The Ship Model Forum

FrancisMcN wrote:I would certainly advocate the technique of feeding the resin in from underneath as the best low tech solution that improved the proportion of good castings for me.

I was trying to figure out a good way to do just that as well. But this was one of my first-ever attempts at casting molds, and a two-piece mold at that. In the end I couldn't come up with a solution so in my typical fashion I kept recasting it, figuring at some point I would get a workable result. I think I did?!? I'm hoping to have them mounted to the hull soon and will post the photos to my build thread once they're all painted up.

I would certainly advocate the technique of feeding the resin in from underneath as the best low tech solution that improved the proportion of good castings for me.

Francis Macnaughton

Umi, that is an impressive technique. I have to admit that's beyond my needs and means at the moment, but sounds like that is the way to go for pro results.

I am often chastised for how I do things, but some of the things I do,

Cast the part at an angle. (uses more RTV to make a mold)
But it causes the resin to fill up one side and force the air to the high point and out of the casting.

Run the filling sprue to the bottom of the part... (uses more RTV to make a mold uses more resin)
Again it allows the resin to force the air up and out of the part by coming in at the bottom and
filling to the top.

The two methods combined can work wonders for simple casting techniques.

$$ method... pick up a paint pressure pot, and modify it for pressure casting.
Placing the mold under pressure forces the resin into the voids, and crushes small
air bubbles in the resin. 35-40 psi is plenty for pressure casting... Higher pressures
only makes the process more risky it doesn't make better parts.

$$$ method Vacuum casting. If you are heading toward pressure castin, it will serve you
well to also create a vacuum chamber. When making the RTV mold, air bubbles are also
caught in the mold as it cures. Pressure casting will also crush the air bubbles in your mold.
If the air bubbles are near the surface of your part, then "pimples" will appear on the part
where the air bubble was hiding.
Vacuum de-airing the mold material before and a little bit after covering the part will make
for nice solid molds and excellent pressure cast parts.

The talcum advice is great... I found I got much smoother surfaces when I started applying talcum powder
to the molds prior to casting any parts.

Vaccum Casting...(again) would actually work with the anchor mold discussed above.
The resin could be mixed, then placed in the vacuum chamber to de-air for a few seconds.
Then the resin can be taken and poured into the mold.
The mold could then be placed into the vacuum for a few more seconds, and the air bubbles
trapped in any undercuts will expand, and be pulled out of the mold.
The mold would then be topped off, and allowed to cure.

Because of the time it takes to do alot of the steps, I use a resin with a 15 minute working time.
But I get great parts with less loss for the time..

Here's two photos of one of the demolded anchors with mold vent tubes still attached. I deemed this one as a keeper.

I have two vents, both 3/16" tube about 1/2" long each and 1/2" apart. They're located on the underside of the anchor baseplate on either side of the arm hinge point. I'm molding it in the upside down position.

I initially tried the same technique but right-side up with the vents running from the flukes, this produced far worse results. I could not figure out another way to mold it other than vertically because the arm is functional with a pass-thru in the anchor to the hinge point.

The tip for using talc is probably going to prove invaluable! Thanks to all for that!!! I will be molding the myriad of vents, chocks, and bits in the near future (not to mention the a/a guns) and I was dreading having to make a dozen castings to get one acceptable result.

frank2056 wrote:Using talcum powder in the molds before casting helps. All you need is a very light dusting of talcum powder; it helps draw the resin towards the surface of the mold, eliminating many bubbles. The powder doesn't affect the resin (at least not the Smooth-on and Alumilite resins I've tried).

What the talc does- and only talc, not corn starch like some powders- is change the surface tension of the silicone in the mold, so the air bubbles don't form.

How many vents, and where are they, on your mold?

Using talcum powder in the molds before casting helps. All you need is a very light dusting of talcum powder; it helps draw the resin towards the surface of the mold, eliminating many bubbles. The powder doesn't affect the resin (at least not the Smooth-on and Alumilite resins I've tried).

yes, that's what I did with troublesome casts from moulds... filling each half up as much as poss and then bringing both halves together.

Works for me.

Guess I'll answer my own post as I've finally managed partial success. It seems that "priming" both halves of the mold with the casting resin first and then mating them together is working with good results, then adding a little more resin after they are joined and "teasing" any remaining air bubbles out. I've managed 2 viable casts and waiting on a 3rd to cure enough to be de-molded as we speak.

Hoping it turns out even better than the last one which was about 95 plus percent to my satisfaction. Thankfully I only need 2 for my BB as I can't take much more of the aggravation!

I'm hoping for any tips for problems I am having in casting a 1/192 scale 30,000 lb stockless anchor. I've tried two different molding techniques-both two part molds with entry and exit holes for the resin and air. I continue to have major air pockets even after vibrating the mold and trying to "tease" out any air pockets after 5 attempts.

I'm thinking of next trying to only pour a little bit of the resin at a time, filling up the mold gradually over 3-4 pours. But is this painfully slow process going to give a better result?

Any tips/techniques are appreciated.