Introduction
Extruder Parameters
Barrels – Grooved/Smooth
Extruder Screws
Melt Filters and Melt Pumps
Dies
Coextrusion

The basics of foam extrusion consist of mixing a chemical foaming agent (CFA) with the polymer
to be extruded.

The heat generated to melt the polymer decomposes the chemical foaming agent resulting
in gas being liberated.

This gas is dispensed in the polymer melt and expands upon exiting the die.

Basically, all common extruders can be used for foaming if the following requirements are met:

1. The melt temperature must be high enough to guarantee a total decomposition of the foaming agent
2. The pressure of the melt must be kept high enough to keep the gas -- generated by the decomposition of the foaming agent -- dissolved in the polymer melt until the melt exits the extrusion die.

If the melt temperature is too low, the decomposition of the foaming agent will be incomplete, resulting in an uneconomical process. And, un-decomposed foaming agent particles can lead to agglomerates, which can clog the melt filter or cause voids, poor cell structure, or poor surface appearance.

A pressure profile that is not high enough can lead to “pre-foaming”. Even with a subsequent pressure increase, the gas cannot be “re-dissolved”, resulting in a large irregular cell structure with broken and collapsed cells.

The coarse foam produced this way leads to holes in flat films, while profiles and sheets get a rough surface (shark skin); during blown film production, the blown film tubing can collapse.

The use of “degassing” or vented extruders (used for PSand PMMA) is possible, but the
following is recommended:

The vent opening should be plugged or sealed to prevent escaping of the gas generated
by the foaming agent.

Use a three-zone screw or a degassing screw, with a not too deep cut screw shaft, and respectively
not too high flights in the degassing zone. 

In the de-volatilizing zone, a higher free volume is available. This can lead to slight pressure decrease, potentially leading to pre-foaming.  For thicker screw shafts, this pressure decrease can be compensated by a higher screw speed or by pressure increase in the die.

Foamazol™ chemical foaming agent, the foaming agent system developed by Bergen International for extrusion, is formulated in a way that, when used properly, ensures complete decomposition and a very consistent and fine foam structure, within a well-defined temperature range.

As the foaming agent masterbatch melts in an early stage and reaches the decomposition temperature earlier, the resulting gas will dissolve the melt, due to the high pressures present at the beginning, and can be very well distributed in a short period of time.

Foaming agent masterbatches with a low melting point can melt too early at the barrel wall, and the resulting gas can partially or completely escape through the hopper. 

In this case the temperature of the feeding zone should be adjusted to a lower temperature, to prevent the premature decomposition of the foaming agent.

Screw Geometry
Basically, all common screws can be used for foam extrusion, as long as there is no large pressure decrease in the single zones of the screw, which leads to unwanted pre-foaming in the melt. Established screws for processing are three-zone screws (Feeding-Compression-Metering/Mixing).  Good results have also been achieved with degassing screws (PS), as long as you keep in mind the conditions mentioned at the beginning of this report.

Barrier Screws
Some new problems developed with the introduction of barrier screws.  One of them is an increased tendency to pre-foam, caused by a high pressure decrease between the barrier flight and the driving pitch.  When using short barrier segments, the pressure decrease can be compensated by higher screw speeds.

The use of fine mesh screen and screen changers are generally not necessary in foam extrusion.  Due to foam structure, impurities, gels or additive agglomerates are usually not visible and do not affect the foam product. 

If melt filters are used it is important to use screens that are rather coarse.  This can help prevent pressure drop after the screens, which can possibly result in pre-foaming.

The use of melt pumps can improve the extrusion process in the following ways:

•  the mass flow is stabilized

•  the pressure in the barrel can be increased

•  less work-intensive material must be transported in the barrel

For foam extrusion it is very important to place a pressure gauge where the melt enters the melt pump.  The pressure reading of this gauge is used to control the screw speed, insuring a uniform melt output at the end of the extruder.

If the pressure setting is too low, pre-foaming may occur.  In this case, it is necessary to increase the pressure setting.

Experiments have shown that the following minimum pressures can be used as a guideline to yield acceptable foam extruded products:

Slot Dies
Slot dies are used for the production of thin films (with a chill-roll calendar stack), or thicker films such as sheets (with a calendar).  When using slot dies with a restrictor bar (for a better dispersion of the melt), the restrictor bar should not be closed too much.  If the restrictor bar is closed too much, a pressure drop can occur right after the restrictor bar, which can result in pre-foaming. 

Ideally, you can detect a slight area in the melt just exiting the die that is free of foam bubbles.  Just after this, the expansion of the gas can be seen.  This is called a “halo”.  When a “halo” can be seen, you can be sure that there is no foaming or expansion occurring in the die or equipment.

During the cooling process of the foamed melt, the following problems can commonly be encountered:

•  Foam formation time is not sufficient –  the distance between the die and the cooling
  roll may be too short.
  
•  The foam structure is crushed – the distance between the rolls of the chill-roll system
  may be too short.
   
•  When producing a thinner film, the foam bubbles become highly stretched and as a result foam film density may increase – the haul-off speed can be too high.  However, for some processes this stretch or orientation is wanted, e.g. decorative ribbons or packaging tapes.

When adjusting the distance of the opening in the slot die, the weight reduction has to be considered.  For 30% weight reduction, the opening should be reduced by roughly 30% compared to a compact film of the same thickness (to compensate for the expansion of the foam).

Profile Die, Tubular Nozzle

Compared with the cross section of a slot die, the cross sections of profile dies generally cannot be changed or adjusted.  When designing and machining the die, the desired foam product needs to be considered. 

Generally, it is best to design dies with short land lengths to maintain high melt pressure right up to the die lip.  This is also valid for the production of foamed blown film.  It must also be noted, that the pressure characteristics of a given die can be affected by many factors, such as resin, resin viscosity, temperature, desired density reduction, output rate, and actual product cross section.

Many coextruded foam products are produced with a foamed inner layer, and solid, unfoamed outer layers.  In this case, it is very important, to select the right material.  For the outer layer, a “softer” material is recommended, while a material that is somewhat harder is recommended for the inner, foamed layer.  This type of structure is suggested due to the fact, that a foamable melt has better flow characteristics (lower apparent viscosity) compared to a solid melt of the same resin.  If the layers differ in viscosity, it can result in poor or destroyed foam structure.

Tests have shown that the solid layers can utilize resin with a 2 – 3 times higher melt flow value than the foam layer to give good foam structure.