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High impact foam question(s)
13 replies to this topic
Posted 20 March 2009 - 08:35 PM
Well, I bought some polyurothane foam today. Pretty cheap. about US$40 for a two gallon kit (1 galon each part). I did a quick test with a old RTV mold I had liing aroung and I'll have toi say, so far a little disapoiunted. The samples I was shown were picture frames and crown molding and they were sturdy and had tight cells. What I tested is brittle and has larger cells. I plan on calling for support in the morning, but does anybody here have any experience with this stuff? All the threads I found on it were pretty old and are probably out dated.
Posted 21 March 2009 - 09:32 AM
Well, the silence speaks volumes. Nobody uses this stuff?
Posted 21 March 2009 - 10:43 AM
I am not even sure what that is. I do know that some of the synthetic moldings I see in stores are not the strongest things in the world. maybe it is stronger once attached to a wall or something.
Posted 21 March 2009 - 12:02 PM
You should look for the number representing the cubic ft. per pound expansion. that also represents the hardness of the foam.
The higher the number, the harder it is.
My opinion is that anything under 8 cfp is too soft & many prefer a 15 to 18 cfp to mimic hardwoods. 8 cfp would mimic balsa IMO.
My guess is that your foam is a lower cfp to be so inexpensive. I know most architectural duty foams usually are.
You should typically be paying double what you are for the material.
Posted 21 March 2009 - 01:06 PM
From my limited experience with the material, I think it is all in the stirring, temperature and the venting. Unlike other casting materials, were bubbles are to be avoided, this stuff needs to be stirred with a vengeance. This, I found, gave a finer bubble spread.
Room temp can make a big difference, by affecting the rate of reaction, ie cool = slow, warm = fast. So, try to be consistent with the stirring and temperature for consistent results.
The venting is important, as it has to allow the extra to escape. If not adequately vented, pressure will build up as the stuff expands and it will compress/distort the mold from the inside and create flash from hell.
Do not expect to master this material in a couple of tries, you have to learn how it behaves and get a feel for it. Husky did a lot with foam a while back, so maybe a PM to him might be informative. But make sure you have read all the foam posts first.
Posted 21 March 2009 - 06:31 PM
Thanks for info. You guys always come through for me.
I did some more tests this morning and each time, the results get better. I made a pretty hefty RTV mold with a wooden box to hold it tight. So far I'm not venting it. I'm trying to determine the right amount of foam to put in. (reducing the number variables) I'm measuring the mix with a seringe. I have produced a couple of products approaching the density that I want. It still seems a bit light. I'm going to try to add some buck shot and let the lure cure in a strategic position to se if I can weight it al in one step. I'll let you know how it goes.
Posted 21 March 2009 - 07:10 PM
Well 16Lb foam is supposed to be light, this converts to specific gravity 0.25 (water = 1.00), which is just slightly heavier than a dense balsa.
Posted 21 March 2009 - 09:57 PM
One Thing I've done is: If you are using a thru-wire design, use a split shot above the belly hanger or on another appropriate spot on the wire.
Since your casting into RTV the wire can be placed in the mold without modification before casting in exchange for a small amount of flashing near the hook hangers/line tie to clean
if your mold is not vented or sprued, I usually fill one cavity with material, close/clamp the mold & stand it up.
A small vent will help relieve the pressure & avoid capturing large air pockets in the bait.
Posted 22 March 2009 - 12:21 AM
After about 19 attempts I reached a density/hardness that I'm happy with. The next step is to add the vent holes starting as small as possible and continuing until the bait maintains the consistancy and doesn't deform. I did think about the split shot thing. I just thought I might circumvent a step and at the same time lower the center gravity of the bait. It worked. I just don't know if I can consistently place the balast in the same place. Anyway, I made some good progress. I'll have to put it away now till next weekend.
Posted 31 March 2009 - 12:07 AM
Un update oin what I've learned so far...
1. Dave was absolutely right about the mixing issue. It does much better when you whip it up into a lather. The texture is more consistent and the final product is much harder.
2. I made a couple new molds and this time I parted the lure on the horrizontal axis so that when I pour the foam, it is in the same position that i want it to dry in. It makes it easier to hang the ballast and set the wire harness when you can just close the top when finished pouring. you still have to share the mold around a little bit to get the sides coated before it begins to expand. We'll see in the long run if i'd rather deal with the set up or with the line it makes across the sides.
3. I tried paining the inside of the mold before adding the foam and seeing if it would prime the lure as suggested in a thread about 5 years ago, but I wasn't really happy with the results. The paint didn't adhear the bait very well and ti craked in a few places. I tried it two or three time and gave up. Then I tried adding some white pigment to the A part of the foam before pouring and it turned a light bone color. Better than the brown color it normally turns. It's drying now. I'll see how it turns out in the morning.
4. From the beginning I've wanted to cast a lure that had the bill integrated in the body -- where the bill is made of the same foam as the body. I had just about given up on the idea since the buills kept snapping off without much effort. Then I asked the question here about "blades for bills" and got a few ideas. Since I hadn't made a mold for integreting the blade-bill yet, I just stuck a #2 colorado blade in the cavity of the bill thinking I would just scrape off the foam and give it a test before making a new mold. Tha foam actually swallowed up the blade. The blade is imbedded in the foam and it streagthed it enough where I think it's going to work. Then I thought about cutting off o piece of the popsickle stick that I'm using for mixing and fitting it into the buill cavity. It works pretty well. The foam adhears extremely well to the wooden stick and the stick provide the streagth I needed. So, if you can imagine, the bill is a piece of popsickle stick coated with foam.
5. Another thing I wanted to do was to make some torpedo-type lures. The problem is that you really need to use screweyes to attach the propand screweyes don't hold well in foam.
The solution that I am testing is imbedding a couple of plastic faceted beads in the tail of the bait, alligning them in the mold with a screw eye that theards well in their hole. The screweye holds really well in the beads, I just don't know who well the beads will hold in the lure. I'll let you know how it turned out in a coupl of days.
That's all for now.
Posted 25 April 2009 - 06:39 PM
My baits are casting great. Great density, very little flashing. They look good.
The only proble is that the finish is bubbling. I use a two part epoxy similar to flexoat. The paint goes on fine. The finish looks good initially, but as it cures blisters develope. Any ideas as to why?
BTW, I applied the same batch of finish to some wood and some plastic lures with the same paint, with no problems. only the foam baits developed the blisters.
Posted 25 April 2009 - 08:08 PM
How long do you wait between casting and finishing?
Foam continues to degas for quite a while. after it's cast. Try letting the casting sit for at least 3 days after casting then run it through the dishwasher with soap before painting. Give it a prime coat of an epoxy resin, then finish. OR you can use a Urethane Resin with Microspheres which is a much kinder medium.
Posted 26 April 2009 - 12:00 AM
-- All of the baits were washed with dish detergent and a toothbrush. They spanned from 3 hours to 10 days old with no noticabe difference in the finish.
-- I doesn't make sense to me to seal the bait with epoxy, since it is the epoxy that forces the bubles out.
-- Is there any solvent that might get rid of any silicone residue without eatng into the foam?
-- The reason I'm going with the foam is because it is readily available here. If the project is going to be sustainable it has to be sourced locally.
Posted 26 April 2009 - 07:22 AM
That's understandable. The kicker is that 16lb foam makes a great bait when it performs right.