This Extrusion Guide applies to the use of Endex Chemical Foaming Agents in a variety of processes including:

Extruded thermoplastics can be foamed to 50% or more, of their original densities. Successful extrusion depends upon adjusting temperature & pressure profiles of the extruder to use Endex most efficiently. Pressure must be maintained on the melt to prevent foaming in the extruder. This can be done by using a high compression screw, fine screen packs, or temperature reduction in the front extruder zones to produce back pressure. Pressure can also be controlled by adjusting screw speed. The die temperature is normally lower than for solid extrusions to enhance surface appearance.

Use Level: Endex use levels will vary with the application, but 0.5% level is a good starting point. The use level can then be optimized for maximum efficiency and cost effectiveness.

Dies: Use a die with minimum land-length ratio, preferably no larger than 10:1, to ensure high pressure on the extrudate, thus preventing premature cell formation. If the pressure drops too low within the die land, cells can form while there is still shear stress on the melt, causing irregular cell formation and poor surface appearance.

Screws: A conventional single-stage, unvented screw is normally used. To ensure adequate mixing of polymer and Endex the Length:Diameter (L/D) ratio of the screw should be at least 16:1. If the screw has a vent it should be sealed; if this is not possible then zone temperatures prior to the vent must be kept below the decomposition temperature of Endex (less then 300°F, 149°C),
which may not be practical on short L/D screws.

Temperature Profile: The temperature profile should be set to achieve a suitable melt temperature for the polymer, however the rear zone/hopper temperature should not be so high as to cause premature decomposition of Endex, and thus loss of gas back through the hopper. A temperature of 360° F (193° C) at the rear zone/hopper is suitable for most polymers.

Take-off Equipment: It is essential to use take-off equipment capable of achieving the desired quality of extrudate. Normal take-off equipment may be used, with extra cooling being advantageous. The distance from the die lip to the take-off equipment is critical to maximum expansion consistent with the desired surface appearance.

Foamed sheets should be passed through chill rolls to polish the surface and set the dimensional stability, and the roll spacing should be adjusted to avoid over-compression of the sheet. Drawdown and vacuum sizing (if used) should be regulated for consistent foam.

Use of Polymer Regrind: The decomposition products of Endex do not affect the use of regrind, since there is no residual activity in the regrind.