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Torsten’s Training Tip: Deformation Behavior and Reversing PVT Curves

by Torsten Kruse

Plastic part designers have an abundance of polymer materials to choose from, ranging from commodity to technical or engineered polymers. Selecting the optimal material for each design is key to a successful molding project. Polymer materials must be heated to their melt temperature ranges to be injection molded. Polymer strength, stiffness, and deformation behavior are all dependent on temperature, chemical and molecular structure, molecular chain branching, and the secondary or inter-molecular bonding forces.

Deformation behavior is different for amorphous and semi-crystalline materials. It is also affected by additives in polymers such as colorants, heat stabilizers, UV protection, fillers, reinforcements, and others.  Data for polymer material deformation behavior is generally available from material suppliers but there is another way to predict this behavior using PVT curves.

The upper line represents what happens to the material if it under zero pressure.

By reversing the line, the graph now indicates that at a lower temperature the density is higher and at a higher temperature the density is lower. Lower density means the molecules have moved further away from each other.

The exercise can be repeated a PE semi-crystalline material with a freeze temperature of about 120 degrees Celsius as indicated by the red line (again, this is not a fixed temperature but fluctuates with pressure.)

The reversed PVT curves for the amorphous and semi-crystalline clearly indicate the significant differences in temperature deformation behavior between these two materials.

Selecting the optimal polymer material for each design is key to a successful molding project. Polymer deformation behavior is an important aspect in material selection. When data is not available from a material supplier, PVT curves are a good way to determine polymer temperature deformation behavior because they are closely linked.

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