Shake, rattle, and scroll

Whether you know it or not, injection molding is a part of your everyday life. A Lehigh University professor has now developed a simple, software-directed instrument that can vibrate molten plastic in an injection mold to produce stronger, more environmentally beneficial products.

Injection molding involves injecting molten plastic into a mold. Only a small amount of recycled plastic is used in this process because recycled plastic is poor in quality and would reduce the product's strength, said John Coulter, Lehigh associate professor of mechanical engineering and mechanics. Coulter's vibration-- assisted process enables manufacturers to recycle once useless and environmentally unfriendly plastic and still produce stronger products than those using new plastic that is not vibrated.

Researchers have tried to vibrate heated plastic, but most methods require costly machine and/or mold changes. Coulter found that by vibrating the feed screw device used to deliver the liquefied plastic material into the mold, they can achieve the same results without vibrating the mold. Making simple alterations to the screWs hydraulic system, Coulter designed software that directs the device to vibrate at low frequencies once the plastic has been fed into the mold.

Coulter said plastics strengthen through vibration because the motion changes their molecular orientation or alignment. Molecules in nonvibrated plastics may stick to the wall of the mold or become folded over, but vibration jiggles the molecules, mixing them more uniformly and allowing them to stretch out to take advantage of their entire length.

Coulter wants to learn the optimal vibration protocols to apply to specific materials and applications.

Manufacturers could cheaply and widely adopt Lehigh's vibration technique, Coulter said. "If this process takes off, any company involved in injection molding could duplicate what we are doing inexpensively and probably do it better. Essentially, all you are doing is changing the signal that controls the hydraulics."

Coulter found that by adding 50% waste polystyrene and vibrating the mixture, he can produce a stronger product than if he used 100% new polystyrene without the vibration process. He believes the ability of the vibration process to use large amounts of recycled plastic material may turn out to be of greater significance than the fact that it increases the strength of plastic products.