Compressive Stress vs. Orientation Stress

Yesterday, I received this question from a frequent blog contributor…

What is the difference between molded-in compressive stresses and molded-in orientation stresses?

My Response
I will first define the two concepts in practical terms, and then address the differences between the two…
Molded-In Compressive Stress – As the polymer fills the mold and cools, the polymer begins to shrink. As the polymer shrinks, additional polymer is then forced, or packed, into the mold to compensate for this shrinkage. As additional polymer is packed into the mold, internal pressure can build up causing general compression. Some molders will refer to a part with too much molded-in compressive stress as over-packed. These stresses most-often occur when the mold temperature, melt temperature, or packing pressure is too high allowing additional packing to occur. 
Molded-In Orientation Stress – As the polymer fills the mold, the long polymer chains tend to orient in the direction of flow. Basically… more of the polymer chains are aligned in the direction of flow than are aligned perpendicular to the direction of flow. Immediately after the polymer fills the mold, the polymer chains try to orient themselves randomly (their preferred state). Since the polymer chains themselves tend to shrink more along the length of the chain, than perpendicular, the differential shrinkages tend to cause stress within the part. Additionally, the polymer chains are not in the preferred random orientation, so there will always be some internal stress due to orientation. Even though these stresses will always exist, they are made worse by factors such as low mold temperatures and high injection speeds.
Basically, compression stress is created during packing while orientation stress is created during mold filing. As a result, you can have a part with both molded-in compressive stress as well as molded-in orientation stress. Although both can cause a bad part, a good molding process is typically a balance of minimizing these affects and maximizing productivity.
Additional Thoughts
There are many applications where a molded-in compressive or orientation stress is beneficial to the products performance. For example, hinges and cable ties get their strength and longevity from a high degree of molded-in orientation. Likewise, many molders improve the useful life and performance of shock absorbing components when they have internal compressive stresses.
In some engineering applications, the molder will heat treat, or anneal, the molded parts to allow the polymer chains to align themselves in a more random state to reduce the internal stresses.

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