proper "plumbing" is essential to a bubbleless casting. Parts molded at angles rather than flat, undercuts (parts) turned upside down, sprues at the low end for material pouring, and vent(s) at the high end for air release. Mold keys to align the mold halves, and "dams" to prevent the mold from leaking during a pour.

There is a pretty big learning curve, but by attempting your own castings at all, you are well ahead of the game.  

Another question comes to mind.  How long do you put your RTV under vacuum before you pour it into your master?  I don't have a vacuum but I would imagine that putting it under pressure might do the same thing.  Let me know your thoughts.

Thank you

Dave

Thanks for the information Umi,

One thing I'm thinking about doing is not using a mold release.  My first casting I did without a mold release and it came out fine.  On the second pour I used the mold release and bits and pieces of the mold stuck to the casting. 

Now you said you used a spray mold release?  The stuff I was using was a brush on.  And I trust the powder is to absorb any wet material?  I noticed on my second pour that I lost some of the fine parts of the casting.  powder may do the trick.

I also didn't try to remove the bubbles before the pour.  I'll give that a try too.  Although my molds and castings are coming out bubble free.  I pressured to 45 lbs as well.     

Thanks again!

Dave

It shouldn't matter if you leave the mold overnight.

I will put my rtv under a vacuum to pull the air bubbles out, and then pour my rtv over the part and clay. I then pull another vacuum, and make sure all the bubbles are away from the part, and let it cure. The same thing is done for the second half.

I will let the mold cure for at least 24 hours. Then after I pull the master part out, I spray the mold with mold release and let it set for at least 12 hours. When I am ready to make a part. I wipe any excess mold release out of the mold, and then sprinkle baby poweder over the part area. I brush the powder around to distribute it evenly, and "smack" the mold halves together once to "poof" any excess powder out of the mold. The mold is set up for a pour, the casting material is mixed, the mold is filled, and placed in the casting chamber. The pot is pressurized up to 45 psi, and then left to cure.

Whether the part is left for half an hour, or overnight depends on the temperature. On hot days, half an hour is fine.  Other days, up to six hours(depending on your casting material formula).  I often have an "excess" pour on the table, something open faced, one sided mold, poured at the same time. If that casting is still soft, I will sometimes pull the molds out of the pressure chamber, but leave them in the molds to cure longer.

For thin castings, I will preheat the casting pot, and molds in a box under a light bulb, and use that same light to keep the pot warm during casting. Again, the need often varies with the formula of the casting material you are using.

I usually don't see any tears out of the mold until the molds are well into use.

Well I had a failure with the 2nd pour and thought I would share with you what went wrong so you don't do what I did.  Because the resin from the first pour was a little soft I thought I would leave the casting in the mold overnight and give it a little more time to set up.  This was a mistake.  Small parts of the mold stuck to the tube even though I had used a mold release.  The lesson learned is to de-mold as early as possible....i think. 

Fortunately the master is in excellent shape so I'll build a new mold.    The igniter came out nice though! 

Dave 

It would probably make more sense if I had a good picture.  The vertical lines you drew in there are actually part number 11 from the WEM kit.  They only go half way around the tube on the top.  I included them on the master so I wont have to add them on each tube.  I would simply mount this tube on part number 12 and it's complete.  The horizontal lines on the breach are supposed to be there too.  I used the WEM breach plate on the master as well.  There are notches molded in there for the turnbuckles on part 10.  I'll nip off each turnbuckle, give them a little twist here and there, and glue it into place.  What are also hard to see are the inspection plates or whatever those little plates are all over the top of the tube.  They are in there with rivits.  I'll post some more pictures once I get them painted and mounted. 

This ended up being one expensive torpedo tube by the time I pony'd up for the pressure pot, the WEM kit and the molding material.  But I think the results were worth it.  I'm getting perfect molds and castings now.

I'll work on the photo techniques too. 

Superb job!

Though, to work with WEM PE set, some parts must still be sanded (in red in the picture)

Michel