Producing Protective
Polymer Padding

Foam liner material presents manufacturing challenges

Initially developed for padded liners used in single-impact helmets (e.g. motorcycles), as well as multiple-impact designs (e.g. football, hockey and snowboarding), this foam incorporates polyethylene or polypropylene spheres. Producing the foam creates spheres of different densities, which can be formed into discreet layers or a homogeneous mix. This construction enables a helmet liner or other protective pad to dissipate both high- and low-energy impacts more effectively than single-density foams, thereby effectively reducing the linear and rotational forces responsible for both catastrophic and chronic injuries.

Brock USA, Boulder, CO, developed and patented this foam bead technology using closed-cell polymer spheres ranging in size from 2 to 6 mm in diameter, then encapsulating them with an elastomeric coating during manufacture. The resulting padding construction can be tailored to meet specific requirements (including Snell impact tests), producing protective foam with enhanced mechanical and physical properties for attenuating impact. In a close-up look of the design, the thousands of tiny spheres making up a helmet liner or other protective pad appear to be in direct contact with one another. In reality, they are bonded by the elastomeric coating with air pockets in between them, permitting the foam to absorb energy in two distinct ways — by using both plastic and elastic deformation.

“When a pad made with Brock foam is subject to an impact, the initial ‘give’ or deformation occurs at the elastic interface,” Brainbridge explains. “As the elastomeric binder begins to compress between the spheres and adhesion points elongate, the air channels collapse. Finally, the spheres themselves deform and further absorb the blow, giving the entire construction the benefits of both a high-volume, low-density foam and a low-volume, high-density pad in the same material.” In other words, the greater the impact force, the harder the liner becomes. Because a soft liner is desirable for preventing non-catastrophic, multiple-impact injuries caused by low energy forces and a firmer liner is preferred for high-energy impacts, a padding material exhibiting this dilatancy is well suited for sports padding.

Unlike conventional foam padding, these designs actively cool the body instead of increasing unwanted thermal insulation. The abundant air channels promote convective and evaporative cooling. The pads have been field tested among athletes as well as by the Human Performance Laboratory of St Cloud State University. Further applications include automotive passenger seating, medical products and outdoor products.

However, producing the foam presented several design challenges.

“In order to meet the rigorous standards such as those established by Snell and ASTM protocols, we needed very precise control over our mix ratios and dispensing rates,” said Dave Bainbridge, vice president of product development at Brock. “It was critical that we be able to deliver both components of the 2-part formulation in exact quantities, even in small volumes dispensed at just a few grams per second.” Bainbridge adds, “We manufacture several different types of foam and each one can be made to different densities.”

The company selected the Advantage II system from Fluid Research Corp, Tustin, CA. The new equipment design is based on a progressive cavity pump, which eliminates phasing errors and other issues faced by conventional piston pumps. The machine features high accuracy, and programmable controls simplify changeovers from one production run to the next. Says Bainbridge, “This system offers a very short turnaround time for material supply changes, and has the ability to purge lines and recall a different program in just a few seconds. Because the material chemistries in this application are similar enough to prevent contamination issues, it’s typically ready to resume production in a few minutes.”

The base material in the foam is a 2-part urethane with one component of the proprietary formulation having a filler. “Control is provided by a Material Management Unit, which heats and recirculates the formulation to maintain stability and constant dispersion,” explains Steven Gordon, FLP director of engineering. Even when the machine has been idle for some time, on startup it recirculates all the material back into the feed tank to be re-agitated, so each component is thoroughly mixed and the fillers are properly suspended.

“The MMU is also designed to allow the exchange of the material supply containers without introduction of air into the system, which is a major cause of phasing errors and inaccuracy in metering or flow rate,” says Gordon. “Supply containers can be disconnected and replaced without stopping production or the need to ‘de-air’ the entire dispense system.”

High flow rate equipment is currently under joint development between the two companies that will permit dispense rates in gallons per minute — instead of grams. “This is a big leap in dispensing technology,” adds Malcolm Bishop, regional manager for FRC. “Instead of dispense rates in the grams per second range, we’ll be able to deliver up to five gallons per minute, with the same accuracy and control.” Bishop says he expects the new technology to be of particular interest in fiber-reinforced plastic applications, such as boat building and other pre-preg molding operations.

—SG

Fluid Research Corp
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