Meaning
The permanent, non-reversible change in the shape of a polymer specimen occurs when the applied stress exceeds the yield point of the material. This mechanical behaviour, known as plastic deformation, involves the sliding of polymer chains past one another, preventing them from returning to their original positions. It represents the transition from elastic, recoverable strain to permanent, permanent structural alteration.
This property is essential in forming processes like blow moulding and stretch film production, where the material must maintain its new shape.
Structural Mechanism
At the molecular level, plastic deformation occurs as the folded chains within crystalline lamellae untangle and align. Under tensile stress, the amorphous regions stretch first before the crystalline regions deform. This alignment increases the tensile strength of the polymer in the direction of the draw.
If the strain continues past this point, localized necking occurs, leading to ultimate failure of the part.
Part Performance
Designing load-bearing moulded components requires keeping the operating stresses well below the threshold of plastic deformation. If a snap-fit joint experiences stress beyond this limit, it will lose its retention force and fail to function. Engineers use finite element analysis to identify areas of stress concentration where permanent deformation might occur.
Modifying the wall thickness and adding reinforcing ribs can prevent this type of failure.
Processing Application
Warm drawing and blow moulding processes exploit this behavior. Plastics are heated close to their melting point. This lowers the yield stress.
Permanent deformation is achieved with less force.
