meanmf

Hi guys,

I am trying to make a plug, and I really want to use urethane foam.

Does anyone know the sources for fairly high density uretane foam? I seem to remember that this type of foam is used to make billboards?

Thanks again for your expeert advice,

Mark

Bill Vargas

My Feedback: (5)

Try The Home Depot or try the ph book under Foam Insulation for your local area.

Hope that works for ya

BV

ptxman

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I bought 10 pcf urethane foam from Aircraft Spruce & Specialty (see link below). Its called last-a-foam & is great stuff. Its dimensionally & temperature stable, med/high density, no grain issues whatsoever for shaping, takes any kind of resin, fillers & finshes. I bought 1" sheet thickness & laminated what I required with urethane glue (probond equivalent) so the joints sand consistent with the material. There are pictures of my plug & mold elsewhere on rcu. Personally Im not as fond of 'other' foams like dow, sm etc. Their density is nominally only 2-3 pcf so you have to build up a glass cloth shell. Theres also the risk that auto paints or polyester based fillers find their way to attack the foam.

http://www.aircraftspruce.com/catmai...hartmaster.php

Ollie

You could also make your own polyurathane foam from a two-part liquid mix. It is sold by Aircraft Spruce and Specialty and Sig. The texture and density is determined by the size of the mix and ambient temperature.

Polyurathane wood glue foams remarkably when mixed about ten to one with water. The resultant rigid foam has a very fine cell size.

ptxman

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You can but... Its more difficult to contain the liquid state 2-part mix between contour formers without some means of daming or containment. You have to gauge how much to fill a particular cavity to allow for expansion. The foaming action has enough pressure to displace formers out of alignment. Depending on the thickness & shape, Ive seen varying density gradations. The density of the 2-part from what Ive seem is still lower than 10 pcf sheet & it varies by one other parameter, the shape of the cavity itself. The equal distance sheet form adds another benefit as you are shaping compound curves particularly, the edges give visual feedback on the iso-contours. Later on I mixed a bit of carbon black in the glue & it shws up the contour very well. The closer the shaping is initially, the less filler work & blocking later.

foxx

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Hi Ollie,

Is it 10 part glue or one part water or the other way around. Have you tried the insulation foam that come in a spray can I was wondering if any of you guys might have experience with this stuff please share it with us.

Ollie

The polyurathane wood glue and water just needs a little water. The less water, the denser the foam and the slower it cures. One part water in 10 parts glue is the most water I would use. The cell size isn't as uniform as I first thought. I wouldn't try to use it for large volumes. It works best to fill low spots in a foam plug or as a light weight glue to fasten wood to foam.

I have used the pressurized can of polyurathene caulking foam to secure control rod tubes in fiberglass tail booms. It is messy stuff until it cures. It can develop a lot of pressure as it expands and has to be used with caution inside a structure. Too much and it will blow the structure apart.

For building plugs, the high density sheets, laminated together, are by far the best way to go.

foxx

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Ollie what I am triying to do is build a fiberglass replica of my plastic cowl. I have never done this before and trying to figure out how to get started. I thought may be it would not a bad idea to use the foam insteated of trying to sand the solid foam.

Ollie

The best foam to use for making a plug for a fiberglass cowl is closed cell (blue, pink or gray) polystyrene foam. It is cheap and usually available in the local building supply or do-it-yourself outlet. This foam can be rough cut with a hot wire, band saw, hand saw or vibrating carving knife. It can be smoothed with sandpaper using light pressure. It shapes as easily as very soft balsa.

Make the foam plug slightly undersized to allow for the thickness of the fiberglass cowl. Cover the foam plug with mylar packing tape to keep the inside of the cowl clean and to facilitate removal of the plug material from the finished cowl. Cover the tape with several layers of light glass cloth in laminating epoxy to build up the strength and stiffness without the uneveness of heavy weight cloth at the overlaps. Stagger the overlaps of succeeding layers to keep the surface fair and minimize the need for filler. Remove the sacrificial foam plug by cutting chunks out untill the tape and foam can be pulled out leaving a clean interior.

For one or two finished parts this requires less work and less material than making female molds. The lost form method also results in seamless parts. It doesn't involve a consideration of drift angles ot the complex molds to achieve them.

foxx

My Feedback: (2)

Thank you so much for the info. The part that I am not clear is the mylar packing tape. It is like sorren wrap (SP?). how do tape the outside to get a nice smooth surface.

Ollie

I was referring to the plastic wrapping tape used to hold cardboard boxes closed for shipment. With a little care and ingenuity it can be applied smoothly enough to do the job. Where you wnat it to go around compound curves you can slit it or cut darts in it and let it overlap itself. The resultant lack of smoothness is of little consequence on the inside of the cowl. Plastic kitchen wrap could also be used as a parting film. It is thinner and stretchier than the mylar packing tape and will go around compound curves a little better. When the kitchen wrap starts to bunch up, that part can be cut away and a fresh piece applied to overlap the edges where the first piece was cut away. Neither the tape nor the kitchen wrap are essential. They just make the foam removal easier. You can lay up the glass cloth and epoxy directly on the foam plug. Then the foam can be dissolved or removed mechanically. I prefer not to use messy solvents. I cut away chunks of the foam until only small bits are left clinging to the fiberglass shell. Then I remove the last bits with coarse sandpaper and a coarse file, used lightly so that it doesn't harm the fiberglass.

foxx

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One more question please. How do you get such a nice surface on the FB parts. I made some landing gear covers with 2 OZ cloth
it came out ok but on the edges you could see the epoxy run (Z-poxy finishing resin and 5 layer of FB cloth) now you can imagine such a part as cowl which is totally curved it would be hard to get a nice surface with runs., but it has been done and there most be a technique to it .
Ollie if we save these thread then we have the whole course. Thanks a million.

Soar Head

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After all of the fiberglass work, as Ollie described, you need to use a filler. AUto body filler is fine. Then sandpaper, patience, filler, sandpaper, and patience will get you a completely smooth outer surface.

If you want the perfect smooth outer surface, then a female mold is required. But this is 10 times more work then making a one-shot male plug out of foam.

foxx

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Thanks Soar Head for clearing things up. Now I know how it is done.

Ollie

There is more than one way to skin a cat. Instead of filler and primer, I put on an extra layer of 1.5 ounce glass cloth to be sacrificed to fairing and to allow a perfectly smooth surface after wet sanding. I just wet sand the texture of the weave away leaving a surface that is ready to paint!

BTW, if you blot off the excess epoxy with paper towels before the epoxy starts to gel you will have a stronger, lighter part that requires less wet sanding. West Systems epoxy sands easier than MGS epoxy but, MGS epoxy is harder and stronger.

foxx

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I was thinking what if we make a wooden box and put our plastic cowl with top and bottom sealed inside the center of the box the fill the box with spray type insulation foam with a passage for the extra foam to escape then do we have a female mold ? or I am losing my marbles

chrisbarker

Take a look at this site:
http://www.cstsales.com/articles_for_model_builders.htm
Good supplier too.

Chris

foxx

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Thanks Chris, I have bought few thing from this company. You are right they are very nice people.