For posterity sake, thought I'd post the link to my Halloween contest entry as it was intended in part to demonstrate some of the limits and limitations of LF, at least in my hands. The web strands at 1/4" x 1/4" and the casting would fit inside a 12" diameter circle. http://forums.thehomefoundry.org/in...-a-spider-halloween-casting-contest-2017.142/ Best, Kelly
Interesting discussions for this newbie. One thing that made me go "Oh Wow" when I originally saw Kelly's first post of his spider web, and it goes along with the discussion here on detail ... In the casting you can see the lines that Kelly originally drew onto the foam with a sharpie.
Kelly, I know you’ve probably tried a bunch of different products to fuse / weld your foam pieces together. I’ve started using cheap acrylic spray paint to weld my foam pieces together and it works like a charm. Lightly dust the surface to be joined and press together. It literally bonds in about 10 seconds with no visible deformation of the foam piece. What it’s actually doing is ever so mildly, melting the 2 mating surfaces together. Try it, you’ll like it.
What kind of Acrylic and what is the solvent? Have you tried hot wiring a joint yet? Might be good for laminating if so. Best, Kelly
No, I haven't tried heat welding anything together. I picked up some E6000 spray adhesive (Water based) 1-3 minute setup time, in a pump bottle which is "suppose" to be for foam and other material adhesion bonding. Turns out that stuff isn't worth the bottle they package it in. Just looked at the paint can. It's actually Rust-Oleum aerosol enamel, which contains acetone, toluene and xylene. Like I said previously, I only very lightly misted the mating surfaces and immediately matted them which results in the foam fusing together. A regular spray bottle would discharge to heavy of a dosage so that wouldn't work if you tried to fill it with any type of solvent. Any solvent would have to be atomized and applied with pressure, (hence, aerosol / pressurized) application or possibly a touch up (jamb) spray gun with a very small tip in it.
Hey OCD, does your spray-paint-glued foam cut up well on your new hot wire table, or does the wire get hung up on the seam? Jeff
IMO, when it comes to gluing foam it's a matter of "horses for courses" because I've found that one size doesn't fit all. If all you are doing is sticking parts together and you don't care how well or consistently it machines, sands, hotwires, and burns out compared to the base foam, then there are many, many, suitable choices. It's more about the finer points for me. Sometimes you need close control of how and where you apply the adhesive and sprays aren't very good for that. Sometimes you need more working time. (Low temp) Hot melt glue is great for short work time, controlling the application of glue, and quickly sticking things together. The joints aren't very thin but it is instant gratification. If you touch any hot melt glue joint on a power sander it will instantly reactivate the hot melt, load the sanding belt/drum, and transfer the glue to the remainder of the surface being sanded. For longer working time and close control of application I use wood glue with a small brush or tooth pick, and thin it with water to keep the joint thin, and then just usually tape the pieces together in compression. It actually sets fairly fast and burns out relatively clean. It's easy to aerosolize about any solvent. The problem with any solvent that attacks the foam is it also densifies the foam. If you try sanding across the joint, the foam on either side will abrade away faster than the joint. If the joint is perpendicular to your sanding direction, that lessens the affect. If it is a feathering shallow angle through the joint, that is the worst. If you can keep the joint very, very thin, that helps. That's why I was using shellac. The solvent is alcohol which does not attack polystyrene. You can laminate sheets with it by spraying a thin mist like you describe. If you let both sides get tacky it works like contact cement and you can achieve very thin joints. You can also brush it and it dries fast....very fast with a heat gun or hair drier. It doesn't burn out as clean as some of the other things but there is very little of it. Most of the adhesives made for foam are latex's. Ammonia will reduce latex and does not attack polystyrene. This is probably what you are calling water based spray. The vapor pressure of water is low so it doesn't dry fast nor make a very good propellant. It's hard to keep latex joints thin. You can use conventional spray contact adhesive if you use it very sparingly as it has the same solvents as your acrylic. I don't like it because the joints make a mess when machining or sanding. Best, Kelly
Jeff, Cuts great with no hang ups what so ever. Guess the trick is to spray / apply a sufficient, yet minimal amount of solvent (spray paint) to the surface just to get it to fuse together. Of course we all know it's not the actual paint solids which is creating a bond of the foam pieces but the foam itself which is being melted from the solvents incorporated with the solids in the rattle bombs. A mixture of toluene and xylene sprayed in a jamb gun at high pressure would probably work just as well and be the cheapest route. Have to play around with that one to see what the results would be.
Kelly, A lot of good points in your reply. And like you said, "horses for courses", not everything works for all situations. Like those micro eyeballs for your Halloween spider. Fine detailed components definitely need precision application of adhesives and parts.
Necro post: Some of the things I have planned to cast with LF do have some thin sections that would be flexible in foam. The thought occurred to me today that I might be able to reinforce the foam patterns with aluminum welding rod stabbed into the foam. A quick Google didn't find anything, but I may not have used the proper GoogleFu. Thoughts about the value of this idea?
Never tried that so can't really say for sure, but unless the aluminum wire/rod was anchored in the sand I suspect it would migrate. The wire mass would have to be very small compared to the molten metal mass so it doesnt cause localized freezing or mess up how the pattern evaporates. If I have a fragile pattern, I fill the mold slowly, like a quart cup of sand at a time, while vibrating, keeping the sand depth the same on all sides of the pattern so the sand pressure remains equal. Maintaining a uniform sand depth and vibe while packing is good practice for any foam pattern IMO. It mininimizes pattern distortion and in most cases at least for me, eliminates it. I've learned not to get too concerned about it because as cast, the castings are soft and can easily be straightened in a few minutes which is much more practical than elaborate means to avoid minor movement. Best, Kelly
I hadn't considered it's quenching effect on the foam. It could be a significant factor. My original thought was not so much to keep things in place in the sand but to help prevent distortion/breakage due to buoyancy during dipping. I had been remembering the video of the WWII Pratt and Whitney cylinder heads being cast and the use of wire in the thin sand to keep it in place.
Since aluminum is such a good conductor, it may actually have the opposite affect and cause the foam to melt away from the rod ahead of the advancing metal front, which would be very undesirable. In my experience, whenever the molten metal front encounters an air pocket nothing good comes from it. If it doesnt first cause localized mold collapse, it often produces fold and other defects. This certaily can be a problem. I consider it when spruing and gating to add pattern strength. Dipping "on edge" with the least projected area helps minimize deflection, as does dipping from both ends. Ladling the coating on the exposed portion is common practice too, but with all the above the pattern still must be able to withstand it's own weight with the wet slurry, and that can be quite a challenge since I typically prefer to hang the parts and allow it to shed some coating. That puts a lot of stress and risk of breakage if done from a single location, typically the sprue, which is why I add the sprue after coating. I've also resorted to building in "feet" for the coated pattern to rest on while drying which can also minimize distortion due to coating weight. If it's a very high risk part, it can be coated in several steps. Harder to do with dip coating but easy with brush....just more laborious. Once the patterns are coated, they become much stiffer andgenerally easier to mold without distortion. That is a neat video and an example of going to extremes to cast a part. Desperate times require deperate measures. One significant difference is those wires/needles were reinforcing the mold not the pattern or cavity where they would be imbedded in the casting. Best, Kelly
