Resin casting bubbles

[VonRyan]

I'd like to poll the experts.
Since folks recommend staying away from the Micro Mark starter kit, what materials should I get that costs about the same?

Also, I'm looking to start off on the best of the right feet so that means pressure casting... Where should I go to find a reasonably sized pot that won't break the bank. With my first pay check coming this Friday, I'm looking to spend it wisely.


-Cody F.

[LV LOU]

Pete,I've done some long castings with my Harbor Freight pot.I made 2-2X4 cradles for it,and lay it down like an autoclave.You can do something almost 2 feet long then..

[LV LOU]

Cody,Smooth on makes some of the better resins & mold rubbers,and cheaper than Micro Mark.I actually think they are one and the same.Stay away from 1:1 rubbers.They are very fragile,tear easily,and don't last long.The difference between 1:1 and 10:1 is the difference between cheese and bubble gum.You should have an accurate scale.
   I use Smooth Cast 322 resin,but I'm looking right now at other companies for a cheaper resin.I use Mold Max 30 rubber.I'm happy with the Smooth On stuff,but I've seen resins online that are WAY cheaper,almost 1/2 the price,and since I'm starting to do commercial casting,cost is a big factor.
  Smooth on resins are more expensive than most others I've seen,but,surprisingly,their rubber is usually cheaper..
 You may be able to find a used pot on Ebay,ETC,the Harbor Freight one works just fine,but,still not cheap.
 

[pnolan48]

Pete,I've done some long castings with my Harbor Freight pot.I made 2-2X4 cradles for it,and lay it down like an autoclave.You can do something almost 2 feet long then..

Lou,

I can't visualize this.

[VonRyan]

Cody,Smooth on makes some of the better resins & mold rubbers,and cheaper than Micro Mark.I actually think they are one and the same.Stay away from 1:1 rubbers.They are very fragile,tear easily,and don't last long.The difference between 1:1 and 10:1 is the difference between cheese and bubble gum.You should have an accurate scale.
   I use Smooth Cast 322 resin,but I'm looking right now at other companies for a cheaper resin.I use Mold Max 30 rubber.I'm happy with the Smooth On stuff,but I've seen resins online that are WAY cheaper,almost 1/2 the price,and since I'm starting to do commercial casting,cost is a big factor.
  Smooth on resins are more expensive than most others I've seen,but,surprisingly,their rubber is usually cheaper..
 You may be able to find a used pot on Ebay,ETC,the Harbor Freight one works just fine,but,still not cheap.
 

I'll have to look at the harbor freight one since there is an HF near me.
Not looking for cheap where pressures are concerned, but my paycheck is only 8 hrs at $7.35...

I love how they tell me I'll be working 16-20hrs per week and they only schedule me for 8hrs this past week... 12 if you count my orientation...

[randgust]

You always want to design your molds - squish molds, whatever, so that escaping air has someplace to go.    You need to design in wastage, extra sprues, whatever.    Nearly everything I do is cast upside-down, so that any internal bubbles are away from the visible surface.

But the 'million-dollar idea' that was given to me is using disposable fiberglass boat syringes for INJECTION of resin into molds.   See West Marine.com for those supplies.  I will partially fill a mold that is designed with an injection gate, close it up, and then finish it off with a hit on the syringe, which displaces any air right up the sprue and bleed gates.   No pressure pot, too slow.   With multiple molds and the syringe injection method, I'll make from 20-50 parts per run, with a 7-minute cure time.   Scrap rates dropped to near zero.   It won't stop the internal foaming (swiss cheese) from excess humidity, but it does solve the trapped air problem on a properly designed mold.  And the syringes make precise control into open-molds a breeze as well.

If you design your molds to be done with an injection gate (usually at the bottom, so that the resin comes from the bottom-up) and lots of air bleeds at the top, you never squish, you inject.    This will dramatically change how you design molds, and originals, when you realize how much better this works. 

http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&storeId=11151&partNumber=318311&langId=-1&searchKeyword=syringe

I pass this one on because it was given to me on a forum, and that was the 'Eureka!' moment for me, so I have to pay it forward.  Until I tried this, I was lucky to get 50-60% good parts, and I was always limited by the cure time on how fast I could fill molds accurately.  I have a big countdown stopwatch above the pour bench with warning timers set.

I coat the plunger with rubber-to-rubber mold release, and any excess resin just pops out after it hardens.  I'll get 20-30 pours out of each syringe unless I get forgetfull or careless.   Material wastage has has dropped to about zero as I can always beat the cure time to work with that many molds with the syringes, although the 'trim scrap' on feed gates and sprues uses up a lot.   I have some parts/molds that are effectively 'resin dumps' that are more tolerant of thicker resin, and I keep them handy at all times so that whatever material is left in the cups can be injected rapidly into them and still get usable parts out of every last drop of mixed material.

[VonRyan]

I'm only looking at doing some boxcars and then some specialty passenger cars.
As for molds, I'm familiar with single piece rubber molds, but I don't want solid blocks, so that means a two piece mold...

My grandmother ran a successful ceramics business out of the studio wing and the basement is filled with over 2000 plaster molds of all shapes, sizes, and subject matter.


-Cody F.

[randgust]

A good example of that is my CF7 sideframes, two-piece mold with left and right frame pairs built onto sprues; single feed gate at the bottom and a HUGE air drain at the top.  A pair of frames eats about 2 oz. of mixed material per cast.   The syringe fits snugly into the bottom of the closed mold, mold his held vertical, and resin is slammed into the bottom gate until it comes out the top, and continued to push until it runs clear (no bubbles) out at the top.  The mold is then re-oriented horizontally to cure, so that both the injection and air drain gates are at the top and nothing leaks.  I've experimented successfully with resin plugs, too, made out of leftover sprue scrap.

Takes about 5-10 seconds to fill the mold that way.   Unless I get goofy and try to inject near-cured resin, it's one of the most reliable molds I've ever made for quality.

I have nothing against pressure pots, I don't doubt they work better, but if I can do this without them I certainly will.   

[VonRyan]

A good example of that is my CF7 sideframes, two-piece mold with left and right frame pairs built onto sprues; single feed gate at the bottom and a HUGE air drain at the top.  A pair of frames eats about 2 oz. of mixed material per cast.   The syringe fits snugly into the bottom of the closed mold, mold his held vertical, and resin is slammed into the bottom gate until it comes out the top, and continued to push until it runs clear (no bubbles) out at the top.  The mold is then re-oriented horizontally to cure, so that both the injection and air drain gates are at the top and nothing leaks.  I've experimented successfully with resin plugs, too, made out of leftover sprue scrap.

Takes about 5-10 seconds to fill the mold that way.   Unless I get goofy and try to inject near-cured resin, it's one of the most reliable molds I've ever made for quality.

I have nothing against pressure pots, I don't doubt they work better, but if I can do this without them I certainly will.

So basically you are doing injection molding just like plastic models?

I'm only doing a single piece casting, so a sprue is not necessary, correct?

[randgust]

Lets think like a boxcar body shape.

Assuming a two-piece mold.... you can go one of two ways.   You can either do it upside down as a 'plunge' mold, or do it endwise as an injection mold.    Either way, you need to think how the material is going to react.   

Plunge molds almost naturally trap air during the plunge, it's hardly ever the same place twice in a row where the bubble ends up.   It either drifts up to land somewhere near the top (the bottom sill of the side) or gets stuck against the bottom of the roof as it never made it out.

So, you can slope the inside of the roof edge like a "v" to help the bubbles go somewhere (and also make it deliberately thicker) , and you can also graft on more material to the edge of the car - say like a .010 additional side rail - so that any air that does drift up and make a bubble goes into a sacrificial area that is going to get cut off anyway as scrap.  That approach works well.  I'll also graft on additional bleed sprues all along the edge of .035 styrene rod to help it along there.    I usually put in one fairly thick sprue on one ends so that after the mold is closed I'll hit it one more time with the syringe and force air out of all the .035 air bleeds.

OR, you can orient it on end with a two-piece mold that stays closed, with a fill gate on one end and a series of air bleeds on the other that get cut off later as scrap.  A good location for a sprue would be the handbrake gear, as when that is cut off with nippers it's pretty flush anyway...  You'd also pepper the end with .035 air bleeds at the seam.   You'd then inject the closed mold until resin comes out the other end (top), then reorient horizontally.   If you asked me to make a car body today, that's probably the way I'd go.

The mold design and master design to make this method work is a whole new world; I find that you have to think backwards from material behavior on casting and injecting and figure out the pour method and orientation of every part, then start scratchbuilding or making the masters.   The 'master' usually ends up so dressed-up with sprues and air bleeds that it would never be used as a model again, that's why those that look at my finished projects and ask if they could be used as a master get an amused expression.

The other thing you have to consider (going back to the boxcar concept) is that a two-piece plunge mold is going to take a BEATING on a typical boxcar, because of the corrugated ends.   Once it is hardened, you'll have to pull and stretch the mold to get the part out.  If you're only trying to make a couple parts, the mold doesn't have to take this repeatedly, but if you're trying to make 50, you want to do everything you can to reduce mold stresses on part removal. I'd probably cut the ends off of the mold and make it into a four-piece mold so that after it cured I'd pull the ends back off rather than try to wrestle the car out of the mold tearing it up.    The trick, and the art, is working with the model looking for places to hide the mold seams, just like on a well-designed injection mold.  My first cabs had, and still have, a mold seam on the top.   The ones I've done lately don't and for the most part, defy attempts to figure out just how I did them.

[VonRyan]

A lot to digest. Since I have Konrad Richter assisting Christian and I with our casting, I'm not without an experienced person to make sure that it doesn't end up a total flop.

If Konrad's name doesn't ring a bell, he owned and operated Camden & Amboy Models. He still does casting for himself these days, albeit with Micro Mark products, but he gets good results.

Luckily since I'm casting double-door B&O wagontops, the ends are fairly smooth.


-Cody F.

[randgust]

I had a chance to compare notes with a couple forum members here on resin casting; I've been really surprised how little the Micro-Mark CR600 shrinks in comparison to some of the other materials I've seen.   I made no provision for shrinkage on my masters; the parts I made fit back on the mechanisms or with other parts like nothing ever happened.   

I went through the 1:1 mold stage (blue stuff) and rapidly graduated to the 10:1 (pink) stuff, but you'll need a good digital scale to use it.   And the rubber thinner isn't a bad idea, along with a lot of the other supplies they sell.  Most of the stuff they sell is definitely 'full retail' and there have to be better places to get it, but they do a fine job on concealing it.

The only thing I got from Micro-Mark that I considered a complete blind alley that I invested a lot of time and money into learning and using and turned out to be relatively useless was the photoetching system.   Casting lured me into a false sense of success and security.     

But other stuff with Micro-Mark casting material, like the weird behavior with humidity - is all part of the hard school of learning that they simply don't tell you about. 

Oh, a couple more tips:   Get yourself a box of disposable gloves; this stuff is murder on skin and likely toxic; coat every surrounding area in some kind of disposable paper as the spills, drips, etc. make a horrific mess, wear eye protection on plunge molds (I've found it on my train room ceiling in hardened drips), get a good supply of disposable dosage cups from your local pharmacy to measure and mix the stuff in.   The stuff does not adhere to soft vynl plastics after it hardens as a mix, but the raw "Part A" stuff is NASTY, it will attack just about everything it touches including cured mold rubber, soft plastic, even metal.

[VonRyan]

I had a chance to compare notes with a couple forum members here on resin casting; I've been really surprised how little the Micro-Mark CR600 shrinks in comparison to some of the other materials I've seen.   I made no provision for shrinkage on my masters; the parts I made fit back on the mechanisms or with other parts like nothing ever happened.   

I went through the 1:1 mold stage (blue stuff) and rapidly graduated to the 10:1 (pink) stuff, but you'll need a good digital scale to use it.   And the rubber thinner isn't a bad idea, along with a lot of the other supplies they sell.  Most of the stuff they sell is definitely 'full retail' and there have to be better places to get it, but they do a fine job on concealing it.

The only thing I got from Micro-Mark that I considered a complete blind alley that I invested a lot of time and money into learning and using and turned out to be relatively useless was the photoetching system.   Casting lured me into a false sense of success and security.     

But other stuff with Micro-Mark casting material, like the weird behavior with humidity - is all part of the hard school of learning that they simply don't tell you about. 

Oh, a couple more tips:   Get yourself a box of disposable gloves; this stuff is murder on skin and likely toxic; coat every surrounding area in some kind of disposable paper as the spills, drips, etc. make a horrific mess, wear eye protection on plunge molds (I've found it on my train room ceiling in hardened drips), get a good supply of disposable dosage cups from your local pharmacy to measure and mix the stuff in.   The stuff does not adhere to soft vynl plastics after it hardens as a mix, but the raw "Part A" stuff is NASTY, it will attack just about everything it touches including cured mold rubber, soft plastic, even metal.

All the molds I have seen Konrad using are indeed pink, so he must be using the 10:1 stuff.

Christian and I intend to purchase all our own materials new since this is only the first project of what could very well be many.
We are doing the best we can to insure we do it correctly from the start so as to provide the best product we can possibly make.

-Cody F.

[jimmo]

Pipe bomb? Considering what kind of pressure it takes to actually burst a pipe or a pressure pot, I seriously doubt that we are getting anywhere near that capacity with our 60 to 80psi range. My wife was afraid I'm going to blow the roof off of our house with the one I got--until I explained the realities of what we do.

If you're that concerned about it, install a safety valve.

[peteski]

I've been having bubble trouble this week too and don't understand why.

I've been casting various details this week, and use a pressure pot with no bubble problems.  Until I decided to try to fill in some windows in the side of a passenger car body... with resin.

All I'm doing is putting Tamiya masking tape on the outside of the body, and using some resin to fill in the window itself.  I had bubbles just letting it sit on the workbench to cure.  So I pulled those plugs out and tried the next pour in the pressure pot.  Somehow I still got a handful of bubbles!  At 70psi! 

And other molds in the pressure pot for that cycle turned out bubble free as usual. 

I don't get that one.

Maybe there is some sort of reaction between the tape's adhesive and the resin which generates gas?  Since the gas will get generated anyway, the 70 pis is simply ambient pressure at that time and the gas still gets generated, creating bubbles. Not sure if that makes sense but that's all I can think of.  Maybe try Scotch frosted tape instead?