Making a game accurate functional ODST ballistic rucksack backpack…
or…
Costume Prop making with hand-carved solid foam and vacuum forming.

It’s been a millennia since I’ve posted a build thread here. Please accept this with my gratitude.

I’ve long promised to make game accurate backpacks available for my ODST costume and others. The ODST ‘rucksack’ is such an iconic and integral part of the character design, however it is often overlooked by costumers. Due to the bulk of the part and the potential to design something that could function practically for carrying around gear and swag at conventions I have been taking my time and collecting ideas/requests for my build.

While there are a few good fan made 3-D files for the backpack available I felt obliged to fabricate my own model instead of using pepakura. Because of the wide planes and lines my opinion was that there was too great a risk for warping. Also I figured that if I would have to hone and straighten the edges of the backpack in bondo, I’d rather make something entirely from scratch..

Years ago I saved a block of high density urethane foam left over from an architectural restoration project. Frequently used for dimensional insulation and rapid prototyping in industry this dense, easy to carve material is becoming more available to artists as ‘carving foam’. The denser weights of foam can be rapidly shaped, cut, carved, sanded and hold fantastic detail. The resulting object can be sealed and molded or hard coated with resin and sanded.

I began by sketching a rough side profile of the core body of the rucksack onto tracing paper and transferring the outline to the foam. I rough cut the block leaving plenty of space for shaving down to my dimensions. Once the block was cut I used ‘Surform’ planers and shavers to square it up then sketched the front profile directly onto it, then planed and cut down to my profile. I repeated this process for the left and right side compartments that attach to the body of the backpack.

Now that I had rough geometric shapes to work with I made successive shaping passes over the backpack parts, first honing and beveling, then carving in the fine details of the design. I used a variety of tools to shape the surface: rasps, sandpaper files, planes. Through this process I frequently re-checked the symmetry using calipers and a bevel gauge. I fully detailed the main body before moving on to the side parts so that I wouldn’t have to keep re-adjusting their fit to each other. I removed the top and bottom panels of the backpack to prevent damage to their finished surfaces. As all these pieces would need to be formed separately, I bisected the main body of the backpack when all detailing was complete..

To prep the pieces for reproduction, I had a choice to make between hard-coating the foam forms, or molding the parts and casting them in a more durable material. If I were only making a single backpack, and it didn’t need to be hollow for functionality, or if the goal was to be a silicone mold, I could have just hard coated the foam. The hardening process could lose some detail that was carved, but it could be sanded and re-defined with more surfacing work. I wasn’t totally confident how the hard coated foam would stand up to vacuum forming, so I decided to mold and recast the backpack parts in a more durable material.

Since I only needed a single cast of each part, I opted to make simple plaster waste molds. I painted the forms with two coats of latex house paint to seal the porous foam, then lightly greased the surface with petroleum jelly to prevent the plaster from adhering to the paint. I hand applied the plaster to form the molds, building up a shell that was an inch or two thick. I embedded burlap into the mold for reinforcement. When the plaster was fully hardened, I flipped the molds over and carefully scraped and crumbled out the foam from inside the mold. Since the surface had such a good barrier coat between the foam and the plaster, the foam popped out fairly well once I cored out the undercut areas.

To cast into these plaster molds I again lightly greased the surface then brushed in successive coats of Bondo/Resin mixture and alternating layers of fiberglass matte. To reduce shrinkage I used the minimum amount of hardener in my batches. I built up the casting to about an even 3/4″ thickness then chipped away the plaster mold. I was left with a very durable duplicate of the foam backpack elements that could easily withstand the rigors of repeated vacuum forming. To prepare the parts for reproduction I smoothed out the models, fixed a few warps, then added MDF risers and structural attachments.

Part Two:

I individually vacuum formed the parts in .080 black HIPS thermoforming plastic. I then carefully trimmed the excess material from all the parting lines. I straightened out the cut seams with an emory board until all the parts neatly fit back together. I cut the opening for the top lid and hinged front hatch and assembled the main body of the backpack with pop rivets, and fastened the bottom together with the end cap. The side compartments of the backpack then were riveted to the structural risers. Once the main body and left and right compartments are together, the final ‘Compartment Rail’ cover parts are glued onto the side pieces with high temp hot glue (or epoxy).

To hinge the front and top lids of the backpack, I used A few strips of 3″ wide elastic that were hot glued to the inside lip of the openings. Once glued into place, these elastic hinges not only flex to allow the lids to open, but also snap back, automatically pulling the lids closed. I wasn’t expecting that feature, but it turned out to be very helpful! All the closure needed was a male tab on the top lid cut from scrap styrene and a little velcro and the lids were secure.

The last elements the backpack needed were some urethane cast handles and decorative pieces. I made these parts by fabricating them in metal and plastic and making silicone molds. The molded parts were cast in gray urethane, trimmed and fitted to the cut out holes in the side compartments of the backpack. I also made a cast faux closure buckle for the lid attachment tab just as a finishing touch. These cast parts would be glued in place 5 minute epoxy after being painted separately.

The paint application for the backpack was fairly standard for ODST gear. The entire backpack assembly was painted with a brushed nickel silver basecoat, then a paint resist was dry brushed on edges to simulate wear and abrasion, then the top coat of Duplicolor Gunmetal Gray Metallic was applied. The resist was removed after the topcoat dried, exposing the bare silver coat below. After painting the entire backpack with this process I airbrushed detail accents and grime in brown and black tones. The center body was painted a little brighter than the Gunmetal, and the round central button was painted nickel silver as per the in-game coloration.

With the paintjob complete, the backpack was strapped for attachment to the ODST shoulder pads, and EVA foam pads and the ODST buckles were attached over the left and right side compartments. I used black parachute buckles to hang the backpack from the shoulders of the costume, and a EVA foam receptacle glued to the backpack to cushion against the raised detail on the back of the ODST back plate. This foam piece secures the backpack from bouncing while moving around in the costume and reduces wear and noise from the parts rubbing together.

As is the backpack is pretty functional. however I might install an EVA foam liner and some sub-compartments to keep stuff from shifting around in the packs. I figure as a base model they’ll be pretty useful for carrying around batteries, cameras, tool kits, small prop weapons, and other convention swag.. The parachute buckle attachment is nice for easy access and removal. For my own personal costume I’m considering installing an amplified sound system for use with helmet mic’s, Ipods, etc. I’m also thinking about making some standard backpack straps that you can switch out so you can wear the backpack when you’re not in costume too.

To read the full interview please visit http://www.solidsmack.com/blue-realm-studios-prop-building-solidworks-3d-cnc-devin-white/2009-10-01/