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Post by cablebutcher on Jul 27, 2018 14:58:55 GMT -5
I am trying to address the issue of relieving the air trapped in a mold while pouring. I am making cannon barrels for a ship model. I used your "starter kit" from Hobby Lobby as I am new to this. I created three one piece molds (I have attached a photo to show the scale I am working with)(I cut one of the one piece molds in half to see how the mold of the master went -perfectly). I did "dust" the molds with talc, heated the molds before the pour,and I attempted to agitate the pour with a wire to release any trapped air. I am pretty sure the short open time of the RC-3 resin kept the agitation from being too effective. I am looking for suggestions for a product with a greater open time and, if possible, thinner consistency to allow the use of a small diameter tube to "flow" the resin into the mold from the bottom up, allowing the air to escape as I back the tube out as the cavity fills. Knowing any filling device would probably be a one use only item, I would prefer not to resort to disposable syringes due to cost. As the photo shows, the silicone mold was excellent, and the one, almost complete, reproduction tube is also excellent, so I know the issue is my not getting the air out, not any product issues. When I begin to produce the barrels (18 in total) I will introduce a black dye to the resin for the pours. The molds seems to have held up well to theextraction of the tube through the pour opening, and I'm hopeful it will last through 6 pours. Any suggestions you may have would be appreciated.  |
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Post by Brian on Jul 27, 2018 22:09:03 GMT -5
Good looking mold.... Since it is a one piece mold the fix is rather easy...... You've done everything right in prepping the mold...powder and heat..here's what works for me..I pour RC-3 as well. You can stir/mix part "A" and part"B" a good solid 30 seconds with out taking away too much time from the work time...which gives you aprox. 2 minutes and 30 seconds to complete a pour....mix in the same cup that you pour to save precious time. I have a transmission that I pour...close to the same length and circumference as your cannon ...its a 1 piece mold as well. What I do is pour and squeeze with my hand to "burp" the air out. The only difference in your mold and what I have , is that I do one transmission at a time so that I can firmly grip the mold with one hand and squeeze while I pour with the other....so its pour for a few seconds, give it 2 or 3 squeezes, pour for a few more seconds, squeeze it a few times, repeat. If you are still having evidence of air pockets in you cast in the same spot(s), add vent gates that angle up towards the top of your mold in the area of the air pockets... they don't have to be as large as the pour gate, just enough to allow air to escape...keep in mind that there will be more flash/clean up to deal with. You have to be absolutely ready to pour with no distractions...2 and a half minutes can pass by rather quickly if your slow on the pour. |
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Post by notoes on Jul 28, 2018 9:43:27 GMT -5
Vents are good, I'd also like to add that any vibration will help too - I use a tapping motion on them myself. Personally I'd cut that mold up into three instead of keeping it one piece, I've done action figure joints (using RC-3) in a multi cavity like that and in single cavity ones and the singles came out better on the no bubbles than the multi ones as I pour one at a time and tap them until the 2 and a half minute mark. For me, I can pour three at a time with smaller pours and get a better pour per mold. You can do a 1oz pour (1/2 oz each of A and B) and do each mold separately with little problems, most of my small pours are 1oz or a little less. I also like to use "dedicated" mold boxes to hold each mold stable when I can. Please keep us updated on how things turn out for you.
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Post by cablebutcher on Aug 5, 2018 19:34:49 GMT -5
Update I made second casting with good results. I made a wooden base so I could use the mold as a "free form" mold, allowing me to flex and squeeze as I poured.I used a needle to burst air bubbles and agitate as I poured, allowing the resin to flow a little better. I also tapped the mold while pouring.The only prep change was that I heated the mold in the microwave rather than the oven, so I am not sure if it reached the recommended 140 degrees. The barrels turned out well. The perforation at the muzzle of one of the barrels was probably due to me trying to remove the casting too quickly (I use a small diameter rod to push the tubes up from the base to help remove them from the mold. Extracting the barrels stresses the mold, so I'm not sure if it will survive six additional pours. If I need to made new molds, I will make single barrel molds as notoes recommended. The original has an oddly shaped cascabel at the large end of the barrel which makes extraction a little tricky. Since the shape is not historically correct, and that end of the cannon will be not be visible (hidden below decks), I'm not too concerned about it. Thank you for your input and advice, I now know I can use the existing resin to get a good pour. 16 more tubes to go. The single tube below the original is from my first pour attempt (only one of the three cavities in the mold that took any resin).  |
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Post by notoes on Aug 6, 2018 7:34:26 GMT -5
Sounds and looks like things came out pretty good then. Glad we could help. Depending on which of the rubbers you use, it shouldn't tear very easily. I just looked it up and the Quick Set (in the kits) has a higher "shore hardness" but the High Strength ones have a higher tear strength. I've yet to use the Quick Set but I have used both HS-3 and HS-2 and I know it's not that easy to tear either one unless you have an under 1/4" thick area and it looks like you've got more than that around each edge of the tubes. I normally have to grab my scissors instead of trying to tear them - and I am fairly rough on my molds simply because I know they can take it. And by "shore hardness" I mean how dense/hard the mold feels, HS-3 is softer than HS-2. If you end up having to make another mold, you could always do the new one as a two piece mold (your first picture in the thread is a two piece) and that would make the castings even easier to remove without any pressure on the mold. But you'll also need to be sure to do "registration keys" which are the places where one side locks into the other and might need to have supports on either side to keep the keys tightly together.
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Post by cablebutcher on Aug 6, 2018 13:13:37 GMT -5
Thanks. The mold shown in my original started out as a one piece that I cut in half to see how well it copied the original tubes. The 3 barrel one piece is the mold I am using. I remember dealing with registration keys on two piece molds from my casting lead soldiers as a youngster and remember seam flash was an issue, although I don't think molten lead and two part resins are a good one to one comparison. I will probably stick with a one piece mold if I need to make a new one, but will restrict it to one or two barrels per mold as you recommended. I did a third pour last night, and the three barrels came out fine. I am also learning to fine tune the amount of resin I need per pour to avoid waste - looks like about 2 1/2 to 3 ml per pour. Thanks again for your input.
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Post by notoes on Aug 14, 2018 8:27:07 GMT -5
When I need to figure out an amount to pour, I like to use water to measure how much to put in. But I also make sure it's completely dry afterwards so it doesn't interfere with the resin pour. When it's something I can't be sure I can get it dry completely (like a complex design), I just mix up a little bit at a time and go from there to find out how much to pour. And as for the flashing, if you demold as soon after it hardens then the flashing is easy to remove. I usually demold at 10 to 20 minutes (depending on the thickness of the casting) and until it hits the full hour or more mark, I've easily snipped thin bits off with a set of cutters or scissors. If you can keep the seams tight, that will cut down on the flashing you get.
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