April 06, 2022 4 min read
Part III: The Construction of the Modern UHMWPE Helmet
Producing fiber composite helmets is no simple matter. The material is inherently expensive, and the processing methods used to turn flat sheets into a finished helmet are complex and unforgiving. Aramid helmets, when they were first introduced, cost the military a few orders of magnitude more than the old steel pot. And aramid is both cheaper and easier to work with than UHMWPE. When it was introduced, the ECH cost the military 3x as much as the ACH.
The complexity generally stems from the fact that fiber composite sheets can’t easily be pressed into shape. When that’s tried, the result is almost always wrinkled and uneven. It can’t be used, and can’t even be recycled -- it has to be trashed.
The most common way of sidestepping this problem involves cutting fabric or fiber composite sheets into darted pinwheel patterns, then laying them up so that the seams are evenly distributed.
(Images from Dangora, Lisa & Mitchell, Cynthia & Sherwood, James & Parker, Jason. (2016). Deep-Draw Forming Trials on a Cross-ply Thermoplastic Lamina for Helmet Preform Manufacture. Journal of Manufacturing Science and Engineering. 139. 10.1115/1.4034791. )
This pinwheel method is very effective at reducing wrinkling and waste, but it is not without its problems. It’s very obviously labor-intensive, for one thing. For another,the seams themselves reduce ballistic performance. This is, in part, why body armor plates and soft armor panels out-perform helmets at an equal weight and thickness.
An interesting side-effect of this process is that the crown of the helmet -- which is seamless -- is considerably stronger than the front, sides, and rear of the helmet. This can safely be considered anegative side-effect, because the crown is the location on the helmet that’s least likely to be struck by a high-velocity projectile.
After fiber composite sheets are pinwheel-darted and laid-up properly, they’re consolidated under heat and pressure. This generally takes place in a matched-metal or silicone mold, at an elevated temperature, in a process that typically takes 20 minutes from start to finish. When the semi-finished helmet shells come out of the mold, they’re trimmed, painted, edge-trim is applied, and then they’re ready for pads and a retention system.
This process is how Kevlar helmets have been made for nearly 50 years, with extremely little variation between manufacturers. It has enabled the industrial production of aramid helmets on massive scales -- millions of helmets have been built in this manner. It’s also how many UHMWPE helmets are made today.
But with UHMWPE there are several points of added complexity. First, UHMWPE degrades at elevated temperatures, so although all consolidation processes employ heat, they’re run at relatively cool temperatures -- often a hundred degrees cooler than the temperatures employed in the production of aramid helmets. Second, end-users are beginning to demand superior ballistic performance from their UHMWPE helmets -- including, in many cases, the ability to stop direct hits from rifle rounds -- so the pinwheel method is quickly falling out of favor. UHMWPE helmetscan be manufactured via other means, and often these means are exotic and proprietary.
One such way to manufacture a UHMWPE helmet without pinwheeling is called “deep drawing.” This involves drawing a UHMWPE composite through a die via a punch, with the assistance of binders. Done properly -- and there’s a steep learning curve -- this can result in a helmet that’s free of both wrinkles and internal seams. The major issue is material waste, which can be considerable. In comparison with the pinwheel method, there’s a curious counter-phenomenon: Because all of the pressure is applied to the crown, a “deep drawn” UHMWPE helmet’s crown is perhaps the weakest and most stretched-out part on the shell, and there can be resin flow from the crown to the sides of the helmet, resulting in variable helmet shell thickness.
Other known methods involve high-pressure autoclaves, multi-stage deep-drawing processes, or hydroclaves. Ultimately, every method boils down to the same thing: Applying heat (as little as possible) and pressure (as much as possible) to press flat UHMWPE sheets into a resin-bound helmet geometry, without pinwheeling, seams, wrinkles, grossly variable helmet shell thickness, or any other structural feature that would make for reduced or inconsistent ballistic performance.
Other materials may be used to reinforce or stiffen the shell, but they’re ancillary, and they’re usually fairly easy to work with. Carbon fiber is by a wide margin the most common reinforcing material, and it’s famously easy to form into complex geometries.
By 2009, lightweight all-polymer armor plates that were capable of providing protection from rifle rounds were commonplace. Just a few years later, with the ECH, lightweight UHMWPE-based helmets began to offer similar, if slightly reduced, performance. Today we’re beginning to see helmets that are competitive with armor plates on a performance to weight basis. The technologies which enable this go beyond fibers and resins; high-pressure, low-temperature seamless helmet consolidation methods have everything to do with it.