Vibration technique produces stronger, environmentally friendly plastics

Because recycled plastic is poor in quality, injection moulding recycled plastic leads to products with reduced strength. Hence only a small amount of recycled plastic can be used in the process.

However, thanks to the recent development of a 'vibration-assisted process' by John Coulter, an associate professor of mechanical engineering and mechanics at Lehigh University, once-useless and environmentally unfriendly plastic can now be used to produce stronger products than those produced using new plastic that is not vibrated.

Researchers have tried to vibrate heated plastic, but most methods require costly machine and/or mould changes. Coulter and his Lehigh colleagues have found that by vibrating the feed screw device used to deliver the liquefied plastic material into the mould, they can achieve the same results without the need to vibrate the mould.

Making simple alterations to the screw's hydraulic system, Coulter designed software that directs the device to vibrate at low frequencies once the plastic has been fed into the mould.

Coulter says plastics are strengthened through vibration because the motion changes their molecular orientation or alignment. Molecules in non-vibrated plastics may stick to the wall of the mould 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 hopes to learn the optimal vibration protocols to apply to specific materials and applications. He notes, for example, that further studies may show that too much vibration will actually weaken a product.

Lehigh's vibration technique could be cheaply and widely adopted by manufacturers, Coulter says. 'If this process takes off, any company involved in injection moulding 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 percent waste polystyrene and vibrating the mixture, he can produce a stronger product than if he used 100 percent 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.

Coulter, a specialist in intelligent manufacturing, received a Presidential Faculty Fellowship from the National Science Foundation in 1993, a five-year grant, to study this field. He says he will continue to study vibration-assisted injection moulding until the process is fully understood. 'My plan and hope is that Lehigh will take the lead in introducing and explaining its use and benefits,' he said.