Styrafoam plugs

General Composites Discussions Composites Talk
#1

I’m hoping to make a relatively large plug, part of which has to meet pretty strict dimension requirements to mate with other parts. As far as I’ve found, the only way to economically machine a big plug is to make it out of styrafoam, and then to coat it with fiberglass/gel coat to give it a surface, but doing this will add thickness and mess with the machined dimensions. Is there any way to coat styrafoam for mold use without adding significant height to the mold? Machining wax would solve all the problems, but this would be pretty expensive. Any other ideas would be great. Thanks for the help.

-Brian

#2

DON"T USE PER on styrofoam…you won’t have anything left as Polyester resin will melt anything of Styrene. If you cheap out on the PS foam, then you have to use the more expensive epoxy resins.

Sounds like this is a precision item (CNC machined), then get a high density PUR (Polyurethene) foam sheets and then you can use any resin matrix!

You can spray like Duratec gel coats or other things to seal the foam, but what ever you do, you will probally will be doing some sanding and polishing before making the master mold from a foam plug.

In my classroom, we are trying to use a PUR foam plug as the mold by shaping it, gel coat it, sanding it, polishing it and then mold releasing before make the finish part from it. The finish part will be like a bath tub…but for mounting in your closet floor as extra storage area for off season clothing items or whatever!

#3

Polystyrene probably is the least expensive way to make a pattern. I’ve made fullsize car patterns this way. Instead of carving the whole pattern out of the foam as if it were a sculpture, consider making “stations” (or bulkheads) every 5 or 10 inches. Make the bukheads .06 smaller than the finished piece to allow for covering with one layer of fiberglass, polyester filler(bondo)and paint. Make templates (finish size) at those stations and check and adjust at the filling stage.
This picture shows the armature (or skeleton)on a rigid melamine board marked out x and y for future reference. Notice the prints glued to the bulkheads.

The polystyrene is glued in place. Consider spacing the stations to correspond with the thickness of the foam sheets;

Cut and sand the foam down to the bulkheads. This pattern I used aluminum bulkheads, usually I use MDF,plywood or masonite

Cover the entire pattern with one layer of 7500 series 10oz tooling fiberglass using epoxy resin.Cover with peel ply. this helps the fiberglass stay down on the styrofoam and gives a smoother finish.

(see next post for more pictures)

#4

After checking pattern with templates, start filling and smoothing. Always sand using a block such as a piece of wood as using snand paper in the hand or fingers alone will leave “tram” lines and will just follow the high oan low spots. the long board sander will show the low spots which require filling.


Use a rag or paper towel in your palm to feel low spots. Somehow it makes your hand more sensitive that way.


When you’re happy with the dimensions (using your templates) and the surface finish, paint using a surface primer, such as duratech.