How Envelopes are Manufactured

(The following excerpts comes from Chapter IX of Free and Captive Balloons) Note to readers:  The information provide below is not specific to traditional spherical balloons.  It was written for a type M balloon.  Some of the details about vertical stabilizers, ballonets, and suspension lines have been omitted.   The basic building techniques outlined were applied to traditional gas balloons.  The author of this site makes no representations that the following information will work with modern fabrics. None of these techniques have been tried or tested by anyone at this site. Users should use the information at their own risk.

Cutting Panels.  After the fabric has been inspected and tested it is ready to be cut into panels for assembly.  The fabric was laid out in 50 foot lengths. Cardboard patterns are then placed on the fabric in a manner to give the greatest economy of fabric.  The outline is marked for cutting, after which a three-quarter inch seam lap allowance is marked.  Curved edges are placed across the fabric and straight edges along the fabric.  The panels are then cut with an electrically driven cutting machine known as the universal reciprocating cutter, type Agile.

The cutter consists of a razor blade which moves up and down at a very high rate of speed, the holding frame of which is moved forward by hand.  After the panels have been cut, they are marked for assembly.  Edges of the panels which are to be cemented together are marked similarly so that when assembling the envelop it is only necessary to match up the assembly marks on the panels.  The panels are then buffed along the seam lines with a wire brush in order to remove any loose flakes and dust to provide a good base for the cement.  The buffing is done by hand and extreme care is taken not to buff so deep as to expose the raw cloth or cut away the rubber coating.

The panel seams are swabbed with a cleaner to remove any foreign matter, and then given three coats of cement, each coat being allowed to dry thoroughly before applying the succeeding coat.  The third coat of cement is not permitted to dry thoroughly but only until the cement becomes “tacky” The cemented edges are then lapped together, allowing for 1 inch overlap, and thoroughly rolled down.

After the gore has been cemented together, the laps are sewn by two rows of locking stitching, six to eight stitches per inch, spaced one-fourth of an inch apart and equidistant from the center line of the seam.  The machine used is a one-fourth-inch gauge, lock-stitch Singer sewing machine, type II-30 and the thread is Richardson’s size “D” silk thread lightly coated with glycerin.

Seams:  After sewing, the seams are covered with single bias tape on both sides, the outside tape being 1 7/8 inch wide and the inside 2 5/8 in. wide.  The seams should have three coats of cement before applying the tape.  The tape is then applied thoroughly rolled down, and soap stoned.  It is not stretched but laid loosely upon the seam, which is pulled tight.  It is essential that all taping be evenly applied so as to cover the stitching and edge of the lap by a good margin, since the purpose is to prevent diffusion at the seams due to the holes of the stitching.

After all the gores have been cemented, sewed and taped.  The next operation is to assemble the gores into sections.  The sections consist of two halves dividing the envelope one panel off it top and bottom longitudinal center lines, making seven gores in one section and nine gores in the other.  The gores for a half section are laid out on a long table, given three coats of cement along the meridian lab edges and assembled together.  The half sections are then spread on a canvas-covered floor, and the locations of any suspension lines are marked on thereon. 

The two halves are then cemented and taped and sewn.  The final seam is sewed by the sewing machine through the top valve opening on the envelope.  Half of the rip-panel is assembled on the nine-gore section, before final assembly of the two half sections. The other half of the rip panel is assembled to the envelope after the final top seam is closed and is accomplished in a similar manner as the sewing of the final seam.