Orientation - PowerPoint PPT Presentation

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Orientation
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Orientation

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  1. Orientation

  2. Orientation • Almost all injection molded parts have some degree of frozen-in molecular orientation • The degree is determined by the molecular weight, relaxation characteristics, and processing conditions • Orientation greatly affects the properties of the part • Shrinkage • Strength • Residual stresses

  3. Orientation • Mold filling related orientation can be affected through process variables that affect mold filling pressure requirements • Flow direction and speed • Channel dimensions • Temperatures • Residual Orientation = Orientation due to flow - relaxation

  4. How Molecular Orientation Occurs • Molecular orientation develops during mold filling as the plastic is injected through the nozzles, runner, gate and cavity • The polymer chains become stretched out due to velocity gradients • The orientation tends to be in the direction of flow

  5. How Molecular Orientation Occurs • The blunted shape of most polymer melt velocity profile causes most of the orientation to occur toward the surface. • The molecules at the core remain random • Extreme in injection molding where the melt adjacent to the cold mold will freeze first, leading to high interfacial shear stresses and not allowing for relaxation • Problems are most significant for higher molecular weight plastics and fiber reinforced plastics

  6. How Molecular Orientation Occurs

  7. Effects of Molecular Orientation • Orientation creates different directional properties • Stronger is the flow direction • Weaker in the transverse direction

  8. Effects of Molecular Orientation • Typical directional property of an injected molded part

  9. Orientation • The degree of orientation caused by mold filling is influenced by processing conditions, material properties, mold design and part design • Large diameter runners, sprues, gates along with shorter flow lengths will reduce orientation • Faster fill rates and higher melt temperatures tend to promote molecular relaxation