Ok, previously we looked at simple ducts. It is also possible to build assemblies of moderate complication with simple foam block male forms.
This is the start of a filtered airbox. The idea was to package the maximum possible filter media area into the limited volume forward of an AF servo (standard cowl without a prop extension).
Here's the basic form. Sometimes a section of a form requires more detail than you can easily shape in foam, or the shape of the detail is critical. In this case the box needed a careful bellmouth radius at the servo entry. A few minutes on the lathe resulted in a nylon block form with the required shape, which was then grafted to the foam block. Epoxy will not stick to waxed nylon.
The interior of the box needed "rails" to constrain the filter. The key here is to realize the form is really a mirror image of the inside of the finished part; cutting into the block results in a protrusion later. In this case I simply fired up a router and cut slots in the block. The inside of the slots are skim-coated with epoxy to seal the foam, just like the rest of the form, and get the same wax and PVA treatment before layup.
The basic layup to form the box is conventional. In this case there is one addition. The slots were spooned full of epoxy/flox mix and squeegeed off flush with the surface of the form. Then the shell layups were applied while the flox was still wet. This "wet-on-wet" method makes the filter rails integral with the shell; no bonding issue.
The box needs to come apart for filter maintenance. When cured the shell was cut into two sections without removing the foam core. All it takes is a fine cutting wheel on a Dremel tool. The cut is just inside one of the rails, which you can easily see through the glass shell.
Next step is an overlapping flange to seal and attach the two halves. One half of the box got a layer of slick packing tape to form a no-stick surface (a bit of vinyl electrical tape covered the corners where packing tape wouldn't lay down neatly). The other half of the box was scuffed for bond prep. Three plies of glass were impregnated between plastic sheets. The flange shape was drawn right on the plastic, then cut to size with a rotary knife (the "pizza cutter"). Peel the plastic off one side, place the bulk layup, peel the other side, and stipple with a brush to remove air.
When cured, dig out the foam. The wax and PVA coated epoxy "skin" placed on the foam form earlier will peel off the inside of the parts like a bad sunburn. A few sharp raps with a rubber hammer, a bit of compressed air blown under the flanges, and the two sections pop apart. Here we are after a bit of rough trim....
....and with the filter inserted on one half. Part number is K&N 33-2124
Continued next post.