Advanced Thermoforming, 2nd Edition

Chapter 42Understanding the Brittle Behavior of Polystyrene Cups

Written in cooperation with Rudi Salmang, Terneuzen, The Netherlands

A dairy packaging producer reported a brittleness in 500‐mL cups under fast squeezing rates, with either supplier A or supplier B polystyrene materials. PS‐HI (A) from supplier A was approved and implemented for the production of the cups for a period of 3 months. During the first 3 weeks of use of PS‐HI (A), good cups were produced; thereafter the cups produced were brittle. However, the same lots of PS‐HI (A) at other producers were not brittle.

The polystyrene cups were made from extruded films and thermoformed offline. The extrusion process consisted of the continuous melting, mixing, and pressing of plastic pellets into a homogeneous film of the desired thickness.

Thermoforming consisted of heating the extruded film until it was stretched into a cavity by means of a plug. Then the hot film was forced against the mold’s wall by use of a vacuum or air pressure. After cooling, the film retained the mold’s shape and detail. Determining an optimum processing window in thermoforming is critical to achieving high‐quality cups, while inconsistent thermoforming usually gives extremely unbalanced orientation and, hence, defective cups.

Orientation is defined as a time‐dependant recoverable elastic deformation, and it is the result of stress buildup and stress relaxation. The orientation can be affected by material properties as well as the processing ...