Molding Thermosets With Gas Assist

Thermoplastic molding with gas injection is already widely practiced on both sides of the Atlantic. Thermoset molding with assistance of gas is much less widely practiced; in fact, we don't know of any applications in this country as yet. But it is already commercial in Europe, for several good reasons:

  • Material savings
  • Cycle time reduction
  • Minimized shrinkage and warping
  • Reduced clamp force

Let's look at the results of some tests run in Germany. They were presented by Tom Betts of Battenfeld of America (West Warwick, RI), subbing for the author, Klaus Schroder, of Meinerzhagen, Germany; at this year's Thermoset Molding Conference. The materials were compounds with glass fillers, thus demonstrating a tendency to viscous flow.

A weight saving of 30 to 40 percent is typical. The cycle time is largely determined by the cure time--the time required to crosslink the resin. The factors involved are section thickness (thicker parts need longer curing times), material temperature, and resin type. Parts with a hollow center obviously have much lower cycle times. Mold packing is done by the gas, which usually means a pressure low enough to produce very little flash.

Tests run on eight parameters showed what affects cycle time or other variables. Some parameters show virtually no effect: injection speed and gas holding time do not affect bubble cross section or length; cross section of the mold does not affect wall thickness. Others do have impact, showing, for example, that percentage of initial filling has a large effect on bubble length, although much less on cross section. The size of the bubble cross-section is little affected by gas pressure, and the mold fills very quickly. The length of the void is determined by gas pressure during hold.

The time between initial partial fill and the beginning of gas injection is critical. A long delay means the bubble cross section decreases and the length increases. Similarly, increasing the mold temperature also decreases the gas bubble diameter and increases its length. Finally, the choice of molding compound affects the ease of flow, level of precondensation, speed of reaction, and degree of crosslinking.

The delay time programmed into the machine for the injection of gas is the critical parameter for determining wall thickness; long delays mean thicker walls. Mold temperature changes between 160 and 190C yield wall thickness changes of 0.1 to 0.8 mm, with splash marks possible at the upper end. Mold temperature should not be used to control wall thickness . . . but with the material usually already fixed, small delta t increments have been used.

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