# Casting Resin Problems



## bnsfconductor (Jan 3, 2008)

To the expert resin cast makers,
I've having trouble finding the source of the problem when I cast a mold. I've made a few resin casts but in every one but the first one, I'm getting a void in the resin. I'm casting in a pressure pot with two pieces of polypropylene ( the stuff that doesn't stick to resin) with a rubber band to hold everything together. In the first couple of resin casts the void was small enough (around the size of a pencil eraser) so it shouldn't make a difference. In my last to casts the void was much larger (around 1"). What am I doing wrong? I'm fulling the mold with resin to the top so it heaps, and then placing a piece of polypropylene on top to create a smooth backing. The mold is about 6" wide and about 3" high.

Any suggestions?
Thanks,
Craig


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## Totalwrecker (Feb 26, 2009)

you might add a second sprue to let the air out. The first sprue is the pour hole.... 

Not sure about the pressure pot... I have a vacuum assist casting table...for metals, sucks the air out! 

John


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## virgal (Sep 25, 2009)

2 suggestions

1 use a slow cure resin. I use a resin that takes about 30 minutes to setup enough to de-mold. Plenty of time to flow out in the mold.

2 after filling the mold, put it in a sealed chamber and pull a vacuum on it. It is not very complicated to do. I use a plastic storage container with a couple of pin holes in it and use a battery operated pump I got in the grocery store to vacuum seal ziplock type bags. I think it was from Reynolds. It cost about 9 bucks. it will remove any air in the mold. I hold the vacuum for only a few minutes. works well. no voids or surface pin holes.

alan


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## Burl (Jan 2, 2008)

I've seen what you're talking about in some of my molds. Certain molds I have found are tricky to cast because of the volume of resin. Going in, there's no air bubble, but coming out... there it is. Drives me crazy! 

This is what I finally figured out: due to the amount of resin, when it begins to cure it, gives off a lot of heat. The heat was making the polypropylene warp, thus a corner lifted and let an air bubble in. Using thicker (1/4") polypropylene helped. 

Also, try to use a piece of polypropylene that isn't drastically bigger than the face of the mold. When you rubber band it, that can aggravate the warping. I also put a flat aluminum block on top of the mold right before I close up the pressure pot (not so heavy that it deforms the mold). This hadn't occurred to me till just now, but it may also be acting as a heat sink. 

Hope some of this helps you.


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## bnsfconductor (Jan 3, 2008)

Burl, 
Thanks for the suggestion. I'm now out of resin but when I get some more I'll try to add some weight to the top of the mold. The piece of polypropylene I'm using isn't much bigger then the mold so I think I can rule that problem out. I've never felt the mold when it was curing so I wasn't aware of how much heat it gave off. I'll try stacking another piece of polypropylene on top to see if that helps with the problem. I thought maybe when I was placing the 'lid' on the mold I was some how getting air between the resin and the 'lid' but from your explanation it seems as if the heat is a bigger factor. 
Any way to save the molds that have these air pockets? I was thinking of filling the smaller ones with puddy, but I'm not to sure about the largest one. Would it be worth trying to put new resin in to fill the void? 

In a related question, what are the classic signs when the resin is too old? 

Thanks again, 
Craig


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## Totalwrecker (Feb 26, 2009)

If you are woking with flat molds get two aluminum plates and a c-clamp instead of weights. Then you can stand the mold on end with the fill hole up. Hopefully any bubbles will migrate upwards into the sprue. 
Keep the clamp centered, too tight distorts, count your turns. 

Hard to say about saving the old ones, you won't get the detail where patched. 

John


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## David Leech (Dec 9, 2008)

Hi Craig,
It would really help if you could take some photos of the original piece, and the mold so that one can get a better idea what it all looks like.
I don't know what type, or brand of resin that you are using.
I use Smooth-On products, which I find really don't need any 'fancy' vacuum or pressure systems to get good castings. 
I don't think that I have ever seen voids that you are experiencing.
Where they are 'open' molds (most of mine are) I just pour in the resin and let it produce it's own nice flat back which makes life nice and easy. 
As far as resin 'going off', as soon as I start to see any sediment settling at the bottom of the container, I know that it is time to use it ALL up pretty soon.
All the best,
David Leech, Delta, Canada


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## bnsfconductor (Jan 3, 2008)

David, 
I think Burl's answer and solution might be similar to what I'm experiencing. I'll try and get some pictures to help explain the problem. The mold is a single flat back mold that is about 6" by 3" so the size might be contrubiting to the problem with the resin heating up. 
I was using resin from Mirco-Mark, but I have since ran out. It seemed to work fine, but the demold time was quite long 30-60 minutes, or even longer. I'd like to work with something a little shorter the next go around. 

Craig


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## FH&PB (Jan 2, 2008)

Try taking an X-acto knife and making a slit from the air pocket to the outside of the mold. Just plunge the blade in until the tip comes out into the place with the void. It doesn't have to be big, just enough to let air seep out. Gravity should do the rest.


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## Burl (Jan 2, 2008)

If the air bubbles are forming next to the polypropylene sheet, I usually just let it set up, then fill the void. Sometimes you get lucky and the shape of the void itself is enough to hold the next pour when it sets up. If its shallow, I'll take an X-acto knife and scratch up the surface so the new resin will have something to bite into. The only reason I say that is with mine the voids tend to have a glassy surface and sometimes, even though resin tries to stick to everything, it will sometimes delaminate later if I don't do anything to it. 

I didn't see where you said what kind of resin you are using (I apologize if you did and I just missed it). Why I ask is I have heard different brands act a little differently with age. I use Alumilite personally. It tends to foam as it gets old.


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## rkapuaala (Jan 3, 2008)

If you are casting using a pressure pot here are some things you need to make sure happen. 
- Make sure there are plenty of vents or a large enough fill hole to handle the pour and venting. 
- Make sure that thin areas are no less than 3/64. The can come to a point or a smaller thickness like on the brim of a hat or an ear, but you need a thick enough void around it to keep the mold from eventually collapsing. 
- Make sure there is absolutely no moisture in your resin. Polyurethane resin will foam up if you put enough water in it. Just a lot of moisture in the air is enough to make it bubble during the cure, so make sure you tightly seal your resin container right after you mix. 
- Always use a vacuum to pull air bubbles out of the mold you are making. Any air bubble left in the mold will be compressed by the resin and the air in the pressure pot. It will cause pimples on your piece and some times huge voids. 
- Don't completely cover your mold with a top or another piece of mold. The vents should be open not just during pouring but when the mold is in the pressure pot. If all the vent and pour holes are covered, your results will be unpredictable because the pressurized air won't be able to get inside the mold, so it will push on the outside of the mold. That means any gases produced during the curing (Resins emit small amounts of gas as they cure) will build up on the inside of the mold.


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## rkapuaala (Jan 3, 2008)

I can't stress the use of a vacuum to make molds enough. It has to be a pump capable of pulling at least 28 HG. Any less and your results will be unpredictable. 
Of course if you're not going to use a pressure pot to cast then you don't need to vacuum the mold.


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## bnsfconductor (Jan 3, 2008)

I'm casting the molds under pressure too because I don't have a vacuum pump. It seems to work fine as I'm not getting air bubbles on the mold itself, but on the cast with the resin. I'm not at home right now, but when I do I'll take some pictures of what's going on. 
Craig


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## rkapuaala (Jan 3, 2008)

I ran into problems with Silicon molds made under pressure. The pressure seem to make bubble in them. I've never made polyproplene molds before so perhaps they don't have the same issue. I know you can make molds from softer polyurethane and they are made under pressure, so perhaps poly propalene is the same.


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## bnsfconductor (Jan 3, 2008)

Here's a link to some photos of the resin castings and the mold. 
http://www.facebook.com/album.php?a...50f723ab0e

Thanks for all the help so far.
Craig


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## Burl (Jan 2, 2008)

That's a pretty substantial air bubble. A void that size would be easily seen if it was there before the resin kicked. I'm still suspecting something is causing your polypropylene backing to curl up and let air in.


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## rkapuaala (Jan 3, 2008)

Craig, 
I don't think that bubble is coming from a mold bubble. It looks like you have cast your wall flat, which is okay if you don't put something on top of the mold to flatten the back; which is what it looks like you are doing. If you are indeed covering the mold with a second half, what is happening is that you are trapping any air that got in there when you put on the second half of the mold. To compound the problem you are not letting any pressurized air in or any surface air to escape. 
So you are just subjecting the mold itself to the pressure and the pressure on the mold is flattening out the little bit of air you have trapped. 
There are two things you can do to correct this. 
1. Don't make a horizontal mold (like yours) if you need both sides and edges to be molded. Make a vertical mold and create some good vents that allow the air to escape. 
or 
2. Don't cover the back of your mold, leave it open and then after you part the casting from the mold use a disc sander to flatten the back. 
I've seen evidence of molds like yours being poured with out pressure. A horizontal mold has so much open space it doesn't really need pressure just careful pouring and mixing. Then just before the resin gels take and screed the back flat with a plastic screed.


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## Totalwrecker (Feb 26, 2009)

I'm not familiar with this system. How is the pressure pot used? 
I do have 20+ years injecting wax into molds and know how to help them breathe as they fill. 

If Burl is correct, then the two metal plates and a C-clamp will keep your mold true and prevent air intake. (as noted above) 
I think there needs to be a vent hole cut in too. The size of the void suggests trapped air, too much for a leak. 
Resin is to thick to allow air to pass through it, up the sprue as you pour down it... 

John


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## David Leech (Dec 9, 2008)

Hi Craig,
I would agree with Richard. 
As I said earlier, I do the majority of my parts as an 'open mold' and just let it self level the back side and sand it if needed.
The only time I use multi part molds is when there is detail on all the sides. 
You talked about wanting a resin that set up a little faster.
I use SMOOTH-CAST 300 and find that it works great for me.
Pot life of 3 minutes, and demold time of 10 minutes, and it pours like water with practically no bubbles.
Good luck
David Leech, Delta, Canada


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## bnsfconductor (Jan 3, 2008)

I think next time I pour the mold (still have to order more resin, and waiting for approaval from the wife), I'll add some weight on top the polyprolyene backing to see if that helps. If Burl's right I could noy only draw the heat away from the casting, but also hold down the backing better. 
What puzzles me is that the void is not in the center of the casting, but rather on one of the edges. This would make since to me if the polyprolyene backing was lifting up. 
Craig


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## David Leech (Dec 9, 2008)

Posted By bnsfconductor on 28 Oct 2009 01:57 PM 
I think next time I pour the mold (still have to order more resin, and waiting for approaval from the wife), I'll add some weight on top the polyprolyene backing to see if that helps. If Burl's right I could noy only draw the heat away from the casting, but also hold down the backing better. 
What puzzles me is that the void is not in the center of the casting, but rather on one of the edges. This would make since to me if the polyprolyene backing was lifting up. 
Craig 

Craig,
I would suggest that perhaps the mold was not sitting dead level, so the air bubble just went to one side.
I would also suggest that you try once with the mold open, and see what happens. 
All the best,
David Leech, Delta, Canada


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## rkapuaala (Jan 3, 2008)

No, no, no, 
The bubbles are not caused by either of those. You want air to get into the mold, other wise there is no point in pressurizing it. 
Remember, FLUIDS can not be compressed. The compressed air pushes on the fluid and all the air in the fluid gets compressed, but the fluid does not. 
Take your back off and let the air hit the resin. 
More than likely the air is being introduced when you cover the mold. Unless you are covering the mold in a vacuum you can't get all that air out. Pressing it harder won't get rid of the air, it will just make it spread out.


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## Budd (Mar 22, 2008)

Looks to me like you have not poured enough resin into the mould, are you over filling and relying on squish factor when you put the back on? and don't forget you will loose a bit of capacity when you pressurise it , the same applies if you vacuum the mould, ie, a quantity of resin will have a percentage of bubbles in it (the more care when mixing and pouring equals less percentage of trapped air), if you pressurise the bubbles or suck the little suckers out you will reduce the "quantity" of resin you have poured in. Clear as mud? 

Well that's my opinion and I'm sticking to it.
Wayne


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## BEEJAX (Apr 7, 2011)

Resin casting questions,
I am casting Smooth-Cast 321 urethane resin in 3" diameter x 6" tall round rods that will be machined into final shapes.
The resin is an ivory color without any pigment added and that is the color I want. 

When the resin cures it develops unwanted white flecks throughout the casting. the resin left over in the mixing pail and on the stir stick have perfect color. I am using a double mix method by mixing in one pail, pouring the mixture into another pail and mixing again, then pouring into the mold and put under 60 PSI for 40 minutes. Also, parts A and B are mixed individually before putting them together. the casting has no air bubbles in it. I am wondering if the problem is caused by the heat generated by the large mass of resin. The folks that sell the resin don't seem to have any experience with deep pours.

If anyone has experience pouring blocks of urethane I would love to hear from you.

Thanks

Brian


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## kormsen (Oct 27, 2009)

do you use something white as moldrelease? like talcum?


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## Burl (Jan 2, 2008)

I would think pouring a 3"x6" object would generate a lot of heat when it kicks. I have heard of larger pours catching fire. Maybe a slower setting resin would be better for you?


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## Jerry Barnes (Jan 2, 2008)

I use talcum, works fine.


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## BEEJAX (Apr 7, 2011)

I'm using a tiny bit of the smooth-on spray release. It is clear.

Thanks

Brian


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## SE18 (Feb 21, 2008)

my main problem casting has been lack of proper curing (the parts come out sticky and it's impossible to get the stick away). I don't know what I'm doing wrong but I've used probably 12 types of resins in the past. Occasionally they will come out ok.

I've used 1:10 and 1:1 etc and use scientific weigh scale.

I find metal casting so much easier. If you screw something up it can be reused 


Dave


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## David Leech (Dec 9, 2008)

Posted By BEEJAX on 07 Apr 2011 12:41 PM 
Resin casting questions,
I am casting Smooth-Cast 321 urethane resin in 3" diameter x 6" tall round rods that will be machined into final shapes.
The resin is an ivory color without any pigment added and that is the color I want. 

When the resin cures it develops unwanted white flecks throughout the casting. the resin left over in the mixing pail and on the stir stick have perfect color. I am using a double mix method by mixing in one pail, pouring the mixture into another pail and mixing again, then pouring into the mold and put under 60 PSI for 40 minutes. Also, parts A and B are mixed individually before putting them together. the casting has no air bubbles in it. I am wondering if the problem is caused by the heat generated by the large mass of resin. The folks that sell the resin don't seem to have any experience with deep pours.

If anyone has experience pouring blocks of urethane I would love to hear from you.

Thanks

Brian 


Brian,
Although you say NOT, are you sure that you are not seeing very small air bubbles?
Putting it under pressure will only 'squash' any bubbles really small, not remove them.
I am sure that you have looked through a magnifier of some sort, so what do the 'flakes' look like.
I use 325 in fairly large amounts sometimes, but not as large as yours, and have seen bubbles, which is not a concern in my parts, but no 'flakes'!
All the best,
David Leech, Delta, Canada


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## David Leech (Dec 9, 2008)

Posted By SE18 on 08 Apr 2011 05:18 AM 
my main problem casting has been lack of proper curing (the parts come out sticky and it's impossible to get the stick away). I don't know what I'm doing wrong but I've used probably 12 types of resins in the past. Occasionally they will come out ok.

I've used 1:10 and 1:1 etc and use scientific weigh scale.

I find metal casting so much easier. If you screw something up it can be reused 


Dave 


Dave,
What type of mold are you casting in?
I have had the same happen the odd time and is normally due to getting the mix wrong, or too small an volume so that the exotherm does not kick in hard enough. 
All the best,
David Leech, Delta, Canada


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## SE18 (Feb 21, 2008)

Hi, 1:10 mold; I forget the company. Sometimes the results are good and sometimes no. Maybe it's the humidity and temperature (cold) of the house. That's my best guess. 

Dave V


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## silverstatespecialties (Jan 2, 2008)

I always refrigerate my resins; this doubles the working time before curing, allowing more time for air bubbles to work their way out. I picked up a small fridge from a friend's moving sale several years ago for free, and it works like a champ...it also keeps my wife happy by not allowing resins around our food. 

I use both pressure pots and vacuum for resin casting. I've found results can vary by pressure and resin brand. What pressures are you using on your pots for resin casting? The lowest I've used is 15psi, I usually get best results around 35 psi, but some I've had to go as high as 50psi. 

You can also get a cure-slowing agent for silicone when creating your molds, to allow slower set-up times in hopes that more air bubbles will escape before curing, if all you have is a pressure pot for silicone molds. 

And if you find yourself mixing in too little hardener with your silicone, and the silicone is still "tacky" or "wet" or not hardening after several days, put the whole thing in an oven at 180-200 degrees for an hour or two....it will harden right up. Let it fully cool at room temperature before you try to remove the pattern master (I've actually lost a couple due to the heat, but simply cast another one from the saved mold). 

Hope this helps.


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