Q+A: Designed to win

Invented by Native Americans hundreds of years ago, today’s lacrosse game relies on the latest plastics design and technology to add a competitive edge.

?If you live in Canada, New England, the Eastern seaboard, or parts of the Midwest, you or your children have likely played lacrosse. Other parts of the United States are now catching up fast as the sport grows in popularity. It is the oldest team sport in North America, yet now operates with some decidedly modern conveniences.

This full-contact sport features a long-handled racket (or stick) called a crosse. The crosse consists of a shaft, typically a hollow, high-strength metal or composite, and an injection molded plastic head that anchors a mesh net, or pocket. Players use the pocket to catch, pass, and shoot a solid rubber ball into a goal to score. Ball speeds can rival those of the best pitchers in major league baseball, more than 100 mph.

As the sport grows, plastic head designs continue to increase in sophistication. Although they must conform to size limits imposed by NCAA and Major League Lacrosse rules, designers have a fair amount of freedom within the “envelope.” That’s important, because it is the shape, rigidity, and physical dynamics of the head that help to determine how fast the ball can be shot and whether the ball stays in the pocket as the player runs full speed during game play.

To find out more about the intricacies of designing this equipment, IMM spoke with Kevin Vititoe, senior industrial designer at Priority Designs (Columbus, OH), who regularly devises new lacrosse heads for the well-known STX brand.

IMM: Which polymers are most commonly used to injection mold lacrosse heads and why? Do the resins vary for men’s and women’s versions?

Vititoe: Most competitive lacrosse heads for professional, college, and high school athletes are made from PA66. Some children’s toy heads or women’s entry-level heads are molded from polypropylene, but PA66 is used most often because of its weight-to-stiffness ratio, its memory, and its ability to retain properties at extreme temperatures. We specify a DuPont Zytel grade for STX, because whether the stick sees high heat in a car’s trunk or extremely cold weather during the early spring games, it stands up to the test.

IMM: Describe some of the physics involved in the motion of the ball as it is flung out of the pocket in a men’s lacrosse stick. How does the head design attempt to control and optimize this motion?

Vititoe: The path the ball takes leaving a lacrosse head is related primarily to the throwing motion, pocket, and throwing strings. However, the last two factors are controlled by the design of the head. The shape of the head and location of the holes used to attach the pocket control the path and release point—the point at which the ball leaves the head when throwing—as a player shoots or passes the ball.

To optimize performance, we start designing on paper, go to CAD drawings, and then to a rapid prototype. STX then reviews the prototype and refines the design. Every phase has revisions.

Some heads have what we call an offset, which can be seen looking at the side of a head. The offset allows the ball to stay in the head a little longer before it releases, giving a player more power. The offset and wall depth also put the center of gravity a little farther away from the shaft handle, which aids in cradling and keeping the ball.

The shaft has a centerline, and the top of the head is parallel to it. With an offset, the top edge of the rail dips down so the sidewall can drop down, pushing the pocket down and lowering the center of gravity. The more time a ball spends in the pocket during a shot, the more energy it has upon release.

IMM: What is the function of the sidewalls, which are perhaps the most variable part of the design?

Vititoe: The sidewalls vary from men’s to women’s lacrosse according to NCAA rules. The men’s head has taller sidewalls and the pocket is allowed to be deeper, so the sidewalls help protect the ball when players run and cradle. This actually doesn’t make it easier for a male player to keep the ball, as you might think. The deeper pocket actually forces a defender to be more aggressive in trying to “steal” the ball by knocking it out of the head, called “stick checking.” Men’s lacrosse is full contact, and the head design makes for a rougher sport, while women’s lacrosse is noncontact and relies on passing and intercepting rather than cradling and aggressive defense.

Holes to attach the pocket are usually placed on the lower rail of the sidewall. We are able to gain stiffness with a truss design, and also optimize the weight-to-stiffness ratio.

IMM: What type of impact and performance requirements must be met by the molded head?

Vititoe: Impact requirements are set by the individual manufacturers based on research and finite-element analysis (FEA) as well as older heads used as a benchmark. We optimize the designs for specific classes of players: attackers, midfielders, defenders, and goalies. Lacrosse heads are also purchased based on performance and aesthetics. Performance features include both weight and stiffness, and these two properties are directly related. Also, the shape of the head itself makes a difference. An advanced head is more narrow and pinched to protect the ball, while entry-level heads are more open for easier catching and throwing.

Typically, assuming the geometry is optimized, a head that weighs more is stiffer for better performance, but players also want less weight so they can shoot faster. We use FEA to evaluate stiffness by putting loads in strategic directions on the head and documenting the amount of deflection.

Once a design is in production, the manufacturer takes the first parts off the line, attaches them to shafts, and simulates the roughest play to test durability. This process can also expose flaws in the manufacturing process, such as knitlines or thin wall sections that may cause failure. If this occurs, we may explore tweaking the mold conditions or the mold geometry somewhat so that the knitline flows into a reservoir where it is trimmed off.

IMM: What purpose does overmolding PA66 with TPE serve?

Vititoe: Designers engineer details into the heads to control ball spin and rattle, and to help direct the ball into the pocket. We design these details as a TPE overmold. In fact, Priority Designs helped develop the first overmolded lacrosse head for STX. Our goal was to control spin, remove rattle, guide the ball toward the pocket when catching, and absorb some of the impact when a player is stick checked.
The first overmolded model was a challenge, because we had to build mechanical fastening areas in addition to using chemical adhesion. Today, we have resolved adhesion issues.

IMM: What are a few rules to remember when designing molded lacrosse heads?

Vititoe: Remember that this is not traditional molding because the walls have varying thicknesses. At the high end, they can be 0.5 inch thick; at the low end, 0.08 inch. Shrink rates are affected and warpage must be controlled.

Avoid extremely thin wall sections. With an infinite possibility of deflections and load conditions, the material may fail at the thinnest points. Be aware of where knitlines will be, as I mentioned before, and design them out if possible.

Further, the dimensional specs of the head have to fall within NCAA or professional league rules. If they don’t, it can void the head from being legal. It’s therefore important to design to control shrink. Because the wall thicknesses are not constant, there is always a potential for shrinkage to vary in different directions. Knowledge about material shrink rates will ensure the head does not fall outside the rules or existing competitors’ patents. Know the rules and pad the tolerances accordingly to allow for a margin of error.

?DuPont | plastics.dupont.com
Priority Designs | www.prioritydesigns.com
STX | www.stx.com