The Impact of Melt Temperatures on Open Cell Percentage

by Kurt Bartosch, Engineering Manager

Proper melt temperature is critical in any plastic extrusion process, and polystyrene (PS) foam extrusion is no exception. This is a very simple concept, in that as the polymer melt temperature rises, the open cell percentage of the foam also increases. This is graphed in the figure below.

PS foam open cell and melt temperature graph

The importance of this graph is not the temperature, since the graph will shift left or right depending on which blowing agent is being used. Also, the graph must be derived for each specific plants process. The important part of the graph is the shape. Notice how steep the slope is over such a small temperature change. Over a four degree (Celsius) temperature change, the open cell percentage goes from less than 10% to over 70%. How does this increase in open cell percentage affect PS foam sheet quality?

FOAM SHEET WITH 5% OPEN CELL

TRAY BOTTOM WITH 70% OPEN CELL

Open Cell foam under microscope

As you can see, the difference in open cell percentage significantly alters the structure of the foam sheet. Both cell size and cell wall thickness uniformity have been compromised in the 70% open cell sample. This will result in a significant reduction in tray strength.

How does this come to play in real life situations? Polystyrene foam extruders that are running too high of a melt temperature usually produce sheet at a higher basis weight to compensate for the strength reduction caused by the open cell, thereby increasing the material cost per tray. Most PS foam lines are easier to operate with higher melt temperatures, so there is a natural tendency for the process to drift towards these higher melt temperatures. If you are not checking the open cell count you may not realize that you have a problem.

Contact us to learn more about the how to measure open cell percentage as part of your quality process. With Commodore's help it is easy to implement, and producing stronger, lighter trays will increase your profitability and reduce customer complaints. Most importantly, your customers will also receive the benefits.