Electron beam has foam in stitches

Sound blocking pads can be designed so that they expand to fit the space available. Tom Shelley reports

Foam pads can be made to expand to any shape by running an electron beam to and fro across them and selectively stitching them with cross links.

In mass production, the technique is likely to cost only a few pence per pad and is highly suitable for making sound absorbing pads for car body cavities.

Foam proves to be useful material for attenuating sound passing along air paths inside hollow body channels in cars. It may be produced in place, like cavity wall insulation, or simply stuffed into the hole. But a better method, according to Dr RJ Chang of Raychem Corporation, is to use pre-programmed foam, designed to expand to the optimum shape.

Speaking at the Institution of Mechanical Engineer’s European Conference on Vehicle Noise and Vibration, he revealed that programmed foam pads could be made which expanded completely isotropically, instead of rising vertically like a loaf of bread, or made to expand and fit a shape.

The pads would expand when cars go into the E-coat paint bake oven. Unlike conventional in-place foams, the new material expands by almost the same amount, about 175 per cent, regardless of whether the baking temperature inside the channel is 145C, 180C or 200C (175 per cent linear expansion translates to a 536 per cent volume expansion).

It is also possible to make a pair of pads expand to form successive barriers. This is useful because two separate 6mm thick barriers work much better than a single 12mm thick barrier, reducing sound transmission by 40 to 50dB over the range 500Hz to 10kHz, and causing substantial sound attenuation at lower frequencies.

Delegates at the conference asked the speaker whether the foams might act as traps for water, especially if they broke down as a result of long term use. Chang said the foam met all criteria demanded of it by the American car industry. Raychem has considerable experience in designing cross linked polymers which repel water and can be used as moisture barriers in electrical equipment.

The company declines to be drawn on the exact chemistry.

Institution of Mechanical Engineers


Design Pointers
Pre-programmed, heat activated foams can be made to expand by the same amount in all directions, or in such a way as to fill a peculiarly shaped cavity. Linear expansion is about 175 per cent over a wide range of paint oven baking temperatures

Single baffles of the material work well as sound blockers, but two successive baffles work even better



Back to Automotive