problem casting with resin

dropshipbob

Master Member
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The parts in question are pretty small, my problem is that when I cast them, they have hundreds of little dimples all over them. Not air bubbles, just tiny little "dents". And these dents are in the same pattern on all the parts cast from this one mold. Was it a problem with the mold while it was curing? Should I have coated the master with something to prevent this from happening? I have other molds and the parts cast with them come out just fine, but one or two molds seem to have this issue.
 
It's possible you have air bubbles trapped in the mold rubber - they would expand as the air inside is heated by the curing resin, leaving an uneven surface on the part. Just how big is "pretty small" though? Small/thin resin parts shouldn't heat up very much so it might not be this.
 
Maybe try talcing your molds before the next round, see if it helps? A cheap suggestion anyways, maybe the mold is degassing a little for some reason? I'm just spitballing here lol :lol
 
All the points above as well as the possibility that there is moisture in the resin.

A few years ago I found out how easy it is to contaminate a pot of resin with moisture in the air. The last pour of the batch actually foamed up.

I was using a 1:1 quick cure and had prepoured two cups because it was quicker and less messy to mix into a third cup. But because it was exposed to the air between pours, it was taking in moisture from the air.

As mentioned air bubbles just under the surface can rupture and cause surface issues. Normally you get pimples, not dents.

Even when I degas my silicone, I like to coat the part with a "print layer" and run a heat gun over it to ensure there are no bubbles. Then back fill and again, hit it with the heat gun. Layering is useful if you work with quick cure silicone.

I generally use a 6 hour cure time product and I degass first.

Sent from my CPH1701 using Tapatalk
 
I think the problem occurred when I was pouring the mold material. When I pull casts from this mold, the dimples are all in the same pattern. How does one "de-gass" silicone?
 
Vacuum pump and pressure stable enclosure that is large enough ($200-300). I just pour the silicone from a meter height so that it thins out to a very narrow stream ( 2-5mm thick)
I think the problem occurred when I was pouring the mold material. When I pull casts from this mold, the dimples are all in the same pattern. How does one "de-gass" silicone?
 
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Did you do both mold and casting in ambient room temperature / air pressure?

If for example the mold was not cured under pressure but your cast are, the pressure cast will get into all air bubbles.

About degassing silicone, my vac chamber cost me about 300$ on ebay and is huge. Not sure where the 2-3K comes from? Vac chambers expands air until all bubbles are bursting out of the material, that’s the first step to a bubble free mold.

My pressure pot cost me about 200$ as it started life as a paint pot that was converted. 5 gallons of pure love crushing the air this time making bubbles microscopiy small, and curing material leaving them invisible/unaffecting the mold and cast.

All my molds go in the vac + pressure pot while all my cast go in the pressure pot {if size allows}.
 
Just my two pennies here.

i suspect that when you cast the original, it was either dusty, or had oil spots, which have pulled into the silicone. What silicone did you use? Tin cure or addition?
Im doubting its going to be the resin you are using as you say the subsiquent pulls all have the same issue in the same area.

As a rule, (and i learned this very early on) theres no substitute for a vacuum chamber and a pressure pot. Especially on small intricate parts.
The first year i started, i probably wasted twice as much silicone through poor practices.
Its VERY easy to get a little moisture, or some detritus in a mold, and im sure we have all done it.

the problem you have now is that the mold may well be set like this, and theres little you can do to an already cured mold.
Choices are
1- remake the mold, using degassed silicone, in a dry atmosphere, and pressure set it as well.
2- continue with the mold you already have, and fix the areas with stopper putty.

dave.
 
You didn't evac your RTV . You need to invest in building a chamber. Easy to do.. Go find a surplus place and get a used VAC Pump.
 
I think the problem occurred when I was pouring the mold material. When I pull casts from this mold, the dimples are all in the same pattern. How does one "de-gass" silicone?

Hope this video helps. I actually "burp" the silicone a few times as I must have had more than my bucket could handle.

https://www.facebook.com/cavx1/videos/1438123279610452/?t=9

Note what the silicone does at the end.

You need a chamber that can handle your pot of silicone. Your pot needs to allow about 10x the volume of the silicone you are degassing. So I usually degass 500g silicone in a 5000g bucket.

You want a pump large enough to reach vacuum of the chamber in less than 60 seconds.

You need to keep the pump running the entire time.

When I started doing this, the videos on YouTube did not show the full process. The silicone really only needs to rise and fall once, but I find I get best results from letting it run a bit longer and in my video, it rises and falls a few times as it pulls all the air out.

You need a re-pressure valve on your chamber or hose and you never want to leave the pump under load of vacuum when it is not running. The suction will suck the oil out of the pump and that oil will probably end up in your silicone. A dry pump is a dead pump. Never let it run dry.
 
I appreciate this information. To answer an earlier question-I'm not sure what kind of silicone I have, I just pick it up at Hobby Lobby. Also, both the silicone and resin are being used in the same temperature/environment. I have no idea what the humidity is but the temp is around 76-77 degrees.
 
I appreciate this information. To answer an earlier question-I'm not sure what kind of silicone I have, I just pick it up at Hobby Lobby. Also, both the silicone and resin are being used in the same temperature/environment. I have no idea what the humidity is but the temp is around 76-77 degrees.

You will need to know about the product you are using or you might find this gets expensive from fails. You need to know your pot life and cure times and it pays to other details like the shore hardness, if it platinum or tin based (how forgiving it is to everything) and what percentage the cat is.

So the stuff I use is called "Vario". It is a Platinum based addition cure at 10:1. Part A comes in two types - A40 (pretty hard) and A15 (quite soft). The really cool part about this product is that you can mix the two shore hardness silicones to make a custom shore. So if you mix 1 part Vario 40 and 1 part Vario 15, you get shore hardness of about A27.5 (middle of the range).

The normal potlife is 15 minutes and the cure time is 6 hours at 25 degrees C. It might take longer at lower temps and I know it cures faster at higher temps. During a recent heat wave, I saw this stuff cure in just 2.5 hours! If it is cold, I'd leave cure for 24 hours.

If I need a faster than 6 hour cure, I can add Vario part B "F-Type" for a fast cure. Still 10:1, but now pot life is just 6 minutes and cure time is just 15 minutes. NO TIME TO DEGAS!

Like the Part A, you can mix the Part B at different ratios to make a custom cure time. With 1 part of normal Part B and 1 part of F-Type, Potlife is just under 10min and cure time is 2.5 hours at 25 degrees C. I can degas as long as I don't mess around.

So I like it because it is customizable to the job I am doing.

I discovered it in late 2014 and to date, I have mixed over 100KGs of the stuff. Yet it is the little things like what type of super glue can I use and not have a reaction? As it turns out, NO air curing products at all. Want to use super glue for a mold wall, use kicker to make sure it is cured.
 
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It sounds to me like you have bubbles in the silicone next to your part. Sometimes the heat generated by your resin curing can cause the air trapped in those bubbles to expand , creating dimples in your casting. If you vacuum cast your parts, it'll be even worse. The dimples showing up in the same pattern is indicative of this problem.

With a mold problem like this, a pressure pot for casting will cause the bubbles to contract - causing bumps on your casting. The opposite problem.

You could try slower setting resins. Since they create less heat as they cure, they hopefully won't expand the trapped air. Rapid-set resins get hotter. You could also try popping the bubbles, which might allow the pressure to equalize, but that could lead to getting either air bubbles in your part or little balls on the surface of your part (if the resin gets in to the bubble voids).

If you want to really solve the problem, you'll probably need to re-mold you part.
 
The resin I use is a quick setting type, maybe a minute? I usually let the part sit for a good while longer just to make sure AND to let it cool off.

When I was molding the part, I probably should have brushed on a coat first, which I have done in the past. As for re-molding it, I probably won't do that. I'm not casting a lot of parts, nor do I have a sizeable operation churning stuff out, I just do this for fun and to make extra parts. It's for this reason I also probably won't buy a $200-300 pressure pot.
 
The resin I use is a quick setting type, maybe a minute? I usually let the part sit for a good while longer just to make sure AND to let it cool off.

When I was molding the part, I probably should have brushed on a coat first, which I have done in the past. As for re-molding it, I probably won't do that. I'm not casting a lot of parts, nor do I have a sizeable operation churning stuff out, I just do this for fun and to make extra parts. It's for this reason I also probably won't buy a $200-300 pressure pot.
Sounds like Bee Queen. A product many people use here because it is fast cure and pretty cost effective.

Painting some into the mold is actually a good idea as some resins have a weird surface tension thing going on when they hit the mold and either trap air or even retract on thin pours.

When I pour my sole parts, I learned that painting the inside and degassing the mold in the chamber with the pot of poly was the best way of avoiding trapped air.

My pressure pot is still sitting in the box. It was only $108, but the compressor, air filter etc are what make it get expensive.

Sent from my CPH1701 using Tapatalk
 
I think "1 minute" was a bit false, it's more like 4 or 5 minutes, but you can see it "kick" and start turning white within a minute. I think the stuff I'm using is made by Alumilite. Sometimes it does foam up a little bit and it also sometimes expands a tiny bit. I'm always casting in the same spot with the same temp and humidity, so I don't know why it does this at random. And after I have poured out equal amounts, I'm not letting the stuff sit there for hours or anything, allowing it to absorb whatever moisture is in the air. I pour out A and B and then both directly into another cup together, stir and directly into the mold.
 
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